i 
 
PRODUCTION OF ORGANIC FERTILIZER FROM BLACK SOLDIER FLY 
FRASS FOR IMPROVED SOIL HEALTH AND MAIZE PRODUCTIVITY IN 
NAIROBI CITY COUNTY, KENYA 
 
 
 
 
BEESIGAMUKAMA DENNIS (MSc)  
A99EA/25000/2018 
 
 
 
 
 
 
A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE 
REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN 
SOIL SCIENCE OF KENYATTA UNIVERSITY 
 
 
 
 
 
JUNE 2021 
 
 
ii 
 
DECLARATION 
I Beesigamukama Dennis declare that this thesis is my original work and has not been 
presented for the award of a degree in any other university or any other award. 
Signature………………………………        Date………………………… 
 
SUPERVISORS’ APPROVAL 
We confirm that the work reported in this thesis was carried out by the candidate under 
our supervision and has been submitted with our approval as university supervisors. 
Prof. Benson Mochoge  
Department of Agricultural Science and Technology 
School of Agriculture and Enterprise Development 
Kenyatta University, P.O. Box 43844 - 00100 Nairobi, Kenya. 
 
Signature………………………………         Date…………………………… 
       
Dr. Nicholas K. Korir  
Department of Agricultural Science and Technology 
School of Agriculture and Enterprise Development 
Kenyatta University, P.O. Box 43844 - 00100 Nairobi, Kenya. 
 
Signature……………………………          Date……………………………. 
Dr. Chrysantus M. Tanga  
Plant Health Theme 
International Centre of Insect Physiology and Ecology (icipe) 
P. O. Box 30772 - 00100 Nairobi, Kenya. 
Signature……………………………           Date…………………………...   
iii 
 
DEDICATION 
To my beloved wife (Mrs. Marion Kabarungi), daughter (Faith Asiimwe), and the entire 
family for their love, sacrifice, encouragement, and perseverance during my academic 
career. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
iv 
 
ACKNOWLEDGEMENTS 
First, I thank the Almighty God for giving me good health, wisdom, and 
protection during the pursuit of my PhD degree. It was an honour to be supervised by a 
strong team led by Prof. Benson Mochoge and Dr. Nicholas K. Korir from Kenyatta 
University (KU), and Dr. Chrysantus M. Tanga, Dr. Martha W. Musyoka and Dr. Komi 
K.M. Fiaboe from the International Centre of Insect Physiology and Ecology (icipe). 
Your intelligent supervision has left a positive imprint on me.  
I am grateful to icipe technicians (Mrs. Faith N. Wamurango, Mr. Shem 
Ondiaka, Mr. Erick Rachami and Mr. Joshua Wambua) for maintaining the insect 
colony and offering support during data collection. Mr. Kennedy Kilel, Mr. Mathew 
Theuri, Mrs. Rachael Kimani, Ms Kallen Gacheri and Mrs. Lucy Wangu (Agricultural 
Science and Technology Laboratories, KU) provided technical assistance during 
laboratory analysis. Mr. Evans M. Wanzetse and Mrs. Miriam Mbaya from Animal 
science lab granted me access to the oven and refrigerator. Mr. Benjamin Gichohi 
(Environmental Science laboratory, KU), Mrs. Jane Karambu and Mr. John Gachoya 
(Food nutrition and dietetics laboratory) and Mr. Stanley Kariuki (Chemistry lab of KU) 
provided some laboratory reagents and equipment. Prof.  Jayne N. Mugwe provided 
some field tools and encouragement throughout my PhD journey. The help offered by 
the KU farm manager (Mr. Ezekiel Chepkwony), farm supervisor (Mr. Anderson N. 
Maina), and Mr. Barunaba M. Mmbaya and Ms. Eunice S. Macharia during field 
experiments is highly appreciated.  
 
v 
 
In a special way, I express gratitude to my dear wife and daughter, mother, 
brother, and sisters for their support during the pursuit of my PhD. Gratitude to my 
fellow postgraduate students at icipe and KU, and other friends who might have 
rendered any help during the entire period of study. My fellow PhD students, especially 
the ARPPIS-DAAD class of 2017 kept me going in times of despair. I acknowledge Dr 
Mohamed A. Mkiga for his assistance during data analysis.  
I would like to appreciate the entire icipe community especially the Capacity 
Building and Institutional Development Programme led by Dr. Robert A. Skilton, M/s 
Vivian A. Atieno and Mrs. Margaret A. Ochanda for coordinating my scholarship 
funding, immigration status, and offering a conducive environment during experiments 
and thesis preparation. In the same way, I am grateful to Kenyatta University through 
the School of Agriculture and Enterprise Development for providing an enabling 
environment during my entire PhD journey. 
Lastly, I acknowledge the German Academic Exchange Service (DAAD) In-
Region Postgraduate Scholarship for paying my tuition fees, stipend, and part of 
research funds. I thank the International Development Research Centre (IDRC), 
Australian Centre for International Agricultural Research (ACIAR) (INSFEED-phase 2: 
Cultivate Grant No: 108866-001), the Netherlands Organization for Scientific Research, 
WOTRO Science for Global Development (NWO-WOTRO) (ILIPA–W 08.250.202), 
and The Rockefeller Foundation (SiPFeed—2018 FOD 009) through icipe for providing 
research funds. 
 
 
vi 
 
TABLE OF CONTENTS 
DECLARATION ............................................................................................................................ ii 
DEDICATION ............................................................................................................................... iii 
ACKNOWLEDGEMENTS ........................................................................................................... iv 
TABLE OF CONTENTS ............................................................................................................... vi 
LIST OF TABLES ......................................................................................................................... xi 
LIST OF FIGURES ..................................................................................................................... xiii 
ABSTRACT ................................................................................................................................. xvi 
CHAPTER ONE: INTRODUCTION ............................................................................................. 1 
1.1 Background to the study ........................................................................................................ 1 
1.2  Statement of the problem ..................................................................................................... 4 
1.3 Justification ........................................................................................................................... 6 
1.5 General objective ................................................................................................................... 9 
1.5.1 Specific objectives ........................................................................................................ 10 
1.6 Hypotheses .......................................................................................................................... 10 
1.7 Conceptual framework ........................................................................................................ 11 
CHAPTER TWO: LITERATURE REVIEW ............................................................................... 13 
2.1 The black soldier fly and its role in agriculture ................................................................... 13 
2.2 Organic wastes as a resource in agriculture ........................................................................ 14 
2.3 Composting ......................................................................................................................... 16 
2.4 Nutrient changes during composting ................................................................................... 17 
2.5 Effect of compost C/N ratio and pH on nitrogen transformation during composting ......... 18 
2.6 Strategies for conserving nitrogen during composting ........................................................ 20 
2.6.1 Regulation of pH ........................................................................................................... 21 
2.6.2 Ammonium precipitation .............................................................................................. 22 
2.6.3 Ammonium adsorption and absorption......................................................................... 22 
2.6.4 Adjustment of substrate C/N ratio ................................................................................ 24 
2.7 Effect of organic amendments on soil nitrogen mineralization .......................................... 24 
2.8 Nitrogen fertilizer equivalence of organic fertilizers .......................................................... 26 
2.9 Economic value of BSF farming and frass fertilizer ........................................................... 27 
CHAPTER THREE: MATERIALS AND METHoDS ................................................................ 28 
vii 
 
3.1 Description of experimental sites ........................................................................................ 28 
3.2 Black soldier fly colony establishment and maintenance.................................................... 33 
3.3 Experiments carried out ....................................................................................................... 34 
3.3.1 Determining the critical C/N ratio for BSF larvae growth and nutrient 
conservation in frass fertilizer................................................................................................ 34 
3.3.1.1 Sources of experimental substrates ............................................................................ 34 
3.3.1.2 Black soldier fly larval growth and yield on various substrates ................................ 36 
3.3.1.3 Quality of frass and frass compost yield .................................................................... 38 
3.3.1.4 Laboratory analysis methods ..................................................................................... 39 
3.3.1.5 Phytotoxicity test on mature frass compost extracts .................................................. 43 
3.3.2  Evaluating the effectiveness of biochar and gypsum amendments on nitrogen 
retention in BSF frass fertilizer .............................................................................................. 44 
3.3.2.1 Experimental diets ..................................................................................................... 44 
3.3.2.2 Black soldier fly larval growth and yield on various substrates ................................ 45 
3.3.3 Determining the comparative performance of BSF frass fertilizer and 
commercial fertilizers on crop production ............................................................................. 46 
3.3.3.1 Source of fertilizers .................................................................................................... 46 
3.3.3.2 Treatments and experimental set up .......................................................................... 47 
3.3.3.3 Maize growth, yield, and mineral nitrogen ................................................................ 51 
3.3.3.4 Nitrogen use efficiencies ........................................................................................... 53 
3.3.4  Determination of soil moisture storage, nitrogen fertilizer equivalence and 
synchrony of N mineralization from BSF frass and commercial organic fertilizers ............. 54 
3.3.4.1 Nitrogen fertilizer equivalence .................................................................................. 54 
3.3.4.2 Synchrony of nitrogen mineralization for maize production ..................................... 56 
3.3.4.3 Soil sampling and incubation ..................................................................................... 56 
3.3.4.4 Mineral nitrogen release ............................................................................................ 58 
3.3.4.5  Synchrony of nitrogen mineralization and nitrogen uptake...................................... 59 
3.3.5  Economic value of frass fertilizer production alongside BSF rearing and 
profitability of crop production using BSF frass fertilizer .................................................... 59 
3.3.5.1 Economic  value of frass fertilizer as a second product from BSF farming .............. 59 
3.3.5.2 Economic evaluation of maize production using BSF frass fertilizer ....................... 62 
3.3.5.2.1  Economically optimum fertilizer rates for maize production ................................ 63 
viii 
 
3.4 Data analysis ........................................................................................................................ 65 
CHAPTER FOUR: RESULTS AND DISCUSSION ................................................................... 67 
4.1 Introduction ......................................................................................................................... 67 
4.2 Critical carbon to nitrogen (C/N) ratio for optimal BSF larval growth and nutrient 
conservation in frass fertilizer ................................................................................................... 67 
4.2.1 Black soldier fly larvae yield ........................................................................................ 67 
4.2.2 Waste reduction and biomass conversion rate .............................................................. 69 
4.2.3 Frass compost yield ...................................................................................................... 71 
4.2.4 Nutrients flow in BSF larvae and frass compost .......................................................... 72 
4.2.5 Temperature, pH, and electrical conductivity during frass composting ....................... 74 
4.2.6 Ammonium and nitrate concentrations, and ammonium nitrate ratio .......................... 78 
4.2.7 Phytotoxicity of frass composts .................................................................................... 82 
4.2.8 Total organic carbon and total nitrogen ........................................................................ 83 
4.2.9 Total phosphorus and potassium .................................................................................. 87 
4.2.10 Ratios of carbon to nitrogen during experiment ......................................................... 90 
4.3 Effect of biochar and gypsum amendments on BSF larval performance and 
nitrogen retention in frass fertilizer ........................................................................................... 91 
4.3.1 Black soldier fly larval yields and biomass conversion rates ....................................... 91 
4.3.2 Waste degradation and frass compost yield .................................................................. 96 
4.3.3 Effect of biochar and gypsum on nitrogen retention and use by BSF larvae ............... 98 
4.3.4 Effect of biochar and gypsum on temperature, pH, and electrical conductivity 
during frass composting ....................................................................................................... 100 
4.3.5 Ammonium and nitrate concentrations during frass composting ............................... 105 
4.3.6 Effect of biochar and gypsum on total organic carbon and total nitrogen 
during frass composting ....................................................................................................... 109 
4.3.7 Total phosphorus and potassium during frass composting ......................................... 115 
4.3.8 Effect of biochar and gypsum on ratios of carbon to nitrogen during frass 
composting ........................................................................................................................... 118 
4.3.9 Phytotoxicity of mature frass compost extracts .......................................................... 121 
4.4 Comparative performance of BSF frass fertilizer and commercial fertilizers on the 
growth, yield, and nitrogen use efficiency of maize ............................................................... 123 
4.4.1 Plant height ................................................................................................................. 123 
ix 
 
4.4.2 Chlorophyll concentration .......................................................................................... 127 
4.4.3 Soil mineral nitrogen content ...................................................................................... 131 
4.4.3.1 Sole organic fertilizer treatments ............................................................................. 131 
4.4.3.2 Combined fertilizer treatments ................................................................................ 133 
4.4.4 Nitrogen uptake by maize ........................................................................................... 136 
4.4.4.1 Sole organic fertilizer treatments ............................................................................. 136 
4.4.4.2 Combined fertilizer treatments ................................................................................ 139 
4.4.5 Phosphorus and potassium uptake at harvesting......................................................... 142 
4.4.5.1 Sole organic fertilizer treatments ............................................................................. 142 
4.4.5.2 Combined fertilizer treatments ................................................................................ 145 
4.4.6 Nitrogen use efficiency ............................................................................................... 146 
4.4.6.1 Agronomic nitrogen efficiency (AEN) ..................................................................... 146 
4.4.6.2 Apparent nitrogen recovery (ANRN) ....................................................................... 146 
4.4.6.3 Agro-physiological nitrogen efficiency (APEN) ...................................................... 149 
4.4.6.4 Nitrogen harvest index (NHI) .................................................................................. 150 
4.4.7 Maize grain and stover yields ..................................................................................... 151 
4.4.7.1 Sole organic fertilizer treatments ............................................................................. 151 
4.4.7.2 Combined fertilizer treatments ................................................................................ 155 
4.5 Soil moisture storage, nitrogen fertilizer equivalence value and synchrony of N 
mineralization from BSF frass and commercial organic fertilizer for maize production........ 158 
4.5.1 Effect of BSF frass and SAFI organic fertilizers on soil moisture storage ................. 158 
4.5.2 Effects of BSF frass and SAFI fertilizers on N mineralization .................................. 160 
4.5.3 Effects of BSF frass and SAFI fertilizers on mineral N distribution in soil ............... 165 
4.5.4 Effects of BSF frass and SAFI fertilizer on maize N uptake ...................................... 168 
4.5.5 Effect of BSF frass and SAFI fertilizers on degree of N synchrony .......................... 170 
4.5.6 Nitrogen fertilizer equivalence values of BSF frass and SAFI fertilizers .................. 173 
4.6 Economic value of frass fertilizer production alongside BSF rearing and 
profitability of crop production using BSF frass fertilizer ...................................................... 176 
4.6.1 Larval and frass fertilizer yields, gross and net income from BSF farming ............... 176 
4.6.2 Gross margin, benefit-cost ratio and return on investment from BSF farming .......... 181 
4.6.3 Economic value of BSF frass fertilizer on maize production ..................................... 185 
x 
 
4.6.4 Economically optimum frass fertilizer rates and stochastic dominance analysis ....... 191 
CHAPTER FIVE: SUMMARY, CONCLUSIONS AND RECOMMENDATIONS ................ 195 
5.1 Summary of the key findings ............................................................................................ 195 
5.2 Conclusions ....................................................................................................................... 197 
5.3 Recommendations ............................................................................................................. 198 
5.4 Areas for future research ................................................................................................... 199 
REFERENCES ............................................................................................................................ 201 
APPENDICES ............................................................................................................................ 219 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
xi 
 
LIST OF TABLES 
Table 1: Selected physical and chemical characteristics of the soil at the experimental 
site. ................................................................................................................................................ 32 
Table 2: Selected characteristics of substrates used in black soldier fly rearing. ......................... 35 
Table 3: Percentage weights of sawdust and brewery spent grain waste mixed to produce 
feedstocks of varying carbon to nitrogen ratios. ........................................................................... 36 
Table 4: Selected characteristics of substrate and additives used in the experiment. ................... 44 
Table 5: Chemical characteristics of the organic fertilizers used during experiments. ................ 48 
Table 6: Treatments considered during the study. ........................................................................ 49 
Table 7: Actual amounts of macro nutrients supplied by organic fertilizer treatments. ............... 51 
Table 8: Parameters used in calculating economic returns from frass fertilizer. .......................... 61 
Table 9: Parameters used to calculate economic returns for different fertilizer inputs. ............... 62 
Table 10: Effect of C/N ratio adjustment on nutrient accumulation by BSF larvae and 
nutrient retention in frass fertilizer ................................................................................................ 73 
Table 11: Effect of substrate carbon to nitrogen (C/N) ratio adjustment on seed 
germination and seed germination indices of mature BSF frass composts. .................................. 82 
Table 12: Ratios of organic carbon and water-soluble carbon to total nitrogen at selected 
periods during the experiment. ...................................................................................................... 91 
Table 13: Waste degradation, biomass conversion rates and frass compost yield by BSF 
larvae reared on biochar and gypsum amended substrates. .......................................................... 95 
Table 14: Effect of biochar and gypsum amendments on nitrogen retention and uptake 
by BSF larvae during experiment set 1 and 2. .............................................................................. 99 
Table 15: Concentrations of nutrients in mature BSF frass composts during experiment 
set 1 and 2. .................................................................................................................................. 114 
Table 16: Ratios of carbon (C/N) and water-soluble carbon (WSC/N) to total nitrogen of 
amended BSF frass composts. .................................................................................................... 120 
Table 17: Seed germination and germination indices of mature BSF frass composts 
amended with biochar and gypsum. ............................................................................................ 122 
xii 
 
Table 18: Mineral nitrogen distribution (kg N ha
-1
) in the top 40 cm of soil treated with 
sole BSF frass and SAFI fertilizers. ............................................................................................ 132 
Table 19: Mineral nitrogen distribution (kg N ha
-1
) in the top 40 cm of soil treated with 
combined fertilizers ..................................................................................................................... 134 
Table 20: Effects of BSF frass fertilizer, commercial organic and mineral (urea) 
fertilizers on phosphorus, and potassium uptake at maturity. ..................................................... 143 
Table 21: Effects of BSF frass fertilizer, commercial organic and mineral (urea) 
fertilizers on nitrogen use efficiency of maize. ........................................................................... 148 
Table 22: Effects of BSF frass fertilizer, commercial organic and mineral (urea) 
fertilizers on maize stover yield. ................................................................................................. 154 
Table 23: Nitrogen fertilizer equivalence values of BSF frass and SAFI fertilizers. ................. 173 
Table 24: Economic returns from BSF production using rearing substrates amended with 
sawdust. ....................................................................................................................................... 178 
Table 25: Economic returns from BSF production using rearing substrates amended with 
biochar and gypsum. ................................................................................................................... 179 
Table 26: Economic returns from maize production if BSF frass fertilizer is bought at 
same price as the commercial organic fertilizer (SAFI) and applied in terms of nitrogen 
rates. ............................................................................................................................................ 188 
Table 27: Economic returns from maize production if BSF frass fertilizer is bought at 
same price as the commercial organic fertilizer (SAFI) and applied in terms of tonnage. ......... 189 
Table 28: Economic returns from maize production if BSF frass fertilizer is made from 
the farm. ...................................................................................................................................... 190 
Table 29: Two sample Kolmogorov-Smirnov statistical test for first-order stochastic 
dominance. .................................................................................................................................. 194 
 
 
 
 
 
xiii 
 
LIST OF FIGURES 
 
Figure 1: Conceptual framework. ................................................................................................ 12 
Figure 2: Cumulative daily rainfall (a), average daily temperature (b), cumulative monthly 
rainfall, and average monthly temperature (c) during field experiments at Kenyatta University 
Teaching and Demonstration farm. .............................................................................................. 30 
Figure 3: Maize grain yield response curves used to determine fertilizer equivalences of BSF 
frass and SAFI organic fertilizers during the short (a) and long rain (b) season experiments. 
Note: Grain yields for BSF frass fertilizer. .................................................................................. 55 
Figure 4: Yield response curves used to calculate economically optimum rates of BSF frass (a 
and d), urea (c) and SAFI (b and e) fertilizers for maize production. .......................................... 64 
Figure 5a and b: Effect of substrate C/N ratio on black soldier fly larval yields. ........................ 68 
Figure 6: Effect of substrate C/N ratio on waste reduction and biomass conversion rates of black 
soldier fly larvae. ......................................................................................................................... 70 
Figure 7: Effect of substrate C/N ratio on frass compost yields .................................................. 71 
Figure 8: Effect of substrate C/N ratio on temperature (a), pH (b) and electrical conductivity 
(EC) (c), during BSF larvae rearing and frass composting phases. ............................................. 75 
Figure 9: Effect of substrate C/N ratio on concentrations of ammonium (a), nitrate (b) and 
ammonium/nitrate ratio (c) during BSF larvae rearing and frass composting phases. ................ 79 
Figure 10: Effect of substrate C/N ratio on concentrations of total organic carbon (a), total 
nitrogen (b) during BSF larvae rearing and frass composting phases.......................................... 85 
Figure 11: Effect of substrate C/N ratio on concentrations of total phosphorus (a) and potassium 
(b) during BSF larvae rearing and frass composting phases. ....................................................... 88 
Figure 12: Effect of biochar and gypsum amendments on fresh (a) and dry (b) black soldier fly 
larval yields during experiments. ................................................................................................. 92 
Figure 13: Effect of biochar and gypsum amendments on temperature during experiment set 1 
(a) and set (2) of BSF frass composting. .................................................................................... 101 
Figure 14: Effect of biochar and gypsum amendments on pH of BSF frass compost during 
experiment set 1 (a) and 2(b). .................................................................................................... 103 
Figure 15: Effect of biochar and gypsum amendments on electrical conductivity (EC) of BSF 
frass compost during experiment set 1 (a) and 2(b). .................................................................. 104 
Figure 16: Effects of biochar and gypsum amendments on concentrations of ammonium during 
experiment set 1 (a) and 2(b) of frass composting. .................................................................... 106 
xiv 
 
Figure 17: Effects of biochar and gypsum amendments on concentrations of nitrates during 
experiment set 1 (a) and 2(b) of frass composting. .................................................................... 107 
Figure 18: Effect of biochar and gypsum amendments on total organic carbon during experiment 
set 1(a) and 2 (b) of BSF frass composting. ............................................................................... 110 
Figure 19: Effect of biochar and gypsum amendments on total nitrogen during experiment set 
1(a) and 2 (b) of BSF frass composting. .................................................................................... 111 
Figure 20: Effect of biochar and gypsum amendments on total phosphorus of BSF frass compost 
during experiment set 1(a) and 2(b). .......................................................................................... 115 
Figure 21:Effect of biochar and gypsum amendments on total potassium of BSF frass compost 
during experiment set 1(a) and 2(b). .......................................................................................... 117 
Figure 22: Effects of BSF frass fertilizer (a and b), commercial organic (SAFI) (a and b) and 
mineral (urea) (c) fertilizers on maize plant height during the short rain season experiments. . 124 
Figure 23: Effects of BSF frass fertilizer (a and b), commercial organic (SAFI) (a and b) and 
mineral (urea) (c) fertilizers on maize plant height during the long rain season experiments. .. 125 
Figure 24: Effect of BSF frass fertilizer (a and b), commercial organic (SAFI) (a and b) and 
mineral fertilizer (urea) (c) on maize leaf chlorophyll concentration during the short rain season 
experiments. ............................................................................................................................... 128 
Figure 25: Effect of BSF frass fertilizer (a and b), commercial organic (SAFI) (a and b) and 
mineral (urea) (c) fertilizers on maize leaf chlorophyll concentration during the long rain season 
experiments. ............................................................................................................................... 129 
Figure 26: Effects of BSF frass fertilizer (a and b), commercial organic (SAFI) (a and b) and 
mineral (urea) (c) fertilizers on maize nitrogen uptake during the short season experiments. .. 137 
Figure 27: Effects of BSF frass fertilizer (a and b), commercial organic (SAFI) (a and b) and 
mineral (urea) (c) fertilizers on maize nitrogen uptake during the long rain season experiments.
 ................................................................................................................................................... 138 
Figure 28: Effects of sole BSF frass and SAFI organic fertilizers on maize grain yields during 
the short (a) and long rain (b) season experiments. ................................................................... 153 
Figure 29: Effects of BSF frass, SAFI organic and urea fertilizers on maize grain yields during 
the short (a) and long rain (b) season experiments. ................................................................... 156 
Figure 30: Soil moisture content in the topsoil (0 – 20 cm) depth of soil amended with BSF frass 
and SAFI organic fertilizers during the short (a) and long rain (b) cropping seasons. .............. 158 
Figure 31: Soil moisture content in the subsoil (20 – 40 cm) depth of soil amended with BSF 
frass and SAFI organic fertilizers the short (a) and long rain (b) cropping seasons. ................. 160 
xv 
 
Figure 32: Mineral nitrogen released by soil amended with BSF frass and SAFI fertilizers during 
the short (a) and long rain (b) cropping seasons. ....................................................................... 162 
Figure 33: Mineral nitrogen release rate by soil amended with BSF frass and SAFI fertilizers 
during the short (a) and long rain (b) cropping seasons. ............................................................ 163 
Figure 34: Soil mineral nitrogen content in the topsoil depth (0 – 20 cm) following amendment 
with BSF frass and SAFI fertilizers during the short (a) and long rain (b) cropping seasons. .. 166 
Figure 35: Soil mineral nitrogen content in the subsoil depth (20 – 40 cm) following amendment 
with BSF frass and SAFI fertilizers during the short (a) and long rain (b) cropping seasons. .. 167 
Figure 36: Nitrogen uptake by maize grown using BSF frass and SAFI fertilizers during the 
short rain (a) and long rain (b) cropping seasons. ...................................................................... 169 
Figure 37: Nitrogen uptake rate by maize grown using BSF frass and SAFI fertilizers during the 
short rain (a) and long rain (b) cropping seasons. ...................................................................... 170 
Figure 38: Nitrogen flux differences between N released by soil amended using BSF frass and 
SAFI fertilizers and N uptake by maize during the short (a) and long rain (b) cropping seasons.
 ................................................................................................................................................... 171 
Figure 39: Gross margins (a), benefit-cost ratio (b) and return on investment (ROI) (c) on two 
BSF farming scenarios using rearing substrates amended with sawdust. .................................. 183 
Figure 40: Gross margins (a), benefit-cost ratio (b) and return on investment (ROI) (c) on two 
BSF farming scenarios using rearing substrates amended with gypsum and biochar. .............. 184 
Figure 41: Stochastic dominance analysis of impact of BSF frass fertilizer on maize production.
 ................................................................................................................................................... 193 
 
 
 
 
 
 
 
 
xvi 
 
ABSTRACT 
Use of organic wastes for Black soldier fly (BSF) farming presents an avenue that could 
contribute to improved crop yield and sustainable soil health. A major waste from BSF 
farming is frass which, however, generates organic fertilizer with low nutrient levels. In 
addition, and the agronomic performance of BSF frass fertilizer (BSFFF) is not known. 
This study aimed at producing high quality and cost-effective organic fertilizer from 
BSF frass for improved soil health and crop productivity. The study evaluated the 
effects of C:N ratio adjustment (C/N ratios of 11, 15, 20, 25 and 30) and substrate 
amendment with biochar (0, 5, 10, 15 and 20%) and gypsum (0, 5, 10 and 15%) on 
nutrient conservation in organic fertilizer. Comparative performance of BSFFF and 
commercial organic (SAFI) and mineral (urea) fertilizers on maize production was 
carried out under field conditions. The first experiment involved application of BSFFF 
and SAFI at sole rates of 0, 2.5, 5 and 7.5 t ha
-1
., while in the second experiment, 
BSFFF, SAFI and urea were applied at rates equivalent to 0, 30, 60 and 100 kg nitrogen 
(N) ha
-1
. Furthermore, field incubation studies involving BSFFF, and SAFI applied at 5 t 
ha-1 were also undertaken to determine synchrony of N mineralization for maize 
production. Finally, data obtained from BSF rearing, frass composting, and agronomic 
experiments were used to perform economic evaluation. The data obtained were 
analyzed using R software version 3.6.0. Results indicated that an amended substrate 
with C:N of 15 increased N and phosphorus (P) retention in frass compost by 21% and 
15%, respectively compared to the unamended substrate, and did not cause significant 
decrease in larval yields. The highest frass compost yield and potassium (K) retention 
were achieved in amended substrate with a C:N ratio of 20. Amended substrate with 
biochar and gypsum enhanced N conservation in frass compost, but gypsum was more 
efficient than biochar. Amendment with 20% biochar significantly increased BSF larval 
yields (88%) relative to the control substrate while amendment with > 5% gypsum 
decreased larval yields. The inclusion of 20% biochar generated the highest frass 
compost yield, gave a 21% increase in N retention and significantly higher K content in 
frass fertilizer compared to the unamended substrate. Initial composting of organic 
wastes using BSF larvae significantly shortened compost maturity time to 5 weeks, 
compared to 8 – 24 weeks in conventional composting methods. Field experiments 
revealed that plots treated with 7.5 t ha
-1
 of BSFFF achieved 14% higher maize grain 
yields than plots treated with a similar rate of SAFI. There was a 27% and 7% increase 
in grain yields in plots treated with 100 kg N ha
-1
 of BSFFF compared to those treated 
with equivalent rates of SAFI and urea, respectively. The agronomic N use efficiency of 
maize grown using 30 kg N ha
-1
 of BSFFF was 27% and 116% higher than the values 
attained using equivalent rates of SAFI and urea, respectively. The BSFFF achieved 
higher nitrogen fertilizer equivalence values (105 – 229%) than SAFI. Application rates 
of 2.5 t ha
-1
 and 30 kg N ha
-1
 of BSFFF were found to be effective in improving maize 
yield, while double rates of SAFI would be required. Economic assessment revealed that 
generating organic fertilizer from BSF frass increased the net income from BSF farming 
by 5 – 15 times compared to sole larvae production. At equivalent rates, the benefit-cost 
ratios from maize grown using BSFFF was higher than those of SAFI. Also, the gross 
margins of maize grown using BSFFF (75 – 78%) were comparable to those of urea (76 
– 78%). The findings of this study are crucial in reducing heavy reliance on the costly 
xvii 
 
mineral fertilizers, by adopting high quality organic fertilizers such as BSFFF for 
improved soil health and productivity. 
1 
 
CHAPTER ONE: INTRODUCTION 
1.1 Background to the study 
Improving food security requires strategies that can match future food demands 
with increasing population growth while conserving the soil resources. High levels of 
hunger and food insecurity dominate most regions of Africa, and the situation continues 
to worsen due to increasing soil degradation (FAO, 2017). Forty percent  of soils in Sub-
Saharan Africa (SSA)  are low in nutrient stocks, with 25% affected by aluminum 
toxicity, 18% prone to leaching and 8.5% characterized by phosphorus fixation (Tully et 
al., 2015). Most soils in SSA are deficient in nitrogen (N), phosphorus (P) and 
potassium (K) (Cobo et al., 2010) while most farm lands in East Africa have negative 
macronutrient balances (Gachimbi et al., 2005; Ebanyat et al., 2010) and yet, very little 
(≤ 10 kg ha-1) or no mineral fertilizer is used (FAO, 2017). Even in the few farms where 
mineral fertilizers are used, low nutrient use efficiencies and crop yields have been 
reported (Ebanyat et al., 2010; Kihara et al., 2016; Liverpool-Tasie et al., 2017), mostly 
attributed to low soil organic matter levels, many micronutrient deficiencies and high 
acidity (Vanlauwe et al., 2015; Musinguzi et al., 2016). 
Like in many SSA countries, most soils in Kenya are low in organic matter, with 
levels even below the critical value of 3% (Gachimbi et al., 2005), and exhibit high 
acidity (Keino et al., 2015). This has led to high P fixation, making P a limiting factor to 
crop production. Furthermore, uptake and utilization of macronutrient (mostly N, P and 
K) is affected by the low availability of secondary nutrients [calcium (Ca), magnesium 
(Mg) and sulphur (S)] and most micronutrients in soils (Tittonell et al., 2008a; 
Wortmann et al., 2019).  
2 
 
Past research efforts on soil fertility management in SSA have recommended 
combined application of mineral and organic fertilizers to improve and sustain soil and 
crop productivity (Tittonell et al., 2008b; Vanlauwe et al., 2014, 2015). When applied 
across farms in Kenya, combined organic and mineral fertilizers have improved crop 
yields, nutrient use efficiencies and soil fertility (Mucheru-Muna et al., 2007; Mucheru-
Muna et al., 2014; Musyoka et al., 2017). Despite this situation, most farmers do not 
apply adequate quantities of organic matter, which could help to replenish some of the 
macro- and micro-nutrients into the soil through mineralization (Baligar et al., 2001; 
Grigatti et al., 2015; Ch’Ng et al., 2016) and at the same time improve the soil structure 
which is an important soil physical fertility property (Mucheru-Muna et al., 2014). The 
major challenge hindering the use of organic fertilizers is the limited sources of organic 
matter, since most organic resources have other competing uses such as feeding 
livestock on the farm (Rufino et al., 2011; Ndambi et al., 2019). Furthermore, most 
organic fertilizers have low nutrient levels (Ndambi et al., 2019; Ebanyat, 2009) which 
limit their effectiveness.  
Insect frass has promising potential to provide alternative high-quality source of 
organic fertilizer. The increasing demand for animal feed has led to black soldier fly 
(Hermetia illucens L.) (BSF) mass rearing using organic waste (van Huis, 2013; Makkar 
et al., 2014), presenting an avenue for organic waste management that could also 
contribute to soil fertility improvement. The BSF larvae have a high waste degradation 
efficiency (65 – 79%) (Diener et al., 2011) and can significantly reduce pathogens 
present in the waste (Lalander et al., 2015).  
 
3 
 
The frass, which is a byproduct (a combination uneaten substrate, feaces, and 
exuviae) from BSF rearing contains substantial amounts of nutrients (Lalander et al., 
2015; Oonincx et al., 2015) that could be useful in crop production if converted into 
organic fertilizer. The frass fertilizer generated would also increase income from insect 
farming through the sale of organic fertilizer as a second product from BSF rearing or 
save the farmer from incurring inorganic fertilizer purchase costs.  
The effectiveness of organic fertilizers highly depends on the source, nutrient 
content, stage of mineralization and storage method (Rufino et al., 2007; Ebanyat, 2009; 
Ndambi et al., 2019). Nutrients availability in manure is greatly influenced by the 
source, mineralization status and C/N ratio (Grigatti et al., 2015; Musyoka et al., 2019a). 
In most homesteads, manure is heaped in open areas whereby most nutrients are lost 
through leaching and volatilization (Ebanyat, 2009; Ndambi et al., 2019). Manures with 
low nutrients require high application rates, which are not affordable by most farmers.  
Application of immature and unstable compost causes nutrient immobilization, 
and phytotoxicity which affect seed germination, crop growth and yield (Emino & 
Warman, 2004; Teresa & Remigio, 2011; Luo et al., 2018; Musyoka et al., 2019a). 
Therefore, in addition to the quest for feasible sources, good handling and treatment 
practices are required to harness the full potential of manure in improving crop 
production and soil fertility. Conversion of BSF frass into quality organic fertilizer is a 
promising technology which has been established in this study. 
4 
 
1.2  Statement of the problem 
The BSF frass generates organic fertilizer with low nutrient levels due to high N 
volatilization during larval feeding, which could reduce the agronomic potential of the 
fertilizer produced. Since most BSF farmers mainly target production of high larval 
biomass, substrates with low C/N ratio (< 25:1) are normally used (Lalander et al., 
2017). A low C/N ratio encourages faster release (mineralization) of N from organic 
matter compared to the degradable carbon, causing an excess of N which may be lost in 
form of ammonia gas especially when the temperature (> 45 ºC) and pH ( > 7.5) are 
high (Bernal et al., 2009). Furthermore, for optimal growth, BSF larvae require  neutral 
to alkaline pH (7 and above) (Ma et al., 2018); yet, pH above 7 causes nitrogen loss 
through ammonia volatilization (Liang et al., 2006). This is because high pH has been 
reported to increase the dissociation of ammonium (NH4
+
) to ammonia gas (NH3), thus 
shifting the equilibrium to NH3 which eventually evaporates (Sánchez et al., 2017). This 
leaves the frass with low nutrients since in addition to the nutrients which are 
accumulated in the larval biomass, more N is lost through volatilization. Lalander et al. 
(2015) reported total N losses of 44% in form of ammonia volatilization while using the 
BSF larvae to compost dog food, pig manure and human feaces. Furthermore, Oonincx 
et al. (2015) demonstrated that 23 – 78% of substrate N from chicken, pig and cow 
manure is lost during BSF larvae-assisted composting. 
 
 
5 
 
On the contrary, the high C/N ratio (> 25) required for production of nutrient 
rich compost does not favour fast growth of BSF larvae. The low concentration of N, P 
and K left in the BSF frass lowers the quality of the organic fertilizer generated. The 
critical C/N ratio for optimal BSF larval growth and nutrient conservation in BSF 
rearing waste is not known. Information on organic or inorganic additives that can 
effectively control N volatilization through ammonium adsorption, absorption and 
precipitation (Sánchez et al., 2017) without being detrimental to BSF larvae growth is 
lacking. Also, it is not known whether by BSF larval harvesting stage (5
th
 instar stage); 
the frass will have fully mineralized to release all the nutrients for crop use. There is 
inadequate information on the time needed to convert the BSF frass into mature and 
stable compost. Furthermore, the economic value of organic fertilizer production 
alongside BSF rearing is not known; and such information is useful in improving the 
adoption of BSF frass composting as a new concept of organic fertilizer production.  
The use of BSF frass as organic fertilizer is a relatively new concept. Adoption 
of a new concept or product as fertilizer in any farming system requires information on 
its performance in terms of how it influences crop growth, yield, nutrient uptake, and 
nutrient use efficiency in comparison to existing fertilizers. Most research efforts on use 
of insect frass as a fertilizer have been conducted under controlled conditions (Kagata & 
Ohgushi, 2012; Poveda et al., 2019; Houben et al., 2020). Those that have involved BSF 
frass (Choi & Hassanzadeh, 2019) have been performed under potted conditions, 
without assessing the economic yield and nutrient utilization. Most research outputs 
from greenhouse or potted experiments cannot be directly transferred to field conditions 
without being tested due to variations in production environments.  
6 
 
Knowledge on the influence of BSF frass fertilizer on crop production in terms 
of nitrogen mineralization, nutrient uptake, and nitrogen use efficiency under field 
conditions is not known. The optimum application rates and comparative performance 
of BSF frass fertilizer in relation to existing commercial organic and mineral fertilizers 
are not documented. Furthermore, the nitrogen fertilizer equivalence of BSF frass 
fertilizer – an important index that would provide information on the amount mineral N 
fertilizer saved while using N from BSF frass fertilizer to achieve the same crop yield 
(Hijbeek et al., 2018) is largely unknown.  
1.3 Justification  
Black soldier fly rearing without frass fertilizer production is less profitable, 
especially if the frass is not good for other uses. Generating organic fertilizer from the 
BSF frass can increase profits and improve soil fertility, which is a major challenge to 
crop production. Furthermore, recent studies  have recommended combination of 
organic and mineral fertilizers to reduce the effects of acidic, saline, and highly 
weathered soils to mineral fertilizer inputs (Tittonell et al., 2008a; Vanlauwe et al., 
2015).  
Composting process has been used to recycle nutrients in organic wastes into 
organic fertilizers. When the process is managed well, a nutrient-rich, mature and stable 
compost is generated (Epstein, 1997). Combining wastes with high-carbon substrates 
such as sawdust and crop straws help to achieve favourable C/N ratios that improve 
nitrogen conservation through the control of ammonia volatilization during composting 
(Sánchez et al., 2017; Bernal et al., 2009).  
7 
 
High carbon materials such as sawdust have been reported to enhance ammonia 
immobilization and binding onto phenolic compounds, which in turn reduce the amount 
of free ammonia, resulting in lower ammonia volatilization (Lim et al., 2017). 
Composting of cattle manure with sawdust has been reported to reduce total nitrogen 
loss by nearly 35%, and ammonia volatilization by up to 71% (Lim et al., 2017).  
Inclusion of materials such as biochar has been found to adsorb and absorb 
ammonium ions on the highly charged surfaces, thus preventing their loss as ammonia 
gas during composting (Awasthi et al., 2016, Awasthi et al., 2017a; Awasthi et al., 
2017b; Sanchez-Monedero et al., 2018). These mechanisms reduce ammonium ion 
concentration in liquid phase and minimizes nitrogen loss as ammonia gas and other 
forms (Wang et al., 2018).  It has been demonstrated that biochar inclusion during 
composting can reduce ammonia volatilization by up to 60% (Agyarko-Mintah et al., 
2017), thus increasing total N content in the compost.  
Conserving N in composting substrates can also be achieved through the 
addition of gypsum to stabilize ammonium ions through ammonium precipitation 
(Sánchez et al., 2017) to form stable compounds such as ammonium sulphate and 
ammonium phosphate (Wang et al., 2013; Li et al., 2018). Through reducing N diffusion 
and converting ammonium carbonate to ammonium sulphate (Sánchez et al., 2017), 
gypsum amendment aids in controlling ammonia volatilization during composting 
(Yang et al., 2015). Addition of gypsum to compost substrates has been found to 
improve P and ash contents, while reducing N loss by between 17% (Yang et al., 2015) 
and 94% (Tubail et al., 2008; Sheng, et al., 2015), thereby improving nutrient 
concentrations  in the compost.  
8 
 
Unlike the conventional composting process which is mediated by 
microorganisms only (de Bartoldi, 1983), the use of BSF to compost organic waste 
amended with sawdust, biochar and gypsum is a new innovation. The effective rates of 
inclusion of these amendments for optimal growth of BSF larvae growth and yield, and 
nutrient conservation have not been documented. Therefore, before BSF frass fertilizer 
is integrated into existing farming practices, the challenge of nutrient losses and the 
knowledge gaps on its agronomic performance mentioned above should first be 
addressed. 
1.4 Significance of the study 
This study sought to develop strategies for obtaining both high quality larval 
biomass and organic fertilizer by recycling organic wastes using the BSF. Results from 
the study will provide recommendations for optimal fertilizer production from BSF 
frass, fertilizer application rates and efficient soil nutrient management. The BSF larval-
based waste management technologies are crucial in converting the huge quantities of 
nutrients contained in waste streams into nutrient-rich larval biomass for incorporation 
as alternative protein additive in animal feeds, and high-quality organic fertilizer for 
improved soil health and crop productivity.   
The integration of frass fertilizer into cropping practices would help to return 
nutrients back into the soil and increase farm productivity through reduced expenses in 
chemical fertilizer inputs. Consequently, the adoption of high-quality and affordable 
organic fertilizers such as the BSF frass fertilizer would sustainably increase farm 
productivity (Ndambi et al., 2019; Rufino et al., 2007) and lift farmers out of poverty.  
9 
 
This would go a long way in improving food and nutrition security among the 
vulnerable segments particularly women and children in the rapidly growing populations 
in most developing countries. With low initial capital investments, smallholder insect 
farmers have good opportunities to increase productivity, improve their livelihood and 
contribute to food security and a circular economy. The frass fertilizer can also be sold 
directly, thus increasing the household income of insect farmers. Therefore, BSF 
farming has the potential to create jobs for the women and youths involved in the insect 
production chain, while conserving the environment by using the BSF larvae as 
biowaste converters (Chia et al., 2020). 
The development of inclusive business models involving organic fertilizer as 
additional value-added product from insect farming would significantly contribute to 
solving socio-economic and environmental problems in developing countries, aligning 
with the United Nations’ Sustainable Development Goals: namely, 1: Zero hunger 
through improved crop and animal yield, 2: Zero poverty through increased household 
income, 5: Gender equality by creating opportunities for women and girls, 6: Clean 
water and sanitation through waste recycling, 9: Industry, innovation and infrastructure 
through use of novel technologies for job creation, 11: Sustainable cities and 
communities through improved waste management, 12: Responsible consumption and 
production through cleaner production methods, 13: Climate action through climate 
change adaptation, and 15: Life on reducing land degradation and biodiversity loss.  
1.5 General objective 
To produce high quality and cost-effective organic fertilizer alongside BSF rearing 
without being detrimental to insect yields. 
10 
 
1.5.1 Specific objectives 
i. To determine the critical carbon to nitrogen (C/N) ratio for optimal BSF larval 
growth and nutrient conservation in frass fertilizer.  
ii. To evaluate the effectiveness of biochar and gypsum amendments on nutrient 
retention in BSF frass fertilizer. 
iii. To compare the performance of BSF frass fertilizer and commercial fertilizers on 
the growth, yield, and nitrogen use efficiency of maize. 
iv. To determine soil moisture storage, nitrogen fertilizer equivalence value and 
synchrony of nitrogen mineralization from BSF frass fertilizer and commercial 
organic fertilizer for maize production. 
v. To determine the economic value of frass fertilizer production alongside BSF 
rearing and profitability of crop production using BSF frass fertilizer. 
1.6 Hypotheses 
1. Optimal growth and nitrogen conservation during BSF rearing is not 
significantly affected by the C/N ratios of the substrates.  
2. Amending rearing wastes with ≥ 10% of biochar or gypsum significantly 
increases nutrient levels of BSF frass fertilizer. 
3. The BSF frass fertilizer can perform equally good as commercial fertilizers in 
terms of maize yield and nitrogen use efficiency. 
4. The BSF frass fertilizer has significantly higher soil moisture storage, nitrogen 
fertilizer equivalence values and better synchrony of nitrogen release for maize 
growth than the commercial organic fertilizer.  
11 
 
5. Generating frass fertilizer as a second product from BSF rearing significantly 
improves the profitability of BSF rearing and crop production. 
 1.7 Conceptual framework 
The challenge of low agricultural productivity can be addressed by using the 
BSF larvae to recycle organic wastes in the urban areas of SSA into organic fertilizer for 
soil fertility improvement and using the BSF larvae as source of proteins in animal feeds 
(Figure 1). This strategy can save farmers the burden of expensive animal feeds and 
inorganic fertilizer inputs. However, this can only be achieved if the BSF rearing 
process is optimized to produce nutrient-rich organic fertilizer and high larval biomass. 
Integration of BSF frass fertilizer into existing farming practices and market structures 
requires information on its agronomic performance and the economic value on BSF 
farming and crop production. These knowledge gaps have formed the research focus of 
this study. The study was based on the hypothesis that in addition to BSF larvae for 
animal feeding, quality and cost-effective organic fertilizer can be generated from the 
BSF frass if nutrient losses are minimized during the rearing process. 
 
 
 
 
 
 
12 
 
 
 
 
 
 
                                                                                                            
 
 
 
 
 
 
 
                                                                                                              
 
Figure 1: Conceptual framework. 
The problem  
Intervention 
Output 
 Economic value of frass 
fertilizer on BSF farming 
 Profitability of maize 
production using frass fertilizer 
Impact 
Low crop and 
animal yields 
leading to hunger 
and poverty 
Environmental 
degradation due 
to poor waste 
management 
Soil degradation due to 
nutrient and organic 
matter losses, and high 
acidity.  
Low organic fertilizer use.  
Poor-quality organics  
 Optimum application rates for crop 
production 
 Nitrogen fertilizer equivalence of 
frass fertilizer 
 Soil N mineralization & distribution 
 Synchrony of N mineralization 
 Macro nutrient uptake 
 Nitrogen use efficiency 
 Comparative agronomic 
performance  
 Substrate C/N ratio 
adjustment 
 Substrate amendment 
with biochar & 
gypsum 
 Frass compost 
maturity 
 Nutrient 
concentrations in frass 
compost  
Increased soil fertility and crop yields 
Improved food security, household income and 
environmental health  
BSF farming using organic wastes to 
produce larvae for animal feeds and high-
quality organic fertilizer for improved crop 
yields 
Key variables 
13 
 
CHAPTER TWO: LITERATURE REVIEW 
2.1 The black soldier fly and its role in agriculture 
The black soldier fly (BSF) (Hermetia illucens L.) is a two-winged black or blue 
fly with a wasp like appearance that belongs to the family Stratiomyidae (Diclaro & 
Kaufman, 2012). It is native to tropical, sub-tropical and temperate regions of America 
but has been dispersed by man to other parts of the world between 45
 
ºN and 40 ºS 
(Caruso et al., 2014). This non-pest and non-vector fly completes most of its life cycle in 
moist or decaying organic matter. The adult BSF mate three days after emergence and 
their life span ranges from 8 – 14 days (Caruso et al., 2014). The female black soldier 
fly lays pale yellow or creamy eggs near decaying wastes in batches of about 320 – 1000 
(Caruso et al., 2014). At optimum conditions of temperature (20 – 30 ºC) and relative 
humidity (30 – 90%), the entire life cycle from egg to adult takes 40 – 43 days 
(Tomberlin et al., 2002). Eggs hatch into larvae in four days after which the larval stage 
stays for 22 – 24 days. 
The BSF larval growth and yield are greatly influenced by the nature of rearing 
substrates/wastes (Oonincx et al., 2015; Chia et al., 2018a; Lalander et al., 2019). Waste 
materials with low proteins, carbohydrate levels and high level of chemical pollutants 
slow down BSF growth and yield (Diener et al., 2015; Cammack & Tomberlin, 2017). 
While the BSF pupae and adult flies do not feed and therefore not associated with 
decaying matter, the larvae stages have high feeding rate and hence the high waste 
degradation efficiency (40 – 80%) (Diener, 2010; Diener et al., 2011). The BSF larvae 
are rich in crude proteins, fats, and minerals (Caruso et al., 2014) and as feed, the BSF 
14 
 
larval meal has been reported to improve the performance of pig (Makkar et al., 2014), 
poultry (Al-Qazzaz et al., 2016; Schiavone et al., 2017) and fish (St-Hilaire et al., 2007).  
The BSF larval meal has thus been recommended as a potential replacement for 
conventional animal protein sources such as fish and soybean, that are not only 
expensive but also scarce and unsustainable (van Huis, 2013; Rana et al., 2015; van 
Huis & Oonincx, 2017). The black soldier fly larvae have high reduction efficiency of 
pathogens (Lalander et al., 2015), heavy metals (Diener et al., 2015), pesticides and 
antibiotics (Lalander et al., 2016) contained in organic wastes. Moreover, the BSF larvae 
do not accumulate these pathogens or chemical pollutants in their biomass, leaving them 
hygienically safe for animal feeding. This is because BSF larvae gut secretes 
antimicrobial agents (Park et al., 2014; Zdybicka-Barabas et al., 2017) and digestive 
enzymes (Kim et al., 2011) that are effective in reducing pathogens and chemical 
pollutants that may be present in organic wastes. The high waste degradation efficiency 
of BSF larvae mentioned above is not only useful in environmental cleanup but has also 
been utilized to recycle organic wastes into organic fertilizers (Oonincx et al., 2015; 
Lalander et al., 2015; Lalander et al., 2017; Choi & Hassanzadeh, 2019).  
2.2 Organic wastes as a resource in agriculture  
In most African cities, large volumes of wastes are produced which give rise to 
large quantities of organic wastes (Muniafu & Otiato, 2010; Okot-Okumu, 2012; 
Komakech, 2014; Friedrich & Trois, 2016 Somorin et al., 2017; Jambeck et al., 2018). 
Organic wastes constitute the highest percent of solid wastes collected in East Africa 
whereby 65, 71 and 77% are for Nairobi, Dar es Salaam and Kampala, respectively 
(Okot-Okumu, 2012).  
15 
 
 
Approximately, 4,788 metric tonnes of wastes are generated in Nairobi per day 
and 51% are decomposable organic wastes (von Blottnitz et al., 2010). Projections show 
that the volume of wastes generated in Nairobi city will be 8,000 metric tonnes per day 
by 2035 (von Blottnitz et al., 2010). These wastes have caused serious pollution 
problems where they have not been properly managed (Kimani et al., 2007; Nweke & 
Sanders, 2009). On the other hand, organic wastes have been used for crop production, 
biogas production and animal feeding (Tumuhairwe et al., 2009; Amoding et al., 2011; 
Komakech, 2014) .  
Organic wastes are used as rearing substrates for insects that have been found to 
contribute to animal feed requirements such as the BSF larvae (Lalander et al., 2019).  
Land filling is the common waste disposal method in East Africa (Okot-Okumu, 2012) 
yet, this locks most of the nutrients that would have been used for crop production. 
Amoding (2007) established that about 30, 50 and 130 metric tonnes of N, P and K 
respectively, are likely to be locked in market crop wastes of Kampala city.  
Several studies have investigated the methods of treating organic wastes for 
agricultural  use. Tumuhairwe et al. (2009) recommended the above ground pile method 
for composting of market crop wastes while Diener et al. (2011) and Dortmans (2015) 
found the BSF larvae very efficient in waste valorization. Komakech et al. (2016) 
reported that vermicomposting as a waste management strategy could also contribute to 
animal feeds. When converted into organic fertilizers, organic wastes are useful in 
integrated soil fertility management to enhance utilization of mineral fertilizers (Zingore 
16 
 
et al., 2007; Vanlauwe et al., 2015; Musyoka et al., 2017) and are sometimes applied 
singly by smallholder farmers who cannot afford synthetic fertilizers.  
In Nairobi, organic wastes have been proven effective in supporting peri-urban 
farming (Onyango et al., 2012). Combined application of organics with synthetic 
fertilizers is already a necessity in East Africa since most soils have low levels of 
organic matter, micronutrients and high acidity (Gachimbi et al., 2005; Nkonya et al., 
2008).  
2.3 Composting  
Composting is the biological decomposition and stabilization of organic 
substrates, under conditions that allow the development of thermophilic temperatures as 
a result of biologically produced heat, to produce a final product that is stable, free of 
pathogens, free of plant seeds and can be beneficially applied to the land (de Bartoldi, 
1983; Epstein, 1997). Composting involves microorganisms such as heterotrophic 
bacteria, archaea, and fungi. The microorganisms break down the organic substrates 
using extracellular enzymes materials to release nutrients through the process of 
mineralization (Sylvia et al., 2005). Soil animals, such as worms may be involved in the 
breakdown of the raw organic materials piled for composting. Insects such as the BSF 
larvae have also been found useful in accelerating the composting process (Diener et al., 
2011; Lalander et al., 2015; Ermolaev et al., 2019).  
The composting process involves three stages: the mesophilic (15 – 40 ºC), 
thermophilic (45 – 80 ºC) and the curing phases (15 – 40 ºC) (de Bartoldi, 1983). The 
mesophilic phase involves oxidation of soluble substrates like proteins, starch, and 
sugars. Here, the composting process is mainly done by bacteria and fungi. However, 
17 
 
because those microorganisms sparingly survive at high temperature, their population is 
reduced drastically as the temperatures approach the thermophilic limit (Epstein, 1997).  
In the thermophilic phase, temperatures rise to about 70 – 80 ºC depending on 
the nature of the substrates, where energy in form of heat is produced. Some of the 
energy that is  not biochemically trapped by microorganisms during catabolism is 
dissipated to the environment as heat (Epstein, 1997) and this is evidenced by increase 
in temperature of the compost pile. This high temperature is important in eliminating 
harmful microorganisms present in the pile (Wichuk & McCartney, 2007). Archaea and 
thermophilic bacteria dominate the thermophilic phase. The curing phase is 
characterized by a decrease in temperatures back to the mesophilic phase due to 
reduction in microbial population and activity because of exhaustion of readily 
oxidizable materials (Epstein, 1997).  
2.4 Nutrient changes during composting 
During the initial stage of composting, N losses occur as gaseous emissions 
(Barrington et al., 2002; Bernal et al., 2009) but as the process progresses, stable N 
compounds which are less susceptible to  volatilization, denitrification and leaching are 
produced; thus realizing higher N levels during maturation as reported by Goyal et al. 
(2005). Composting the water hyacinth with cattle manure increased N, P, Na, Ca, and 
Mg levels of the compost (Chandra et al., 2012), and at the same time enhanced 
microbial activities and P mineralization. Likewise, small scale composting of bio-
filtration waste with cow manure increased total N, P, K, Ca, and Na in the compost 
(Wilde et al., 2010).  
18 
 
 
 
Composting human feaces, pig manure and organic wastes using the BSF larvae 
significantly increased the total P content of frass compost by 45% but decreased N 
content (Lalander et al., 2015). In the same study, most of the N in the frass compost 
remained in the ammonium form. Nitrogen losses of 23 – 78% have been reported on 
composting chicken, pig, and cow manure using the BSF larvae (Oonincx et al., 2015). 
During the same study, the P content increased by 50% in frass compost. Most N losses 
during BSF rearing have been attributed to ammonia volatilization, arising from high pH 
and low C/N ratio of the rearing substrates (Lalander et al., 2015; Oonincx et al., 2015).   
2.5 Effect of compost C/N ratio and pH on nitrogen transformation during 
composting 
The adequate C/N ratio for composting ranges between 25 – 35 (Bernal et al., 
2009). A C/N ratio higher than 30 results in N immobilization and the composting 
process takes a longer time to reach maturity (Guo et al., 2012). A low C/N ratio (< 20) 
encourages faster release (mineralization) of N from organic matter compared to the 
degradable (available) organic C; causing an excess of N which may be lost by 
volatilization in form of ammonia or denitrification   of nitrates especially when the 
moisture capacity is more than 60% (Bernal et al., 2009; Guo et al., 2012). Lalander et 
al., (2015) reported 44% nitrogen loss through volatilization in BSF frass compost after 
using substrates with a low C/N ratio in this case 16.  
19 
 
The highest concentrations of ammonium ions occur during the first 2 – 3 weeks 
of composting when organic matter degradation is most intense (Sánchez-Monedero et 
al., 2001). During composting, N can be lost through leaching, volatilization and 
denitrification (Sánchez et al., 2017; Hao & Benke, 2008).  
Volatilization is the most dominant pathway of N loss (Liang et al., 2006). In 
addition to reducing fertilizer value of manure, ammonia is harmful to human and 
environmental health. Exposure to ammonia gas can cause respiratory infections, eye 
irritation and dyspnea (Palashikar et al., 2016).  
The nitrification process in which ammonium (NH4
+
) is oxidized to nitrates 
(NO3
-
) via nitrites acidifies the medium. By contrast, an increase in pH of a medium is 
caused by the ammonification process when organic N is converted into the ammonia 
(NH3) gas that can be lost or reacts with water to form ammonium hydroxide (NH4OH) 
which is an alkaline, thus increasing the pH (Sánchez-Monedero et al., 2001). The 
NH4OH is not stable and can easily be oxidized to nitrates. Moisture abundance and 
limited oxygen supply promote denitrification (reduction of NO3
-
 to N2 gas) (Sánchez et 
al., 2017).  
As a rule, and when the breakdown occurs on the surface, N is lost by 
volatilization as NH3 gas (Sánchez-Monedero et al., 2001). Should the breakdown occur 
inside the pile, the NH3 gas is converted into NH4
+ 
as follows [(NH3 + H
+
 = (NH4)
+
] and 
the subsequent (NH4)
+
 is then bound to a negatively charged surface such as a nitrate, 
carbonate, chloride and sulphate, and retained in the substrate. Ammonium sulphate is 
more stable than ammonium carbonate and thus less susceptible to leaching or 
20 
 
volatilization (Wang et al., 2013). Ammonia volatilization takes place when ammonium 
(NH4
+
) dissociates to ammonia gas (NH3)  and ranges from 13 – 70% (Hao & Benke, 
2008). The rate of volatilization is controlled by temperature, pH and ammonium 
concentration (Hao & Benke, 2008; Wang & Schuchardt, 2010).  
Most ammonia volatilization takes place during the early stages of composting at 
thermophillic temperatures and at high pH levels (> 7)  (Sánchez-Monedero et al, 2001; 
Bernal et al., 2009). This is because, increase in pH increases dissociation of NH4
+
 to 
NH3 gas and this shifts equilibrium to NH3. Thermophillic temperature also increases 
dissolution of NH4
+ 
to NH3, where diffusion of NH3 reduces solubility of NH3; thus, 
increasing its evaporation. Ammonia emissions increase when moisture and initial C/N 
ratio are low (<  25) (Sánchez-Monedero et al., 2001) and when ambient temperatures 
and aeration are high (Hao & Benke, 2008).  
2.6 Strategies for conserving nitrogen during composting  
Strategies for conserving N during composting can be biological (addition of 
ammonia oxidizing enzymes and bacteria), chemical (precipitation, adsorption) or 
physical-chemical (regulation of temperature, pH, particle size and moisture) (Sánchez 
et al., 2017). Ammonia volatilization can be reduced by reducing the turning frequency, 
using smaller piles, covering the compost pile and using bulking agents to widen C/N of 
the composting materials (Ogunwande et al., 2008; Bernal et al., 2009; Sánchez et al., 
2017).  
21 
 
2.6.1 Regulation of pH 
Reducing compost pH to values below 7 has been reported to reduce ammonia 
loss (Liang et al., 2006). Additives such as acids (sulphuric, hydrochloric, nitric and 
phosphoric) and base precipitating salts (chlorides and sulphates of Ca and Mg and 
aluminium) (Hao & Benke, 2008) can also be used. Acids are more efficient than bases, 
but they are limited by cost and hazards of use. Use of elemental sulphur and phosphor-
gypsum has also been found effective (Hashemimajd et al., 2012; Li et al., 2018).  
A dose of 0.5% (weight/weight) elemental sulphur was found effective for 
decreasing compost pH by one unit within two weeks and nitrogen conservation during 
composting of olive mill wastes (Roig et al., 2004). Adjusting initial pH to values below 
7 using phosphate salts was capable of  N retention in wheat straw and dairy manure 
compost by 82 – 90% (Liang et al., 2006). 
Addition of 8% aluminium sulphate, and 4% aluminium chloride reduced 
ammonia volatilization from poultry litter by 94 and 92%, respectively and improved N 
levels in mature compost but increased electro conductivity to values higher than 10 
mS/cm (Choi & Moore, 2008a). Use of 2.5% aluminium sulphate reduced ammonia 
emissions from slurry by 58% and also reduced extractable P levels (Lefcourt & 
Meisinger, 2010). While using aluminium salts, nitrogen conservation is achieved 
through pH lowering and formation of a foam on manure surface that act as a physical 
barrier to NH3 volatilization. A 75% reduction in NH3 loss was observed when the 
initial pH of broiler litter decreased from 8.3 to 6.6 using aluminium sulphate (Ekinci et 
22 
 
al., 2000).  Aluminium sulphate, therefore, has been found to be more effective in N 
conservation than calcium and magnesium chloride additives (Koenig et al., 2005).  
2.6.2 Ammonium precipitation 
Ammonium precipitation happens when salts are added to chemically combine 
with ammonium ions in composting and from stable compounds such as struvite 
(MgNH4PO4.6H2O), ammonium sulphate and ammonium phosphate (Wang et al., 2013; 
Li et al., 2018). Addition of 17% gypsum to dairy cow manure improved P and ash 
contents, while it reduced ammonia volatilization and N loss to 6.4% (Tubail et al., 
2008).  
Use of 20% zeonite, biochar and flue gas desulfurization gypsum during chicken 
litter composting reduced ammonia loss by 68, 40 and 21%, respectively, but  increased 
N, P, K, Ca, Mg and micronutrient levels in the compost (Sheng et al., 2015). Co-
composting of kitchen waste with 10% phosphogypsum and super phosphate conserved 
N by 16.7 and 9.9%, respectively, and improved levels of P, K, Ca and Mg in final 
compost (Yang et al., 2015). The additives also reduced ammonia volatilization by 23.5 
and 18.9% for phosphogypsum and super phosphate, respectively.   
2.6.3 Ammonium adsorption and absorption 
Ammonia adsorption happens when materials with high surface charge such as 
biochar, peat zeolite and clay adsorb ammonium ions and prevent further loss by 
reducing the activities of nitrifying bacteria (Xiao et al., 2017). The adsorption of 
ammonium on negatively charged materials reduces its concentration in liquid phase, 
and consequently potential for loss as ammonia gas and other forms (Wang et al., 2018).  
23 
 
Through the adsorption and absorption processes of ammonium into its pore spaces, 
biochar has been found to reduce ammonia volatilization by 44 and 60% during 
composting of poultry manure (Janczak et al., 2017) and poultry litter  (Agyarko-Mintah 
et al., 2017), respectively. Use of 6.25% zeolite achieved 47% reduction in slurry 
ammonia emissions, through the adsorption process (Lefcourt & Meisinger, 2010). Such 
mechanisms described above reduce ammonium mineralization but later enhance 
conversion of ammonium into nitrates during the maturation stage, which improves N 
content in the compost (Sanchez-Monedero et al., 2018).  
Addition of biochar has been reported to improve microbial activity by providing 
habitat and enhancing the physicochemical properties of compost (Sanchez-Monedero et 
al., 2018). It has been established that biochar also reduces heavy metal toxicity through 
adsorption, thereby reducing the mobility and bioavailability of heavy metals (Li et al., 
2017). Addition of 2% co-composted biochar reduced N leaching and improved crop 
growth through increased nitrate capture in pores and sorption on anion exchange 
surfaces (Kammann et al., 2015). In addition to improving the agronomic value of 
compost, biochar also reduces greenhouse gas emissions (Sánchez-García et al., 2015; 
Awasthi et al., 2017a); thereby  safeguarding the environment. Biochar doses of 5 – 
10% have been found effective while rates higher than 20% have been found to 
negatively affect microbial activity by reducing enzymatic activity, thus slowing down 
the composting process (Sanchez-Monedero et al., 2018).  
24 
 
2.6.4 Adjustment of substrate C/N ratio 
 Adjustment of C/N ratio of composting materials to a favourable range (25 – 35) 
has been reported to improve N conservation in compost (Bernal et al., 2009; Wu et al., 
2010). High carbon materials such as sawdust have enhanced NH4
+
 immobilization and 
binding onto phenolic compounds of sawdust, which in turn reduces the amount of free 
ammonia and thus occurrence of ammonia volatilization (Lim et al., 2009).  
Composting of cattle manure with saw dust and phosphogypsum or zeolite has been 
reported to reduce total N loss from 44% to < 35% and ammonia volatilization by up to 
71% (Lim et al., 2017).  
2.7 Effect of organic amendments on soil nitrogen mineralization  
 The effectiveness of organic fertilizers for crop production largely depends on 
their nutrient content, especially N, and rate of nutrient release (Bowden et al., 2007; 
Cabrera et al., 2005; Musyoka et al., 2019a). Plant available N from organic fertilizers is 
highly dependent on their composition, organic N fractions, C/N ratio, appropriate 
timing, rate, and method of application (Li and Li, 2014; Cai et al., 2016). Organic 
materials with high levels of lignin and polyphenols are resistant to microbial 
decomposition (Rovira and Vallejo, 2002; Kleber, 2010), and consequently cause N 
immobilization (Kimetu et al., 2004; Musyoka et al., 2019a). Soils with low total N 
concentration have also been reported to stimulate N immobilization (Gómez-Muñoz et 
al., 2015), thereby, reducing the quantity of available N in soils. 
 Nitrogen mineralization rate is the single most important factor which 
determines the quantity and period in which N from organic fertilizers is available for 
plant uptake (Cabrera et al., 2005; Musyoka et al., 2019a). For optimal crop growth and 
25 
 
yield, N mineralization patterns should match fluctuations in crop nutrient demands to 
cause synchrony, i.e., the balance between N supply and N demand (Johnson et al., 
2012). The mineralization and synchrony of N from different organic resources have 
been widely studied (Bowden et al., 2007; Alizadeh et al., 2012; Li and Li, 2014; 
Musyoka et al., 2019). However, some studies have not incorporated the manure into the 
soil (Adin Yéton et al., 2019), thereby ignoring the soil factors that influence N 
mineralization (Friedel et al., 2000; Cai et al., 2016; Osterholz et al., 2017).  
 
 Mineralization studies which do not involve a crop make it difficult to know 
whether the N mineralized from BSF frass fertilizer can match the crop N demand, 
because, although the N content of the organic fertilizer could be high, its release could 
be hampered by soil factors such as pH, bacterial regime, and C/N ratio of the substrate 
after incorporation in the soil (Johnson et al., 2012; Musyoka et al., 2019a). 
 While using organic fertilizers, the likelihood of meeting crop N demand during 
critical growth stages depends on the ability of the crop to compete for the mineralized 
N with other consumption pathways such as immobilization by microbes (Osterholz et 
al., 2017), losses through volatilization, and leaching beyond the rooting system 
(Musyoka et al., 2019b). Therefore, knowledge of the dynamics and synchrony of N 
release from organic fertilizer for crop production in any cropping system is crucial in 
guiding recommendations on timing and rate of application for efficient N management 
(Musyoka et al., 2019a). 
26 
 
2.8 Nitrogen fertilizer equivalence of organic fertilizers 
 In addition to N mineralization, the quality of an organic resource is also 
assessed based on its performance as compared with a standard mineral fertilizer, also 
known as the N fertilizer equivalence (NFE) (Bowden et al., 2007; Lalor et al., 2011). 
Nitrogen fertilizer equivalence also known as nitrogen fertilizer replacement value, is 
the amount of mineral fertilizer N saved when using organic amendment to produce the 
same yield (Hijbeek et al., 2018). Nitrogen fertilizer equivalences of organic fertilizers 
have been found to vary with the type (Kimetu et al., 2004) and application rates 
(Hijbeek et al., 2018).  
 Higher NFE values have been reported while using organic fertilizers with 
higher total N such as tithornia and calliandra (Kimetu et al., 2004), lucerne (De Notaris 
et al., 2018), poultry litter and yard waste compost (Bowden et al., 2007), cattle manure 
(Lalor et al., 2011; Cavalli et al., 2016; De Notaris et al., 2018) and pig manure (van 
Middelkoop and Holshof, 2017).  
 In addition to nutrient supply, previous studies have reported additional benefits 
associated with certain types of organic fertilizers (Kagata and Ohgushi, 2012; Debode 
et al., 2016; Choi and Hassanzadeh, 2019; De Tender et al., 2019). For example, higher 
crop improved drought tolerance, disease suppression, and higher crop growth yield, 
while using insect frass fertilizer (Kagata and Ohgushi, 2012; Poveda et al., 2019; 
Houben et al., 2020). Furthermore, the chitin contained in insect frass fertilizers has 
been reported to improve plant health by stimulating disease resistance in crops 
(Quilliam et al., 2020).  
27 
 
2.9 Economic value of BSF farming and frass fertilizer 
 Insects such as the BSF are expected to contribute immensely to boosting 
agricultural productivity because of their multipurpose nature (Chia et al., 2019a; 
Dobermann et al., 2017; Makkar et al., 2014). In Kenya, socio-economic studies have 
shown that majority (70 – 80%) of poultry, fish and pig farmers are aware that insects 
can be used as a feed ingredient while 60 - 70% of fish farmers considered insects as a 
good feed ingredient (Chia et al., 2020), and 90% of the farmers were ready and willing 
to use insect-based feeds (Okello et al., 2021). The potential macroeconomic impact of 
adopting BSF larval meals in Kenya’s poultry sector has been estimated at between 16 
and 687 million USD per year (Abro et al., 2020), not forgetting other benefits such as 
job creation and environmental sanitization.  
The BSF frass fertilizer is a by-product that is gaining popularity due to its high 
nutrient content (Lalander et al. 2015; Oonincx et al. 2015) and potential for use as 
organic fertilizer. Abro et al. (2020), using a replacement cost approach estimated the 
macroeconomic gains of BSF frass fertilizer at 9 – 85 million USD in Kenya.  
 Previous studies have reported improvement in crop growth and yield while 
using the BSF frass fertilizer (Choi and Hassanzadeh, 2019; Quilliam et al., 2020). 
Although, information on the economics of using BSF frass fertilizer for crop 
production is still limited, it is expected to increase profits like any other organic 
fertilizer. For example, application of organic fertilizers has been reported to increase 
profitability of maize production in Central Kenya (Mucheru-Muna et al., 2014), with 
higher profits recorded in farming systems where manure is locally available on the 
farm (Adamtey et al., 2016).  
28 
 
 In the long term, the residual benefits of organic fertilizers on soil fertility are 
expected to reduce the need for fertilizer application (Adamtey et al., 2016), thereby 
saving the farmer from incurring high fertilizer costs. Therefore, the use of organic 
fertilizers such as BSF frass fertilizer is envisaged as a sustainable way of increasing 
agricultural productivity in soils with multiple degradation challenges (Tilman et al., 
2002; Vanlauwe et al., 2014). 
 
 
 
 
CHAPTER THREE: MATERIALS AND METHODS 
3.1 Description of experimental sites  
The black soldier fly (BSF) rearing and frass composting experiments were 
carried out at the animal rearing and quarantine unit of the International Centre of Insect 
Physiology and Ecology (icipe), Nairobi Kenya (S 01° 13' 14.6''; E 036° 53' 44.5'', 1612 
m above sea level). The field experiments were carried out for two seasons (April – 
September 2019 and October 2019 – March 2020) at the Kenyatta University Teaching 
and Demonstration farm (1° 10′ 59″ S, 36° 55′ 34″ E), located in Nairobi County, Kenya 
at an elevation of 1580 m above sea level. Nairobi County receives bimodal rainfall with 
annual averages of 925 mm. The first rainfall season starts from March to June while the 
29 
 
second season runs from October to December. The mean monthly temperatures of 
Nairobi ranges between 21 and 28 ºC (www.meteo.go.ke).  
During the field experiments, daily temperatures and rainfall data were sourced 
from Kenyatta University weather station, located about 0.5 km from the experimental 
site. Cumulative rainfall totals of 252 and 281 mm were received during the short and 
long rain season, respectively (Figure 2a). Mean daily temperatures of 22 – 29 ºC and 22 
– 28 ºC were recorded at the experimental site during the short and long rain season, 
respectively (Figure 2b). The long rain season received higher mean monthly rainfall 
(31 – 102 mm) than the short rain season (40 – 87 mm) but had lower average monthly 
temperatures (25 – 26 ºC) than the short rain season (25 – 27 ºC) (Figure 2c). 
 
 
30 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2: Cumulative daily rainfall (a), average daily temperature (b), cumulative 
monthly rainfall, and average monthly temperature (c) during field experiments at 
Kenyatta University Teaching and Demonstration farm. 
31 
 
Soils in the study site are Acric Ferralsols (Gachene & Kimaru, 2003), which are 
highly weathered soils characterized by shallow depths, low organic matter, low P, and 
low pH levels. Before field experiments, soil sampling (0 – 20 cm) was done for 
determination of total organic N, total organic carbon, available P, exchangeable cations 
(K, Ca and Mg), pH, electrical conductivity and soil texture using methods described in 
(Okalebo et al., 2002). Table 1 shows selected physical-chemical characteristics of the 
soils used in the experiment.  
32 
 
Table 1: Selected physical and chemical characteristics of the soil at the experimental site 
Parameter pH 
(1:2.5 
water) 
ρb Min. N TN TOC SOM Av. P 
(ppm) 
Exchangeable cations 
(cmol kg
-1
) 
% 
sand 
% clay % 
silt 
Textural 
class 
(g cm
-3
) (ppm) (%) K Ca Mg 
Test 
value 
5.9 1.35 1.81 0.04 1.3 2.3 9.7 2.07 0.91 0.07 63 20.3 16.7 Sandy loam 
Critical 
value* 
5.5 NA NA 0.25 NA 3 15 0.22 4 0.2 NA NA NA  
*Source: Okalebo et al. (2002) Key: ρb = bulk density, Min.N = mineral nitrogen (ammonium and nitrate), TOC = total organic 
carbon, SOM = soil organic matter, Av. P = available phosphorus, NA = Not available. 
 
33 
 
3.2 Black soldier fly colony establishment and maintenance  
The colony of BSF was established and maintained at the animal rearing and 
quarantine unit at the International Centre of Insect Physiology and Ecology (icipe), 
Nairobi Kenya following the methodology described by Chia et al. (2018b). The colony 
was initiated from eggs collected from wild adult BSF populations using a combination 
of 2-week-old fermented chicken and rabbit manure mixed with fruits and household 
food wastes as baiting materials for adult flies.  
Wild adult BSF were trapped at Kasarani, Nairobi County, Kenya. The baiting 
materials were placed inside 6-litre plastic buckets designed with six openings, 5 cm 
from the lid to facilitate entry of adult flies for oviposition on the flutes of cardboard 
boxes attached on the sides of the containers. Thereafter, the buckets containing the 
baits were hung on metallic stands under the shades around homesteads or close to 
garbage dump sites.  
To prevent intruders (ants, reptiles, rats, and others) from accessing the content 
of the traps, the metallic stands holding the buckets were smeared with Tanglefoot 
(Tangle-trap; The Tanglefoot Company, Grand Rapids, MI, USA) insect barrier paste. 
The buckets were checked regularly (2 – 3 times/week) for deposited egg clusters in the 
cardboard flutes. Thereafter, the egg clusters were transferred into new containers with 
suitable substrates (kitchen waste, brewers’ spent grain and pig manure) for rearing the 
newly hatched neonates. Conditions in the rearing room were maintained at 28 ± 2°C, 
60 – 70% relative humidity and a photoperiod of 12 hours of light and 12 hours of 
darkness. Portable digital thermo-hygrometers were placed inside each of the rearing 
rooms to monitor temperature and relative humidity.  
34 
 
The pre-pupal stages were harvested from the rearing trays and placed in 
transparent rectangular plastic containers (Kenpoly Manufacturer, Nairobi, Kenya) 
containing 60% moist wood shavings (sawdust). The prepupae were allowed to grow to 
pupal stages and the emerged flies were transferred into large outdoor cages (1 × 1 × 1.8 
m) holding approximately 2000 – 2500 adult flies per cage.  The adult flies were 
provided with 60% sugar solution on cotton wools.   
3.3 Experiments carried out  
The study comprised of five experiments. The first experiment determined 
critical C/N ratio for optimal BSF growth, and nutrient conservation in frass fertilizer. In 
the second experiment, effectiveness of biochar and gypsum on nitrogen retention 
during BSF rearing was assessed. The third experiment compared the performance of 
the BSF frass fertilizers and commercial fertilizers on maize production and N use 
efficiency. In the fourth experiment determined the N fertilizer equivalence value and 
synchrony of N mineralization from BSF frass fertilizer for maize production. The fifth 
study assessed the economic value of organic fertilizer production alongside BSF 
rearing and profitability of crop production using BSF frass fertilizer. 
3.3.1 Determining the critical C/N ratio for BSF larvae growth and nutrient 
conservation in frass fertilizer 
 
3.3.1.1 Sources of experimental substrates  
Before the start of the experiment, fresh brewery spent grains (BSGs) were 
sourced from Kenya Breweries Ltd. (Nairobi, Kenya), while sawdust (from blue gum 
tree, Eucalyptus globulus L. belonging to the family Myrtaceae) was collected from a 
local carpentry workshop located in Kasarani, Nairobi, Kenya.  
35 
 
Detailed analyses of the nutrient levels, moisture content, pH, electrical 
conductivity, and C/N ratio of BSGs and sawdust were conducted using recommended 
laboratory methods as described in section 3.3.1.4. Results from the analyses are 
presented in Table 2.   
Table 2: Selected characteristics of substrates used in black soldier fly rearing 
Substrate Moisture 
content 
(%) 
pH 
(1:10 
water) 
EC 
(mS/cm) 
TOC Total 
N 
Total 
P 
Total 
K 
Total 
Ca 
Total  
Mg 
C/N 
ratio 
  (%) 
BSG 68.5 4.2 1.52 30.5 2.76 0.44 0.11 0.45 0.19 11.1 
Saw dust 21.3 5.4 0.45 53.3 0.18 0.016 0.043 0.14 0.024 296.1 
 
Based on the results presented in Table 2, a combination of BSGs and sawdust 
was formulated to obtain five different substrates with varying C/N ratios (15, 20, 25 
and 30). The control treatment was not amended with sawdust. The amount of sawdust 
required to achieve the above C/N ratios was calculated using equation (1) (Richard & 
Trautmann, 1996). Table 3 shows the percent weight of sawdust and BSGs used to 
obtain the experimental substrates 
    
       (   
  
  
) (      )
    (
  
  
  ) (      )
 
Where, 
C1 and C2 - represent total organic carbon contents of brewer’s spent grain and sawdust, 
respectively 
N1 and N2 - represent total nitrogen contents of brewery spent grain and sawdust, 
respectively 
M1 and M2 represent moisture contents of brewery spent grain and sawdust, respectively 
(1) 
36 
 
Q1 - represents the amount of brewery spent grain (fresh weight) used 
Q2 - represents the amount of sawdust (fresh weight) required to adjust C/N ratio of 
brewery spent grain to the desired value 
R - represents the desired C/N ratio of the rearing substrate (brewery spent grain and 
sawdust mixture). 
Table 3: Percentage weights of sawdust and brewery spent grain waste mixed to 
produce feedstocks of varying carbon to nitrogen ratios. 
Feedstock 
Formulations 
Brewery 
spent 
grain 
(%) 
Sawdust  
(%) 
Cost of 
brewery 
spent grain 
Cost of 
sawdust 
Total cost 
per kg of 
feedstock 
US dollar 
Control (C: N 
11) 
100 0.0 0.36 0.0 0.180 
C/N 15 91 8.5 0.33 0.003 0.167 
C/N 20 80.2 19.8 0.29 0.008 0.149 
C/N 25 68.5 31.5 0.25 0.012 0.131 
C/N 30 56.1 43.9 0.20 0.018 0.109 
Exchange rate: 1USD = 100 Ksh. 
 
3.3.1.2 Black soldier fly larval growth and yield on various substrates 
The BSF larvae rearing facility was equipped with a wooden stand (180 cm high 
× 66 cm wide × 420 cm long). The stand had three shelves separated from each 
other by 30 cm space, where the metallic trays used in rearing the BSF larvae 
were fitted. Metallic trays used during the experiment measured 76 cm long × 
27.5 cm wide × 10 cm deep. The bottom of each tray measured 52 cm in length 
by 27.5 cm in width, which allowed for both edges of the tray to be inclined at 
an angle of 35º. Both ends of each tray were provided with a collar of 5 cm long 
to allow it to sit smoothly on the edge of each trough.  
37 
 
In the experimental room, one wooden stand was used, to accommodate 5 trays 
per shelf, totaling to 15 trays arranged in a complete randomized design with three 
replicates. At the start of the experiment, two thousand (2000) 5-day old BSF larvae 
(equivalent to 10.7 g fresh weight) were obtained from the stock colony (section 3.2) 
and introduced into each tray containing 6.85 kg of the fresh formulated substrates. 
Lumpsum feeding and the recommended feeding rate of 125 mg (dry weight) per larva 
per day was used (Diener et al., 2009; Banks et al., 2014).  
 Each rearing substrate was hydrated to approximately 70.0 ± 2% moisture by 
weight according to the protocol described by Cammack & Tomberlin (2017) which was 
confirmed using a moisture sensor with two 12-cm-long probes (HydroSense CS620, 
Campbell Scientific, Logan, USA). Each experimental substrate was monitored as often 
as necessary until the last larval stage (5
th
 instar) was completed.  
The experimental room was fitted with heater and subsequently heated using fast 
moving dry hot air at 28.0 ± 2 °C (using Xpelair heater: WH30, 3KW Wall Fan Heater, 
UK) with thermoregulators. Portable digital thermo-hygrometers were placed inside 
each of the rearing rooms to monitor temperature and relative humidity. Environmental 
conditions in the experimental rearing room were maintained at 28 ± 2 °C, 60 – 70% RH 
and a photoperiod of L12:D12. The BSF larvae were harvested at fifth instar stage (13 
days after hatching) as indicated by the appearance of cream-like color on the 
exoskeleton of the larvae (Dortmans et al., 2017). At harvesting, hand sieving with a 2 
mm diameter sieve was used to separate the larvae from the frass. Thereafter, the larvae 
from each rearing tray were harvested and weighed to determine the yield per tray. 
38 
 
Larvae from each treatment was oven dried at 105 ℃º for 24 hours to determine the dry 
larval weight. 
The amount of substrate consumed throughout the larval development phase and 
the frass were used to determine waste reduction efficiency on dry weight basis 
(equation 2) (Diener et al., 2009).  
%WR = 
(                           )
              
                 (2) 
 Bioconversion rate (BCR) which indicates the efficiency of conversion of waste 
by BSF larvae into usable energy was also calculated using equation (3) (Dortmans, 
2015). 
      *
      
   
+                                                  
Where,  
Mi - represents the dry weight of the larvae at start of experiment,  
Mpp - represents the dry weight of larvae at 5
th
 instar,  
Fin - represents the initial dry weight of the rearing wastes used at the start of 
experiment. 
3.3.1.3 Quality of frass and frass compost yield 
After larval harvesting, the frass from each treatment was returned to respective 
trays for further composting until compost maturity attained. The moisture content of 
the frass from the different treatments after harvesting the BSF larvae was maintained at 
70% as described in section 3.3.1.2. Compost maturity was monitored both during the 
larvae rearing and frass composting phases. During the larvae rearing and frass 
composting phases, changes in temperatures were carefully monitored daily using a 
(3) 
39 
 
composting digital thermometer. The metallic probe of the digital thermometer was 
inserted at a depth of 7 cm into the substrate and four readings from four different points 
in each tray were recorded daily.  
From each tray, 50g of the substrate was collected weekly for monitoring 
compost maturity. This process was repeated for each treatment and parameters for 
monitoring compost maturity such as water soluble carbon to total nitrogen ratio (0.55), 
pH (6 – 8), ammonium to nitrate ratio (< 0.16) (Bernal et al., 2009), electrical 
conductivity (< 4 mS cm
-1
) (Huang et al., 2004). The C/N ratio (< 20) (Goyal et al., 
2005) of the frass was monitored on a bi-weekly basis, while seed germination index (> 
80%) was determined at maturity (Teresa & Remigio, 2011).  
At maturity, frass compost from each treatment was weighed and oven dried at 
105 
o
C for 24 hours to determine the dry frass compost yield. In addition, 50 g of the 
frass compost were collected from each treatment, air dried for five days and ground 
into powder (< 0.25 mm) to determine total organic carbon, nitrogen, phosphorus, 
potassium, and C/N ratio at a different time of each experiment (weeks 1, 3 and 5). 
3.3.1.4 Laboratory analysis methods 
Fifty (50) grams of fresh frass compost from each treatment were analyzed to 
determine their pH, electrical conductivity (EC) and water-soluble carbon using aqueous 
extracts of 1:10 (w/v) compost to distilled water (Teresa and Remigio, 2011). The 
content of each treatment was shaken for 30 minutes, and the pH and EC values were 
recorded directly using a pH meter (AD1000, Romania) and an EC meter (AVI, India), 
respectively. The solution was then filtered using a Whatman No. 1 filter paper and the 
40 
 
filtrate was used for further analysis of water-soluble carbon by complete oxidation 
using potassium dichromate and concentrated sulphuric acid, which was heated at 150 
ºC for 30 minutes.  
Later, the solution was allowed to cool, and the un-neutralized potassium 
dichromate was titrated against ferrous ammonium sulphate and water-soluble carbon 
concentration was determined following standard procedures (Okalebo et al., 2002). The 
nitrate and ammonium from compost of each treatment was determined using 0.5 M 
potassium sulphate at 1:10 (w/v). The entire content was shaken for 1 hour using a 
reciprocating shaker (KOS – 3333/KCS – 3333, MRC, UK) and the solution was filtered 
through a Whatman No. 1 filter paper and the filtrate used for further analyses. 
Ammonium and nitrate concentrations were determined colorimetrically according to 
the methods described by Okalebo et al. (2002).  
The ammonium in the filtrate was complexed with sodium hydroxide, sodium 
hypochlorite, sodium nitroprusside, sodium salicylate, sodium citrate and sodium 
tartrate while nitrate was complexed with sodium hydroxide and 5% salicylic acid. The 
complexed solutions were allowed to stand for 1 hour to enable blue and yellow colour 
development for ammonium and nitrate, respectively. Standard solutions for ammonium 
and nitrate were prepared using ammonium sulphate and potassium nitrate, respectively. 
Reagent blanks and the standard series for ammonium and nitrate were complexed 
following the procedures described above for colour development.  The absorbencies of 
the standards, samples and blanks were read using the atomic absorbance 
spectrophotometer (G10S UV-Vis, USA) at 419 and 655 nm for nitrate and ammonium, 
respectively.  
41 
 
 
Standard calibration curves of absorbance against concentration of ammonium 
and nitrate were plotted. Slopes from the calibration curves were used to calculate the 
ammonium and nitrate concentrations using the absorbance values obtained above. 
Thereafter, the final concentrations of ammonium and nitrate (on dry weight basis) in 
the samples were calculated while considering the extraction volume, sample weight and 
moisture content as described in Okalebo et al. (2002).  
Total organic carbon was determined using the wet oxidation method (Nelson & 
sommers, 1982). Total N, P, K, Ca, and Mg were extracted using acid digestion methods 
as outlined in the procedures described by  Okalebo et al. (2002). Total nitrogen in the 
digestate was determined using the Kjeldahl distillation method (Jackson, 1973).  
Total P was determined using the Ultraviolet-visible (UV-Vis) spectroscopy 
method by complexing the digestate with the solution containing sulphuric acid, 
ammonium molybdate, antimony potassium tartrate and ascorbic acid (Okalebo et al., 
2002). The standard series for P were prepared using potassium dihydrogen phosphate. 
The complexed solutions were allowed to stand for 1 hour for complete reaction (i.e., 
development of blue colour). Thereafter, the absorbances of the standard solutions, 
samples and blanks were read at 880 nm using atomic absorbance spectrophotometer 
(G10S UV-Vis, USA). Standard calibration curve of the absorbance versus 
concentration was plotted. Sample absorbances were converted into P concentration 
using the slope calculated from the standard curve. The final P concentration was 
42 
 
calculated as described by Okalebo et al. (2002) while considering the sample weight 
and volume of the digest . 
Total K concentration in the digested sample solution was determined using 
flame photometry  (Okalebo et al., 2002). Standards for K were prepared using 
potassium chloride. The K standards, digested sample solutions, and blanks were 
sprayed into flame photometer (410 flame photometer, UK) and the flame transmissions 
were recorded. The amount of K present in the sample was read from the standard 
calibration curve, prepared by plotting transmission readings against K concentration. 
The sample weight and volume of the digest were used in calculating final K 
concentration  (Okalebo et al., 2002).  
The Ca and Mg contents in the digested solutions were determined by atomic 
absorbance spectrometry  (Okalebo et al., 2002). Standard solutions were prepared using 
calcium carbonate and magnesium sulphate for Ca and Mg, respectively. During Ca 
determination, 10 ml of 0.15% Lanthanum chloride were added in the standard 
solutions, sample solutions and the blanks to avoid ionic interference. Magnesium 
samples were prepared by diluting 5 ml aliquots of digested solution to 50 ml using 
distilled water in a 50 ml volumetric flask.  
The standard solutions, sample solutions and the blanks for Ca and Mg were 
aspirated into atomic absorption spectrophotometer (iCE 3000 series AA spectrometer, 
China) to obtain absorbances at 422.7 and 285.2 nm for Ca and Mg, respectively. The 
calibration curves of the standard series readings were constructed for both Ca and Mg, 
which were later used to determine the concentrations of the samples and the blanks. 
43 
 
The final concentrations of Ca and Mg in the samples were calculated as described by 
Okalebo et al. (2002).  
3.3.1.5 Phytotoxicity test on mature frass compost extracts 
The germination index (GI) was determined by growing cabbage seeds in each 
of the prepared compost extracts, obtained by dissolving 10 g of fresh frass compost in 
100 ml of water. Thereafter, the mixtures were shaken for 30 minutes and filtered using 
Whatman No. 1 filter paper. Ten cabbage seeds were randomly selected and placed on 
petri dishes lined with filter paper moistened with 10 ml of 10% compost extracts for 96 
hours at 25°C in a dark chamber. The same procedure was repeated using distilled water 
as a positive control. Germinated seeds were counted, and their radicle lengths 
measured. Cabbage seeds were selected due to their high photosensitivity (Teresa & 
Remigio, 2011).  
Germination index (GI) was calculated using equation (4) (Emino & Warman, 
2004). Frass composts with GI values below 50% were considered highly phytotoxic, 
values between 50% and 80% indicated moderate phytotoxicity; and values above 80% 
indicated the absence of phytotoxicity (Emino & Warman, 2004; Teresa & Remigio, 
2011). 
   
(          )
   
                         (4) 
 Where, 
 RSG (%) represents the relative seed germination calculated as: 
     
                                             
                                          (              )
      
 
44 
 
RRG (%) represents the relative root growth calculated as: 
     
                                   
                                (              )
     
3.3.2  Evaluating the effectiveness of biochar and gypsum amendments on nitrogen 
retention in BSF frass fertilizer  
3.3.2.1 Experimental diets 
The brewery spent grains (BSG) was used as the rearing substrate. Biochar made 
from rice husks at pyrolysis temperatures of 350 °C was sourced from SAFI Organics 
Limited, located in Mwea, Kirinyaga County, Kenya. Gypsum was sourced from the 
Kenya Farmers’ Association stores, Nairobi. Detailed analyses of the nutrient levels (C, 
N, P, K, Ca, and Mg), moisture content, pH, EC of the BSG, biochar and gypsum were 
conducted using laboratory methods described in section 3.3.1.4. The results are 
presented in Table 4. 
Based on the analyses described above, the BSG was amended with biochar at 
different inclusion rates of 5, 10, 15 and 20% dry weight (weight /weight) to obtain four 
treatment regimes, denoted as 5Biochar, 10Biochar, 15Biochar and 20Biochar, 
respectively. The BSG was also amended with gypsum at three different inclusion rates 
of 5, 10 and 15% dry weight (weight /weight), represented as 5Gypsum, 10Gypsum and 
15Gypsum, respectively. The control treatment was neither amended with biochar nor 
gypsum. 
Table 4: Selected characteristics of substrate and additives used in the experiment. 
Substrate Moisture 
content 
(%) 
pH 
(1:10 
water) 
EC 
(mS/cm) 
TOC Total 
N 
Total 
P 
Total 
K 
Total 
Ca 
Total  
Mg 
C/N 
ratio 
 (%) 
45 
 
BSG 71.4 3.5 0.86 33.9 2.65 0.54 0.38 0.48 0.20 12.7 
Biochar 8.9 7.7 0.51 22.6 0.46 0.014 2.67 0.14 0.86 49.1 
Gypsum 0.62 8.0 4.24 na 0.00 0.003 0.05 25.1 0.43 na 
na = not applicable 
3.3.2.2 Black soldier fly larval growth and yield on various substrates 
The conditions of rearing facility, wooden stand and rearing trays were like those 
described in section 3.3.1.2. The wooden stand had three shelves separated from each 
other by 30 cm space, where the metallic trays used in rearing the BSF larvae were 
fitted. In the experimental room, 8 trays were fitted per shelf of the wooden stand, 
totaling to 24 trays arranged in a complete randomized design with three replicates. 
At the start of the experiments, eggs of BSF were hatched as described in section 
3.2 and larvae reared on each of the eight substrate treatments as described in section 
3.3.2.1. After 5 days, 2,000 neonates (equivalent to 10.7 g fresh weight) from each 
treatment were collected and transferred directly to their respective treated substrates 
(7.10 kg) in metallic trays. The effect of substrate amendment with biochar and gypsum 
on BSF growth was monitored by determining wet and dried larval yields as described 
in section 3.3.1.2 and N accumulated by BSF larvae from each treatment. Data on waste 
reduction efficiency (equation 2) and biomass conversion rate (equation 3) were 
recorded at harvesting stage.   
Samples of the dried BSF larvae from each treatment were ground into powder 
and used for determination of total N concentration (section 3.3.1.4). The fraction of 
initial N accumulated by BSF larvae (BSFL) biomass was determined using equation 
46 
 
(5), while the fraction of initial N retained in the raw frass and mature frass fertilizer 
were determined using equation (6). 
                                
(
         
   
)              
                             
                        
(5) 
             
(
   
   
)           
                             
                                                            (6) 
Where, 
                              
  
                                                  
   
 
Dry matter represents the dry weight of raw frass or mature frass compost 
The effect of biochar and gypsum amendments on frass compost quality was 
determined by assessing compost maturity, nutrient concentrations and phytotoxicity of 
the frass composts generated (equation 4), following procedures described in sections 
3.3.1.3, 3.3.1.4 and 3.3.1.5, respectively. 
3.3.3 Determining the comparative performance of BSF frass fertilizer and 
commercial fertilizers on crop production  
3.3.3.1 Source of fertilizers 
The experiment involved three fertilizers: BSF frass fertilizer, commercial 
organic (SAFI), and urea fertilizer (45% N). The BSF frass fertilizer was produced as 
described in section 3.3.1. The SAFI fertilizer was sourced from Safi Organics Limited 
(http://safiorganics.co.ke/) located in Mwea town, Kirinyaga County, Kenya. It was a 
mixture of chicken manure, biochar, and rock phosphate. Urea was sourced from Kenya 
47 
 
Farmers’ Association stores, Nairobi. Table 5 shows selected physical-chemical 
characteristics of the organic fertilizers used in the experiments.  
 
 
 
48 
 
Table 5: Chemical characteristics of the organic fertilizers used during experiments. 
Parameters 
 
Moisture 
(%) 
pH 
 
EC 
(mS/cm) 
NH4
+
 NO3
-
 TOC  N P K Ca Mg C/N 
ratio 
…. (mg/ kg) …     …………………  ( %)  ………………….. 
BSF frass 30.1 7.7 2.7 74.4 1.39 35.2 2.1 1.16 0.17 0.19 0.16 16.8 
SAFI 29.8 6.4 6.1  39.4 92.3 45.1 3.0 1.23 1.49 0.29 0.43 15.0 
Key: TOC = total organic carbon, EC = electrical conductivity, NH4
+
 = ammonium, NO3
- 
= nitrate,  
SAFI = commercial organic fertilizer. 
 
 
 
 
 
49 
 
3.3.3.2 Treatments and experimental set up 
The study consisted of two experiments, each repeated for two seasons: (1) the 
first experiment involved the use of two organic fertilizers (BSF frass and SAFI organic) 
applied at sole rates in tonnes per hectare (t ha
-1
) and (2) the second experiment involved 
the utilization of three fertilizers (BSF frass, SAFI and urea) applied to supply different 
levels of nitrogen. The two experiments were set side-by-side at the Kenyatta University 
teaching and demonstration farm, using the randomized complete block design (RCBD) 
with three replicates per treatment. Plots measured 4 x 4 m with border widths of 0.5 m 
and 1 m between the plots and blocks, respectively. The distance between experiments 
was 2 m.  
In the first experiment, the BSF frass fertilizer and SAFI were applied singly at 
2.5, 5.0 and 7.5 t ha
-1
 to determine the most effective rate for crop production (Table 6). 
These were denoted as control, 2.5BSF, 5BSF and 7.5BSF for BSF frass fertilizer, and 
2.5SAFI, 5SAFI and 7.5SAFI for SAFI treatments. The control treatment consisted of 
unfertilized soil. This gave a total of seven treatments, which are referred to as sole 
organic fertilizer treatments in this document. In the second experiment, the organic 
fertilizers were applied at three rates equivalent to 30, 60 and 100 kg N ha
-1
 to determine 
the optimum N rate (Table 6). These were denoted as 30N BSF, 60N BSF and 100N 
BSF for BSF frass fertilizer treatments, and 30N SAFI, 60N SAFI and 100N SAFI for 
SAFI treatments. The synthetic N fertilizer (45% N urea) was applied at equivalent rates 
(30, 60 and 100 kg N ha
-1
) as the organic fertilizers.  
 
50 
 
Table 6: Treatments considered during the study. 
Experiment Fertilizer Fertilizer rates 
Experiment 1 BSF frass  Each fertilizer applied at four uniform rates 
equivalent to 0, 2.5, 5 and 7.5 t ha
-1
 
SAFI organic  
Experiment 2 BSF frass Each fertilizer applied at four uniform rates 
equivalent to 0, 30, 60 and 100 0 kg N ha
-1
 
SAFI organic 
Urea 
 
To remove any limitation in the mineral N and organic N fertilizer treatments, 
inorganic P [supplied as triple super phosphate – TSP (46% P2O5)] and K [supplied as  
muriate of potash (60% K2O)] were applied at uniform rates of 60 kg P ha
-1
 and 50 kg K 
ha
-1 
(Tittonell et al., 2008a). Table 7 shows the amounts of N, P and K supplied by the 
different organic fertilizer treatments. The control treatment consisted of unfertilized 
soil.  
The second experiment, where fertilizers were applied in terms of N rates and 
supplemented with P and K consisted of seven treatments which are referred to as 
combined fertilizer treatments in this document. Maize hybrid H513, which is 
recommended for low and medium altitude areas of Kenya, was used as the test crop. 
The TSP fertilizer was applied at planting while urea and muriate of potash were applied 
in two splits: 50% at 4 weeks after planting and another 50% at 7 weeks after planting.  
 
 
 
51 
 
Table 7: Actual amounts of macro nutrients supplied by organic fertilizer treatments. 
Organic 
fertilizer 
Treatment Amount 
applied  
(kg ha
-1
) 
Nutrients supplied (kg ha
-1
) 
Nitrogen Phosphorus Potassium 
Black soldier 
fly -
composted 
manure 
 
30 kg N ha
-1
 1,429 30 16.57 2.43 
60 kg N ha
-1
 2,858 60 33.14 4.86 
100 kg N ha
-1
 4,762 100 55.24 8.09 
2.5 t ha
-1
 2,500 52.5 29 4.25 
5 t ha
-1
 5,000 105.0 58 8.50 
7.5 t ha
-1
 7,500 157.5 87 12.75 
Safi organic 
fertilizer 
 
30 kg N ha
-1
 1,000 30 12.3 14.9 
60 kg N ha
-1
 2,000 60 24.6 29.8 
100 kg N ha
-1
 3,334 100 40.99 49.7 
2.5 t ha
-1
 2,500 75 30.75 37.25 
5 t ha
-1
 5,000 150 61.50 74.50 
7.5 t ha
-1
 7,500 225 92.25 111.75 
 
3.3.3.3 Maize growth, yield, and mineral nitrogen 
Plant height and chlorophyll contents were measured from 10 plants per plot 
which were randomly selected and tagged for repeated measurements at early vegetative 
[35 days after planting (DAP)] (8 – 10 leaves), late vegetative (55 DAP) (13 – 16 
leaves), tasseling (70 DAP) and silking stages (91 DAP). Plant heights were measured 
using a tape measure from the ground level up to the apex of the topmost leaf while 
chlorophyll content was measured using a SPAD meter placed on the fourth fully 
opened leaf from the top (Yuan et al., 2016). Biomass amounts and nitrogen uptake 
were determined at early vegetative, tasseling, silking and maturity (125 DAP) 
(harvesting) stages. Two plants were randomly selected from the two outer rows of each 
plot, cut at ground level and their fresh weights determined.  
52 
 
Plant subsamples were collected from each plot, and air dried for five days 
before oven drying at 60 ºC for 72 hours for complete drying without affecting 
nutritional quality. After cooling the dry weights were determined. The dried samples 
were ground into powder (< 0.25 mm) which was used for the determination of total N.  
Grain yield data was collected at harvesting period from each plot area after all
 
the ears had dried. Plants in the harvested area were cut at ground level and their ears 
threshed to determine grain weights and weight of residues using a weighing scale. 
Grain and stover samples were taken to the laboratory and air-dried to 12.5% moisture 
content for determination of grain and residues yields per plot and on a hectare basis (t 
ha
-1
). Part of the grain and stover sample from each treatment was ground into powder 
for determination of N, P and K concentrations. 
During plant sampling, soil samples were also collected at two depths (0 – 20 cm 
and 20 – 40 cm) to determine mineral N (nitrate-N and ammonium-N) content in the 
top- and sub-soil layers during crop growth. A soil auger was used to collect sub-
samples from eight spots within the inner four rows of each plot. The subsamples were 
homogenized by the quarter sampling approach (Okalebo et al., 2002) to obtain 
representative samples. The soil samples were placed in air-tight polythene bags and 
carried to the laboratory using cool boxes containing ice blocks to reduce microbial 
activities during transportation.  
 
53 
 
3.3.3.4 Nitrogen use efficiencies 
The following N use efficiencies were calculated: the total N uptake by plants as 
measured in plant tissues and grain, agronomic efficiency, apparent N recovery 
efficiency, agro-physiological N efficiency, and N harvest index. Part of the air-dried 
samples were ground using an analytical mill for analysis of N in grain and plant tissues, 
which were used to measure the amount of N in grain and plant residues per treatment 
(equation 7). Agronomic efficiency (AEN), which is a measure of economic yield 
produced per unit N supplied from each treatment, was calculated using grain yields 
from each treatment (equation 8). Apparent N recovery efficiency (ANRN) was 
calculated to determine the ability of the plant to acquire the N supplied from different 
treatments (equation 9).  
Agro-physiological N efficiency (APEN) was calculated to determine the 
economic yield per unit N accumulated from each fertilizer treatment (equation 10). 
Nitrogen harvest index (NHI) was calculated to determine the fraction of nitrogen 
accumulated in the grain (equation 11). All the N use efficiency indices were calculated 
according to Baligar et al. (2001). 
         (       )   
(            )
   
                            (7) 
   (     
  )  
(               )
                    
                                            (8) 
    ( )  
(                    )
                    
                                        (9) 
    (     
  )  
(              ) 
*(
            
                 
) (
           
                   
)+
          (10) 
54 
 
   ( )   
                  
                            
                                (11) 
Where, 
F represents plots that received fertilizer treatments,  
C represents the control plots. 
3.3.4  Determination of soil moisture storage, nitrogen fertilizer equivalence and 
synchrony of N mineralization from BSF frass and commercial organic fertilizers 
3.3.4.1 Nitrogen fertilizer equivalence  
The grain yield data obtained from the second field experiment where fertilizers 
were applied in terms of N rates (0, 30, 60 and 100 kg N ha
-1
) (section 3.3.3) was used 
in the determination of the nitrogen fertilizer equivalence values (NFEs) of BSF frass 
and SAFI fertilizers. Nitrogen fertilizer equivalence value represents the amount mineral 
N fertilizer saved when using organic fertilizer N to achieve the same crop yield 
(Hijbeek et al., 2018). Nitrogen fertilizer equivalence value is also referred to as 
nitrogen fertilizer replacement value and is expressed as kg kg
-1
, but mostly converted 
into percent for comparison purposes (Kimetu et al., 2004; Cavalli et al., 2016; van 
Middelkoop & Holshof, 2017). The grain yields from the urea fertilizer treatment were 
used to draw the N response curves (Figure 3) that were used to calculate the NFEs of 
the two organic fertilizers  (equation 12) (Lalor et al., 2011).  
    (       )  
      
     
                                   (12) 
Where, 
NA f =GY represents the mineral fertilizer N required to obtain grain yield equivalent to 
those of organic fertilizer treatments (kg ha
-1
).   
55 
 
NA org represents the total N supplied by organic fertilizer inputs (kg ha
-1
). 
 
The NAf = GY for each organic fertilizer treatment was estimated using the grain 
yield response curve specific to each season. Since the response assumed a quadratic 
function with the equation Y = aNAf
2
+ bNAf + c, the equations were solved using the 
quadratic formula. The constants a, b and c were obtained from the N response curves.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 3: Maize grain yield response curves used to determine fertilizer equivalences of 
BSF frass and SAFI organic fertilizers during the short (a) and long rain (b) season 
experiments. Note: Grain yields for BSF frass fertilizer.  
 
56 
 
3.3.4.2 Synchrony of nitrogen mineralization for maize production 
In this study, the BSF frass and SAFI fertilizers were applied at rates of 0 and 5 t 
ha
-1
, a rate that has been previously used  in central Kenya (Mucheru-Muna et al., 2014; 
Musyoka et al., 2017, 2019a). The establishment and management of experiments were 
done following procedures described in section 3.3.3. Data was collected on maize 
biomass amounts, N uptake at different maize growth stages (early seedling, tasseling, 
silking and maturity) and mineral N in bulk soil determined at early seedling, tasseling, 
silking and maturity stages, following procedures described in section 3.3.1.4. 
3.3.4.3 Soil sampling and incubation  
From each plot, soil was collected from 0 – 20 cm depth before application of 
organic fertilizers. The soil was manually sorted to remove objects, stones, and big clods 
bigger than 2 mm. The soil was then homogenized by hand mixing in a basin. The two 
organic fertilizers were mixed with the soil at same rates as mentioned in section 3.3.4.2 
and the moisture content of the mixture (soil-organic fertilizer) was adjusted to 60% soil 
water holding capacity using distilled water. Two hundred grams of the mixture were 
then placed in an air permeable zip lock bag that was sealed to prevent water entry. The 
bags were then buried at 10 – 20 cm depth in respective plots. Similarly, 200 g of 
unamended soil from each plot (as control) were placed in zip lock bags and buried at 
the same depths (10 – 20 cm) in each of the respective plots.  
 
 
57 
 
Five bags were randomly buried per replicate, giving a total of 15 bags per 
treatment at the beginning of each cropping season. The bags were retrieved at 0, 35, 70, 
91 and 125 days of the incubation period which corresponded to planting, early 
vegetative, tasseling, silking, and harvesting stages of the maize crop. The positions of 
the bags were marked using pegs to avoid disturbance during weeding and for ease of 
retrieval.  
On each sampling date, the retrieved bags were labelled, placed in airtight 
polythene bags, and carried in a cool box containing ice blocks to maintain low 
temperatures and reduce microbial activities pending analysis. The samples were used 
for determination of mineral N content. Also, on each sampling date, other soil samples 
were collected in top (0 – 20 cm) and subsoil (20 – 40 cm) from the respective plots for 
the determination of soil moisture and mineral N content during crop growth (early 
seedling, tasseling, silking and maturity). A soil auger was used to collect sub-samples 
from eight spots within the inner four rows of each plot. 
 The subsamples were homogenized by quarter sampling approach to obtain 
representative samples. The soil samples were also placed in airtight polythene bags and 
carried to the laboratory using cool boxes containing ice blocks to reduce microbial 
activities during transportation. Soil moisture was determined using the gravimetric 
method (Okalebo et al., 2002). 
 
 
58 
 
3.3.4.4 Mineral nitrogen release  
Mineral N released was calculated on per hectare basis (kg N ha
-1
) by using bulk 
density and soil depth as shown in equation 13. 
                 (        )
 
                       (       )                     (  )
         
              (  ) 
Where, 
                   (  )
                   (     )                       (  )  
                     (  )
                     (  )                      ( ) 
 The mineral N released from unamended soil and from soil amended with 
organic fertilizers at  each sampling time during mineralization was determined using 
equations (14) and (15) (Musyoka et al. (2019a). The N release rate (kg N ha day
-1
) was 
calculated by dividing  mineral N released at each sampling date by the number of 
incubation days (Loecke et al., 2012). 
                  (        )           
                                                                                          (14)                                                                                        
 
                  (        )             
                                                                                   (15) 
 
Where, 
 ti represents sampling times i = 0, 1, 2, 3… 
t+k is ti plus time k intervals where k = 1, 2, 3, 4… 
59 
 
3.3.4.5  Synchrony of nitrogen mineralization and nitrogen uptake  
To determine the N synchrony, N flux was calculated as the difference between 
plant N uptake and mineral N released from the amended soils per day (equation 16) 
according to  Musyoka et al. (2019a). A positive N flux meant that the N released was 
larger than the crop’s N demand while a negative N flux meant that mineral N released 
was insufficient to meet the crop’s N demand. 
       (            ) 
                                                                  (  ) 
Where,  
              (           )  
(                          )      
  
(        )
  
N uptake represents the quantity of N taken up at each maize growth stage (equation 7). 
3.3.5  Economic value of frass fertilizer production alongside BSF rearing and 
profitability of crop production using BSF frass fertilizer 
3.3.5.1 Economic  value of frass fertilizer as a second product from BSF farming  
A cost-benefit analysis of frass fertilizer as a by-product of BSF farming was 
carried out, where gross margins (equation 17), the benefit cost ratio (equation 18), and 
return on investment (equation 19) were calculated as shown in the three equations 
(17,18 &19) (Chia et al., 2019b).  
            ( )  
          
             
                                                           (  ) 
                    
          
                    
                                                     (  ) 
60 
 
                      
          
                    
                                      (  ) 
Where, 
                                               
 For comparative purposes, the cost-benefit analysis considered two options for 
the outputs of BSF farming: (1) BSF farming solely for larvae production and (2) BSF 
farming for both larvae and frass fertilizer production. Under the first option, the BSF 
production process ends at the 5th instar larval stage when the larvae are usually 
harvested for animal feeding purposes. The variable costs of insect production included 
cost of transporting rearing substrates, water utilized, electricity consumed, and labour 
for egg collection, feeding and harvesting the BSF larvae (Table 8).  Labour costs were 
calculated by recording the number of hours spent on each activity, at a wage rate of 
1.25 USD per hour (Kenyan Government, 2019). Gross income was measured using the 
revenue generated from BSF larvae. 
Under the second option, after larvae harvesting, frass was collected and 
composted to convert it into an organic fertilizer, which is referred to as frass fertilizer 
following procedures described in section 3.3.1 and 3.3.2 above. The additional costs 
incurred in transporting the frass to the composting site and turning the frass to convert 
it into an organic fertilizer were added to the fertilizer production costs (Table 8). The 
quantity of dry frass fertilizer and dry BSF larvae as obtained from each waste 
combination, were used to measure the gross income at current market prices in Kenya.  
 
61 
 
Table 8: Parameters used in calculating economic returns from frass fertilizer. 
Parameter  Unit price 
Cost of labour 1.25 USD hour
-1
 
Cost of sawdust 0.02 USD kg
-1
 
Cost of brewery waste 0.0522 USD kg
-1
 
Cost of biochar 0.3 USD kg
-1
 
Cost of gypsum 0.48 USD kg
-1
 
Price of dry BSF larvae 0.9 USD kg
-1*
 
Price of BSF frass fertilizer 0.3 USD kg
-1
 
Dollar exchange rate: 1USD = 100 Kshs 
*Source: Sanergy limited (http://www.sanergy.com) 
The costs of substrate amendments (sawdust, biochar, and gypsum) were 
included in insect production costs under the second option. After 14 days of rearing (5
th
 
instar stage), the BSF larvae were harvested, and quantity of dry larvae obtained per 
treatment was recorded. 
The performance of BSF larvae on brewery spent grain is comparable with those 
of other wastes such as chicken manure and kitchen waste (Shumo et al., 2019) that can 
be readily available both on- and off-farm. Therefore, three scenarios of rearing 
substrate in establishing the economic benefits of frass fertilizer were considered: (1) 
BSF farming when the main rearing substrate is freely available but has to be 
transported to the farm which is the case currently; (2) BSF farming when the main 
rearing substrate is purchased and transported to the farm; and (3) BSF farming when 
the main rearing substrate is available on the farm, i.e., no purchase or transport costs 
incurred. In all options, it was assumed that the farmer is producing BSF farming for 
commercial purposes and the frass was not utilized in any sector e.g. crop fields where it 
could have served as an organic fertilizer. 
62 
 
3.3.5.2 Economic evaluation of maize production using BSF frass fertilizer 
This study utilized the data collected from the two field experiments as described 
in section 3.3.3 to determine the economic return to using frass fertilizer for maize 
production. Cost benefit analysis was carried out by calculating gross margin, benefit-
cost ratio and return on investment using equations 17 – 19 (section 3.3.5.1). The 
economic return to frass fertilizer was compared with that for mineral fertilizer (urea) 
and commercial organic fertilizer (SAFI).  
Prices of fertilizers, and maize seed were obtained from Africa fertilizer 
(Africafertilizer.org) and Kenya Farmers’ Association, respectively (Table 9). Labour 
expenses (for land preparation, planting, weeding, fertilizer application, and harvesting) 
were estimated by recording the time taken to perform each activity and valuing labour 
at the rate of 1.25 USD per working hour (Kenyan Government, 2019). Revenue was 
calculated using harvested maize grain and stover yields per fertilizer treatment (in kg 
per hectare). Price of maize grain was obtained from human data exchange 
(https://data.humdata.org). Since stover is used as an animal feed in Kenya, it was 
considered an additional output and valued at 22.1 USD per tonne (Mucheru-Muna et 
al., 2014).  
Table 9: Parameters used to calculate economic returns for different fertilizer inputs. 
Parameter  Unit cost 
Price of urea (46% N) 0.54 USD kg
-1a
 
Price of Safi organic  0.3 USD kg
-1
 
Price of maize seed 1.8 USD kg
-1
 
Labour cost 1.25 USD hour
-1
 
Price of maize  0.37 USD kg
-1b
 
Dollar exchange rate: 1USD = 100 Ksh.  
Source:
 a
Africafertilizer.org, 
b
https://data.humdata.org/ 
63 
 
3.3.5.2.1  Economically optimum fertilizer rates for maize production  
The values of grain yield were regressed against the fertilizer rates and response 
curves were drawn for each fertilizer treatment (Figure 4). Since the regression 
equations  for N rates (Figures 4a – d) were quadratic in nature, a quadratic function 
equation (equation 20) was used to calculate the economically optimum fertilizer rates 
(N) for maize production using equation (21) (Naher et al., 2011).  
                                                                       (20) 
  
    
  
                                                             (21) 
Where, 
  is maize grain yield in kg per hectare,  
  is rate of nitrogen fertilizer (kg ha-1),  
a, b and c are parameters estimated by the quadratic function  
En represents the ratio of fertilizer price (  ) to maize price (  ),    
  
  
  
Pf   represents the price of fertilizer (urea, FF, and SAFI) per kg  
Py represents the price of maize per kg 
 
64 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 4: Yield response curves used to calculate economically optimum rates of BSF 
frass (a and d), urea (c) and SAFI (b and e) fertilizers for maize production. 
 
3.3.5.2.2 Stochastic dominance analysis 
Plots of cumulative density functions for grain yields obtained while using BSF 
frass fertilizer, SAFI organic fertilizer and urea fertilizer were constructed to compare 
yield distributions obtained using the three fertilizer treatments. 
 
65 
 
3.4 Data analysis  
Prior to analysis, data was tested for normality using the Shapiro-Wilk test. 
Analysis of variances was performed using the linear mixed-effect model with ‘lmer’ 
function from the package ‘lme4’ in R statistical software. The substrate and sampling 
time were kept as fixed effects, while replication was a random effect. The effects of the 
substrate C/N ratios, biochar and gypsum amendments and composting time on pH, 
electrical conductivity, water soluble carbon, ammonium, nitrate, ammonium/nitrate 
ratio, total organic carbon, C/N ratio, total N, P and K were determined. The linear 
mixed-effect model was used for analysis of data collected from field experiments on 
plant height, chlorophyll content, nitrogen uptake, and mineral N content, whereby 
fertilizer treatments and maize growth stage were considered as fixed effects while 
replication was considered as a random effect.  
Data on fresh and dry BSF larval yields, waste reduction, biomass conversion 
rate, frass compost yield, seed germination, germination index, initial substrate N 
content, nutrient uptake by BSF larvae, N retention in frass, maize grain and stover 
yields, P and K uptake and N use efficiency, and NFE were analyzed using one-way 
analysis of variance. Computation of least squares means was done using ‘lsmeans’ 
package. Significant means were separated using Tukey’s test at p < 0.05. For objective 
two (biochar and gypsum amendments) data was analyzed per experimental set. Also, 
data from field experiments (objectives 3 and 4) was analyzed per season.  
 
66 
 
For economic analysis, the two-sample parametric Kolmogorov-Smirnov test for 
first-order stochastic dominance was done to assess for the vertical distance between the 
cumulative density functions of the maize grain yield obtained. All the statistical 
analyses were conducted using R software version 3.6.0 (R Core Team, 2019). 
 
 
 
 
 
 
 
 
 
 
 
 
 
67 
 
CHAPTER FOUR: RESULTS AND DISCUSSION 
4.1 Introduction  
This chapter is arranged in the following order. Section 4.2 presents results on 
the critical carbon to nitrogen (C/N) ratio for optimal black soldier fly (BSF) growth and 
nutrient conservation in frass fertilizer. Section 4.3 evaluates the effect of biochar and 
gypsum amendments on BSF larval performance and nitrogen retention in frass 
fertilizer. In both sections (4.2 and 4.3), the effects of BSF larvae on compost maturity 
and stability were determined by assessing temperature, pH, electrical conductivity, C/N 
ratio, water soluble carbon to nitrogen ratio and seed germination index. Section 4.4 
describes results on the comparative performance of BSF frass fertilizer and commercial 
fertilizers on the growth, yield, and nitrogen use efficiency of maize.  Section 4.5 
presents results on the N fertilizer equivalence value and synchrony of N mineralization 
from BSF frass and commercial organic fertilizer for maize production. Section 4.6 
assesses the economic value of frass fertilizer production alongside BSF rearing and 
profitability of crop production using BSF frass fertilizer. 
4.2 Critical carbon to nitrogen (C/N) ratio for optimal BSF larval growth and 
nutrient conservation in frass fertilizer 
4.2.1 Black soldier fly larvae yield  
There was a significant effect of rearing substrates on the fresh (p < 0.001) and 
dry (p = 0.001) BSF larval yields (Figure 5a). Fresh larval weight gain of BSF reared on 
the control substrate (unamended) and substrate with C/N of 15 did not vary 
significantly. Rearing substrates with C/N ratios of 30 and 25 had the least fresh and dry 
larval yields, respectively.  
68 
 
There was significant percentage reduction in fresh (p < 0.001) and dry (p = 
0.014) BSF larval yield across the different substrates when compared with the control 
treatment (Figure 5b). The lowest percentage reduction for both dried and fresh BSF 
larvae was observed on substrates with C/N ratio of 15 while the highest percent 
reduction in dried and fresh larval yields were recorded on substrate with C/N ratios of 
25 and 30, respectively.  
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 5a and b: Effect of substrate C/N ratio on black soldier fly larval yields. 
Key: C/N = total organic carbon to total organic nitrogen ratio; Control = unamended substrate (brewery 
spent grain); C/N 15, C/N 20, C/N 25, and C/N 30 = amendment of brewery spent grain using sawdust to 
obtain substrates with C/N ratios of 15, 20, 25 and 30, respectively. For each parameter and panel, means 
(± standard error) followed by the same letters are not significantly different at p ≤ 0.05. 
 
69 
 
The significantly higher fresh and dry BSF larval yields in the control treatment 
with C/N ratio of 11 and the substrate with C/N ratio of 15 (Figure 5a and 5b) could be 
attributed to higher nitrogen content of both substrates. Results from the current study 
are consistent with those reported by Lalander et al. (2019) and Spranghers et al. (2017), 
who demonstrated that high nitrogen content of rearing substrates was one of the major 
factors capable of enhancing faster growth rates of BSF larval and biomass 
accumulation. Furthermore, the findings of this study are supported by Jucker et al. 
(2017), who observed smaller larvae sizes from substrates with low nitrogen content.  
4.2.2 Waste reduction and biomass conversion rate 
The waste reduction (p = 0.234) and biomass conversion rates (p = 0.178) did 
not vary significantly across the different substrates (Figure 6). The waste reduction 
ranged between 24 and 34 % while biomass conversion rates varied between 9 and 14% 
(Figure 6).  Results from the current study are consistent with those reported by Rehman 
et al. (2017) who found that substrates with high carbon to nitrogen ratios such as 
soybean residues resulted in low biomass conversion rates by BSF larvae, which is in 
accordance with the larval yields recorded from substrates with C/N ratio of 30. On the 
contrary, substrates with lower carbon to nitrogen ratios in the present study had higher 
biomass conversion rates (Figure 6). This implies that substrates with lower carbon to 
nitrogen ratios or substrates with higher nitrogen should be used in BSF rearing to 
achieve higher larval biomass conversion rates (Shumo et al., 2019). 
 
70 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 6: Effect of substrate C/N ratio on waste reduction and biomass conversion rates 
of black soldier fly larvae. 
Key: C/N = total organic carbon to total organic nitrogen ratio; Control = unamended substrate (brewery 
spent grain); C/N 15, C/N 20, C/N 25, and C/N 30 = amendment of brewery spent grain using sawdust to 
obtain substrates with C/N ratios of 15, 20, 25 and 30, respectively. For each parameter and panel, means 
(± standard error) followed by the same letters are not significantly different at p ≤ 0.05. 
 
The waste degradation efficiencies obtained in the current study (24 – 34%) 
(Figure 6) could be due to the presence of recalcitrant carbon tissues such as lignin, 
cellulose and hemicelluloses that are difficult to be broken down (Huang et al., 2004). 
The findings of this study are comparable to the findings of earlier studies with rice 
straw (10 – 32%) (Manurung et al., 2016) and cassava peels (28%) (Supriyatna et al., 
2016), which based their arguments on the presence of recalcitrant carbon tissues. For 
farmers, targeting organic fertilizer production using BSF larvae, the low waste 
reduction is advantageous since it would result into higher frass compost yields.  
 
71 
 
4.2.3 Frass compost yield  
Significant differences were observed for both fresh (p = 0.002) and dry (p = 
0.001) compost yields among the substrates used (Figure 7). Substrates with C/N ratio 
of 20 and above showed no significant variations in fresh frass compost yields. Also, the 
frass compost yields generated substrates with C/N ratio of 15 was not significantly 
different from that of the control. Substrates with C/N ratio of 20 generated frass 
compost with the highest weight of fresh (3 kg) and dry (0.89 kg) compost yields, 
followed by substrates with C/N ratios of 25 and 30. Substrates whose initial C/N ratios 
were adjusted to 20 and above produced significantly higher (p < 0.05) frass compost 
yields than the control.  
Figure 7: Effect of substrate C/N ratio on frass compost yields 
Key: C/N = total organic carbon to total organic nitrogen ratio; Control = unamended substrate (brewery 
spent grain); C/N 15, C/N 20, C/N 25, and C/N 30 = amendment of brewery spent grain using sawdust to 
obtain substrates with C/N ratios of 15, 20, 25 and 30, respectively. For each parameter and panel, means 
(± standard error) followed by the same letters are not significantly different at p ≤ 0.05. 
 
72 
 
The control treatment and substrate with C/N of 15 had the lowest fresh (1.03 
kg) and dry (0.58 kg) compost yields, respectively. The significantly higher fresh and 
dry frass compost yields produced from substrates with C/N ratios of 20 and 30 
compared to the control and substrate with C/N of 15 can be attributed to reduced 
nitrogen availability for larval uptake, arising from increased sawdust as a carbon source 
in the various substrates (Huang et al., 2004).  
4.2.4 Nutrients flow in BSF larvae and frass compost 
 Increasing substrate C/N ratios caused significant differences in the initial 
content of K in rearing substrates (p = 0.022) (Table 10). However, increasing C/N ratio 
did not significantly influence the initial contents of total N (p = 0.075) and P (p = 
0.160) of the substrates. Initial total N contents ranged between 31 and 40 g while the 
initial total P was between 6.7 and 8.6 g.  The highest K content was observed in 
substrate with C/N ratio of 15. The fraction of initial nitrogen (p = 0.189), phosphorus (p 
= 0.312) and potassium (p = 0.340) accumulated in BSF larval biomass did not vary 
significantly due to increasing substrate C/N ratio.  
Increasing substrate C/N ratios caused significant differences (p = 0.023) in the 
fraction of phosphorus retained in frass composts (Table 10). Frass composts generated 
from substrates with C/N ratio of 20 retained significantly (p = 0.023) higher P than the 
frass composts generated from control substrates and substrates with C/N ratios of 30. 
The fraction of initial total nitrogen (p = 0.056) and potassium (p = 0.058) retained in 
the frass composts did not vary significantly (Table 10). The total N and K retention 
efficiencies of 12 – 19 g 100 g-1 and 20 – 36 g 100 g-1, respectively, were recorded 
during the study.  
73 
 
Table 10: Effect of C/N ratio adjustment on nutrient accumulation by BSF larvae and nutrient retention in frass fertilizer 
Key: * P < 0.05, ns = not significant (p ≥ 0.05), N = nitrogen, P = phosphorus, K = potassium. C/N = total organic carbon to total organic nitrogen ratio; Control 
= unamended substrate (brewery spent grain); C/N 15, C/N 20, C/N 25, and C/N 30 = amendment of brewery spent grain using sawdust to obtain substrates with 
C/N ratios of 15, 20, 25 and 30, respectively. In the same column, means (± standard error) followed by the same letters are not significantly different at p ≤ 0.05. 
 
  
Rearing 
substrates  
Initial nutrients content per tray 
(g) 
Fraction of initial nutrients content 
accumulated by BSF larvae 
(g 100 g
-1
) 
Fraction of initial nutrients content 
 retained in frass fertilizer 
(g 100g
-1
) 
N P K N P K N P K 
Control  39.7 ± 2.7 8.6 ± 0.1 2.2 ± 0.0ac 6.8 ± 0.7 6.0 ± 1.0 22.2 ± 0.8 33.1 ± 4.6a 12.0 ± 0.4b 35.5 ± 4.4 
C/N 15 30.9 ± 0.6 7.3 ± 0.6 3.07 ± 0.5a 9.0 ± 0.5 8.8 ± 2.0 23.0 ± 6.2 47.0 ± 4.4a 15.8 ± 1.1ab 20.0 ± 2.1 
C/N 20 30.9 ± 2.4 7.5 ± 0.9 3. 05 ± 0.1ab 8.9 ± 0.6 5.5 ± 0.8 14.7 ± 1.2 45.6 ± 6.7a 19.1 ± 2.2a 29.1 ± 2.8 
C/N 25 30.5 ± 2.3 6.7 ± 0.2 2.04 ± 0.2ac 7.4 ± 1.3 6.4 ± 0.3 20.5 ± 1.7 36.2 ± 2.3a 14.4 ± 1.4ab 28.4 ± 1.7 
C/N 30 32.7 ± 0.8 6.9 ± 0.3 1.89 ± 0.2ac 7.4 ± 0.4 6.8 ± 0.4 21.7 ± 0.3 29.2 ± 0.8a 11.7 ± 1.5b 21.7 ± 4.9 
 ns ns * ns ns ns ns * ns 
74 
 
The higher N, P and K uptake by larvae reared on substrates with C/N ratios of 
15 and 20 (Table 10) imply that adjusting C/N ratio of rearing substrates could 
positively influence the nutritional value of BSF larvae obtained (Shumo et al., 2019). 
However, substrates with higher carbon to nitrogen ratios will require higher feeding 
rates to achieve similar results (Manurung et al., 2016). Although, BSF larvae can 
successfully grow and thrive on a wide range of organic wastes that dominate most 
cities of SSA (Okot-Okumu, 2012), this study has demonstrated that the larvae yield 
would largely depend on the substrate qualities such as nutrient content and C/N ratio. 
4.2.5 Temperature, pH, and electrical conductivity during frass composting 
The variation in room and substrate temperature during the larval rearing and 
composting phase (post-larval rearing phase), respectively, is presented in Figure 8a. 
The temperature of the various substrates during the larval rearing phase increased from 
initial values of between 26 and 31 
o
C to peak levels of between 26 and 43 
o
C on the 
third day. The highest initial temperature was recorded in substrate with C/N ratio of 20 
(42 
o
C).  During the composting phase, the room temperature was slightly above that of 
the substrate throughout the experiment.  
The pattern of temperature observed during the study has been previously 
reported by (Tumuhairwe et al., 2009). The highest temperature achieved at the onset of 
composting could be due to high availability of energy sources from simple 
carbohydrates such as simple sugars coupled with amino acids and proteins that are 
easily broken down by microbes to release energy in form of heat that eventually causes 
rise in temperatures (Bernal et al. 2009).  
75 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 8: Effect of substrate C/N ratio on temperature (a), pH (b) and electrical 
conductivity (EC) (c), during BSF larvae rearing and frass composting phases. 
Key: C/N = total organic carbon to total organic nitrogen ratio; Control = unamended substrate (brewery 
spent grain); C/N 15, C/N 20, C/N 25, and C/N 30 = amendment of brewery spent grain using sawdust to 
obtain substrates with C/N ratios of 15, 20, 25 and 30, respectively.  
 
76 
 
As the composting process progresses, the available sources of energy get 
depleted and microbial activities reduce, causing a decline in temperature as observed 
from the 10
th
 day of experiments. Temperature evolution during composting is majorly 
determined by the nature of substrate, size of the compost heap/pile and moisture 
content of decomposing materials (Epstein, 1997).  
A big compost heap/pile conserves temperatures within the heap due to self-
insulating property and this causes rapid heating to thermophilic temperatures (Said-
Pullicino et al. 2007). Therefore, the low temperatures (< 45 ℃) observed in this study 
could partly be attributed to the small size of substrate heaps (≤ 10 cm deep and 50 cm 
wide) required in BSF rearing. Furthermore, since there was no covering of rearing 
trays, most of the heat could have been lost to the environment, thus leading to lower 
temperatures in the rearing substrates (Bernal et al., 2009). Nevertheless, the low 
temperatures (< 45 ℃) observed in this study (Figure 8a) could have been beneficial in 
conserving ammonia (Figure 9a), which is highly volatilized at thermophilic 
temperatures (> 45℃) (Bernal et al., 2009). 
The pH of the substrates during the larval rearing and composting phase varied 
significantly (p = 0.015) (Figure 8b). The pH reached peak values of between 8.1 and 
8.8 in the third week, with the highest value recorded in substrates with C/N ratio of 30. 
At the end of the experiment, the control treatment had the highest pH of 7.6 while frass 
compost from substrates with C/N ratio of 30 had the lowest pH of 6.9. The electrical 
conductivity (EC) values of the substrates differed significantly during the experiment 
(p < 0.001) (Figure 8c). The EC increased to peak values of between 2.7 and 4.2 mS cm
-
1
 in the first three weeks after which the values decreased up to the end of experiment.  
77 
 
The EC of the unamended (control) was significantly higher than those of the 
other substrates throughout the experiments. The increase in pH and electro conductivity 
at initial stages could be attributed to evolution of ammonium (Figure 8), resulting from 
rapid degradation of organic substrates (Said-Pullicino et al., 2007, Azim et al., 2018). 
During the ammonification process, organic N is converted into the ammonia gas which 
reacts with water to form ammonium hydroxide which is alkaline and thus high pH 
(Sánchez-Monedero et al., 2001).  
During nitrification (an oxidation process), the hydrogen ions are formed and 
released into the compost matrix, and this leads to the decrease of pH as observed in the 
fourth and fifth weeks (Figure 8b). Oxidation of one molecule of ammonium to nitrate 
gives rise to two molecules of hydrogen ions  which are the cause of pH reduction 
(Aparna et al., 2008; Sánchez-Monedero et al., 2001). More so, during the maturation 
phase, organic acids, such as humic acids are produced and these also contribute to 
reductions in compost pH (Epstein, 1997).  
In the same phase (maturation), reduction in electro conductivity is mostly due to 
precipitation or adsorption of most cations on to humic substances; thus making the 
cations unavailable in compost matrix (Huang et al. 2004). Substrates with higher 
organic carbon content are most likely to produce more humic acids than those with low 
organic carbon levels. Therefore, the pH values of less than 7 and lower EC values 
observed in treatments C/N20, C/N25 and C/N30 (Figure 8b and 8c) could be attributed 
to higher nitrification and production of organic acids during the maturation phase of 
compost compared to control and C/N15 which experienced low nitrification.  
78 
 
The mature frass compost generated had pH (Figure 8b) and electrical 
conductivity (Figure 8c) values that were also within the recommended ranges (6 – 8 for 
pH and < 4 mS cm
-1
 for electrical conductivity) (Huang et al., 2004; Bernal et al., 2009). 
4.2.6 Ammonium and nitrate concentrations, and ammonium nitrate ratio 
Increasing substrate C/N ratios caused significant (p < 0.001) differences in 
ammonium concentration during the study (Figure 9a). The ammonium concentration 
recorded in the control treatment was significantly higher (p < 0.05) than those of the 
other frass composts, followed by that of substrates with C/N ratio of 15.  
The ammonium concentrations increased steadily in the control experiment up to 
the fourth week and decreased slightly in the fifth week, while those of composts from 
substrates with C/N ratio of 15 increased from the second week up to the end of the 
experiment (Figure 9a). The nitrate concentration changed significantly (p < 0.001) over 
time and was significantly (p < 0.05) influenced by the substrate treatments (Figure 9b). 
The nitrate concentration of substrates with C/N ratio 15 sharply increased after the 
fourth week and peaked at the fifth week (159.6 mg kg
-1
); while for the control 
treatment, the nitrate concentration decreased in the fifth week of experiment. At the end 
of the fifth week, the lowest concentration of nitrate was recorded from frass compost 
derived from substrates with C/N ratio of 25 (59.8 mg kg
-1
).  
 
79 
 
Figure 9: Effect of substrate C/N ratio on concentrations of ammonium (a), nitrate (b) 
and ammonium/nitrate ratio (c) during BSF larvae rearing and frass composting phases. 
Key: C/N = total organic carbon to total organic nitrogen ratio; Control = unamended substrate (brewery 
spent grain); C/N 15, C/N 20, C/N 25, and C/N 30 = amendment of brewery spent grain using sawdust to 
obtain substrates with C/N ratios of 15, 20, 25 and 30, respectively.  
 
 
80 
 
The ammonium to nitrate ratios of compost generated from substrate with C/N 
ratio of 30 were the highest at weeks 2 and 3 (Figure 9c). There was an overall decline 
of ammonium-nitrate ratios among the various frass composts from the third week of the 
experiment. However, the ammonium to nitrate ratio of control treatment increased in 
the fifth week. Mature frass compost generated from substrates with a C/N ratio of 30 
had the lowest ratio of ammonium/nitrate ratio (0.4), while the control treatment had the 
highest (35.2). 
Through some studies, it has been revealed that nitrogen mineralization 
(ammonification and nitrification processes) is controlled by the substrate C/N ratio, 
availability of microorganisms, compost pH and temperature (Huang et al. 2004). The 
high ammonium concentration at initial stages (Figure 9a) could be attributed to the high 
pH (> 8) (Figure 8b) and higher temperature (Figure 8a) that prevailed during the early 
stages of the experiment. The low temperatures (< 45℃) observed in this study (Figure 
8a) therefore, were beneficial in conserving ammonia (Figure 5d), which is highly 
volatilized at thermophilic temperatures (> 45℃) (Bernal et al., 2009).  
As soon as the pH dropped below 8 (Figure 8b), nitrate concentration started 
increasing (Figure 9b), indicating the onset of the nitrification process. Nitrifying 
bacteria (nitrosomonas and nitrobacter) are mainly favoured by neutral pH range and 
mesophilic temperatures (Sánchez-Monedero et al., 2001) which dominated the process 
from the third week up to the end of experiment. Therefore, the surge in nitrate 
concentration observed in fourth and fifth weeks for all treatments where C/N ratio was 
increased could be attributed to favorable pH for nitrification process.  
81 
 
For example, the low pH and EC values observed in substrates with C/N ratio of 
15 and 20 might have been responsible for enhancing the nitrification process, which 
could have had a direct effect on the total nitrogen retained in the frass compost (Table 
9). Substrates with high C/N ratios are difficult to breakdown and this reduces the 
ammonium concentration released from such substrates (Huang et al., 2004). Therefore, 
the significantly low concentrations of ammonium observed in treatments whose C/N 
ratios were adjusted to 20 and above could be attributed to the high amount of resistant 
carbon tissues in form of lignin and cellulose from the added sawdust. Nevertheless, the 
rate of conversion of ammonium to nitrate nitrogen remained constantly low in this 
study (Figure 9c). Future studies should determine the population and species of 
nitrifying bacteria in BSF composted manures to devise microbe-based approaches for 
improving the nitrification process. 
The ammonium to nitrate ratios (0.4 – 35) of the mature frass composts (Figure 
9a) were above the recommended value of < 0.16 (Bernal et al., 1998). High ammonium 
concentration might have reduced the action of nitrifying bacteria by inhibiting possible 
enzymatic activities (Agyarko-Mintah et al., 2017). The findings of this work are in line 
with other studies (Guo et al., 2012), which reported much higher ammonium to nitrate 
ratios (1 – 60) of mature compost.  
 
 
82 
 
4.2.7 Phytotoxicity of frass composts  
Frass composts generated from the various substrates had significant influence 
on the percent seed germination (p = 0.022) and seed germination index (p = 0.033) 
(Table 11). The seed germination percent was above 80% for all substrates tested. The 
frass compost derived from control and substrates with C/N ratio of 20 had the highest 
seed germination rates, while the compost from substrate with C/N ratio of 15 had the 
lowest value (83%). The seed germination indices (GI) of frass composts generated 
from substrates with C/N ratios of 15 and 20 had significantly higher values than other 
composts. No significant differences in GI were observed between the compost derived 
from control substrate and that produced from substrates with C/N ratios of 25 and 30.  
Table 11: Effect of substrate carbon to nitrogen (C/N) ratio adjustment on seed 
germination and seed germination indices of mature BSF frass composts. 
Rearing substrates Seed germination (%) Seed germination index 
(%) 
Control (C/N 11) 100 ±0.0a 62.4 ± 39.5b 
C/N15 83.3 ± 3.3b 160.3 ± 47.3a 
C/N20 100 ± 0.0a 190.7 ± 26.2a 
C/N25 93.3 ± 3.3a 71.5 ± 12.9b 
C/N30 93.3 ± 3.3a 81.6 ± 23.6b 
p value * * 
Key: *p < 0.05; C/N = total organic carbon to total organic nitrogen ratio; Control = unamended substrate 
(brewery spent grain); C/N 15, C/N 20, C/N 25, and C/N 30 = amendment of brewery spent grain using 
sawdust to obtain substrates with C/N ratios of 15, 20, 25 and 30, respectively. In the same column, means 
(± standard error) followed by the same letters are not significantly different at p ≤ 0.05. 
 
 
 
 
83 
 
Germination index is used as a reliable indicator to evaluate the effect of 
compost extract on crop growth (Emino & Warman, 2004; Bernal et al., 2009). 
Germination index values below 50% indicate high phytotoxicity, while GI of 50 – 80% 
indicate moderate phytotoxicity and GI values above 80% indicate the absence of 
phytotoxicity. When the GI exceeds 100%, the compost is considered a phytonutrient or 
phytostimulant (Teresa & Remigio, 2011). The high percentage seed germination and 
germination index values recorded (Table 10) is an indication that all the composts 
generated from substrate with C/N ratios of 15 – 30 were free of phytotoxic substances.  
The germination indices of above 100% for substrates with C/N ratios of 15 and 
20 were a clear indication of complete frass compost maturity and stability (Teresa & 
Remigio, 2011). On the other hand, the lowest germination index registered for compost 
generated from the control (C/N ratio of 11) (Table 11) is an indicator of moderate 
phytotoxicity, probably due to high ammonium concentration and electrical 
conductivity, that have been reported to hinder root growth (Teresa & Remigio, 2011). 
The results of the germination indices reported in the current study for all the frass 
composts were within the ranges  reported by several authors (Guo et al., 2012; Antil et 
al., 2013), which was achieved within five weeks.  
4.2.8 Total organic carbon and total nitrogen 
The total organic carbon concentration of the various treatments varied 
significantly (p < 0.001) throughout the experiment. The trends of total organic carbon 
concentration are presented in Figure 10a. The initial total organic carbon ranged 
between 31 (control treatment) and 49% in substrates with C/N ratio of 30. 
84 
 
The total organic carbon contents of all the experimental substrates declined 
considerably throughout the experiment. Frass compost generated from substrates with a 
C/N ratio of 30 had the highest (30%) total organic carbon content at the end of 
experiments while compost generated from the control substrate had the least (24%).  
When organic matter is aerobically broken down, carbon is lost to the 
atmosphere in form of carbon dioxide gas while nitrogen and other nutrients are retained 
in the residual materials (Tumuhairwe et al., 2009). Such changes cause high carbon loss 
which give way to narrow C/N ratios of the decomposing materials. However, as the 
process goes on, the easily decomposable materials get depleted quickly effecting the 
reduction in microbial activities as indicated by minimal changes in organic carbon 
observed from third week up to the end of experiment (Epstein, 1997). 
Adjustment of substrate C/N ratios also caused significant differences (p = 
0.005) in total N concentrations. A consistent decline in total N concentrations in all the 
treatments was observed until the third week after which there was a gradual increase 
until the fifth week (Figure 10b). The control treatment maintained highest N 
concentration in the first three weeks, while total N concentration of substrate with a 
C/N ratio of 15 increased beyond that of the control in the 5
th
 week. The total N 
concentration of mature frass compost generated from substrates with C/N ratio of 15 
(2.5%) was significantly (p < 0.001) higher than those of frass composts derived from 
substrates whose C/N ratios were increased. 
 
 
85 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 10: Effect of substrate C/N ratio on concentrations of total organic carbon (a), 
total nitrogen (b) during BSF larvae rearing and frass composting phases. 
Key: C/N = total organic carbon to total organic nitrogen ratio; Control = unamended substrate (brewery 
spent grain); C/N 15, C/N 20, C/N 25, and C/N 30 = amendment of brewery spent grain using sawdust to 
obtain substrates with C/N ratios of 15, 20, 25 and 30, respectively.  
 
 
 
 
 
86 
 
There was a significant (p = 0.015) variation in the percent change of nitrogen 
across the different treatments. The substrate with C/N ratio of 15 was the most efficient 
in nitrogen conservation, as indicated by a 21% higher N concentration of frass compost 
generated compared to the control. However, there was an increased reduction of 
nitrogen content in frass compost generated from substrates with C/N ratio of 30 (-21%) 
and the control (-19%).  
The higher N retention in frass composts generated from substrates with C/N 
ratios of 15 and 20 compared to the control substrate (Table 10), indicate the effect of 
C/N adjustment on improving compost quality. Increase in total nitrogen during 
composting has been previously reported by Lalander et al. (2015) and indicate that the 
increase in total N in the frass could be due to reduced N losses during the compost 
maturation phase. There was reduced loss of nitrogen in all the substrates with C/N 
ratios ranging between 15 and 30 as a result of enhanced retention of ammonium 
nitrogen through microbial immobilization (Huang et al., 2004) and the binding of 
ammonium onto phenolic compounds of sawdust which in turn reduces amount of labile 
ammonium ions (Lim et al., 2017). 
Studies have also reported that nitrogen mineralization (ammonification and 
nitrification processes) during composting can be controlled by the substrate’s C/N ratio, 
compost pH and temperature (Huang et al., 2004; Ogunwande et al., 2008). Therefore, 
the low temperature (Figure 8a) and pH values (< 7.5) (Figure 8b) observed for most 
treatments in the fourth week of the experiment could have also been crucial in 
controlling ammonia volatilization; thereby conserving N in the frass compost.  
87 
 
High carbon materials like sawdust also release a lot of organic acids (Chefetz et 
al., 2010) that reduce compost pH during maturation phase, thereby deterring ammonia 
volatilization. In comparison however, the high pH in control treatment during 
maturation phase must have triggered ammonia volatilization, and consequently a 
reduction in total N of mature frass compost. Furthermore, frass compost derived from 
substrates with C/N ratio 15 had higher nitrate concentration in the fifth week than the 
control (Figure 9b), meaning that most of its ammonium was converted to nitrates as 
opposed to the control treatment, which might have led to more nitrogen accumulation 
in mature frass compost than the control treatment. 
4.2.9 Total phosphorus and potassium 
The total P concentration in the different frass composts was found to vary 
significantly (p = 0.004) during the composting period (Figure 11a). Phosphorus 
concentration was observed to decline significantly until the third week of the 
experiment, followed by a slight increase at fifth week. The P concentration of frass 
compost derived from substrate with C/N ratio of 15 was significantly (p = 0.004) 
higher than those of the other treatments until the third week. However, the P 
conservation in frass compost generated from substrate with C/N ratio of 30 remained 
low throughout the experiment compared to the other composts.  
At the end of the composting process, frass compost derived from substrates 
with C/N ratio of 15 had significantly (p < 0.001) higher P concentration (2.0 g kg
-1
) 
than those derived from substrates with higher C/N ratios. However, there was a 
significant (p < 0.037) percent reduction in total P concentration at end of the 
experiment in all the frass composts.  
88 
 
The lowest P reduction was observed in compost from substrate with a C/N ratio 
of 15 (-45%), while the highest percent reduction was recorded in the compost derived 
from substrate with a C/N of 30 (-68%). Total K concentrations were found to vary 
significantly (p = 0.029) among the different experimental substrates tested (Figure 
11b). Substrates with C/N ratios of 20 and 15 had the highest initial total K 
concentrations while the substrate with a C/N ratio of 30 had the least.  
 
 
 
 
 
 
 
 
 
 
Figure 11: Effect of substrate C/N ratio on concentrations of total phosphorus (a) and 
potassium (b) during BSF larvae rearing and frass composting phases. 
Key: C/N = total organic carbon to total organic nitrogen ratio; Control = unamended substrate (brewery 
spent grain); C/N 15, C/N 20, C/N 25, and C/N 30 = amendment of brewery spent grain using sawdust to 
obtain substrates with C/N ratios of 15, 20, 25 and 30, respectively.  
89 
 
 
Total K concentration reduced to minimum levels in the first three weeks and 
then increased from the third week up to the end of experiments. At the end of the 
composting phase, K concentration in frass compost generated from the control 
substrate (1.6 g k
-1
) was higher (p = 0.057) than those of frass composts generated from 
amended substrates. Substrates with a C/N ratio of 30 generated frass compost with the 
least K concentration (0.8 g kg
-1
). 
The percentage change of K content at the end of the composting process 
(mature frass compost) was observed to vary significantly (p = 0.043) among the 
different treatments. Increase in K concentration was observed in composts from the 
control substrate (44%) and compost derived from substrates with C/N ratio of 25 
(1.5%) while the least reduction (-12%) in K concentration was recorded in frass 
compost derived from substrate with C/N ratio of 15.  
The highest decrease in total K (-20%) was recorded in the compost generated 
from substrates with C/N ratio of 30 and this was slightly higher than that of compost 
generated from substrates with a C/N ratio of 10 (-18%). Unlike nitrogen, phosphorus 
(Figure 11a) and potassium (Figure 11b) concentrations showed minimal variation 
during the experiment. This is because phosphate ions are usually bound onto organic 
matter or chemically precipitated to form calcium and magnesium complexes at high pH 
levels (> 8) and therefore not easily lost from the compost matrix (Tumuhairwe et al., 
2009).  
 
90 
 
 In the mature frass compost, K concentration among the substrates did not vary 
significantly (Figure 11b). During composting, K is usually lost through leaching due to 
excessive moisture, which was not the case in the present studies because the moisture 
content was maintained at field capacity, that is between 40 and 65%. Therefore, the 
reduction in phosphorus and potassium (Figure 11b) observed were due to uptake by 
black soldier fly larvae (Table 10). However, much as treatments substrates with C/N 
ratio of 15 and control had the highest larvae yields, their final P and K contents were 
higher (Figure 11) because of the initially high levels in the substrates (Table 1). 
Potassium usually is not much in plant tissues because it is not one of the building units 
like carbon, hydrogen, oxygen, phosphorus, magnesium, iron, and silicon but is a 
nutrient that plays a big role in catalytic-enzymic reactions in the plant. 
4.2.10 Ratios of carbon to nitrogen during experiment 
The ratios of total organic carbon to total nitrogen (C/N ratio) and water-soluble 
carbon to total nitrogen (WSC/TN) are presented in Table 12. The C/N and WSC/TN 
ratios were observed to decrease progressively with composting time in all the substrates 
tested. The highest C/N ratios were recorded from substrate with C/N ratio of 30 where 
values ranged between 23 and 39.  At the end of experiments, frass composts generated 
from substrates with C/N ratio of 15 had the lowest C/N (10.5) and WSC/TN (1.0) 
ratios. The C/N ratios in mature frass compost ranged between 10.5 and 23.4 (Table 12), 
which is within the recommended range (< 25) for field application (Goyal et al., 2005). 
However, the ratios WSC/TN in all mature composts were above the recommended 
value of < 0.55 (Bernal et al., 1998). This could be partly attributed to the recalcitrant 
carbon introduced by sawdust amendment.  
91 
 
Table 12: Ratios of organic carbon and water-soluble carbon to total nitrogen at 
selected periods during the experiment. 
Ratio Time 
(weeks) 
Rearing substrates 
 Control  C/N15  C/N20  C/N25  C/N30 
C/N 
0 11.5 ± 0.95 15 ± 0.35 20.4 ± 0.99 25 ± 1.53 29.8 ± 0.32 
1 13.7 ± 0.68 19.3 ± 2.2 23.7 ± 0.75 31.4 ± 1.51 36.9 ± 3.74 
3 21 ± 3.62 24.8 ± 1.85 30.9 ± 2.85 34.6 ± 1.42 38.9 ± 0.44 
5 11.5 ± 1.47 10.5 ± 0.58 18.7 ± 0.98 17.7 ± 0.64 23.4 ± 1.72 
WSC/TN 
0 4.8 ± 0.57 5.2 ± 0.16 3.5 ± 0.11 4.2 ± 0.37 4.6 ± 0.06 
 
1 5.2 ± 0.88 5.2 ± 1.04 5.4 ± 0.18 6.1 ± 0.48 6.1 ± 0.59 
 
3 4.2 ± 1.16 3.0 ±0.80 3.3± 0.22 1.8 ± 0.67 3.0 ± 1.16 
 
5 1.2 ± 0.25 1.0 ± 0.07  1.7 ± 0.24 1.6 ± 0.29 1.7 ± 0.24 
Key: C/N = total organic carbon to total organic nitrogen ratio; Control = unamended substrate (brewery 
spent grain); C/N 15, C/N 20, C/N 25, and C/N 30 = amendment of brewery spent grain using sawdust to 
obtain substrates with C/N ratios of 15, 20, 25 and 30, respectively. WSC/TN = water-soluble carbon to 
total nitrogen. 
 
4.3 Effect of biochar and gypsum amendments on BSF larval performance and 
nitrogen retention in frass fertilizer  
4.3.1 Black soldier fly larval yields and biomass conversion rates 
There was a significant effect of rearing substrates on the fresh (experiment set 
1: p < 0.001, set 2: p < 0.001) and dry (experiment set 1: p = 0.003, set 2: p < 0.001) 
yields of BSF larva (Figure 12a and 12b). Substrates amended with 20% biochar 
produced significantly higher fresh (experiment set 1: p < 0.001, set 2: p < 0.001) and 
dry (experiment set 1: p < 0.001, set 2: p < 0.01) larval yields than the gypsum and 
control treatments in both experiments.  
 
 
92 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 12: Effect of biochar and gypsum amendments on fresh (a) and dry (b) black 
soldier fly larval yields during experiments. 
Key: Control = unamended substrate (brewery spent grain); 5Gypsum, 10Gypsum, 15Gypsum = substrate 
amended with 5, 10 and 15% gypsum, respectively; 5Biochar, 10Biochar, 15Biochar and 20Biochar = 
substrate amended with 5, 10, 15 and 20% biochar, respectively. For each parameter and panel, means (± 
standard error) followed by the same letters are not significantly different at p ≤ 0.05. 
 
 
 
93 
 
Substrates amended with gypsum resulted in decreased larval yields, whereby 
dry larval yields decreased by 0.6 – 30% and 8 – 25% during experiment set 1 and 2, 
respectively. However, these differences were not statistically significant. The highest 
decline in dry larval yield was recorded on substrates amended with 15% gypsum 
(Figure 12a). Amendment of rearing substrate using biochar significantly increased the 
wet and dry BSF larval yields by up to 89 and 85%, respectively, compared to the 
control substrate. Larval yields increased with increase in the amount of biochar added, 
whereby substrates amended with 20% biochar produced the highest fresh (382 and 398 
g for experiment set 1 and 2, respectively) and dry (125 and 130 g for experiment set 1 
and 2, respectively) larval yields (Figure 12).  
Significant differences were also observed in biomass conversion rates (BCR) 
among the various treatments in experiment set 1 (p = 0.002) and set 2 (p < 0.001) 
(Table 13). Larvae provided with substrates amended with 20% biochar had around 
double (15 and 24% for experiment set 1 and 2, respectively) the BCR recorded in the 
control. Larvae fed on substrates amended with 20% biochar had significantly 
(experiment set 1: p = 0.002, set 2: p < 0.001) higher BCR than those reared on the 
control substrate and substrates amended with gypsum, but not for larvae reared on 
substrates amended with 10 and 15% biochar in experiment set 2. The lowest BCRs 
(6.9%) were recorded from larvae fed on substrates treated with 10 and 15% gypsum.  
 
 
94 
 
The higher yields of fresh and dried BSF larvae and BCR (Figure 12a and 12b) 
from biochar-amended substrates, especially at inclusion levels of 15 and 20% could be 
attributed high availability of N for larval uptake. This is in line with results reported by 
Awasthi et al. (2016), who found that improved nitrogen conservation can be achieved 
through biochar amendment of substrates. Similarly, Lalander et al. (2019) 
demonstrated that BSF larvae fed on substrates with high nitrogen concentration 
developed faster and had higher biomass accumulation. Furthermore, Sanchez-
Monedero et al. (2018) also showed that biochar amendment has the ability to improve 
water holding capacity and porosity of substrate, which enhances larvae mobility and 
feeding and thus resulting into high larval biomass.  
 
 
 
 
 
 
 
 
 
 
 
 
 
95 
 
Table 13: Waste degradation, biomass conversion rates and frass compost yield by BSF larvae reared on biochar and gypsum 
amended substrates during experiment set 1 and 2. 
Substrate 
Formulations 
Experiment set 1  Experiment set 2 
Waste 
degradation 
efficiency (%) 
Biomass 
conversion 
rate (%) 
Frass compost 
yield per tray 
(kg) 
 Waste 
degradation 
efficiency (%) 
Biomass 
conversion  
rate (%) 
Frass compost 
yield per tray 
(kg) 
 Control 57.4 ± 1.1a 7.4 ± 1.08b 0.49 ± 0.02e  40.6 ± 0.31a 8.2 ± 0.5cd 0.75 ± 0.02d 
 5Gypsum 53.0 ± 1.9ab 8.0 ± 0.82b 0.58 ± 0.01de  36.0 ± 0.76ab 9.7 ± 0.3cd 0.89 ± 0.04cd 
 10Gypsum 44.0 ± 1.2bc 6.9 ± 0.78b 0.79 ± 0.03bc  30.4 ± 0.91bc 9.9 ± 0.9cd 1.03 ± 0.03abc 
 15Gypsum 42.7 ± 0.84bc 6.9 ± 0.21b 0.85 ± 0.01b  22.9 ± 1.60d 10.8 ± 1.4cd 1.12 ±0.02ab 
 5Biochar 52.8 ± 2.9abc 9.6 ± 1.40ab 0.59 ± 0.04de  37.2 ± 0.91ab 10.8 ± 1.1cd 0.81 ± 0.04d 
 10Biochar 45.0 ± 4.0bc 11.5 ± 1.61ab 0.63 ± 0.02cde  36.8 ± 0.56ab 14.2 ± 1.1bc 0.89 ± 0.03cd 
 15Biochar 47.9 ± 0.29abc 10.0 ± 0.43ab 0.68 ± 0.02bcd  32.9 ± 1.04bc 18.2 ± 1.4b 0.97 ± 0.01bc 
 20Biochar 42.2 ± 2.9c 14.6 ± 1.68a 1.13 ± 0.08a  26.3 ± 3.10cd 24.2 ± 1.3a 1.13 ± 0.04a 
p value *** ** ***  *** *** *** 
Key: *** p < 0.001, **p < 0.01; Control = unamended substrate (brewery spent grain); 5Gypsum, 10Gypsum, 15Gypsum = substrate amended with 5, 10 and 
15% gypsum, respectively; 5Biochar, 10Biochar, 15Biochar and 20Biochar = substrate amended with 5, 10, 15 and 20% biochar, respectively. In the same 
column, means (± standard error) followed by the same letters are not significantly different at p ≤ 0.05. 
96 
 
Several authors using nitrogen-rich substrates such as dog food, poultry food, 
abattoir waste, food remains, chicken, pig and cow manures
 
have also reported higher 
BSF larval yields (Oonincx et al., 2015; Lalander et al., 2019; Shumo et al., 2019). 
Despite its efficiency in nitrogen conservation in organic substrates (Table 14) (Ren et 
al., 2010), the low BSF larvae yields and BCR associated with gypsum amended 
substrates (Figure 12a and 12b) might be attributed to aggregation and drying properties 
of substrates induced by gypsum inclusion, which might have negatively influenced 
BSF growth performance. BSF larvae have been reported to thrive well in substrates 
with 70% moisture content (Cheng et al., 2017). Hence, the drying properties of 
substrates due to gypsum inclusion further explain the low larvae performance.  
High EC of substrates amended with gypsum was observed in the current studies 
(Figure 15a and 15b). These high EC levels are known to be directly related to high salt 
content added through gypsum, which has been documented to reduce nutrient uptake 
and assimilation by insect larvae (Clark et al., 2004). However, further studies to 
evaluate the effects of high EC on BSF larval performance are warranted. 
4.3.2 Waste degradation and frass compost yield 
The waste degradation efficiency in experiment set 1 (p < 0.001) and 2 (p < 
0.001) varied significantly (Table 13). The rate of waste degradation among the different 
treatments ranged between 23 and 57%. However, larvae fed on the control substrate 
had significantly higher (experiment set 1: p < 0.001, set 2 p < 0.001) waste degradation 
efficiency than those provided with substrates amended with > 5% gypsum and > 10% 
biochar, except for 15% biochar in experiment set 1.  
97 
 
The lowest waste degradation rates were recorded in substrates amended with 
15% gypsum (43 and 23% for experiment sets 1 and 2, respectively) due to reduced 
larval feeding arising from the high electrical conductivity, and aggregation and drying 
of substrates amended with gypsum (Clark et al., 2004; Cheng et al., 2017).  
The frass compost yields varied significantly in both experiments set 1 (p < 
0.001) and 2 (p < 0.001) (Table 13). All biochar and gypsum treatments produced higher 
frass compost yields than the control where frass compost yields increased with increase 
in the amounts of biochar or gypsum added in the substrates. The highest frass compost 
yields (1.13 kg) were recorded from substrates amended with 20% biochar treatment. 
The same treatment produced significantly (experiment set 1: p < 0.001, set 2: p < 
0.001) higher frass compost yields than other treatments, except for compost generated 
from substrates amend\ed with 10 and 15% gypsum in experiment set 2. Substrates 
amended with > 5% gypsum and > 10% biochar generated significantly (experiment set 
1: p < 0.001, set 2: p < 0.001) higher frass compost yields than the control treatment.  
The higher frass fertilizer yields achieved from substrates amended with biochar 
could be attributed to the lower values of waste degradation efficiency associated with 
these treatments (Table 13). This is consistent with previous studies reported by 
Manurung et al. (2016) and Supriyatna et al. (2016) when high recalcitrant carbon 
substrates with high C/N ratios such as rice straw
 
and cassava peels were used for 
rearing BSF larvae. Khan et al. (2014) also demonstrated that substrates amended with 
carbon-rich materials such as sawdust and biochar were extremely difficult to be broken 
down.  
98 
 
The factors that influenced the low frass fertilizer yields in unamended substrates 
(control treatment) compared to those amended with gypsum and biochar are likely due 
to the high waste degradation efficiency observed in control treatment (Table 13). This 
makes biochar a better amendment for organic fertilizer production since it also 
enhances frass fertilizer yields in addition to improving larval performance.  
4.3.3 Effect of biochar and gypsum on nitrogen retention and use by BSF larvae 
At the start of the experiments, the initial total nitrogen content of substrate 
amended with biochar and gypsum were significantly different (experiment set 1: p < 
0.001, set 2: p < 0.001) (Table 14). The control treatment had significantly (experiment 
set 1: p < 0.001, set 2: p < 0.001) higher initial total nitrogen content than substrates 
treated with various inclusion levels of biochar and gypsum. The quantity of nitrogen 
accumulated in larval biomass varied significantly both in experiment set 1 (p = 0.002) 
and set 2 (p < 0.001) (Table 14). Larvae fed on substrates amended with 20% biochar 
accumulated 2- and 3-times greater nitrogen (experiment set 1: p = 0.002, set 2: p < 
0.001) than those fed on control substrates in experiment set 1 and 2, respectively.  
The fraction of initial N retained in the raw frass varied significantly among the 
various substrates in experiment set 2 (p < 0.001) but not in experiment set 1 (p = 0.053) 
(Table 14). In experiment set 2, amendment of substrates with 15% gypsum retained 
significantly (p < 0.001) higher nitrogen in frass than in biochar and control treatments, 
except for 15% biochar. The fraction of initial N retained in mature frass compost also 
varied significantly (experiment set 1: p < 0.001, set 2: p = 0.001) (Table 14).  
99 
 
Table 14: Effect of biochar and gypsum amendments on nitrogen retention and uptake by BSF larvae during experiment set 1 and 2. 
Substrate 
formulations 
Experiment set 1   Experiment set 2  
Initial N 
content in 
substrate per 
tray (g) 
Fraction of 
initial N 
accumulated 
in larval 
biomass 
(g/100g) 
Fraction of 
initial N  
retained in 
raw frass 
 (g/100g) 
N content in 
frass 
fertilizer as 
% of initial 
N content (g/ 
100g) 
 Initial N 
content in 
substrates per 
tray (g) 
 
        
Fraction of 
initial N 
accumulated 
in larval 
biomass  
(g/ 100g) 
Fraction of 
initial nitrogen 
retained in raw 
frass (g/ 100g) 
N content in 
frass fertilizer 
as % of initial 
N content (g 
/100g) 
Control 52.0 ± 0.80a 4.8 ± 0.59b 37.1 ± 2.93 23.4 ±1.4d  43.1 ± 1.32a 4.8 ± 0.40c 55.0 ± 4.02c 33.6 ± 0.9c 
5Gypsum 44.0 ± 0.75bc 6.9 ± 0.81b 49.0 ± 5.48 28.7 ± 1.6d  36.2 ± 0.64bc 6.1 ± 0.35bc 68.6 ± 2.89abc 47.5 ± 2.1ab 
10Gypsum 43.1 ± 0.00c 4.8 ± 0.31b 50.7 ± 1.37 39.9 ± 3.4bc  35.2 ± 0.06bc 5.5 ± 0.64bc 73.9 ± 2.88ab 53.7 ± 2.8a 
15Gypsum 39.8 ± 0.56d 5.1 ± 0.43b 56.1 ± 1.40 41.5 ± 1.7b  33.5 ± 0.72c 4.8 ± 0.58c 83.3 ± 2.88a 52.2 ± 3.0ab 
5Biochar 46.4 ± 0.28b 7.2 ± 1.53ab 39.1 ± 0.95 25.2 ± 0.6d  38.4 ± 0.50ab 6.9 ± 1.05bc 54.2 ± 0.85c 40.7 ± 3.5abc 
10Biochar 43.6 ± 0.53c 8.3 ± 0.95ab 51.5 ± 8.45 28.1 ± 0.4d  38.8 ± 0.18ab 6.1 ± 0.01bc 51.2 ± 0.84c 39.5 ±3.0bc 
15Biochar 44.9 ± 0.64bc 6.4 ± 0.54b 43.3 ± 3.35 30.3 ± 1.4cd  32.6 ± 2.12c 9.4 ± 1.21ab 64.4 ± 6.97abc 48.1 ± 2.8ab 
20Biochar 34.1 ± 0.16e 11.6 ± 1.45a 44.7 ± 2.28 56.2 ± 3.7a  36.3 ± 0.19bc 11.4 ± 1.13a 62.2 ± 5.37bc 49.4 ± 3.5ab 
p value *** ** ns ***  *** *** *** ** 
Key: *** p < 0.001, ** p < 0.01, ns = not significant (p ≥ 0.05); Key: Control = unamended substrate (brewery spent grain); 5Gypsum, 10Gypsum, 15Gypsum = 
substrate amended with 5, 10 and 15% gypsum, respectively; 5Biochar, 10Biochar, 15Biochar and 20Biochar = substrate amended with 5, 10, 15 and 20% 
biochar, respectively. In the same column, means (± standard error) followed by the same letters are not significantly different at p ≤ 0.05. 
 
 
 
 
100 
 
Frass compost from substrates amended with 20% biochar retained significantly 
(p < 0.001) higher N content than other treatments in experiment set 1. In experiment set 
2, the fraction of initial N retained in mature frass composts from substrates amended 
with gypsum, and those amended with 15 and 20% biochar was significantly (p < 0.01) 
higher than that retained in the frass compost produced from the unamended substrate. 
The findings of this study are in line with those reported by Awash et al. (2016, 
2017) and Wang et al. (2018), who demonstrated that improved nitrogen conservation 
can be achieved through biochar amendment of substrates. Biochar amendments 
improve nitrogen conservation through mechanisms such as adsorption of ammonium 
on the negatively charged surface and absorption of ammonium ions into the pore 
spaces of biochar (Sanchez-Monedero et al., 2018).  For gypsum amended frass 
compost, the high N retention could have been influenced by the lower pH values, given 
that pH levels of 7.5 and below do not favour formation of ammonia gas (Liang et al., 
2006, Hao and Benke, 2008; Bernal et al., 2009) but chemically combines to form a 
more stable ammonium sulphate (Yang et al., 2015).  
4.3.4 Effect of biochar and gypsum on temperature, pH, and electrical conductivity 
during frass composting 
The variation in temperature during the BSF rearing and composting phases is 
presented in Figures 13a and 13b. The temperature during this period in both 
experiments peaked between 43 and 47 ºC on the third and fourth day of BSF rearing. 
The highest temperatures were recorded for substrates amended with 20% biochar (43 
ºC) and 15% biochar (47 ºC) on the third day for experiment set 1 and 2, respectively.  
101 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 13: Effect of biochar and gypsum amendments on temperature during 
experiment set 1 (a) and set (2) of BSF frass composting.  
Key: Control = unamended substrate (brewery spent grain); 5Gypsum, 10Gypsum, 15Gypsum = substrate 
amended with 5, 10 and 15% gypsum, respectively; 5Biochar, 10Biochar, 15Biochar and 20Biochar = 
substrate amended with 5, 10, 15 and 20% biochar, respectively.  
 
  
 
102 
 
A sharp decline of temperature was then observed up to the ninth day (28 – 30 
ºC). Thereafter, temperature fluctuations were negligible until the end of the experiment. 
This study reports for the first-time room and substrate temperature patterns during BSF 
facilitated composting (Figure 13a and 13b). Most studies using BSF for waste 
degradation and composting have focused on monitoring the room temperature only 
(Lalander et al., 2015; Isibika et al., 2019), yet it is the  temperature inside the substrate 
that greatly influences microbial activities and nitrogen dynamics (Bernal et al., 2009).  
The mesophilic temperature (45 ºC and below) patterns were observed in the 
current study (Figure 13a and 13b) during the composting process and were found to be 
within the range as reported in previous studies (Dias et al., 2010). The mesophilic 
temperature ranges are crucial in reducing nitrogen volatilization during composting 
(Bernal et al., 2009). The highest temperature values observed during the first week 
could be attributed to the rapid breakdown of available simple protein and carbohydrate 
sources for energy by BSF larvae and associated micro decomposers (Tumuhairwe et 
al., 2009).  
The pH was significantly (experiment set: p < 0.001, set 2: p < 0.001) affected 
by substrate amendments during experiments (Figure 14a and 14b). The pH significantly 
increased from initial values of between 3.6 and 5.0 and reached peak values on the 
third week of composting which ranged between 7.9 and 9.1. Substrate treated with 
gypsum had the lowest pH values (6.7 – 7.1) throughout the rearing and composting 
process in both experiments. On the other hand, substrates amended with 15% biochar 
inclusion had the highest pH value (7.8) at the end of composting process.  
103 
 
The low pH associated with gypsum amended substrates could be attributed to 
the acidity effect caused by sulphate ions in solution. When gypsum dissolves, the 
sulphate ions in solution combine with hydrogen ions to form sulphuric acid, that is 
responsible for all the apparent low pH values observed across all gypsum amended 
substrates (Wang et al., 2013). 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 14: Effect of biochar and gypsum amendments on pH of BSF frass compost 
during experiment set 1 (a) and 2(b). 
Key: Control = unamended substrate (brewery spent grain); 5Gypsum, 10Gypsum, 15Gypsum = substrate 
amended with 5, 10 and 15% gypsum, respectively; 5Biochar, 10Biochar, 15Biochar and 20Biochar = 
substrate amended with 5, 10, 15 and 20% biochar, respectively. 
 
104 
 
The EC of the various treatments also varied significantly (experiment set 1: p < 
0.001, set 2: p < 0.001) during both experiments. The pattern of EC fluctuations 
throughout the experiments are shown in Figures 15a and 15b. Gypsum amended 
substrates had significantly (p < 0.001) higher EC values than biochar amended 
substrates throughout the experiment period.  
 
 
 
 
 
 
 
 
 
 
 
Figure 15: Effect of biochar and gypsum amendments on electrical conductivity (EC) of 
BSF frass compost during experiment set 1 (a) and 2(b). 
Key: Control = unamended substrate (brewery spent grain); 5Gypsum, 10Gypsum, 15Gypsum = substrate 
amended with 5, 10 and 15% gypsum, respectively; 5Biochar, 10Biochar, 15Biochar and 20Biochar = 
substrate amended with 5, 10, 15 and 20% biochar, respectively.  
 
 
105 
 
The highest EC values were recorded in substrates amended with 15% gypsum 
in the third week (3.5 and 4.8 mS cm
-1
) for experiment sets 1 and 2, respectively. In 
mature composts, substrates amended with 15% gypsum had the highest EC value (4 mS 
cm 
-1
), while those amended with 20% biochar had the lowest EC value (0.36 mS cm
-1
). 
The decrease in EC observed in the fifth week could be largely attributed to 
precipitation of salts in solution during the compost maturation phase (Bernal et al., 
2009).  
The significantly high EC in substrate treated with gypsum during the 
experiments (Figure 15a and 15b) is in line with previous studies where the inclusion of 
salts of calcium, potassium and aluminium resulted in significant increase in EC values 
in composts (Choi & Moore, 2008; Yang et al., 2015; Wang et al., 2016). However, the 
EC values of mature compost for substrates amended with gypsum were also within the 
recommended range (< 4 mS cm
-1
) for field application (Huang et al., 2004).  
4.3.5 Ammonium and nitrate concentrations during frass composting 
Ammonium concentration was significantly influenced by substrate amendments 
(experiment set 1: p < 0.001, set 2: p < 0.001), composting time (experiment set 1: p < 
0.001, set 2: p < 0.001), but not their interactions (experiment set 1: p = 0.089, set 2: p = 
0.615) (Figure 16a and 16b). The ammonium concentration in all the treatments had 
similar patterns of increase in both experiments, whereby significant (p < 0.001) 
increases were observed in the first week. Gypsum amended substrates retained higher 
ammonium concentration than biochar treatments.  In mature frass composts, the 
ammonium concentration ranged between 588 and 4516 mg kg
-1
 in all the treatments. 
106 
 
However, the highest ammonium concentrations were observed in substrates amended 
with 15% gypsum (Figure 16a and 16b). 
 
 
 
 
 
 
 
 
 
 
 
Figure 16: Effects of biochar and gypsum amendments on concentrations of ammonium 
during experiment set 1 (a) and 2(b) of frass composting. 
Key: Control = unamended substrate (brewery spent grain); 5Gypsum, 10Gypsum, 15Gypsum = substrate 
amended with 5, 10 and 15% gypsum, respectively; 5Biochar, 10Biochar, 15Biochar and 20Biochar = 
substrate amended with 5, 10, 15 and 20% biochar, respectively.  
 
In the case of nitrate, the concentrations varied significantly due to substrate 
amendments with biochar and gypsum in experiment set 1 (p = 0.03) and composting 
time in both experiments (experiment set 1: p < 0.001, set 2: p < 0.001). The interaction 
107 
 
effect of substrates and composting time was significant in experiment set 2 only (p < 
0.001) (Figure 17a and 17b).  
 
 
 
 
 
 
 
 
 
 
 
Figure 17: Effects of biochar and gypsum amendments on concentrations of nitrates 
during experiment set 1 (a) and 2(b) of frass composting. 
Key: Control = unamended substrate (brewery spent grain); 5Gypsum, 10Gypsum, 15Gypsum = substrate 
amended with 5, 10 and 15% gypsum, respectively; 5Biochar, 10Biochar, 15Biochar and 20Biochar = 
substrate amended with 5, 10, 15 and 20% biochar, respectively.  
 
The peak concentrations of nitrate (71 – 114 mg kg-1) were observed in the 3rd 
and 5
th
 weeks of the experiment set 1 (p < 0.001) and 2 (p < 0.001), respectively. For 
108 
 
both experiments (set 1 and 2), the highest concentration of nitrates (72 mg kg
-1
) in the 
mature frass composts was recorded in substrates amended with 15% gypsum. 
 During experiment set 2, all treatments the ammonium concentration from the 
first week up to the end of experiment, indicating the onset of the nitrification process. 
The fairly high ammonium concentration in gypsum amended substrates could be 
attributed to the lower pH values associated with gypsum amended substrates (Figure 
14a and 14b) that could have reduced ammonium loss by inhibiting formation of 
ammonia gas through shifting the chemical equilibrium towards ammonium form 
(Wang et al., 2013).  
In addition to shifting the chemical equilibrium towards ammonium nitrogen, 
gypsum also chemically combined with ammonium ions to form ammonium sulphate 
(equation 22), a more stable from that is less prone to losses unlike ammonium 
carbonate (Wang et al., 2013, 2016; Yang et al., 2015).  
            (   )        (   )                                           (22) 
 
On the other hand, the higher ammonium retention in the biochar amended 
substrates could be attributed to the high cation exchange capacity, that could have 
caused ammonium adsorption. Furthermore, the high surface area of biochar has been 
reported to enhance absorption of ammonium ions in the pores (Sánchez et al., 2017; 
Sanchez-Monedero et al., 2018). Other studies have also reported higher ammonium 
retention due to amendment of substrates such as poultry litter with biochar (Agyarko-
Mintah et al., 2017).  
 
109 
 
 
On the other hand, the nitrification process in which ammonium is converted to 
nitrate nitrogen was rather slow during the experiments. The low nitrate concentration 
observed during the study  (Figure 17a and 17b) could be attributed to high ammonium 
retention by biochar and gypsum additives, whereby high ammonium has been found to 
inhibit the enzymatic activities of nitrifying bacteria (Agyarko-Mintah et al., 2017). On 
the contrary, the sharp decline in nitrate concentration observed in the fourth week 
during experiment set 1 (Figure 17a) could be largely attributed to nitrogen 
immobilization. 
4.3.6 Effect of biochar and gypsum on total organic carbon and total nitrogen 
during frass composting 
There was a significant impact of substrate amendments (experiment set 1: p < 
0.001, set 2: p < 0.001) on total organic carbon during frass composting. Initial total 
organic carbon concentration ranged between 34 and 40%, whereby substrates amended 
with 20 and 5% biochar had highest carbon concentrations during experiment set 1 and 
2, respectively (Figure 18a and 18b). There was a gradual decrease in total organic 
carbon concentration in compost treatments throughout both experiments. Significant 
decreases (p < 0.001) in total organic carbon were observed in the first and third weeks 
of experiments.  
Mature frass composts generated from substrates amended with 15% gypsum 
had the lowest carbon concentration (26%) while those generated from substrates 
amended with 5% biochar had the highest (35%) (Figure 18a and 18b). The trends of 
110 
 
carbon concentration observed during the study are normal and have been reported in 
other studies (Tumuhairwe et al., 2009).  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 18: Effect of biochar and gypsum amendments on total organic carbon during 
experiment set 1(a) and 2 (b) of BSF frass composting. 
 
Key: Control = unamended substrate (brewery spent grain); 5Gypsum, 10Gypsum, 15Gypsum = substrate 
amended with 5, 10 and 15% gypsum, respectively; 5Biochar, 10Biochar, 15Biochar and 20Biochar = 
substrate amended with 5, 10, 15 and 20% biochar, respectively. 
 
The rapid decrease in concentrations of total organic carbon observed in the first 
three weeks could be attributed to rapid breakdown of organic matter by the BSF larvae 
and microbial decomposers (Epstein, 1997; Tumuhairwe et al., 2009). The total N varied 
significantly due to substrate treatments (experiment set 1: p < 0.001, set 2: p < 0.001) 
111 
 
and composting time (experiment set 1: p < 0.001, set 2: p < 0.001) during experiments 
but the interaction effect was significant during experiment set 2 only (p < 0.001) 
(Figure 19a and 19b).  
 
 
 
 
 
 
 
 
 
 
 
Figure 19: Effect of biochar and gypsum amendments on total nitrogen during 
experiment set 1(a) and 2 (b) of BSF frass composting. 
Key: Control = unamended substrate (brewery spent grain); 5Gypsum, 10Gypsum, 15Gypsum = substrate 
amended with 5, 10 and 15% gypsum, respectively; 5Biochar, 10Biochar, 15Biochar and 20Biochar = 
substrate amended with 5, 10, 15 and 20% biochar, respectively. 
 
Initial N concentration ranged between 1.6 and 2.6%, whereby the control 
treatment had the highest while substrate amended with 20% biochar had the lowest. 
112 
 
The total N concentration for most treatments decreased in the first and third week 
during experiment set 1 and 2, respectively.  
 
The control treatment generated mature frass compost with significantly higher 
(p = 0.003) total N concentration (2.5%) than composts generated from substrates 
amended with 15% gypsum, and 10 and 20% biochar in experiment set 1 (Table 15).  In 
experiment set 2, the total N concentrations of mature frass composts generated from the 
control substrate and substrates amended with 5% biochar and 5% gypsum were 
significantly (p = 0.01) higher than the total N concentrations of frass composts 
amended with 15% gypsum, and 15 and 20% biochar.  
Mature frass composts generated from substrates amended with 20% biochar and 
15% gypsum had the lowest total N concentrations during experiment set 1 (1.7%) and 2 
(1.6%), respectively. A remarkable increase in N retention was observed in frass 
generated from substrates amended with biochar and gypsum (Table 14). For gypsum 
treated frass, N retention could have been influenced by the lower pH values (Figure 14a 
and 14b), given that pH levels of 7.5 and below do not favour formation of ammonia gas 
(Liang et al., 2006; Hao & Benke, 2008; Bernal et al., 2009) but chemically combines to 
form a more stable ammonium sulphate (Yang et al., 2015).  
Nitrogen conservation in biochar amended frass compost (Table 14) can be 
attributed to both adsorption and absorption mechanisms of ammonium ions as 
discussed in section 4.2.5 (Sánchez et al., 2017). However, frass compost generated 
from substrate amended with 20% biochar had the lowest N (Figure 19a and 19b) and P 
113 
 
concentrations (Figures 20a and 20b, Table 14), which could be partially attributed to 
high bio-conversion efficiency (Table 13) and nitrogen uptake of larvae reared on this 
substrate (Table 14) (Lalander et al., 2019; Shumo et al., 2019).
114 
 
Table 15: Effects of biochar and gypsum amendments on the concentrations of nutrients in mature BSF frass composts during 
experiment set 1 and 2. 
Substrate 
formulations 
Experiment set 1   Experiment set 2 
Nitrogen  
(%) 
Phosphorus 
 (g kg
-1
) 
Potassium  
(g kg
-1
) 
 Nitrogen 
 (%) 
Phosphorus  
(g kg
-1
) 
Potassium 
 (g kg
-1
) 
 Control 2.45 ± 0.02a 11.03 ± 0.34a 4.56 ± 0.62bc   1.94 ± 0.087a 10.58 ± 0.37a 0.82 ± 0.004bc 
 5Gypsum 2.18 ± 0.08ab 10.36 ± 0.29ab 2.89 ± 0.62c   1.92 ± 0.044a 8.74 ± 0.14b 0.69 ± 0.00c 
 10Gypsum 2.16 ± 0.15abc 8.44 ± 0.75bc 1.75 ± 0.37c   1.84 ± 0.102a 8.16 ± 0.36bc 0.67 ± 0.069c 
 15Gypsum 1.95 ± 0.05bc 6.79 ± 0.42c 1.55 ± 0.44c   1.55 ± 0.042a 7.24 ± 0.18c 0.56 ± 0.013c 
 5Biochar 1.99 ± 0.14abc 10.54 ± 0.58ab 4.82 ± 1.51bc   1.92 ± 0.120a 9.33 ± 0.25ab 1.12 ± 0.075ab 
 10Biochar 1.95 ± 0.07bc 9.42 ± 0.66ab 7.46 ± 1.26ab   1.72 ± 0.084a 9.11 ± 0.17b 1.35 ± 0.097a 
 15Biochar 1.99 ± 0.13abc 8.65 ± 0.10bc 9.63 ± 0.62a   1.60 ± 0.074a 7.17 ± 0.31c 1.31 ± 0.168a 
 20Biochar 1.70 ± 0.02c 8.34 ± 0.07bc 10.99 ± 0.56a   1.60 ± 0.069a 7.15 ± 0.29c 1.35 ± 0.085a 
p value ** *** ***   ** *** *** 
Key: *** p < 0.001, ** p < 0.01; Control = unamended substrate (brewery spent grain); 5Gypsum, 10Gypsum, 15Gypsum = substrate amended with 5, 10 and 
15% gypsum, respectively; 5Biochar, 10Biochar, 15Biochar and 20Biochar = substrate amended with 5, 10, 15 and 20% biochar, respectively. In the same 
column, means (± standard error) followed by the same letters are not significantly different at p ≤ 0.05. 
115 
 
4.3.7 Total phosphorus and potassium during frass composting 
Total P concentration varied significantly due to substrate amendments during 
both experiments (experiment set 1: p < 0.001, set 2: p < 0.001) (Figure 20a and 20b).  
There were significant (p < 0.001) increases in total P in the first and third weeks of the 
experiment set 1, and throughout experiment set 2.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 20: Effect of biochar and gypsum amendments on total phosphorus of BSF frass 
compost during experiment set 1(a) and 2(b). 
Key: Control = unamended substrate (brewery spent grain); 5Gypsum, 10Gypsum, 15Gypsum = substrate 
amended with 5, 10 and 15% gypsum, respectively; 5Biochar, 10Biochar, 15Biochar and 20Biochar = 
substrate amended with 5, 10, 15 and 20% biochar, respectively.  
116 
 
Amendment of substrates with 5% of either biochar or gypsum produced frass 
composts with higher total P content than the other amended treatments. The frass 
compost generated from the control substrate had significantly (experiment set 1: p < 
0.001, set 2: p < 0.001) higher total P concentration than other treatments, except for 
frass composts amended with 10 and 5% gypsum, and 10 and 10% biochar in 
experiment set 1, and 5% biochar in experiment set 2 (Table 15).  
Total K concentrations of the different compost treatments were also 
significantly affected by substrate amendments (p < 0.001, set 2: p < 0.001) and frass 
composting time (experiment set 1: p < 0.001, set 2: p = 0.023) during experiments. The 
interaction effect of substrate amendments and composting time was only significant in 
experiment set 1 (p = 0.006) (Figure 21a and 21b).  
Biochar amended composts had higher K concentrations than the control as well 
as gypsum treated composts. Substrates amended with 15% biochar had the highest 
initial K concentration (10 g kg
-1
) while those amended with 15% gypsum had the 
lowest (0.7 g kg
-1
). The concentrations of potassium in the frass composts were 
observed to increase with increased biochar rates, while it was the opposite with gypsum 
treatments, whereby potassium concentrations decreased with increased gypsum 
inclusion rates (Figure 21a and 21b). The mature frass composts generated from 
substrates amended with 15 and 20% biochar had significantly (p < 0.001) higher K 
concentrations than the other treatments, except for compost amended with 10% biochar 
in experiment set 1 (Table 15).  
 
117 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 21:Effect of biochar and gypsum amendments on total potassium of BSF frass 
compost during experiment set 1(a) and 2(b). 
Key: Control = unamended substrate (brewery spent grain); 5Gypsum, 10Gypsum, 15Gypsum = substrate 
amended with 5, 10 and 15% gypsum, respectively; 5Biochar, 10Biochar, 15Biochar and 20Biochar = 
substrate amended with 5, 10, 15 and 20% biochar, respectively.  
 
In experiment set 2, frass composts derived from substrates amended with 10 – 
20% biochar achieved significantly (p < 0.001) higher K levels than those generated 
from the control and gypsum amended substrates. The highest K concentration (11 g kg
-
1
) was recorded from compost generated from substrates amended with 20% biochar 
118 
 
while compost generated from substrates amended with 15% had the lowest K (0.56 g 
kg
-1
) (Table 15). 
The considerable variation in the trend in K concentration observed throughout 
the experiments (Figure 21a and 21b) is consistent with previous studies using chicken, 
pig and cow manure as rearing substrates for BSF larvae (Lalander et al., 2015; Oonincx 
et al., 2015). However, the highest K concentrations were achieved at 20% inclusion of 
biochar (Table 15), which can be attributed to the initial high concentrations of K 
introduced through biochar amendment (Table 3) (Sanchez-Monedero et al., 2018). 
Therefore, biochar amendment is also effective for production of potassium-rich organic 
fertilizer.  
The results of this study indicate that the N concentration in frass compost 
generated from all amended substrates (Table 15) were within the recommended 
standards of > 1% N according to the Kenya Bureau of Standards (KEBS) guidelines for 
optimal SAFI (Kenya Bureau of Standards, 2017). Also, the P and K concentrations in 
the frass compost (Table 15) and level of compost maturity achieved meet the required 
international standards and guidelines for compost quality (Brinton, 2000).  
4.3.8 Effect of biochar and gypsum on ratios of carbon to nitrogen during frass 
composting 
Ratios of total organic carbon to total nitrogen (C/N) and water-soluble carbon to 
total nitrogen (WSC/TN) decreased from the start up to the end of experiment (Table 
16).  At the start of the experiments, substrates treated with 15% biochar had the highest 
119 
 
C/N ratio (23.1) while the control substrate had the lowest C/N ratio (13.3). Minimal 
reduction in C/N ratio and WSC/TN ratio were noted across all treatments tested.  
120 
 
Table 16: Ratios of carbon (C/N) and water-soluble carbon (WSC/N) to total nitrogen of amended BSF frass composts. 
Ratios 
Time 
(weeks) 
Experiment set 1 
 Control  5Gypsum  10Gypsum  15Gypsum  5Biochar  10Biochar  15Biochar  20Biochar 
C/N  0 13.3 ± 0.33 15.4 ± 0.27 16.2 ± 0.43 17.1 ± 0.23 15.0 ± 0.43 15.6 ± 0.19 15.1 ± 0.33 21.7 ± 0.07 
 
1 15.2 ± 0.48 13.6 ± 0.49 15.7 ± 0.09 15.3 ± 0.65 16.7 ± 1.23 16.4 ± 1.32 17.5 ± 0.95 25.6 ± 0.12 
 
3 13.5 ± 0.67 13.6 ± 0.30 13.3 ± 0.79 12.5 ± 0.52 16.4 ± 1.07 14.2 ± 1.41 12.6 ± 0.72 19.7 ± 0.33 
 
5 12.8 ± 0.12 13.7 ± 0.10 13.6 ± 1.09 13.6 ± 0.52 15.8 ± 1.06 15.9 ± 0.62 15.2 ± 1.02 17.9 ± 0.15 
WSC/TN 0 0.50 ± 0.02 0.78 ± 0.05 0.72 ± 0.03 0.75 ± 0.02 0.53 ± 0.01 0.58 ± 0.04 0.57 ± 0.01 0.99 ± 0.03 
 
1 0.53 ± 0.06 0.45 ± 0.19 0.66 ± 0.07 0.53 ± 0.21 0.42 ± 0.11 0.39 ± 0.10 0.55 ± 0.06 1.05 ± 0.03 
 
3 0.47 ± 0.11 0.47 ± 0.10 0.52 ± 0.11 0.59 ± 0.02 0.52 ± 0.08 0.49 ± 0.12 0.37 ± 0.07 0.54 ± 0.06 
 
5 0.72 ± 0.15 0.47 ± 0.19 0.66 ± 0.27 0.66 ± 0.05 0.49 ± 0.14 0.55 ± 0.14 0.46 ± 0.03 0.56 ± 0.02 
 Experiment set 2 
C/N 0 18.5 ± 0.69 20.9 ± 0.07 20.2 ± 0.03 21.6 ± 0.72 20.9 ± 0.15 19.9 ± 0.26 23.1 ± 1.54 21.5 ± 0.23 
 
1 18.8 ± 0.66 180 ± 0.49 18.6 ± 0.62 17.6 ± 0.55 22.3 ± 0.46 21.9 ± 0.21 20.9 ± 0.55 20.6 ± 0.80 
 
3 22.2 ± 2.28 17.7 ± 0.66 17.4 ± 0.75 18.0 ± 1.23 23.1 ± 0.95 25.4 ± 3.85 23.4 ± 1.17 22.6 ± 4.00 
 
5 19.3 ± 1.28 17.4 ± 0.48 16.6 ± 0.54 19.3 ± 1.02 18.2 ± 0.93 19.3 ± 1.24 20.3 ± 1.08 18.7 ± 0.83 
WSC/TN 0 0.59 ± 0.02 0.75 ± 0.01 0.87 ± 0.01 0.88 ± 0.02 0.83 ± 0.06 1.14 ± 0.02 1.14 ± 0.07 1.16 ± 0.05 
 
1 0.84 ± 0.07 0.45 ± 0.06 0.46 ± 0.08 0.62 ± 0.06 1.11 ± 0.07 0.75 ± 0.10 0.43 ± 0.09 0.32 ± 0.03 
 
3 0.38 ± 0.12 0.25 ± 0.08 0.21 ± 0.05 0.23 ± 0.04 0.18 ± 0.02 0.40 ± 0.10 0.36 ± 0.02 0.27 ± 0.05 
 
5 0.22 ± 0.08 0.06 ± 0.02 0.07 ± 0.02 0.09 ± 0.03 0.11 ± 0.04 0.15 ± 0.03 0.07 ± 0.03 0.21 ± 0.10 
Key: Control = unamended substrate (brewery spent grain); 5Gypsum, 10Gypsum, 15Gypsum = substrate amended with 5, 10 and 15% gypsum, respectively; 
5Biochar, 10Biochar, 15Biochar and 20Biochar = substrate amended with 5, 10, 15 and 20% biochar, respectively. 
121 
 
In mature frass composts, biochar amended substrates had higher C/N ratios than 
gypsum amended and control treatments. At the end of the composting process, 
substrates amended with 15% biochar had the highest C/N ratio (20.3), while the control 
treatment had the lowest value (12.8). In comparison however, the control treatment had 
the highest ratio of WSC/TN in mature frass compost (0.77) while frass compost 
generated from substrates amended with 5% gypsum had the lowest (0.06). This was 
closely followed by frass compost derived from substrates amended with 10% gypsum 
and 15% biochar, with equal WSC/TN ratios of 0.07 (Table 16). The C/N ratios and 
WSC/TN of mature frass composts generated from the various substrates (Table 16) are 
within the range (< 20) recommended by Goyal et al. (2005) and Guo et al. (2012) for 
mature and stable composts.  
4.3.9 Phytotoxicity of mature frass compost extracts 
The germination rate (experiment set 1: p = 0.649, set 2: p = 0.255) and 
germination index (GI) (experiment set 1: p = 0.899, set 2: p = 0.076) of the cabbage 
seeds were not significantly influenced by the compost treatments for both experiments 
(Table 17). Percent seed germination ranged between 63 and 100%, whereby frass 
compost generated from substrates amended with 15 gypsum had the lowest value while 
compost generated from substrates amended with 5% biochar had the highest seed 
germination rate.  
Frass composts generated from biochar amended substrates had higher seed 
germination indices than the frass composts derived from the unamended substrate 
(Table 17). Similarly, frass fertilizers from substrates amended with biochar had higher 
seed germination rates and GI than those amended with gypsum in experiment set 2.  
122 
 
Table 17: Seed germination and germination indices of mature BSF frass composts 
amended with biochar and gypsum. 
Substrate 
formulations 
Experiment set 1  Experiment set 2 
Germination 
rate (%) 
Germination 
index (%) 
 Germination 
rate (%) 
Germination 
index (%) 
 Control 93.3 ± 3.3 90.2 ± 26.9  90 ± 5.8 232.2 ± 92.6 
 5Gypsum 90 ± 5.8 115.1 ± 13.8  93.3 ± 6.7 56.2 ± 7.4 
 10Gypsum 83.3 ± 6.7 120.8 ± 37.0  86.7 ± 6.7 101.1 ± 45.7 
 15Gypsum 86.7 ± 3.3 123 ± 19.5  63.3 ± 23.3 68.4 ± 29.2 
 5Biochar 83.3 ± 12.0 98.3 ± 29.9  100 ± 0.0 273.5 ± 51.8 
 10Biochar 93.3 ± 3.3 133.7 ± 15.5  90 ± 5.8 158.1 ± 29.8 
 15Biochar 93.3 ± 3.3 108.1 ± 11.6  100 ± 0.0 165.4 ± 61.4 
 20Biochar 96.7 ± 3.3 114.1 ± 9.5  90 ± 5.8 132 ± 30.2 
p value ns ns  ns ns 
Key: ns = not significant (p ≥ 0.05); Control = unamended substrate (brewery spent grain); 5Gypsum, 
10Gypsum, 15Gypsum = substrate amended with 5, 10 and 15% gypsum, respectively; 5Biochar, 
10Biochar, 15Biochar and 20Biochar = substrate amended with 5, 10, 15 and 20% biochar, respectively.  
 
The highest GI was recorded in frass compost generated from substrates 
amended with 5% biochar (274%) while frass compost derived from substrates amended 
with equivalent rate of gypsum had the lowest GI value (56%). This study revealed that 
biochar amendment of substrates did not influence the compost maturity period as 
indicated by the high cabbage seed germination rates (> 90%) and germination index 
values (> 80%) in mature compost extracts for all the treatments (Table 17). This 
implies that mature compost from substrates amended with biochar were free of any 
phytotoxic substances such as organic acids, salts and ammonia (Teresa & Remigio, 
2011), thus ready for field application after only five weeks of composting.  
 
 
123 
 
The low seed germination rates and germination indices observed for frass 
compost generated from substrate amended with 5 and 15% gypsum (Table 17) implies 
moderate phytotoxicity, which might be probably attributed to the high EC (Figure 15a 
and 15b). Moderate phytotoxicity of mature compost has been reported to affect seed 
germination and radical elongation (Emino & Warman, 2004; Teresa & Remigio, 2011). 
This  study demonstrated that the composting time of substrates amended with gypsum 
and biochar can be drastically reduced to 5 weeks using BSF larvae for initial 
breakdown compared to the conventional composting process of 8 – 24 weeks  
(Tumuhairwe et al., 2009 Dias et al., 2010; Awasthi et al., 2017a). 
4.4 Comparative performance of BSF frass fertilizer and commercial fertilizers on 
the growth, yield, and nitrogen use efficiency of maize  
4.4.1 Plant height  
The different sole organic and combined fertilizer treatments showed significant 
differences in maize plant heights at different growth stages during the short (sole 
organic: p < 0.001, combined: p < 0.001) (Figure 22) and the long rain seasons (sole 
organic: p < 0.001, combined: p < 0.001) (Figure 23). Plant heights significantly (p < 
0.001) increased from early seedling stage (27 – 94 cm) (35 DAP) to highest values (173 
– 264 cm) at silking stage (91 DAP) with the smallest increases in plant heights 
observed at silking stage (Figure 22 and 23). Among sole organic fertilizer treatments, 
plots treated with 5 and 7.5 t ha
-1
 of SAFI produced the tallest plants during the short 
rain (215 cm) (p < 0.001) (Figure 22a and 22b) and long rain (264 cm) (p = 0.26) 
season, respectively (Figure 23a and 23b).  
 
124 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 22: Effects of BSF frass fertilizer (a and b), commercial organic (SAFI) (a and 
b) and mineral (urea) (c) fertilizers on maize plant height during the short rain season 
experiments. 
Key: Control = unfertilized soil; 2.5BSF, 5BSF and 7.5BSF = application BSF frass fertilizer at rates of 
2.5, 5 and 7.5 t ha
-1
, respectively; 2.5SAFI, 5SAFI and 7.5SAFI = application SAFI organic fertilizer at 
rates of 2.5, 5 and 7.5 t ha
-1
; 30N BSF, 60N BSF and 100N BSF = application of BSF frass fertilizer at 
rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N SAFI, 60N SAFI and 100N SAFI = 
application of SAFI organic fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N 
UREA, 60N UREA and 100N UREA = application of urea fertilizer at rates equivalent to 30, 60 & 100 kg 
N ha
-1
, respectively; BSF = Black soldier fly frass fertilizer; SAFI = commercial organic fertilizer. 
125 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 23: Effects of BSF frass fertilizer (a and b), commercial organic (SAFI) (a and 
b) and mineral (urea) (c) fertilizers on maize plant height during the long rain season 
experiments. 
Key: Control = unfertilized soil; 2.5BSF, 5BSF and 7.5BSF = application BSF frass fertilizer at rates of 
2.5, 5 and 7.5 t ha
-1
, respectively; 2.5SAFI, 5SAFI and 7.5SAFI = application SAFI organic fertilizer at 
rates of 2.5, 5 and 7.5 t ha
-1
; 30N BSF, 60N BSF and 100N BSF = application of BSF frass fertilizer at 
rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N SAFI, 60N SAFI and 100N SAFI = 
application of SAFI organic fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N 
UREA, 60N UREA and 100N UREA = application of urea fertilizer at rates equivalent to 30, 60 & 100 kg 
N ha
-1
, respectively; BSF = Black soldier fly frass fertilizer; SAFI = commercial organic fertilizer. 
126 
 
For combined fertilizer treatments, plots treated with organic fertilizers produced 
taller plants than urea treated plots (Figure 22a – c and 23a – c). Application of 60 kg N 
ha
-1
 supplied as BSF frass fertilizer produced the tallest plants (208 cm) that were 7 and 
11% (p = 0.53) taller than those produced using equivalent rates of SAFI organic and 
urea fertilizers, respectively, during the short rain season (Figure 22a – c).  
Plots treated with 100 kg N ha
-1
 suppled using SAFI produced the tallest plants 
(263 cm) during the long rain season, which were 3 and 10% (p < 0.001) taller than 
those grown using equivalent rates of BSF frass fertilizer and urea fertilizer, respectively 
(Figure 23a – c). The control treatment and plots treated with 2.5 t ha-1 as well as 30 kg 
N ha
-1
 supplied using the SAFI produced the shortest plants during the two seasons of 
the experiments.  
The higher maize plant height associated with BSF frass fertilizer could be 
attributed to high growth rate arising from the high rate of nutrient release (Adin Yéton 
et al., 2019). Previous studies using frass fertilizer have reported the presence of growth 
hormones (Poveda et al., 2019), microbial strains and enzymes responsible for soil 
nitrogen cycling and plant defense against pests and diseases (Debode et al., 2016; 
Quilliam et al., 2020). The siderophores and chitinase contained in insect frass increase 
nutrients availability, acquisition, and metabolism through ammonia oxidization and 
nitrification (De Tender et al., 2019), thus the higher plant height observed of maize 
growth using BSF frass fertilizer. 
127 
 
4.4.2 Chlorophyll concentration 
The maize leaf chlorophyll concentration varied significantly at different growth 
stages due to different sole organic fertilizer treatments during the short (p = 0.032) 
(Figure 24) and long rain seasons (p = 0.003) (Figure 25). However, the chlorophyll 
concentration was significantly influenced by the different combined fertilizer 
treatments (short rain season: p < 0.001, long rain season: p < 0.001) and maize growth 
stages (short rain season: p < 0.001), long rain season: p < 0.001) only. The interaction 
effects of combined fertilizer treatments and maize growth stage were not significant 
(short rain season: p = 0.589, long rain season: p = 0.059).  
The highest chlorophyll concentrations (45 – 61 SPAD values) were achieved at 
late seedling stage (55 DAP) during the short rain season (Figure 24a – c). In the short 
rain season, chlorophyll concentrations for all treatments significantly decreased and 
reached their lowest values at silking stage. During the long rain season, however, a 
significant increase in chlorophyll concentrations was observed at both late seedling and 
silking stages (Figure 25a – c).  
Plots treated with BSF frass fertilizer applied at 5 and 7.5 t ha
-1
, 60 and 100 kg N 
ha
-1
 as well as urea fertilizer applied at 100 kg N ha
-1
 achieved highest chlorophyll 
concentrations (83 – 93 SPAD values) at silking stage compared to other treatments 
which peaked at late seedling stage. Plots treated with BSF frass fertilizer at 7.5 t ha
-1
 
produced plants with the highest chlorophyll concentrations, which were 18% (p = 
0.032) and 20% (p = 0.003) higher than those achieved using equivalent rates of the 
SAFI in the short and long rain seasons, respectively (Figure 25a and 25b). 
128 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 24: Effect of BSF frass fertilizer (a and b), commercial organic (SAFI) (a and b) 
and mineral fertilizer (urea) (c) on maize leaf chlorophyll concentration during the short 
rain season experiments. 
Key: Control = unfertilized soil; 2.5BSF, 5BSF and 7.5BSF = application BSF frass fertilizer at rates of 
2.5, 5 and 7.5 t ha
-1
, respectively; 2.5SAFI, 5SAFI and 7.5SAFI = application SAFI organic fertilizer at 
rates of 2.5, 5 and 7.5 t ha
-1
; 30N BSF, 60N BSF and 100N BSF = application of BSF frass fertilizer at 
rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N SAFI, 60N SAFI and 100N SAFI = 
application of SAFI organic fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N 
UREA, 60N UREA and 100N UREA = application of urea fertilizer at rates equivalent to 30, 60 & 100 kg 
N ha
-1
, respectively; BSF = Black soldier fly frass fertilizer; SAFI = commercial organic fertilizer. 
129 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 25: Effect of BSF frass fertilizer (a and b), commercial organic (SAFI) (a and b) 
and mineral (urea) (c) fertilizers on maize leaf chlorophyll concentration during the long 
rain season experiments. 
Key: Control = unfertilized soil; 2.5BSF, 5BSF and 7.5BSF = application BSF frass fertilizer at rates of 
2.5, 5 and 7.5 t ha
-1
, respectively; 2.5SAFI, 5SAFI and 7.5SAFI = application SAFI organic fertilizer at 
rates of 2.5, 5 and 7.5 t ha
-1
; 30N BSF, 60N BSF and 100N BSF = application of BSF frass fertilizer at 
rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N SAFI, 60N SAFI and 100N SAFI = 
application of SAFI organic fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N 
UREA, 60N UREA and 100N UREA = application of urea fertilizer at rates equivalent to 30, 60 & 100 kg 
N ha
-1
, respectively; BSF = Black soldier fly frass fertilizer; SAFI = commercial organic fertilizer. 
130 
 
During the long rain season, the highest chlorophyll concentration among the 
combined fertilizers was recorded from plants treated with 100 kg N ha
-1 
supplied as 
BSF frass fertilizer, and this was 32 and 7% (p < 0.001) higher those from plants treated 
with equivalent rates of SAFI (Figure 25b) and urea fertilizer (Figure 25c), respectively. 
The higher maize plant height (Figure 18), chlorophyll concentration (Figure 19) and N 
(Figure 20) and P uptake (Table 20) observed in BSF-treated plots than SAFI organic 
and urea fertilizer treated plots could be due to the better supply and availability of 
nutrients from this organic fertilizer. The high release of nutrients resulting from the 
high mineralisation rate of BSF frass fertilizer (Adin Yéton et al., 2019) might have led 
to better synchrony of nutrients supply with plant growth and chlorophyll formation of 
plants supplied with BSF frass fertilizer. High quality frass fertilizers have been 
associated with improved nutrient availability and plant growth as reported by Kagata & 
Ohgushi (2012).  
On the other hand, high amount of recalcitrant carbon in the biochar used to 
make SAFI organic fertilizer could have caused nitrogen immobilisation (Fornara et al., 
2011; Musyoka et al., 2019a), hence, limiting the amounts of N available for plant 
growth Further, the beneficial microbes associated with insect frass fertilizer have been 
found to promote plant growth and protect the pant against biotic and abiotic stresses 
(Choi & Hassanzadeh, 2019; Poveda et al., 2019; Quilliam et al., 2020). Such additional 
advantages of BSF frass fertilizer could have enhanced higher plant growth and 
nutrients acquisition than to plants treated with SAFI organic fertilizer and urea 
fertilizers. The higher chlorophyll concentration and nutrient uptake observed during the 
131 
 
long rain season could be largely attributed to residual effect of fertilizers applied during 
the short rain season and higher rainfall (Figure 2) received during the long rain season.  
4.4.3 Soil mineral nitrogen content  
4.4.3.1 Sole organic fertilizer treatments 
At 0 – 20 cm soil depth, the mineral N content during the short and long rain 
seasons of maize growth was significantly influenced by different sole organic fertilizer 
treatments (short rain season: p < 0.001, long rain season: p < 0.05) and maize growth 
stages (short rain season: p < 0.001, long rain season: p < 0.001) (Table 18). The 
interaction effect of fertilizer treatments and maize growth stages was significant during 
the short rain season only (p < 0.001).  
The mineral N content significantly increased to highest levels at the early 
seedling stage (6 – 36 kg N ha-1), before decreasing to significantly minimum values at 
tasseling stage. Thereafter, slight changes in mineral N were observed from tasseling to 
maturity stage. At the early seedling stage, the mineral N content in plots treated with 
7.5 t ha
-1
 of BSF frass fertilizer was 4 and 6 times higher than those achieved at 
equivalent rates of SAFI during the short (p = 0.004) and long rain (p = 0.003) seasons, 
respectively. Plots treated with 5 t ha
-1
 of BSF frass fertilizer had 2.4 times higher (p < 
0.05) mineral N content than those treated with equivalent rate of SAFI at tasseling 
stage during the short rain season (Table 17). Increase in mineral N contents was 
observed at the maturity stage during the long rain season for treatments consisting 5 
and 7.5 t ha
-1 
of BSF frass fertilizer, and 7.5 t ha
-1
 of SAFI. 
132 
 
Table 18: Mineral nitrogen distribution in the top 40 cm of soil treated with sole BSF frass and SAFI fertilizers. 
Key: *** p < 0.001, ** p < 0.01, * p < 0.05; ns = not significant (p ≥ 0.05); 0 = control (unfertilized soil); 2.5BSF, 5BSF and 7.5BSF = application of BSF frass 
fertilizers at rates of 2.5, 5 and 7.5 t ha
-1
, respectively; 2.5SAFI, 5SAFI and 7.5SAFI = application of SAFI organic fertilizer at rates of 2.5, 5 and 7.5 t ha
-1
; BSF 
= Black soldier fly frass fertilizer; SAFI = commercial organic fertilizer. In the same column, means (± standard error) followed by the same letters are not 
significantly different at p ≤ 0.05. 
Soil depth 
(cm) 
Rate 
(t ha-1) 
Soil mineral N content (kg N ha-1) 
SEASON 2019A (Short rains) SEASON 2019B (Long rains)  
 Time (days after planting)   
0 35 70 91 125 0 35 70 91 126 
0 – 20 0  4.1 ± 1.1b 9.9 ±2.8b 2.4 ±0.8b 2.9 ±2.2a 6.3 ±0.8 39.0 ±3.8 5.7 ±1.0b 26.1 ±3.1 14.4 ±0.5 31.5 ± 0.8 
2.5 BSF 4.1 ±1.5b 8.8 ±3.2b 5.1 ±1.6ab 3.5 ±2.5a 6.4 ±1.5 39.0 ±3.8 14.5±1.9ab 63.3±9.2 16.5±2.1 32.4±7.9 
5 BSF 13.3±4.7ab 28.9±10ab 12.2 ±2.7a 11.3 ±2.4a 8.6 ±0.6 44.8±6.4 8.6±2.0b 107.4±71 20.7±3.3 51.6±21.9 
7.5 BSF 15.7 ±2.3a 35.7 ±5.8a 8.1 ±3.5ab 9.5 ±0.5a 7.6 ±1.1 40±3.3 38.3±13a 37±6.5 19.5±0.7 44.6±14.1 
2.5 SAFI 2.8 ±0.6b 6.0 ±1.2b 1.6 ±0.1b 5.8 ±0.6a 8.8 ±2.6 38.2±8.3 3.9±0.1b 26.2±4.5 16.4±1.1 40.7±16.6 
5 SAFI 3.4 ±1.4b 7.3 ±3.0b 5.0 ±2.1ab 3.7 ±2.1a 5.9 ±1.2 39.1±4.4 3.8±0.03b 18.8±3.2 17.2±0.4 21.2±6.2 
 7.5 SAFI 3.7 ±1.1b 8.0 ±2.5b 5.4 ±0.8ab 4.8 ±1.6a 6.9 ±0.5 24.8±2.3 6.8±2.8b 27.8±8.2 18.2±2.3 41.9±14 
p value ** ** * * ns ns ** ns ns ns 
20 – 40 0  3.4 ±1.3 7.8 ±2.9 6.4 ±1.7 5.9 ±0.8 3.6 ±0.8ab 16.0 ± 6.3 19.1± 8.7b 35.9± 15 11.6±3.6 12.3±1.2 
2.5 BSF 2.1 ±0.7 4.1 ±1.3 9.0 ±2.0 6.6 ±1.0 3.5 ±1.3ab 24.7 ± 4.4 46.1±0.5b 29.7±16.9 14±4.3 11.3±2.5 
5 BSF 3.2 ±0.3 6.2 ±0.5 12.2 ±2.0 7.8 ±1.2 5.3 ±0.9ab 32.3 ± 21.5 41.5±12b 18.6±5.0 10.6±0.3 17.9± 4.1 
7.5 BSF 3.7 ±0.6 7.4 ±1.0 10.8 ±1.1 6.7 ±0.4 8.2 ±2.2a 29.7 ± 13.4 119±18.5a 43.3±6.4 18±3.9 13.4±5.8 
2.5 SAFI 1.2 ±0.4 2.4 ±0.7 9.0 ±1.8 5.0 ±1.1 4.8 ±0.4ab 22.7 ± 4.5 17.2±7.1b 37.7±1.1 10.1±3.5 6.9±0.4 
5 SAFI 1.6 ±0.1 3.3 ±0.2 9.5 ±1.7 5.3 ±0.6 3.4 ±0.7ab 15.3 ± 3.5 11±1.5b 31.9±9.1 18.4±0.4 20±2.5 
7.5 SAFI 2.6 ±0.5 5.0 ±1.0 8.5 ±2.0 6.0 ±0.8 2.3 ±0.4b 19.1 ± 2.5 32±11.5b 58.9±24.9 10±4.2 24.5±8.5 
p value ns ns ns ns * ns *** ns ns ns 
133 
 
At soil depth of 20 – 40 cm, the mineral N content was also significantly 
influenced by fertilizer treatments (short rain season: p < 0.001, long rain season: p < 
0.001) and crop growth stage (short rain season: p < 0.001, long rain season: p < 0.001) 
(Table 18). The interaction effect of fertilizer treatments and maize growth stages was 
significant (p < 0.001) during the long rain season only. Peak mineral N levels during 
the short rain season were achieved at tasseling stage (70 DAP) with the highest 
recorded in plots treated with 7.5 t ha
-1
 of BSF frass fertilizer. The same treatment had 
significantly (p = 0.044) higher mineral N content at maturity stage than to plots treated 
with 7.5 t ha
-1 
of SAFI. During the long rain season, peak mineral N contents were 
attained at early seedling and silking stages for BSF frass fertilizer and SAFI organic 
fertilizer treatments, respectively. Soil treated with 7.5 t ha
-1 
BSF frass fertilizer had 
significantly (p < 0.001) higher mineral N contents than other treatments at the early 
seedling stage (Table 18).  
4.4.3.2 Combined fertilizer treatments 
For combined fertilizer treatments, mineral N content in the topsoil depth (0 – 20 
cm) varied significantly at different maize growth stages during the long rain season 
only (p = 0.014) (Table 19). The mineral N significantly increased to peak levels 
between early seedling and silking stages before decreasing gradually to the end of the 
short rain season. However, no significant differences in mineral N content were 
observed between the treatments. During the long rain season, mineral N significantly 
decreased from initial values to lowest levels at early seedling stage. Plots treated with 
60 kg N ha
-1 
supplied as BSF frass fertilizer and urea fertilizer had the highest mineral N 
contents in topsoil at tasseling and silking stages, respectively.  
134 
 
Table 19: Mineral nitrogen distribution in the top 40 cm of soil treated with combined fertilizers 
Key: ** p < 0.01, * p < 0.05; ns = not significant (p ≥ 0.05); 0 = Control (unfertilized soil); 2.5BSF, 5BSF and 7.5BSF = application BSF frass fertilizer at rates 
of 2.5, 5 and 7.5 t ha
-1
, respectively; 2.5SAFI, 5SAFI and 7.5SAFI = application SAFI organic fertilizer at rates of 2.5, 5 and 7.5 t ha
-1
; 30N BSF, 60N BSF and 
100N BSF = application of BSF frass fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N SAFI, 60N SAFI and 100N SAFI = application of 
SAFI organic fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N UREA, 60N UREA and 100N UREA = application of urea fertilizer at 
rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; BSF = Black soldier fly frass fertilizer; SAFI = commercial organic fertilizer. Means (± standard error) 
followed by the same letters are not significantly different at p ≤ 0.05. 
Soil 
depth 
(cm) 
Rate 
(kg N ha
-1
) 
Soil mineral N content (kg N ha
-1
) 
SEASON 2019A (Short rains) SEASON 2019B (Long rains) 
 Days after planting   
0 35 70 91 125 0 35 70 91 125 
0 - 20 0   4.1 ± 1.1 9.9 ±2.8 2.4 ±0.8 2.9 ±2.2 6.3 ±0.8 39.0 ±3.8 5.7 ±1.0 26.1 ±3.1a 14.4 ±0.5 31.5 ± 0.8 
30N BSF 3.2 ±1.6 6.9 ±3.6 5.7 ±1.4 5.4 ±2.4 6.7 ±1.3 42.1±4.2 11.4±5.2 37±8.7a 19.9±3.3 23.5±16.5 
60N BSF 2.2 ±0.1 4.8 ±0.2 3.4 ±1.5 8.1 ±1.0 6.3 ±1.0 43.2±7.7 23±9.9 63.0±14.5a 15.4±1.4 23.4±13.1 
100N BSF 4.2 ±1.1 9.3 ±3.0 4.2 ±1.6 2.5 ±2.2 6.7 ±0.6 36.2±7.9 6.4±3.1 50.1±6.1a 27.5±4.7 54.4±7.1 
30N SAFI 1.9 ±1.1 4.4 ±2.6 5.1 ±1.9 6.0 ±0.9 5.8 ±1.0 38.1±4.9 4.0±0.1 28.9±8.5a 17.9±2.0 36.6±4.5 
60N SAFI 1.8 ±0.2 4.0 ±0.2 11.3 ±5.7 0.82 ±0.1 5.4 ±1.8 69.6±10.2 3.9±0.0 62.7±15.0a 12.6±5.6 59.2±10.3 
 100N SAFI 5.7 ±1.7 12.9 ±3.4 7.2 ±1.0 4.8 ±1.8 5.9 ±1.6 46.5±23.3 5.4±1.6 47.1±7.1a 16.0 ±1.1 31.6±1.8 
 30N UREA 1.7 ±0.8 3.7 ±1.9 7.3 ±1.2 5.5 ±2.2 7.1 ±2.1 46.2±1.3 6.8±1.6 23.6±4.7a 14.5±2.4 41.5±3.5 
 60N UREA 2.6 ±1.3 5.9 ±3.1 6.1 ±0.7 8.1 ±0.7 7.5 ±1.2 25.4±5.0 5.2±1.6 45.1±8.3a 32±15.5 49.5±23.0 
 100N UREA 6.1 ±0.5 13.3 ±0.7 6.6 ±1.1 8.2 ±3.9 13.0 ±3.9 39.9±11.4 13.3±2.9 51.1±7.8a 24.4±5.5 62.6±7.6 
 p value ns ns ns ns ns ns ns * ns ns 
20 - 40 0   3.4 ±1.3 7.8 ±2.9 6.4 ±1.7 5.9 ±0.8ab 3.6 ±0.8a 16.0 ± 6.3 19.1± 9b 35.9± 15 11.6±3.6 12.3±1.2ab 
30N BSF 2.0 ±1.1 3.9 ±2.0 11.4 ±0.4 5.0 ±0.4b 3.1 ±0.4a 17.5±4.2 27.9±11ab 44.5±16.4 15.2±6.2 9.1±2.6b 
60N BSF 1.2 ±0.4 2.5 ±0.7 6.2 ±0.8 5.9 ±0.5ab 3.1 ±0.6a 18±11.2 42.5±19ab 37.5±10.8 17.1±6.8 9.1±2.0b 
100N BSF 2.1 ±0.1 4.1 ±0.1 7.8 ±3.3 5.3 ±0.2ab 4.6 ±0.6a 19±4.4 34.5±11ab 49.6±8.1 11.0±2.7 15.9±5.0ab 
30N SAFI 1.2 ±0.4 2.5 ±0.7 7.5 ±1.6 4.5 ±0.3b 3.0 ±0.2a 32.5±12 16.5±8.3b 65.6±1.5 10.2±0.4 7.5±1.0b 
 60N SAFI 0.8 ±0.4 1.8 ±0.7 7.4 ±1.6 5.1 ±0.2ab 3.6 ±0.5a 38.9±15.4 32.4±14ab 42±8.3 11.1±6.8 9.4±1.6b 
 100N SAFI 2.4 ±0.9 4.6 ±1.7 7.3 ±2.8 6.4 ±0.6ab 2.8 ±1.3a 13.4±6.0 8.2±2.1b 35.9±11.2 12.4±4.6 6.5±0.1b 
 30N UREA 2.1 ±0.9 4.0 ±1.6 12.1 ±2.5 5.1 ±0.1ab 5.9 ±0.8a 9.6±2.8 12.4±2.1b 36.8±13.8 16.1±6.4 20.5±4.1ab 
 60N UREA 1.1 ±0.3 2.3 ±0.5 9.9 ±2.4 7.5 ±1.3ab 3.9 ±0.5a 19.7±8.7 89.5±22ab 48.1±14 12.7±3.5 26.1±5.5a 
 100N UREA 2.4 ±1.0 4.6 ±1.8 15.5 ±5.2 8.9 ±1.6ab 6.2 ±0.5a 35.6±6.5 149.3±69a 108.2±41 26.2±6.9 20.6±1.5ab 
 p value ns ns ns * * ns * ns ns ** 
135 
 
In the subsoil depth (20 – 40 cm), mineral N content in soil was significantly 
affected by combined fertilizer treatments (short rain season: p < 0.001, long rain 
season: p < 0.001) and maize growth stage (short rain season: p < 0.001, long rain 
season: p < 0.001) (Table 19). The interaction effect of combined fertilizer treatments 
and maize growth stage was significant during the long rain season only (p = 0.003). 
The soil mineral N content significantly increased to peak levels of between 6.2 and 
15.5 kg N ha
-1
 at tasseling stage, before gradually decreasing towards the end of the 
short rain season. Plots treated with 100 kg N ha
-1
 supplied as urea had significantly (p = 
0.018) higher mineral N contents at silking stage of the short rain season than where 30 
kg N ha
-1
 from SAFI and 60 kg N ha
-1
 supplied as BSF frass fertilizer were applied 
(Table 19). Similarly, plots treated with 100 kg N ha
-1
 supplied as urea achieved 
significantly (p = 0.017) higher mineral N content at the early seedling stage during the 
long rain season than the equivalent rate of SAFI.  
At maturity stage (125 DAP), plots treated with 60 kg N ha
-1 
applied as urea 
fertilizer had significantly (p = 0.001) higher mineral N content than BSF frass and 
SAFI treatments, except for plots where 100 kg N ha
-1
 of BSF frass fertilizer was 
applied. The high soil mineral N content observed in plots treated with BSF frass 
fertilizer (Table 18 and 19) could be majorly attributed to the higher mineralization rate 
of this organic fertilizer (Adin Yéton et al., 2019). The low mineral N observed in urea 
amended plots could be attributed to losses through denitrification and volatilization 
(Bernal et al., 2009), thus reducing the quantity of N available for plant uptake, 
especially during the short rain season when temperatures were high (Figure 2).  
136 
 
On the other hand, biochar was one of the components of SAFI, yet biochar 
consists of recalcitrant organic carbon which causes delays in N release, thus low soil 
mineral N (Kleber, 2010; Rovira & Vallejo, 2002). Biochar consists of compounds such 
as lignin and polyphenols (Sanchez-Monedero et al., 2018) which have been reported to 
cause N immobilization (Kimetu et al., 2004; Musyoka et al., 2019b). Furthermore, the 
high nitrate concentration associated with SAFI organic fertilizer  could have facilitated 
N leaching, especially during the long rain season, when soil moisture content was high 
(Loecke et al., 2012). High nitrate leaching (80% of total N), up to 1 m down the soil 
profile, has been reporting in maize cropping systems of Kenya (Musyoka et al., 
2019ba) and could be attributed to the high sand content and low organic matter levels 
of the experimental soil. 
4.4.4 Nitrogen uptake by maize  
4.4.4.1 Sole organic fertilizer treatments 
The quantity of nitrogen accumulated in maize biomass was significantly 
influenced by the different sole organic fertilizer treatments (short rain season: p < 
0.001, long rain season: p < 0.001) and maize growth stages (short rain season: p < 
0.001, long rain season: p < 0.001) (Figures 26 and 27). The interaction of sole organic 
fertilizer treatments and growth stages was significant during the long rain season only 
(p = 0.012) (Figure 27). Nitrogen uptake significantly (p = 0.012) increased from 9 – 16 
kg N ha
-1
 at early seedling stage (35 DAP) to highest values (40 – 139 kg N ha-1) at 
tasseling (70 DAP) and silking stages (91 DAP). Thereafter, N uptake levels decreased 
gradually to maturity stage.  
137 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 26: Effects of BSF frass fertilizer (a and b), commercial organic (SAFI) (a and 
b) and mineral (urea) (c) fertilizers on maize nitrogen uptake during the short season 
experiments. 
Key: Control = unfertilized soil; 2.5BSF, 5BSF and 7.5BSF = application BSF frass fertilizer at rates of 
2.5, 5 and 7.5 t ha
-1
, respectively; 2.5SAFI, 5SAFI and 7.5SAFI = application SAFI organic fertilizer at 
rates of 2.5, 5 and 7.5 t ha
-1
; 30N BSF, 60N BSF and 100N BSF = application of BSF frass fertilizer at 
rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N SAFI, 60N SAFI and 100N SAFI = 
application of SAFI organic fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N 
UREA, 60N UREA and 100N UREA = application of urea fertilizer at rates equivalent to 30, 60 & 100 kg 
N ha
-1
, respectively; BSF = Black soldier fly frass fertilizer; SAFI = commercial organic fertilizer. 
138 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 27: Effects of BSF frass fertilizer (a and b), commercial organic (SAFI) (a and 
b) and mineral (urea) (c) fertilizers on maize nitrogen uptake during the long rain season 
experiments.  
Key: Control = unfertilized soil; 2.5BSF, 5BSF and 7.5BSF = application BSF frass fertilizer at rates of 
2.5, 5 and 7.5 t ha
-1
, respectively; 2.5SAFI, 5SAFI and 7.5SAFI = application SAFI organic fertilizer at 
rates of 2.5, 5 and 7.5 t ha
-1
; 30N BSF, 60N BSF and 100N BSF = application of BSF frass fertilizer at 
rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N SAFI, 60N SAFI and 100N SAFI = 
application of SAFI organic fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N 
UREA, 60N UREA and 100N UREA = application of urea fertilizer at rates equivalent to 30, 60 & 100 kg 
N ha
-1
, respectively; BSF = Black soldier fly frass fertilizer; SAFI = commercial organic fertilizer. 
139 
 
Maize grown using 7.5 t ha
-1
 of BSF frass fertilizer and 7.5 t ha
-1
 of SAFI 
accumulated the highest N levels at tasseling stage of the short (p = 0.021) and long rain 
(p = 0.2) season experiments, respectively (Figure 26). At the silking stage of the short 
rain season, maize grown in plots treated with 5 t ha
-1 
of BSF frass fertilizer 
accumulated significantly (p = 0.004) higher N at the silking stage than those where 2.5 
t ha
-1
 of either BSF frass or SAFI was applied (Figure 26a). At maturity stage, the N 
accumulation (38 – 112 kg ha-1) varied significantly among the treatments (short rain 
season: p = 0.012, long rain reason: p = 0.006). The N accumulated in maize treated 
with 7.5 t ha
-1
 of BSF frass fertilizer was 17 and 44% higher than N accumulated in 
maize treated with equivalent rates of SAFI during the short (Figure 26a) and long rain 
seasons, respectively (Figure 27a), but the differences were not statistically significant.  
4.4.4.2 Combined fertilizer treatments 
The different combined fertilizer treatments significantly influenced the N 
uptake in maize biomass during the short and long rain seasons (short rain season: p < 
0.001, long rain season: p < 0.001) (Figure 26 and 27). Like with sole organic fertilizers 
(section 4.4.4.1), N uptake also varied significantly at different maize growth stages 
(short rain season: p < 0.001, long rain season: p < 0.001). The interaction effect of 
combined fertilizer treatments and growth stages was significant during the long rain 
season only (p < 0.001). The N accumulation in maize tissues at early seedling stage (5 
– 13 kg N ha-1) significantly (p < 0.001) increased to highest values at tasseling and 
silking stages (40 – 129 kg N ha-1). A significant (p < 0.001) decrease in N uptake was 
observed at maturity stage, except for plots treated with 30 and 60 kg N ha
-1
 supplied 
using SAFI organic fertilizer during the long rain season (Figure 26b and 27b).  
140 
 
Maize grown in plots treated with 60 and 100 kg N ha
-1
 supplied as BSF frass 
fertilizer accumulated higher N levels at tasseling (short rain season: p = 0.181, long 
rain season: p < 0.001) and silking stages (short rain season: p = 0.197, long rain season: 
p < 0.001) than equivalent rates of SAFI (Figure 26 and 27). The N accumulated in 
maize treated with 100 kg N ha
-1
 of BSF frass fertilizer at tasseling stage of the long rain 
season was significantly (p < 0.001) higher than those of maize grown using 30 and 60 
kg N ha
-1
 supplied using SAFI organic fertilizer (Figure 27b). Similarly, application of 
100 kg N ha
-1
 as BSF frass fertilizer caused significantly (p < 0.001) higher N uptake at 
silking stage of the long rain season compared to where 30 kg N ha
-1 
of all fertilizers, 
and 60 kg N ha
-1
 of SAFI organic fertilizer were applied.   
Application of 100 kg N ha
-1
 supplied as BSF frass fertilizer during the short and 
long rain seasons effected significantly (short rain season: p < 0.001, long rain season: p 
< 0.001) higher N accumulation than other treatments at harvesting stage, except for 
maize grown using 100 kg N ha
-1
 of urea fertilizer in both seasons, and 60 kg N ha
-1 
of 
urea during the long rain season (Figure 26 – 27). The higher mineral N content and N 
uptake (Figure 26 – 27) associated with BSF frass fertilizer compared to other 
treatments could be attributed to higher nutrient release (Kagata & Ohgushi, 2012; 
Houben et al., 2020), better timing of application and climatic conditions during 
experiments.  
 
 
 
141 
 
It should be noted that for organic fertilizer treatments, the entire dose was 
applied at planting, while for urea two splits were administered, whereby the second 
split of urea was applied when rainfall was low during the short rain season (63 DAP) 
(Figure 2a – c). The low moisture in soils could have reduced available N concentrations 
in solution for plant uptake, while the high temperatures could have triggered N loss 
through volatilisation (Bernal et al., 2009) and thereby reducing N use by plants. 
Unfortunately, this coincided with tasselling stage, when there was high N demand for 
ear and grain formation by the plant (Figure 26 and 27).  
Similarly, the higher rainfall in second season (Figure 2c) could have triggered N 
loss in mineral fertilizer treatments through leaching and denitrification due to excess 
soil moisture (Musyoka et al., 2019b). These N losses were responsible for the lower 
agronomic N use efficiency in mineral fertilizer than those of BSF frass fertilizer 
treatments (Table 21). On the other hand, organic inputs are effective in moisture 
conservation which consequently improve nutrient availability through minimising 
leaching losses (Baligar et al., 2001; Vanlauwe et al., 2015).  
Since there was enough nitrogen at critical growth stages like tasselling and 
silking (Figure 20), a better crop yield was obtained from BSF frass fertilizer treatments 
(Figure 28 and 29). Findings of the study correspond with Musyoka et al. (2017) who 
also reported higher apparent nitrogen recovery and agronomic nitrogen use efficiencies 
in experiments treated with high quality organic inputs. Furthermore, the high nitrate-N 
contained in SAFI organic fertilizer (Table 5) could have been lost through leaching 
especially during the long rain season (Musyoka et al., 2019b), thereby leaving little 
mineral N for plant uptake.  
142 
 
4.4.5 Phosphorus and potassium uptake at harvesting  
4.4.5.1 Sole organic fertilizer treatments  
There were significant differences in the quantity of P accumulated in maize 
biomass at harvesting stage among the different sole organic fertilizer treatments during 
the short rain (p = 0.032) and long rain seasons (p = 0.006) (Table 20). Total P 
accumulated in maize grown in plots treated with 5 and 7.5 t ha
-1
 of BSF frass fertilizer 
were significantly higher than those of the control treatment in both seasons. The total P 
accumulated in plots treated with 7.5 t ha
-1
 of BSF frass fertilizer was 15 and 27% 
higher than the P accumulated by maize grown using similar rates of SAFI organic 
fertilizer during the short and long rain seasons, respectively.  
Accumulated K also varied significantly in both seasons due to different sole 
organic fertilizer treatments (short rain season: p = 0.019, long rain season: p = 0.044) 
(Table 20). Maize grown using 7.5 t ha
-1 
of SAFI organic fertilizer accumulated 
significantly higher K contents in both seasons than maize grown in the control 
treatments. On the other hand, the total K accumulated in plots treated with 7.5 t ha
-1
 of 
SAFI organic fertilizer was 44 and 23% higher than that accumulated using equivalent 
rates of BSF frass fertilizer during the short and long rain seasons, respectively.  
143 
 
Table 20: Effects of BSF frass fertilizer, commercial organic and mineral (urea) fertilizers on phosphorus, and potassium uptake at 
maturity. 
Fertilizer 
treatments 
SEASON 2019A (short rains) SEASON 2019B (long rains) 
Phosphorus Potassium Phosphorus Potassium 
(kg ha
-1
) 
Sole organic fertilizer rates (t ha
-1
) 
Control  4.2 ± 0.22b 77.6 ± 10.3b 5.4 ± 1.3b 63.6 ± 13.7b 
2.5 BSFF 12.7 ± 2.1a 128.5 ± 25ab 12.7 ± 1.3ab 122.0 ± 14.8ab 
5 BSFF 12.9 ± 1.9a 160.5 ± 15.3ab 16.3 ± 2.5a 163.8 ± 34.5ab 
7. 5 BSFF 13.3 ± 1.2a 118.9 ± 10.9ab 18.6 ± 2.8a 173.4 ± 37.0ab 
2.5 SAFI 10.7 ± 2.2ab 97.4 ± 18.7ab 10.2 ± 1.5ab 123.5 ± 32.5ab 
5 SAFI 11.1 ± 1.2ab 144.0 ±22.8ab 11.8 ± 1.8ab 110.1 ± 21.6ab 
7.5 SAFI 11.6 ± 2.1ab 171.2 ± 10.6a 14.6 ± 1.8ab 212.5 ± 33.5a 
p value * * ** * 
Combined fertilizer rates (kg N ha
-1
) 
30N BSFF 10.6 ± 0.9ab 104.9 ± 20.9ab 11.7 ± 0.6abc 125.1 ± 21.6ab 
60N BSFF 13.6 ± 1.8ab 109.4 ± 12.8ab 14.6 ± 2.4ab 115.5 ± 1.8ab 
100N BSFF 16.4 ± 2.0a 179.0 ± 37.5a 17.4 ± 1.0a 189.2 ± 30.9a 
30N SAFI 8.7 ± 0.6ab 83.3 ± 14.1ab 7.2 ± 0.9bc 69.0 ± 13.5b 
60N SAFI 7.2 ± 1.1bc 98.0 ± 19.9ab 8.1 ± 0.4bc 94.1 ± 16.8b 
100N SAFI 10.8 ± 1.3ab 119.0 ± 4.4ab 11.8 ± 1.3abc 124.7 ± 2.7ab 
30N UREA 10.6 ± 0.8ab 143.2 ±28.5ab 12.7 ± 1.1abc 113.3 ± 7.3ab 
60N UREA 10.0 ± 2.6ab 91.6 ± 11.3ab 12.6 ± 3.3abc 129.5 ± 5.5ab 
100N UREA 12.6 ±2.3ab 121.4 ± 11.1ab 13.3 ± 2.9abc 118.3 ± 13.6ab 
Control  4.2 ±0.2c 77.6 ± 10.3b 5.4 ± 1.3c 63.6 ± 13.7b 
p value ** * ** *** 
Key: *** p < 0.001, ** p < 0.01, * p < 0.05; 2.5BSF, 5BSF and 7.5BSF = application BSF frass fertilizer at rates of 2.5, 5 and 7.5 t ha
-1
, respectively; 2.5SAFI, 5SAFI 
and 7.5SAFI = application SAFI organic fertilizer at rates of 2.5, 5 and 7.5 t ha
-1
; 30N BSF, 60N BSF and 100N BSF = application of BSF frass fertilizer at rates 
equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N SAFI, 60N SAFI and 100N SAFI = application of SAFI organic fertilizer at rates equivalent to 30, 60 & 100 kg N 
ha
-1
, respectively; 30N UREA, 60N UREA and 100N UREA = application of urea fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; BSF = Black 
144 
 
soldier fly frass fertilizer; SAFI = commercial organic fertilizer. In the same column, means (± standard error) followed by the same letters are not significantly different 
at p ≤ 0.05.  
145 
 
4.4.5.2 Combined fertilizer treatments 
The quantity of phosphorus accumulated in maize biomass at maturity stage (4 – 
17 kg P ha
-1
) was significantly influenced by different combined fertilizer treatments 
(short rain season: p = 0.002, long rain season: p = 0.004) (Table 20). There was 
significantly higher accumulation of P in maize grown using 100 kg N ha
-1
 supplied as 
BSF frass fertilizer than in maize grown using 60 kg N ha
-1
 of SAFI organic fertilizer 
and control treatment in both seasons and 30 kg N ha
-1
 of SAFI organic fertilizer during 
the long rain season. Similarly, application of 100 kg N ha
-1
 supplied as BSF frass 
fertilizer caused significantly higher K accumulation in maize than the control treatment 
in both seasons (short rain season: p = 0.048, long rain season: p < 0.001), and where 30 
and 60 kg N ha
-1 
of SAFI organic fertilizer were applied during the long rain season 
(Table 20). 
The increased uptake of phosphorus in the study might have been partly 
responsible for high nitrogen accumulation observed in maize grown in plots treated 
with BSF frass fertilizer (Tittonell et al., 2008b). This can be attributed to the role of 
phosphorus in energy transfer as a component of adenosine phosphates (Fageria, 2001). 
Although, potassium has been equally reported to be important in nitrogen absorption 
and metabolism as well as plant growth (Fageria, 2001), it was taken up in sufficient 
quantities across all treatments. The observations described above are in line with the 
report by Kagata and Ohgushi (2012), who also demonstrated that frass fertilizer is of 
good quality and capable of improving soil nutrient availability, and growth and yield of 
Brassica rapa L. var. rapa. Additional benefits of insect frass fertilizer on soil health for 
146 
 
improved drought and salt tolerance, disease suppression, higher crop growth and yield 
have also been documented by Choi and Hassanzadeh (2019) and Houben et al. (2020).  
4.4.6 Nitrogen use efficiency 
4.4.6.1 Agronomic nitrogen efficiency (AEN) 
The various treatments of sole organic fertilizers (short rain season: p = 0.004, 
long rain season: p = 0.012) and combined fertilizers (short rain season: p = 0.005, long 
rain season: p < 0.001) had significant effects on the AEN of maize during experiments 
(Table 21). Application of 2.5 t ha
-1
 of BSF frass fertilizer increased the AEN of maize 
by 2.4 and 2.2 times compared to the equivalent rate of SAFI organic fertilizer during 
the short and long rain season, respectively. Furthermore, maize grown using 2.5 t ha
-1
 
of BSF frass fertilizer had significantly higher AEN than other treatments, except for 
plots treated with 5 and 7.5 t ha
-1
 of the same fertilizer and 2.5 t ha
-1
 of SAFI organic 
fertilizer during the long rain season.  
During the short rain season, the AEN of maize treated with 30 kg N ha
-1
 of BSF 
frass fertilizer was significantly (p = 0.004) higher than those of maize treated with 100 
kg N ha
-1
 of all fertilizers, and 60 kg N ha
-1
 of SAFI organic fertilizer (Table 21). Also, 
the AEN in plots treated with 30 kg N ha
-1
 of BSF frass fertilizer was significantly (p < 
0.001) higher than those achieved by all SAFI organic fertilizer treatments and 100 kg N 
ha
-1
 of urea during the long rain season.  
4.4.6.2 Apparent nitrogen recovery (ANRN) 
The ANRN of maize was significantly influenced by the different sole organic 
(short rain season: p < 0.001, long rain season: p = 0.028) and combined fertilizer 
147 
 
treatments (short rain season: p < 0.001, long rain season: p < 0.001) as shown in Table 
21.  
148 
 
Table 21: Effects of BSF frass fertilizer, commercial organic and mineral (urea) fertilizers on nitrogen use efficiency of maize. 
Fertilizer 
treatments 
SEASON 2019A (Short rains) SEASON 2019B (Long rains) 
AEN 
(kg kg
-1
 N) 
ANRN  
(%) 
APEN  
(kg kg
-1 
N) 
NHI 
 (%) 
AEN 
(kg kg
-1
 N) 
ANRN  
(%) 
APEN  
(kg kg
-1 
N) 
NHI 
 (%) 
Sole organic fertilizer treatments 
2.5 BSF 43.2 ± 10.8a 52.8 ± 2.4a 81.2 ± 19.6b 52.9 ± 3.3ab 33.0 ± 4.2a 63.8 ± 15.0a 55.5 ± 9.2 55.4 ± 1.9 
5 BSF 18.5 ± 1.8b 31.7 ± 7.0ab 62.5 ± 9.6b 51.3 ± 5.4ab 29.7 ± 2.7ab 50.3 ± 13.5a 69.9 ± 23.2 60.9 ± 4.6 
7.5 BSF 16.7±1.8b 23.5 ± 7.7b 80.9 ± 15.1b 53.4 ± 1.4ab 22.9 ± 3.8ab 46.7 ± 13.7a 56.2 ± 12.5 52.4 ± 6.1 
2.5 SAFI 18.4 ± 1.0b 28.0 ± 1.9b 66.5 ± 7.6b 42.6 ± 4.0b 15.3 ± 7.8ab 15.5 ± 11.1a 35.7 ± 13.8 54.4 ± 9.4 
5 SAFI 15.8 ± 1.9b 9.9 ± 2.8b 177.7 ± 36.9a 67.5 ± 5.6a 11.6 ± 2.1b 14.6 ± 1.1a 81.1 ± 17.5 65.7 ± 5.0 
7.5 SAFI 18.3 ± 1.9b 11.3 ± 1.5b 72.3 ± 6.4b 42.7 ± 2.4b 12.1 ± 0.4b 17.6 ± 3.3a 74.6 ± 15.1 51.9 ± 6.9 
Control  na na na 55.2 ± 2.0ab na na na 51.6 ± 5.8 
p value ** *** ** ** * * ns ns 
Combined fertilizer treatments 
30N BSF 74.4 ± 7.1a 50.9 ± 9.5a 153.4 ± 22.3ab 54.3 ± 5.9 55.0 ± 10.7a 68.2 ± 3.4ab 79.9 ±13.8 55.4 ± 7.0 
60N BSF 47.7 ± 6.7ab 30.2 ± 3.7ab 165.5 ± 34.8ab 52.2 ± 7.0 40.0 ± 4.3ab 32.9 ± 6.3bcd 82.4 ± 10.4 58.6 ± 3.3 
100N BSF 29.9 ± 3.2b 48.7 ± 8.3a 64.6 ± 12.0b 62.8 ± 2.2 26.1 ± 1.0abc 59.2 ± 7.5abc 45.2 ± 4.5 62.9 ± 0.8 
30N SAFI 58.8 ± 17.4ab 27.0 ± 10.7ab 234.7 ± 43.3a 50.3 ± 2.0 4.4 ± 1.3c 7.5 ± 2.7d 98.4 ± 58.2 64.6 ± 1.5 
60N SAFI 21.3 ± 7.4b 14.0 ± 5.3b 156.2 ± 14.7ab 56.0 ± 1.6 15.6 ± 4.6bc 25.1 ± 8.7bcd 66.7 ± 18.0 58.9 ± 5.7 
100N SAFI 17.8 ± 1.5b 11.4 ± 4.0b 182.0 ± 38.8ab 56.4 ± 7.2 15.7 ± 2.0bc 18.2 ± 4.2cd 91.1 ± 11.4 57.0 ± 6.9 
30N UREA 34.4 ± 14.8ab 57.3 ± 1.5a 85.6 ± 2.3b 54.4 ± 8.1 55.3 ± 11.8a 91.4 ± 15.2a 60.3 ± 9.5 67.4 ± 3.3 
60N UREA 32.2 ± 6.0ab 27.7 ± 2.7ab 122.6 ± 35.0ab 56.8 ± 6.8 31.3 ± 0.6abc 60.3 ± 4.8abc 52.5 ± 3.8 51.8 ± 2.4 
100N UREA 26.7 ± 0.9b 30.8 ± 5.7ab 95.8 ± 24.6ab 58.4 ± 1.9 22.4 ± 4.2bc 37.4 ± 9.0bcd 61.7 ± 4.9 60.6 ± 4.4 
Control  na na na 55.2 ± 2.0 na na na 51.6 ± 5.9 
p value ** *** * ns *** *** ns ns 
Key: *** p < 0.001, ** p < 0.01, * p < 0.05; ns = not significant (p ≥ 0.05); AEN = agronomic nitrogen efficiency; ANRN = apparent nutrient recovery; APEN = 
agro-physiological nitrogen efficiency; NHI = nitrogen harvest index; Control = unfertilized soil; 2.5BSF, 5BSF and 7.5BSF = application BSF frass fertilizer at 
rates of 2.5, 5 and 7.5 t ha
-1
, respectively; 2.5SAFI, 5SAFI and 7.5SAFI = application SAFI organic fertilizer at rates of 2.5, 5 and 7.5 t ha
-1
; 30N BSF, 60N BSF 
and 100N BSF = application of BSF frass fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N SAFI, 60N SAFI and 100N SAFI = 
application of SAFI organic fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N UREA, 60N UREA and 100N UREA = application of urea 
fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; BSF = Black soldier fly frass fertilizer; SAFI = commercial organic fertilizer; na = not 
applicable. In the same column, means (± standard error) followed by the same letters are not significantly different at p ≤ 0.05. 
149 
 
Maize treated with 2.5 t ha
-1
 of BSF frass fertilizer had significantly (p < 0.001) 
higher ANRN values than other treatments, except for plots where 5 t ha
-1
 of the same 
fertilizer were applied during the short rain season. The ANRN of maize treated with 2.5 
t ha
-1 
of BSF frass fertilizer was 1.9 and 4.1 times higher than that obtained using 
equivalent rates of SAFI organic fertilizer during the short and long rain season, 
respectively.  
Maize grown in plots treated with 30 kg N ha
-1
 supplied as urea had significantly 
(p < 0.001) higher ANRN values than other treatments in the short rain season, except 
for those treated with 30 and 60 kg N ha
-1
 supplied using SAFI organic fertilizer (Table 
21). Similarly, the ANRN of maize treated with 30 kg N ha
-1 
as urea, was significantly (p 
< 0.001) higher than those of other treatments during the long rain season, except for 
maize grown using 60 kg N ha
-1
 of urea, 30 and 100 kg N ha
-1
 of BSF frass fertilizer.  
4.4.6.3 Agro-physiological nitrogen efficiency (APEN) 
The APEN of maize under different sole organic (p = 0.009) and combined 
fertilizer treatments (p = 0.013) varied significantly during the short rain season only 
(Table 21). Maize treated with 5 t ha
-1
 applied as SAFI organic fertilizer had 
significantly (p = 0.009) higher APEN values than other treatments. During the long rain 
season, the APEN of maize treated with 5 t ha
-1
 of SAFI organic fertilizer was 16% 
higher (p = 0.77) than that obtained using an equivalent rate of BSF frass fertilizer.  
 
 
 
150 
 
On the other hand, the APEN of maize grown in plots treated with 30 kg N ha
-1 
supplied as SAFI organic fertilizer was significantly (p = 0.013) higher than those 
attained by 100 kg N ha
-1
 of BSF frass fertilizer and 30 kg N ha
-1
 of urea fertilizer 
during the short rain season (Table 21). During the long rain season, APEN values of 
maize treated with 30 kg N ha
-1
 of SAFI organic fertilizer was 63 and 23% (p = 0.704) 
higher than those obtained using equivalent rates of urea and BSF frass fertilizer, 
respectively.  
4.4.6.4 Nitrogen harvest index (NHI) 
The NHI of maize grown using different sole organic treatments varied 
significantly (p = 0.006) during the short rain season only (Table 21). Maize grown on 
plots treated with 5 t h
-1
 of SAFI organic fertilizer had significantly (p = 0.006) higher 
NHI values than those treated with 2.5 and 7.5 t ha
-1 
of the same fertilizer treatments. 
Also, the NHI of maize treated with 5 t ha
-1
 of SAFI organic fertilizer was 8% higher 
than that obtained using an equivalent rate of BSF frass fertilizer during the long rain 
season. On the other hand, the different combined fertilizer treatments did not influence 
the NHI of maize significantly (short rain season: p = 0.894, long rain season: p = 0.312) 
(Table 21).  Maize treated with 100 kg N ha
-1
 of BSF frass fertilizer and 30 kg N ha
-1
 of 
urea fertilizer, had the highest NHI values during the short and long rain season, 
respectively.  
 
 
 
151 
 
 
Remarkably, higher apparent nitrogen recovery and agronomic nitrogen use 
efficiency values were observed in maize harvested from plots treated with BSF frass 
fertilizer compared to those from plots treated with commercial fertilizers (Table 21). 
The highest values of agronomic and agro-physiological nitrogen use efficiencies as 
well as nitrogen harvest indices observed at lower application rates of BSF frass 
fertilizer (2.5 t ha
-1 
and 30 kg N ha
-1
) indicate sufficiency in nitrogen supply for maize 
growth. Interestingly, one of the major factors limiting the use of organic fertilizers is 
associated with the high rates of application required (≥ 5 t ha-1) (Mucheru-Muna et al., 
2014; Musyoka et al., 2017) arising from the low-quality of organic resources available 
on the market.  
Our findings demonstrate that BSF frass fertilizer has potential to increase crop 
productivity in most developing countries where mineral fertilizer use is still limited by 
high prices yet less effective due to multiple soil degradation challenges. Findings from 
this study would encourage a shift in attitude towards embracing the use of organic 
fertilizers such as the BSF frass fertilizer. Nonetheless, further studies to determine the 
economically optimum rates of BSF frass fertilizers for maize production are warranted.  
4.4.7 Maize grain and stover yields  
4.4.7.1 Sole organic fertilizer treatments 
The different sole organic fertilizer treatments during the short and long rain 
seasons had significant effects on maize grain yields (short rain season: p < 0.001, long 
rain season: p < 0.001) (Figure 28). Application of BSF frass fertilizer treatments 
152 
 
increased grain yields by between 71 and 96% during the short rain season (Figure 28a) 
and 49 – 101% during long rains compared to the control (Figure 28b).  
On the other hand, grain yields increased by 50 – 87% during the short rains and 
32 – 77% during the long rains season due to SAFI organic fertilizer treatments. For 
SAFI organic fertilizer, significant (p < 0.001) increases in grain yield above the control 
were achieved at rates of 5 – 7.5 t ha-1 during the short rain season, and at 7.5 t ha-1 
during the long rain season. Maize grain yields did not vary significantly at equivalent 
rates of SAFI and BSF frass fertilizers (Figure 28a and 28b). 
Application of BSF frass fertilizer at 7.5 t ha
-1
 produced the highest maize grain 
yields of 5.4 and 7.2 t ha
-1 
during the short and long rain seasons, respectively, which 
were 5 and 14% higher than the highest grain yield obtained using SAFI organic 
fertilizer during the short and long rain seasons, respectively. Grain yield from plots 
treated with 7.5 t ha
-1
 of BSF frass fertilizer was significantly (p < 0.001) higher than 
that obtained from plots amended with 2.5 t ha
-1
 of SAFI during the short rain season. 
Similarly, grain yield from maize grown using 7.5 t ha
-1
 of BSF frass fertilizer was 
significantly (p < 0.001) higher than those of other treatments during the long rain 
season, except for maize treated with 5 t ha
-1
 BSF frass fertilizer and 7.5 t ha
-1 
for SAFI 
organic fertilizer (Figure 28).  
153 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 28: Effects of sole BSF frass and SAFI organic fertilizers on maize grain yields 
during the short (a) and long rain (b) season experiments. 
Control = unfertilized soil; 2.5BSF, 5BSF and 7.5BSF = application BSF frass fertilizer at rates of 2.5, 5 
and 7.5 t ha
-1
, respectively; 2.5SAFI, 5SAFI and 7.5SAFI = application SAFI organic fertilizer at rates of 
2.5, 5 and 7.5 t ha
-1
; BSF = Black soldier fly frass fertilizer; SAFI = commercial organic fertilizer. In the 
same season, means (± stand error) followed by the same letters are not significantly different at p ≤ 0.05 
 
 
 
The different sole BSF frass and SAFI organic fertilizer treatments did not 
significantly influence maize stover yields during the experiments (short rain season: p 
= 0.118, long rain season: p = 0.259) (Table 22). However, plots treated with BSF frass 
fertilizer had higher stover yields (7.9 – 11.5 t ha-1) than those treated with SAFI organic 
fertilizer (6.4 – 10.6 t ha-1) during the long rain season.  
154 
 
Table 22: Effects of BSF frass fertilizer, commercial organic and mineral (urea) 
fertilizers on maize stover yield. 
Fertilizer 
treatments 
SEASON 2019A (short 
rains) 
SEASON 2019B 
(long rains) 
Stover yield (t ha
-1
) 
Fertilizer treatments (t ha
-1
) 
Control  5.9 ± 0.78 5.8 ± 1.2 
2.5 BSFF 8.4 ± 0.63 8.1 ± 0.6 
5 BSFF 9.6 ± 0.76 9.8 ± 2.4 
7. 5BSFF 7.9 ± 1.05 11.5 ± 2.3 
2.5 SAFI 8.5 ± 0.70 7.2 ± 1.9 
5 SAFI 6.4 ± 0.44 7.4 ± 0.6 
7.5 SAFI 8.8 ± 1.49 10.6 ± 1.8 
P value ns ns 
Fertilizer treatments (kg N ha
-1
) 
30N BSFF 6.8 ± 1.78 7.7 ± 0.32abc 
60N BSFF 8.3 ± 0.83 8.9 ± 0.26ab 
100N BSFF 9.6 ± 1.41 10.3 ± 0.10a 
30N SAFI 6.1 ± 0.60 5.0 ± 0.34c 
60N SAFI 7.6 ± 0.97 7.3 ±0.27abc 
100N SAFI 7.7 ± 0.71 8.0 ± 0.20abc 
30N UREA 9.0 ± 1.23 7.1± 0.4babc 
60N UREA 6.4 ± 0.89 9.1± 0.04ab 
100N UREA 8.1 ± 0.54 7.7± 0.42abc 
Control  5.9 ± 0.78 5.8 ± 1.2bc 
p value ns ** 
Key: *** p < 0.001, ** p < 0.01, * p < 0.05; ns = not significant (p ≥ 0.05); 2.5BSF, 5BSF and 7.5BSF = 
application BSF frass fertilizer at rates of 2.5, 5 and 7.5 t ha
-1
, respectively; 2.5SAFI, 5SAFI and 7.5SAFI 
= application SAFI organic fertilizer at rates of 2.5, 5 and 7.5 t ha
-1
; 30N BSF, 60N BSF and 100N BSF = 
application of BSF frass fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N SAFI, 
60N SAFI and 100N SAFI = application of SAFI organic fertilizer at rates equivalent to 30, 60 & 100 kg 
N ha
-1
, respectively; 30N UREA, 60N UREA and 100N UREA = application of urea fertilizer at rates 
equivalent to 30, 60 & 100 kg N ha
-1
, respectively; BSF = Black soldier fly frass fertilizer; SAFI = 
commercial organic fertilizer. In the same column, means (± standard error) followed by the same letters 
are not significantly different at p ≤ 0.05. 
 
 
 
155 
 
4.4.7.2 Combined fertilizer treatments 
The grain yields also varied significantly among the different fertilizer 
treatments during the short and long rain seasons (short rain season: p < 0.001, long rain 
season: p < 0.001) as indicated in Figures 29a and 29b. All plots treated with BSF frass 
fertilizer produced significantly (p < 0.001) higher grain yields than the control. Grain 
yields from plots treated with 100 kg N ha
-1
 of BSF frass fertilizer were significantly (p 
< 0.001) higher than those where 30 kg N ha
-1
 of urea and 60 kg N ha
-1
 of SAFI organic 
fertilizer were applied during the short rain season (Figure 29a). Similarly, plots treated 
with 60 and 100 kg N ha
-1
 of BSF frass fertilizer, and 100 kg N ha
-1
 of urea, produced 
significantly (p < 0.001) higher maize grain yields than where 30 and 60 kg N ha
-1
 of 
SAFI organic fertilizer were applied during the long rain season (Figure 29b).  
Maize treated with 100 kg N ha
-1
 applied as BSF frass fertilizer produced the 
highest grain yield (5.7 t ha
-1
) during the short rain season which was 6 and 27% higher 
than yields obtained using equivalent rates of urea and SAFI organic fertilizer, 
respectively (Figure 29a). Likewise, grain yield from maize treated with 100 kg N ha
-1
 
applied as BSF frass fertilizer (6.2 t ha
-1
) was 7 and 21% higher than those obtained 
using equivalent rates of urea and SAFI organic fertilizer, respectively, during the long 
rain season (Figure 29b).  
The stover yields varied significantly (p = 0.003) during the long rain season 
only (Table 22). Plots treated with 100 kg N ha
-1
 supplied as BSF frass fertilizer 
produced significantly (p = 0.003) higher stover maize yields than the control treatment 
and where 30 kg N ha
-1
 supplied using SAFI organic fertilizer was applied.  
156 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 29: Effects of BSF frass, SAFI organic and urea fertilizers on maize grain yields 
during the short (a) and long rain (b) season experiments. 
Control = unfertilized soil; 30N BSF, 60N BSF and 100N BSF = application of BSF frass fertilizer at 
rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N SAFI, 60N SAFI and 100N SAFI = 
application of SAFI organic fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N 
UREA, 60N UREA and 100N UREA = application of urea fertilizer at rates equivalent to 30, 60 & 100 kg 
N ha
-1
, respectively; BSF = Black soldier fly frass fertilizer; SAFI = commercial organic fertilizer. In the 
same season, means (± standard error) followed by the same letters are not significantly different at p ≤ 
0.05 
 
 
157 
 
The findings of this study revealed higher maize growth, nutrient uptake, and 
grain yields associated with all fertilizer treatments compared to the control (unamended 
soil) (Figures 28 and 29). From the results, it can be concluded that treating the soil with 
fertilizer significantly improved the maize yield and quality in terms of macronutrients 
(nitrogen, phosphorus, and potassium). This is consistent with other studies which 
clearly demonstrated that most parts of central Kenya have low soil fertility (Gachimbi 
et al., 2005; Muchena et al., 2005) and recommended regular fertilizer inputs for soil 
fertility improvement (Tully et al., 2015; Vanlauwe et al., 2015). Therefore, the 
increased maize plant height (Figure 22 and 23), chlorophyll concentration (Figure 24 
and 25), and nitrogen (Figures 26 and 27) and phosphorus uptake (Table 20) observed in 
plots treated with BSF frass fertilizer compared to plots treated with the commercial 
organic and mineral fertilizers could be attributed to better supply and availability of 
nutrients from the newly introduced frass fertilizer. Furthermore, it is suggested that the 
high release of nutrients resulting from the high mineralisation rate of BSF frass 
fertilizer (Adin Yéton et al., 2019) and high availability of mineral nitrogen in the top 20 
cm of soil depth (Tables 17 and 18) might have partly contributed to better synchrony of 
nutrients supply for maize growth, chlorophyll formation and high yields. 
  
158 
 
4.5 Soil moisture storage, nitrogen fertilizer equivalence value and synchrony of N 
mineralization from BSF frass and commercial organic fertilizer for maize 
production  
4.5.1 Effect of BSF frass and SAFI organic fertilizers on soil moisture storage 
 The soil moisture content in the topsoil depth (0 – 20 cm) varied significantly 
during the short (p < 0.001) and long rain season (p < 0.001) experiments (Figure 30a 
and 30b). The soil moisture ranged between 14 and 27 g 100 g
-1 
at 35 days after planting 
(coinciding with early vegetative stage) after that it decreased significantly to minimum 
values at 91 and 70 days after planting during the short (10 g 100g
-1
) and long (12 g 
100g
-1
) rain seasons, as shown in Figures 30a and 30b, respectively. The soil moisture 
content significantly (p < 0.001) increased at the 125
th
 day after planting (corresponding 
to maturity stage) during long rain season. 
  
 
  
Figure 30: 
Soil 
moisture 
content in 
the 
topsoil (0 – 20 
cm) depth 
of soil 
amended with 
BSF frass and 
SAFI 
organic 
159 
 
fertilizers during the short (a) and long rain (b) cropping seasons. 
 
 
 
 
 
The soil moisture content in the subsoil depth (20 – 40 cm), also varied 
significantly during the maize growing period (short rain season: p < 0.001, long rain 
season: p < 0.001). However, after 70 days of planting, the soil moisture significantly (p 
< 0.001) decreased (corresponding to tasseling stage) and leveled off until maturity 
stage of the short rain season (Figure 31a and 31b).  
 
 
 
 
 
 
 
 
 
160 
 
 
 
Figure 31: Soil moisture content in the subsoil (20 – 40 cm) depth of soil amended with 
BSF frass and SAFI organic fertilizers the short (a) and long rain (b) cropping seasons. 
 
Soil amended with SAFI maintained higher moisture contents than that amended 
with BSF frass fertilizer from the 70
th
 to 125
th
 day after planting (Figure 31a and 31b). 
The higher soil moisture content obtained during the long rain season could be largely 
attributed to higher rainfall and lower temperatures experienced than during the short 
rain season (Figures 2a – c).  The findings of this study are consistent with findings of 
Rusinamhodzi et al. (2013) who reported high water infiltration and moisture storage in 
soil amended with organic fertilizers. Furthermore, Vanlauwe et al. (2015) highlighted 
the positive roles of manure application in improving soil moisture conservation, 
fertilizer use efficiency and crop productivity. The increased soil moisture storage 
achieved using organic fertilizers is a positive step towards climate change adaptation 
and could act as a strategy for maintaining crop production in rainfed production 
systems which characterise most regions of SSA. 
4.5.2 Effects of BSF frass and SAFI fertilizers on N mineralization 
Application of BSF frass and SAFI organic fertilizers caused significant 
differences on N mineralization during the two cropping seasons (short rains season: p < 
0.05, long rain season: p < 0.01) as indicated in Figures 32a and 32b. Mineral N released 
from soil amended with BSF frass fertilizer reached peak levels at 70 days of incubation 
which coincided with the tasseling stage of maize crop in both seasons, after which it 
161 
 
decreased until the 125
th 
day of incubation (coinciding with the maturity stage). For 
SAFI organic fertilizer, the peak levels in mineral N release were attained after 91 days 
of incubation which coincided with silking stage of maize crop during the short rain 
season (Figure 32a) while in the long rain season, the N release kept on a decreasing 
trend throughout the season (Figure 32b).  
The soils amended with SAFI organic fertilizer released significantly (p < 0.05) 
higher mineral N at tasseling and at maturity stages than BSF frass fertilizer amended 
soils during the short rain season (Figure 32a). However, the mineral N released from 
soil amended with BSF frass fertilizer was significantly (p < 0.01) higher than that 
released from soil amended with SAFI organic fertilizer at the early vegetative and 
tasseling stages during the long rain season (Figure 32b).  
 
 
 
 
 
 
 
 
 
 
 
 
162 
 
 
 
 
Figure 32: Mineral nitrogen released by soil amended with BSF frass and SAFI 
fertilizers during the short (a) and long rain (b) cropping seasons. 
 
The rates of mineral N release from soils amended with organic inputs also 
varied significantly during the short (p < 0.001) (Figure 33a) and long (p < 0.001) rain 
seasons (Figure 33b). There was N immobilization in soil amended with BSF frass 
fertilizer in the first 91 days of incubation, with highest rates observed after 35 days for 
short rain season (Figure 33a) and 91 days for long rain season (Figure 33b), coinciding 
with early vegetative and silking stages, respectively. 
  
 
 
 
 
 
 
 
 
 
 
 
163 
 
 
 
 
Figure 33: Mineral nitrogen release rate by soil amended with BSF frass and SAFI 
fertilizers during the short (a) and long rain (b) cropping seasons. 
 
On the other hand, net N mineralization of up to 6 kg N ha
-1
 day
-1
 at 125 days of 
incubation was recorded in soil BSF frass fertilizer which coincided with the maturity 
stages. Soil amended with SAFI organic fertilizer immobilized significantly (p < 0.001) 
higher N (-32 kg N ha
-1
 day
-1
) at the tasseling stage of the long rain season than soil 
amended with BSF frass fertilizer (Figure 33b). However, soil amended with SAFI 
organic fertilizer achieved higher and earlier net N mineralization (up to 25 kg N ha
-1
 
day
-1
 after 91 days of incubation) compared to soil amended with BSF frass fertilizer 
(Figure 33). 
 The net N immobilization observed at the active stages of maize growth (Figure 
33a and 33b) has been previously reported (Johnson et al., 2012; Loecke et al., 2012; 
Musyoka et al., 2019a) and could be largely attributed to the quality of organic inputs 
applied (Table 5). Fertilizers generated from insect frass contain high levels of labile 
organic carbon and organically bound ammonium nitrogen (Adin Yéton et al., 2019; 
Houben et al., 2020; Kagata & Ohgushi, 2012; Lalander et al., 2015), which requires 
time to be converted into a plant available form (NO3
−
) through the nitrification process. 
At the same time, ammonium nitrogen is a highly preferred by soil microorganisms 
making it prone to immobilization and denitrification pathways (Fornara et al., 2011). 
  
164 
 
 
 
 
Results from the current study are consistent with those of  Li & Li (2014) who 
established that ammonium nitrogen is one of the major N fractions that influences the 
quantity of N released from organic fertilizers. Due to the high demand for available N 
by microorganisms, there is a high possibility that most of the ammonium N from BSF 
frass fertilizer was rapidly immobilized by microbes immediately after application 
(Fornara et al., 2011; Loecke et al., 2012), thereby, reducing N supply to the plant at the 
early growth stages.  
 Furthermore, some ammonium could have also been lost through denitrification 
(via nitrification) and volatilization especially in soils with pH values (>7.5) and high 
temperature (Bernal et al., 2009), thus reducing the quantity of available N for plant 
uptake at the early stages of maize growth. On the other hand, biochar consists of 
recalcitrant organic carbon that is resistant to microbial decomposition and, 
consequently, causes delays in N release (Kleber, 2010; Rovira & Vallejo, 2002). 
Biochar contains compounds such as lignin and polyphenols (Sanchez-Monedero et al., 
2018) which have been reported to cause N immobilization (Kimetu et al., 2004; 
Musyoka et al., 2019a). Low N content, such as that observed in SAFI amended soils, 
has also been reported to stimulate N immobilization (Gómez-Muñoz et al., 2015), 
thereby, reducing the quantity of available N for plant use.  
 
165 
 
 
 
Furthermore, the low ammonium/nitrate ratio (0.43) and high nitrate 
concentration associated with SAFI organic fertilizer (Table 5) could have led to N 
leaching, especially during the long rain season, when moisture content was high 
(Loecke et al., 2012). High nitrate leaching (80% of total N), up to 1 m down the soil 
profile, has been reported in maize cropping systems in Kenya (Musyoka et al., 2019b) 
especially in soils with high sand and low organic matter levels comparable to the soils 
of this study (Table 1). 
4.5.3 Effects of BSF frass and SAFI fertilizers on mineral N distribution in soil 
The mineral N content in the topsoil depth (0 – 20 cm) varied significantly at 
different stages of maize growth during the short rain season (p < 0.05) (Figure 34a and 
34b) with mineral N content ranging between 3 and 29 kg N ha
-1
 at early vegetative 
stage after which the amount decreased until the end of the season. Soil amended with 
BSF frass fertilizer maintained significantly (p < 0.05) higher mineral N content during 
the short rain season than that amended with SAFI organic fertilizer (Figure 34a) while 
in the long rain season, the mineral N amount increased to peak levels at the tasseling 
stage, with higher values recorded in BSF frass fertilizer treated soils than those treated 
with SAFI (Figure 34b). 
 
166 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 34: Soil mineral nitrogen content in the topsoil depth (0 – 20 cm) following 
amendment with BSF frass and SAFI fertilizers during the short (a) and long rain (b) 
cropping seasons. 
 
Similarly, in the subsoil depth (20 – 40 cm), the organic fertilizers also showed 
significant differences in mineral N content at different maize growth stages during the 
short (p < 0.001) and long (p < 0.001) rain seasons (Figure 35a and 35b). At early 
vegetative stage (35 DAP), the mineral N amount lay between 3 – 42 kg N ha-1 but 
increased to peak levels (9 – 32 kg N ha-1) at silking stage, from thereafter, the amount 
decreased up to the end of the experiments. However, the mineral N content of soil 
167 
 
amended with BSF frass fertilizer decreased throughout the long rain season (Figure 
35b).  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 35: Soil mineral nitrogen content in the subsoil depth (20 – 40 cm) following 
amendment with BSF frass and SAFI fertilizers during the short (a) and long rain (b) 
cropping seasons. 
 
Soil amended with BSF frass fertilizer attained significantly (p < 0.001) higher 
mineral N content than that treated with SAFI organic fertilizer at early vegetative stage 
during both seasons, and at the silking and harvesting stages during the short rain 
season. At the same time, the mineral N content in soil amended with SAFI organic 
168 
 
fertilizer was significantly (p < 0.001) higher than that of soil amended with BSF frass 
fertilizer from tasseling stage to silking stage (Figure 35b) during the long rain season.              
4.5.4 Effects of BSF frass and SAFI fertilizer on maize N uptake  
 There were significant differences in N uptake by maize grown in soil amended 
with organic fertilizers during the short (p < 0.001) and long rain (p < 0.001) seasons 
(Figures 36a and 36b). The N uptake increased significantly to peak levels (82 – 139 kg 
N ha
-1
) at tasseling and silking stages, after which they decreased up to harvesting stage. 
Maize grown in soil amended with BSF frass fertilizer, accumulated significantly (p < 
0.001) higher N levels at the silking stage in both seasons, but only at the harvesting 
stage during the short rain season (Figure 36a).  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
169 
 
 
Figure 36: Nitrogen uptake by maize grown using BSF frass and SAFI fertilizers during 
the short rain (a) and long rain (b) cropping seasons. 
The N uptake rates also varied significantly (p < 0.01) at different growth stages 
during the long rain season only (Figures 37a and 37b). The trends in N uptake rates 
followed those of N uptake, with peak values of between 0.4 and 1. kg N ha
-1
 day
-1
. The 
N uptake rate of maize grown in soil amended with BSF frass fertilizer decreased to 
negative values (-1.2 kg N ha
-1
 day
-1
) at the silking stage of the short rain season and at 
the harvesting stages during both seasons. On the other hand, maize grown in soil 
amended with SAFI organic fertilizer had positive N uptake rate at all stages of growth 
except at the harvesting stage during the short rain season where uptake rates of -0.4 kg 
N ha
-1
 day
-1
 were recorded. 
 
 
 
 
 
 
 
 
 
 
 
 
170 
 
 
Figure 37: Nitrogen uptake rate by maize grown using BSF frass and SAFI fertilizers 
during the short rain (a) and long rain (b) cropping seasons. 
 
4.5.5 Effect of BSF frass and SAFI fertilizers on degree of N synchrony 
The different organic fertilizers showed significant variations in N flux during 
the short (p < 0.01) and long rain (p < 0.001) seasons (Figure 38a and 38b). Nitrogen 
fluxes for maize grown on soil amended with BSF frass fertilizer ranged between -16 
and 7 kg N ha
-1
 day
-1
 while those of SAFI organic fertilizer treatments ranged between -
34 and 24 kg N ha
-1
 day
-1
. The N fluxes for BSF frass fertilizer amended treatments 
increased from early vegetative stage during the short rain season towards positive 
values (Figure 38a), while the N flux during the long rain season decreased with higher 
negative values up to the silking stage (Figure 38b).  
 
  
 
 
 
 
 
 
171 
 
 
 
Figure 38: Nitrogen flux differences between N released by soil amended using BSF 
frass and SAFI fertilizers and N uptake by maize during the short (a) and long rain (b) 
cropping seasons. 
Positive N flux differences for soil amended with BSF frass fertilizer were 
observed at the harvesting stages of both seasons. In comparison however, positive N 
flux differences for soil amended with SAFI organic fertilizer were attained at silking 
stage in both seasons (Figure 38a and 38b). Higher positive N flux differences were 
recorded during the long rain season compared to the short rain season. Soil amended 
with SAFI organic fertilizer produced significantly (p < 0.001) higher positive N flux 
differences than that amended with BSF frass fertilizer in both seasons.  
It was noted that BSF frass fertilizer treatments had less negative N fluxes at the 
tasseling stage, whereas soil amended with SAFI organic fertilizer achieved positive N 
flux differences earlier (silking stage) than where BSF frass fertilizer was applied 
(maturity stage) (Figure 38a and 38b). The negative N fluxes observed during the active 
growth stages could be attributed to N immobilization and leaching, as earlier explained 
in section 4.5.2.  
 Previous research efforts have suggested supplementation with mineral N 
(Johnson et al., 2012; Cavalli et al., 2016; Musyoka et al., 2019a) or application of 
organic fertilizers before planting (Li & Li, 2014) to compensate for N immobilization 
during the early growth stages and periods of peak N demand. On the one hand, the 
present study established that an application of 2 – 16 kg ha−1 of mineral N at periods 
between the early vegetative and silking stages (35 – 91 days after planting) was 
necessary to compensate for the deficits in N release observed, while using the BSF 
172 
 
frass fertilizer for maize production. On the other hand, 6 – 36 kg N ha−1 of mineral N 
fertilizer would be required between the early vegetative and tasseling stages (35 – 70 
days after planting) to cause N synchrony for maize growth, while using SAFI organic 
fertilizer.  
 However, it is expected that with continued organic fertilizer application, 
mineral N release could increase gradually and consequently reduce the period of N 
immobilization. Conversely, the positive N flux difference observed at the silking stage 
(91 DAP) (Figure 38a and 38b) associated with SAFI organic fertilizer amendment 
could be explained by the increase in mineral N release (Figure 32 and 33) at 91 days of 
incubation. Such excesses in N could have resulted into N leaching to deeper subsoil 
layers (Musyoka et al., 2019b). However, the higher N uptake achieved by maize grown 
on soil amended with BSF frass fertilizer as compared with SAFI organic fertilizer 
(Figure 36 and 37) could be partly attributed to the higher mineral N content in the 
topsoil (Figure 34a and 34b).  
 It was also noted that peak N uptake by maize grown on soil amended with BSF 
frass fertilizer did not coincide with periods of positive N fluxes, indicating asynchrony 
(Figure 37). This mismatch has been previously attributed to N uptake from deeper soil 
layers or acquisition from N rich microsites in soil (Loecke et al., 2012; Musyoka et al., 
2019a). Deeper plant root growth and spread due to the presence of plant growth 
promoting organisms and growth hormones, such as auxin and gibberellins, have been 
reported, while using insect frass as a biofertilizer (Poveda et al., 2019). Such behavior 
of root growth could have been crucial in nutrient acquisition by maize grown using 
BSF frass fertilizer. In addition, the higher N release and uptake in the second season 
173 
 
(long rain season) could be attributed to residual effect of inputs applied in the first 
season and higher rainfall during the long rain season (Rovira & Vallejo, 2002; 
Musyoka et al., 2019a).  
4.5.6 Nitrogen fertilizer equivalence values of BSF frass and SAFI fertilizers 
 The N fertilizer equivalence values (NFE) of organic fertilizers applied at 
different N rates varied significantly during the short (p < 0.05) and long rain (p < 0.01) 
seasons (Table 23). At equivalent N rates, BSF frass fertilizer had higher NFE values 
than SAFI organic fertilizer. During the short rain season, the NFE of BSF frass 
fertilizer applied at 30 kg N ha
-1
 was 3.8 and 4.4 times higher (p < 0.05) than that of 
SAFI organic fertilizer applied at 60 and 100 kg N ha
-1
, respectively. Likewise, the NFE 
of BSF frass fertilizer applied at 60 kg N ha
-1
 was significantly (p < 0.01) higher than 
those of all SAFI organic fertilizer treatments which was even 5 times higher than the 
NFE of SAFI fertilizer applied at 60 kg N ha
-1
. 
Table 23: Nitrogen fertilizer equivalence values of BSF frass and SAFI fertilizers. 
Fertilizer  Rate  
(kg N ha
-1
) 
N fertilizer equivalence value (%) 
Season 2019A (Short rains) Season 2019B (Long 
rains) 
BSF frass 
fertilizer 
30 229.4 ± 28.4a 131.8 ± 34.3ab 
60 165.9 ± 32.7ab 151.0 ± 37.5a 
100 105.3 ± 17.7ab 110.6 ± 8.6ab 
SAFI organic 
fertilizer 
30 178.6 ± 59.7ab 3.5 ± 1.3c 
60 61.2 ± 21.9b 30.4 ± 10.9bc 
100 52.1 ± 5.3b 35.6 ± 6.5bc 
 p value  * ** 
Key: ** p < 0.01, * p < 0.05; 30, 60 and 100 = fertilizer application rates equivalent to 30, 60 and 100 kg 
N ha
-1
. In the same column, means (± standard error) followed by the same letters are not significantly 
different at p ≤ 0.05. 
 
174 
 
 
The NFE of BSF frass fertilizer treatments decreased with increase in N 
application rates, with highest values observed at the rate of 30 kg N ha
-1
 (Table 23). 
Increasing N rate of BSF frass fertilizer to 100 kg N ha
-1 
decreased the NFE by 2.2 and 
1.2 times during the short and long rain seasons, respectively. However, NFE of SAFI 
organic fertilizer did not follow a consistent trend, whereby it decreased with increase in 
N rates during the short rain season but varied proportionally with N rates during the 
long rain season.  
The higher nitrogen fertilizer equivalence values associated with the BSF frass 
fertilizer (Table 23) at all N rates than SAFI organic fertilizer are an indication of the 
high quality of this fertilizer as compared with the commercial organic fertilizer, SAFI. 
Our results are supported by previous studies which have reported improved drought 
tolerance, disease suppression, and higher crop growth yield, while using insect frass 
fertilizer (Houben et al., 2020; Kagata & Ohgushi, 2012; Poveda et al., 2019). 
Furthermore, the chitin contained in BSF frass fertilizers has been reported to improve 
plant health by stimulating disease resistance in crops (Quilliam et al., 2020). Such 
additional benefits provided by BSF frass fertilizer could have been responsible for the 
higher grain yields achieved from maize grown in soil amended with this fertilizer (BSF 
frass). The SAFI organic fertilizer is made of biochar and organic inputs with high 
recalcitrant carbon which are associated with N immobilization (Musyoka et al., 2019a). 
Furthermore, the SAFI organic fertilizer has a high nitrate concentration, that could have 
been lost through leaching, especially during the long rain season (Musyoka et al., 
2019b).  
175 
 
 
 The nitrogen fertilizer equivalence values (105 – 229%) of BSF frass fertilizer 
obtained in this study are comparable to those obtained by Kimetu et al. (2004), who 
used tithonia as organic fertilizer in the same study. Furthermore, the nitrogen fertilizer 
equivalence values of BSF frass fertilizer reported in the present study are higher than 
those previously reported for calliandra and senna (Kimetu et al., 2004), lucerne (De 
Notaris et al., 2018), poultry litter and yard waste compost (Bowden et al., 2007), and 
slurry from cattle (Cavalli et al., 2016; De Notaris et al., 2018; Lalor et al., 2011), and 
pigs (van Middelkoop & Holshof, 2017). On the contrary, the low and inconsistent 
nitrogen fertilizer equivalence recorded from SAFI organic fertilizer could be improved 
by combining it with a mineral N fertilizer to increase the N supply (Musyoka, et al., 
2019a). 
 Whereas previous studies where nitrogen fertilizer equivalence values of 
different organic amendments were found to increase with an increase in N rates 
(Hijbeek et al., 2018), this study established that the nitrogen fertilizer equivalence 
values of BSF frass fertilizer decreased with an increase in N application rates (Table 
23). The high value of nitrogen fertilizer equivalence obtained at low N rates implies 
that even at low application rates, the BSF frass fertilizer can perform equally good or 
even better than the mineral N fertilizer, and indication of  its high quality as an organic 
fertilizer input (Quilliam et al., 2020). Findings from the present study are crucial in 
changing the negative attitude towards use of organic fertilizers, with the advantage of 
less bulkiness and high nutrient quality associated with the BSF frass fertilizer. 
176 
 
Subsequently, the heavy reliance on the highly expensive mineral N fertilizers could be 
lessened by adopting high-quality organic fertilizers such as BSF frass fertilizer. 
4.6 Economic value of frass fertilizer production alongside BSF rearing and 
profitability of crop production using BSF frass fertilizer  
4.6.1 Larval and frass fertilizer yields, gross and net income from BSF farming 
The yields of BSF larva, BSF frass and economic output from the two BSF 
farming scenarios using sawdust, biochar and gypsum amended substrates are presented 
in Tables 24 and 25, respectively. Amending the rearing substrate with sawdust and > 
5% gypsum decreased the gross income from BSF larvae by 4 – 29% (Table 24) and 8 – 
24% (Table 25), respectively while substrate amendment with biochar increased the 
gross income generated from BSF larvae by 20 – 88% (Table 25). On the positive side, 
generating frass fertilizer from substrates amended with sawdust, gypsum and biochar 
increased the gross income generated from BSF farming by 4 – 6, 8 – 9 and 7 – 10 
times, respectively, compared to if BSF are reared for larvae production alone. It was 
established that even without sawdust, biochar, or gypsum amendment of the substrates, 
the frass would raise the gross income by 4.5 – 6.6 times if converted into organic 
fertilizer (Tables 24 and 25). This implies that frass fertilizer is an important component 
of BSF farming.  
For a farmer rearing BSF for both larvae and organic fertilizer production, the 
revenue from frass fertilizer accounted for 78 – 86% (Table 24) and 80 – 92% (Table 
25) of the gross income generated from insect farming, with significantly (p < 0.001) 
higher values obtained from frass fertilizer generated from substrates amended with 
20% sawdust, 15% gypsum or 20% biochar. This could be attributed to the significantly 
177 
 
(p < 0.001) higher yields of frass fertilizer generated from substrates amended with 20% 
sawdust (Table 24), 15% gypsum and 20% biochar (Table 25).  
178 
 
Table 24: Economic returns from BSF production using rearing substrates amended with sawdust. 
Rearing 
substrates 
Cost of 
rearing 
substrate 
as % of 
production 
costs 
Larval 
yield per 
tonne of 
dry 
substrate 
(kg) 
Frass 
fertilizer 
yield (kg) 
Income 
from BSF 
larvae 
(USD) 
Income 
from BSF 
frass 
fertilizer 
(USD) 
Gross 
income 
(USD) 
Income 
from frass 
fertilizer as 
% of gross 
income %) 
Net income (USD) 
Substrate 
purchased & 
transported 
Substrate 
transported 
only 
Zero cost on 
substrate 
 BSF farming for larvae production only 
BSG 89.3±0.0a 45.3±0.80a na 40.7±0.7a na 40.7±0.7c na -214.1±0.7c -36.6±0.7c 13.4±7.1c 
BSF farming for larvae and frass fertilizer production 
BSG  86.7±0.0b 45.3±0.8a 472±22.8b 40.7±0.7a 141.7±6.8b 182.5±6.2b 77.6±1.1b -79.9±6.2b 97.6±6.2b 147.6±6.2b 
BSG+9SD 85.7±0.0c 43.2±1.2ab 451±26.6b 38.9±1.1ab 135.4±8.0b 174.3±8.1b 77.6±1.1b -68.8±8.1b 84.0±8.1b 134.0±6.2b 
BSG+20SD 84.3±0.0d 37.4±0.8bc 696±36.8a 33.6±0.7bc 208.8±1.1a 242.4±11.6a 86.1±0.5a 22.4±11.6a 145.9±11.6a 195.9±11.6a 
BSG+30SD 82.6±0.0e 32.1±3.4c 555±23.3b 28.9±0.3c 166.5±7.0b 195.4±6.8b 85.2±1.6a -50.7±6.8a 93.3±6.8b 143.3±6.8b 
BSG+44SD 81.0±0.0f 34.2±0.4c 580± 9.2ab 30.9±0.4c 174±12ab 204.8±12ab 84.8±0.9a 21.1±11.8a 97.2±11.8b 147.2±11.8b 
p value  *** *** *** *** *** *** *** *** *** 
Key: *** p < 0.001. BSG = brewery spent grains; SD = sawdust; BSG+9SD, BSG+20SD, BSG+32SD and BSG+44SD = amendment of brewery spent grain 
with sawdust at inclusion levels of 9, 20, 32 and 44% (weight/weight), respectively; na = not applicable.  The amendments with sawdust at 9, 20, 32 and 44% 
corresponded to C/N ratios of 15, 20, 25 and 30, respectively. In the same column, means (± standard error) followed by the same letters are not significantly 
different at p ≤ 0.05. 
 
 
 
179 
 
Table 25: Economic returns from BSF production using rearing substrates amended with biochar and gypsum. 
Rearing 
substrates 
Cost of 
rearing 
substrate 
as % of 
productio
n costs 
Larval yield 
per tonne of 
dry 
substrate 
(kg) 
Frass 
fertilizer 
yield (kg) 
Income 
from BSF 
larvae 
(USD) 
Income 
from BSF 
frass 
fertilizer 
(USD) 
Gross income 
(USD) 
Income 
from frass 
fertilizer as 
% of gross 
income (%) 
Net income (USD) 
Substrate 
purchased & 
transported 
Substrate 
transported 
only 
Zero cost on 
substrate 
BSF farming for larvae production only 
BSG 89.6±0.0c 34.6±1.7cd na 31.1±1.5cd na 31.1±1.5f na -232.6±1.5c -46.2±1.5c 37.9±1.5c 
 BSF farming for larvae and frass fertilizer production 
BSG 87.2±0.0h 34.6±1.7cd 585±17.1c 31.1±1.5cd 175.4±5.1c 206.5±4.9e 84.9±0.8abc -64.7±4.9b 121.7±4.9b 171.7±4.9b 
BSG+5GY 88.2±0.0f 36.3±1.2cd 696±30.5bc 32.6±1.1cd 208.9±9.2bc 241.6±8.5cd 86.4±0.9abc -53.9±8.5b 132.5±8.5b 182.5±8.5b 
BSG+10GY 89.1±0.0d 31.7±3.5cd 802±25.4ab 28.6±3.1cd 240.6±7.6ab 269.2±4.5bc 89.3±1.3ab -50.6±4.5b 135.9±4.5b 185.6±4.5b 
BSG+15GY 89.9±0.0a 26.1±3.8d 878±15.3a 23.5±3.4d 263.4±4.6a 286.9±2.7b 91.8±1.2a -56.8±2.7b 129.6±2.7b 179.6±2.7b 
BSG+5BC 87.9±0.0g 41.4±4.0bcd 633±34.3c 37.3±3.6bcd 190.0±10.3c 227.2±10.1de 83.6±1.6bc -60.5±10.1b 125.9±10.1b 175.9±10.1b 
BSG+10BC 88.5±0.0e 53.6±4.8abc 696±20.7bc 48.3±4.3abc 208.9±6.2bc 257.2±2.3bcd 81.2±1.8c -47.1±2.3b 139.3±2.3b 189.3±2.3b 
BSG+15BC 89.1±0.0d 63.4±6.3ab 759±8.6b 57.1±5.7ab 227.7±2.6b 284.8±4.8b 80.0±1.7c -36.0±4.8ab 150.4±4.8ab 200.4±4.8ab 
BSG+20BC 89.7±0.0b 64.9±8.4a 876±33.9a 58.4±7.6a 262.8±10.2a 321.1±9.4a 81.8±2.2c -16.1±9.4a 170.3±9.4a 220.3±9.4a 
p value *** *** *** *** *** *** *** *** *** *** 
Key: *** p < 0.001. BSG = brewery spent grains; GY = Gypsum; BC = Biochar; BSG+5GY, BSG+10GY and BSG+15GY = amendment of brewery spent 
grains with gypsum powder at inclusion rates of 5, 10 and 15% (weight/weight), respectively; BSG+5BC, BSG+10BC, BSG+15BC and BSG+20BC = 
amendment of brewery spent grain with biochar at inclusion rates of 5, 10, 15 and 20%(weight/weight), respectively; na = not applicable. In the same column, 
means (± standard error) followed by the same letters are not significantly different at p ≤ 0.05. 
 
180 
 
This study established that for every kg of BSF larvae, 10 – 17, 10 – 19, 19 – 34 
and 12 – 15 kg of frass fertilizer were produced while using the unamended substrate, 
and substrates amended with sawdust, gypsum, and biochar, respectively. The cost of 
rearing substrate accounted for most expenses of BSF farming (81 – 90%) (Tables 24 
and 25), whereby expenses incurred on substrates amended by sawdust were 
significantly (p < 0.001) lower than using unamended substrates. On the contrary, 
substrate amendment using biochar or gypsum significantly increased the cost of rearing 
substrate (Table 25). It was established that cost of purchasing and transporting the BSF 
rearing substrates, would lead to loss as indicated by the negative net income values 
(Tables 24 and 25). Results on net income show that BSF farming would still not be 
profitable even if the farmer only incurred the costs of transporting substrate but does 
not utilize the frass for organic fertilizer production. Contrarily, amending the substrate 
with 20% sawdust and generating organic fertilizer from the frass, can deliver little 
profits even if the BSF farmer incurs expenses of purchasing and transporting rearing 
substrate. 
The net income generated from BSF farming increased positively in values when 
the cost of purchasing the rearing substrate was removed (Tables 24 and Table 25). 
However, this was only achieved in systems designed for larvae and frass fertilizer 
production, indicating that frass fertilizer is crucial for improving the cost-effectiveness 
of BSF rearing. Considering a scenario where the rearing substrate is available on the 
farm, i.e. no incurring expense on rearing substrate, the estimates indicate that 
generating frass fertilizer would increase the net income from BSF farming by 11 – 15, 
181 
 
4.8 – 4.9, and 5 – 6 times for substrates amended with sawdust (Table 24), gypsum and 
biochar (Table 25), respectively, compared to rearing BSF for larvae production alone.  
The significantly (p < 0.001) higher net income achieved from substrates 
amended with 20% biochar could be attributed to the positive impact of biochar on both 
larval and frass fertilizer yields (Table 25). Biochar is a bulking agent and its 
amendment improves nutrients, moisture retention, and porosity of organic substrates, 
which are favourable for BSF larval growth and yield and frass fertilizer production 
(Sanchez-Monedero et al., 2018).  
4.6.2 Gross margin, benefit-cost ratio and return on investment from BSF farming 
When the costs on purchasing and transporting of BSF rearing substrate are 
incurred, this resulted into loss as indicated by negative values of gross margin (Figures 
39a and 40a), benefit cost ratio (Figures 39b and 40b) and return on investment (Figures 
39c and 40c). However, amending the rearing substrates with 20% sawdust and 
generating organic fertilizer from the frass could deliver little profits even if the farmer 
had to incurred expenses of purchasing and transporting rearing substrates.  
Generating frass fertilizer as a second product would significantly (p < 0.001) 
increase gross margin, BSR and ROI compared to larval production alone (Figures 39 
and 40). It was established that without the cost of rearing substrate i.e., incurring 
transportation costs only, there is potential to increase gross margin, benefit-cost ratio 
(BCR) and return on investment (ROI) if the frass is converted into an organic fertilizer 
(Figures 39 and 40). Nevertheless, the highest gross margin of 81 and 83%, BCR of 4.2 
and 4.9 and ROI of 424 and 493% were attained using unamended substrate. Among the 
182 
 
sawdust amended substrates, the substrates amended with 20% sawdust generated the 
highest gross margins (81%), BCR (4.2) and ROI (421%) (Figure 39a and 39c). 
In rearing systems where the farmer incurs no cost or cost of transport alone, 
substrates amended with 20% sawdust generated significantly (p < 0.001) higher gross 
margins than substrates with 32 and 44% sawdust. Also, the BCR and ROI achieved 
using substrates amended with 20% sawdust were significantly higher than those of 
other amended substrates.  On the other hand, substrates amended with 5% gypsum 
produced the highest gross margin (76%), BCR (3.1) and ROI (309%) among gypsum 
amended substrates, while of substrates amended with 5% biochar generated the highest 
gross margins of (77%), BCR (3.4) and ROI (343%) among the biochar amended 
substrates (Figure 40a – c).  
The gross margins were not significantly different when the cost of transport was 
considered while using biochar and gypsum amendments to produce both larval and 
frass fertilizer (Figure 40).  Furthermore, the unamended substrate generated 
significantly higher gross margin than substrates amended with 10 – 15% gypsum, and 
10 – 20% biochar when the costs of substrate purchase and transportation were 
removed. While considering the cost of substrate transportation, unamended substrate 
generated significantly higher BCR and ROI than substrates amended with 10 – 15% 
gypsum, and 15 – 20 % biochar (Figure 40). When the costs of substrate purchase and 
transportation were removed, unamended substrate produced significantly higher BCR 
and ROI than other substrates. It was noted that increasing biochar and gypsum 
amendment rates caused significant decreases in BCR and ROI (Figure 40). 
183 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 39: Gross margins (a), benefit-cost ratio (b) and return on investment (ROI) (c) 
on two BSF farming scenarios using rearing substrates amended with sawdust.  
Key: BSG = brewery spent grains; 9sawdsut, 20sawdust, 32sawdust and 44sawdust = amendment of 
brewery spent grain with sawdust at inclusion levels of 9, 20, 32 and 44% (weight/weight), respectively. 
The amendments with sawdust at 9, 20, 32 and 44% corresponded to C/N ratios of 15, 20, 25 and 30, 
respectively. 5gypsum, 10gypsum and 15gypsum = amendment of brewery spent grains with gypsum 
powder at inclusion rates of 5, 10 and 15% (weight/weight), respectively; 5biochar, 10biochar, 15biochar 
184 
 
and 20biochar = amendment of brewery spent grain with inclusion rates of 5, 10, 15 and 20% 
(weight/weight) gypsum, respectively. Per parameter and per panel, means (±standard error) followed by 
the same letters are not significantly different at p ≤ 0.05. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 40: Gross margins (a), benefit-cost ratio (b) and return on investment (ROI) (c) 
on two BSF farming scenarios using rearing substrates amended with gypsum and 
biochar. 
Key: BSG = brewery spent grains; 9sawdsut, 20sawdust, 32sawdust and 44sawdust = amendment of 
brewery spent grain with sawdust at inclusion levels of 9, 20, 32 and 44% (weight/weight), respectively. 
The amendments with sawdust at 9, 20, 32 and 44% corresponded to C/N ratios of 15, 20, 25 and 30, 
185 
 
respectively. 5gypsum, 10gypsum and 15gypsum = amendment of brewery spent grains with gypsum 
powder at inclusion rates of 5, 10 and 15% (weight/weight), respectively; 5biochar, 10biochar, 15biochar 
and 20biochar = amendment of brewery spent grain with inclusion rates of 5, 10, 15 and 20% 
(weight/weight) gypsum, respectively. Per parameter and per panel, means (±standard error) followed by 
the same letters are not significantly different at p ≤ 0.05. 
The higher values of gross margins (53 – 83%) and returns on investment of  > 1 
obtained when using unamended substrate, and substrates amended with 9 – 20% 
sawdust (Figure 39c), 5 – 10% gypsum, and 5 – 20% biochar (Figure 40c) indicate that 
with or without substrate amendment, generating organic fertilizer as a second product 
from BSF farming has high potential for increasing cost-effectiveness of insect rearing 
systems (Makkar et al., 2014; Chia et al., 2019a; Singh & Kumari, 2019).  
Since the net income generated while using substrate amended with either 20% 
sawdust (Table 24), 10% gypsum or 20% biochar (Table 25) was greater than that 
obtained while using unamended substrate, substrate amendment is recommended for 
higher profit margins. Such innovations in BSF rearing have potential for maximizing 
profits from BSF farming and improving the sustainability of insect farming as a 
business (van Huis, 2013). 
4.6.3 Economic value of BSF frass fertilizer on maize production 
Maize grown using 100 kg N ha
-1
 of frass fertilizer generated higher gross 
income than where the commercial organic (24 – 48%) and mineral fertilizer (urea) (8 – 
32%) were applied (Table 26). However, maize grown using 100 kg N ha
-1
 of urea 
generated significantly (p < 0.001 higher net income than that grown using the BSF 
frass and SAFI organic fertilizers. Also, maize grown using urea fertilizer treatments 
produced significantly (p < 0.001) higher BCR and ROI than those grown using where 
SAFI and BSF frass fertilizers. Maize grown using 30 kg N ha
-1
 of BSF frass fertilizer 
186 
 
achieved significantly higher BCR and ROI than where 100 kg N ha
-1
 of SAFI was 
applied. The gross margin achieved using urea fertilizers was significantly (p < 0.001) 
higher than those of other treatments, except where 30 kg N ha
-1
 of BSF frass fertilizer.  
In comparison however, the gross margin achieved using 30 kg N ha
-1
 of BSF 
frass fertilizer was significantly (p < 0.001) higher compared to other organic fertilizer 
treatments, except where 30 kg N ha
-1
 of SAFI was applied. The findings on net income 
are consistent with previous studies which reported higher net income  from maize 
treated with mineral fertilizer than where organic fertilizers were applied (Mucheru-
Muna et al., 2014). It was noted that maize grown using 30 kg N ha
-1
 applied as BSF 
frass fertilizer produced significantly (p < 0.001) higher net income than all the SAFI 
treatments. The values of net income obtained in the current study are higher than those 
reported by Mucheru-Muna et al. (2007) who used organic fertilizers generated from 
agroforestry trees for maize production in central Kenya. Results of BCR indicated that 
it is profitable to apply frass fertilizer at rates equivalent to 30 kg N ha
-1
 (Table 26).  
Frass fertilizer applied at a rate of 2.5 t ha
-1
 generated the highest net income, 
that was significantly (p < 0.001) higher than the value achieved using 5 t ha
-1
 of SAFI 
by 4.5 folds. Also, the net income generated using 2.5 t ha
-1
 frass fertilizer was 33% 
higher than that obtained using equivalent rate of the commercial organic fertilizer 
(Table 27). Maize grown using 2.5 t ha
-1
 of frass fertilizer generated significantly higher 
BSR and Roi than SAFI treatments, except where 2.5 tha
-1
 of SAFI was applied. Also, 
the profit margin achieved using 2.5 t ha
-1
 of frass fertilizer was significantly (p < 0.001) 
higher compared to other sole organic fertilizer treatments, except where 5 t ha
-1
 of frass 
fertilizer and 2.5 t ha
-1
 of SAFI were applied. The decrease in net income, BCR (< 1), 
187 
 
ROI and profit margins at rates beyond 2.5 t ha
-1
 or 30 kg N ha
-1
 suggest that it may be 
less profitable to apply purchased BSF frass fertilizer at higher rates.  
 
188 
 
Table 26: Economic returns from maize production if BSF frass fertilizer is bought at same price as the commercial organic fertilizer 
(SAFI) and applied in terms of nitrogen rates. 
Treatment Grain yield (kg 
ha
-1
) 
Gross income 
(USD) 
Net income 
(USD) 
Benefit-cost 
ratio 
Return on 
investment (%) 
Gross 
margin (%) 
30N BSF 5,077 ± 180.9 abc 2,038 ± 64.5abcd 1,233 ± 65.5bcd 1.5 ± 0.08b 153.3 ± 8.0b 60.3 ± 1.3ab 
60N BSF 5,768 ± 227.4a 2,324 ± 95.9ab 1,091 ± 95.9cde 0.9 ± 0.08bc 88.4 ± 7.8bc 46.4 ± 2.4c 
100N BSF 5,937 ± 180.8a 2,417 ± 74.6a 548 ± 74.6f 0.3 ± 0.04c 29.3 ± 4.0c 22.3 ± 2.5d 
30N SAFI 4,048 ± 296.2c 1,635 ± 111.8d 959 ± 111.8def 1.4 ± 0.17b 141.8 ± 16.5b 57.8 2.7±b 
60N SAFI 4,242 ± 255.7c 1,733 ± 93.5d 757 ± 93.5ef 0.8 ± 0.10bc 77.5 ± 9.6bc 42.8 ± 3.5c 
100N SAFI 4,809 ± 177.0abc 1,952 ± 69.4bcd 756 ± 69.4f 0.4 ± 0.05c 41.9 ± 5.0c 29.1 ± 2.5d 
30N UREA 4,481 ± 411.4bc 1,836 ± 141.0cd 1,425 ± 141.0abc 3.5 ± 0.34a 346.4 ± 34.3a 76.9 ± 1.94a 
60N UREA 5,039 ± 235.2abc 2,035 ± 93.6abcd 1,589 ± 93.6ab 3.5 ± 0.21a 355.9 ± 21.0a 77.8 ± 1.10a 
100N UREA 5,588 ± 210.4ab 2,242 ± 77.3abc 1,748 ± 77.3a 3.6 ± 0.16a 354.3 ± 15.7a 77.8 ± 0.76a 
p value *** *** *** *** *** *** 
Key: *** p < 0.001. 30N BSF, 60N BSF and 100N BSF = application of BSF frass fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N 
SAFI, 60N SAFI and 100N SAFI = application of SAFI organic fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 30N urea, 60N urea and 
100N urea = application of urea fertilizer at rates equivalent to 30, 60 & 100 kg N ha
-1
, respectively; BSF = Black soldier fly frass fertilizer; SAFI = commercial 
organic fertilizer. In the same column, means (± standard error) followed by the same letters are not significantly different at p ≤ 0.05. 
 
 
 
189 
 
Table 27: Economic returns from maize production if BSF frass fertilizer is bought at same price as the commercial organic fertilizer 
(SAFI) and applied in terms of tonnage. 
Treatment  Grain yield 
 (kg ha
-1
) 
Gross income 
(USD) 
Net income 
(USD) 
Benet-cost ratio Return on 
investment (%) 
Gross margin 
(%) 
2.5 BSF 4,972 ± 188.5ab 2,022 ± 68.6ab 896.1 ± 75.1a 0.80 ± 0.07a 79.6 ± 6.7a 43.9 ± 2.1ab 
5 BSF 5,826 ± 467.3ab 2,370 ± 158.9ab 493.5 ± 174.1abc 0.30 ± 0.09bc 26.3 ± 9.3bc 18.2 ± 7.3bc 
7.5 BSF 6,252 ± 497.6a 2,527 ± 194.1a -98.6 ± 212.7cd -0.04 ± 0.08c -3.8 ± 8.1c -7.5 ± 8.6cd 
2.5 SAFI 4,396 ± 295.6b 1,800 ± 113.2b 673.9 ± 124.0ab 0.60 ± 0.11ab 59.9 ± 11ab 36.2 ± 3.8b 
5 SAFI 5,190 ± a195.5b 2,075 ± 67.8ab 198.6 ± 74.3bcd 0.10 ± 0.04bc 10.6 ± 4.0bc 9.0 ± 3.2c 
7.5 SAFI 5,436 ± 421.3ab 2,226 ± 153.2ab -400.1 ± 167.8d -0.20 ± 0.06c -15.2 ± 6.4c -21.8 ± 10.1d 
p value *** *** *** *** *** *** 
Key: *** p < 0.001. 2.5 BSF, 5 BSF and 7.5 BSF = application of BSF frass fertilizer at rates of 2.5, 5 and 7.5 t ha
-1
, respectively; 2.5 SAFI, 5 SAFI and 7.5 
SAFI = application of BSF frass fertilizer at rates of 2.5, 5 and 7.5 t ha
-1
, respectively; BSF = Black soldier fly frass fertilizer; SAFI = commercial organic 
fertilizer. In the same column, mean (± standard error) followed by the same letters are not significantly different at p ≤ 0.05. 
 
 
 
 
 
 
 
190 
 
Considering, a farmer who produces own frass fertilizer (homemade) and uses it 
for crop production (i.e., incurs labour costs for making the frass fertilizer at 45 USD per 
tonne), the net income would increase by 30, 67 and 232% for maize grown using 30, 60 
and 100 kg N ha
-1
, respectively compared to purchasing the same frass fertilizer (Table 
28). Estimates indicated that this approach of using homemade BSF frass fertilizer could 
generate higher net income from maize production compared to using urea fertilizer. 
Likewise, using homemade frass fertilizer would increase the profitability of maize 
production by 71 and 259% increase in net income from maize grown using 2.5 and 5 t 
ha
-1
 of homemade frass fertilizer, respectively (Table 28).  
Table 28: Economic returns from maize production if BSF frass fertilizer is made from 
the farm. 
BSF frass 
fertilizer 
treatments 
Fertilizer rates (kg N ha
-1
) 
Net income 
(USD) 
Benefit-cost 
ratio 
Return on 
investment (%) 
Gross margin 
(%) 
30  1,598 ± 64.5a 3.63 ± 0.15a 363 ± 14.7a 78.3 ± 0.69a 
60  1,819 ± 95.9a 3.61 ± 0.19a 361 ± 19.0a 78.1 ± 0.97ab 
100  1,817 ± 74.6a 3.03 ± 0.12b 303 ± 12.4b 75.0 ± 0.80b 
p value ns  *  *  *  
                  Fertilizer rates (t ha
-1
) 
2.5  1,534 ± 75.1a 3.1 ± 0.15a 314 ± 15.4a 75.7 ± 0.92a 
5  1,769 ± 174.1a 2.9 ± 0.29a 294 ± 29.0a 73.8 ± 2.33a 
7.5  1,814 ± 212.7a 2.5 ± 0.30a 254 ± 29.8a 70.8 ± 2.33a 
p value ns  ns  ns  ns  
Key: * p < 0.05, ns = nonsignificant (p ≥ 0.05). 30, 60 and 100 = application of BSF fertilizer at rates 
equivalent to 30, 60 & 100 kg N ha
-1
, respectively; 2.5, 5 and 7.5 = application rates of BSF fertilizer at 
2.5, 5 and 7.5 t ha
-1
, respectively; BSF = Black soldier fly frass fertilizer. In the same column, mean (± 
standard error) followed by the same letters are not significantly different at p ≤ 0.05. 
 
 
 
 
 
191 
 
The net income generated at different N application rates of BSF frass fertilizer 
did not vary significantly (Table 28). Furthermore, the net income, BCR, ROI and gross 
margin did not vary significantly when BSF frass fertilizer was applied at sole rates. It 
was noted that application of BSF fertilizer at 30 and 60 kg N ha
-1
 would generate 
significantly higher values of benefit-cost ratio (p = 0.024) and ROI (p = 0.025) than 
using 100 kg N ha
-1
 of the same fertilizer. Also, the gross margin achieved using 30 kg 
N ha
-1
 of BSF frass fertilizer was significantly (p = 0.024) higher than the value 
obtained using 100 kg N ha
-1
.  
The higher BCR (2.5 – 3.1), ROI and gross margins (71 – 76%) (Table 28) 
imply that application of frass fertilizer alone could increase economic gains if the 
farmer makes own organic fertilizer. Results from the current study are consistent with 
other researchers who established that proper utilization of organic resources can boost 
crop productivity (Rusinamhodzi et al., 2013, 2016). The approach of using homemade 
frass fertilizer would contribute to the concept of circular economy, whereby the frass 
from BSF farming is converted into organic fertilizer for use in crop fields. Such 
innovative approaches have potential to improve farm productivity through sustainable 
use of available resources (Vanlauwe et al., 2014). 
4.6.4 Economically optimum frass fertilizer rates and stochastic dominance 
analysis 
The BSF frass fertilizer had a lower optimum N rate (79 kg N ha
-1
) than urea 
(100 kg N ha
-1
) and SAFI fertilizers (120 kg N ha
-1
) (Figures 4a – c). On the other hand, 
the economic optimum rate of sole BSF frass fertilizer (7.4 t ha
-1
) was comparable to 
that of sole SAFI organic fertilizer (7.5 t ha
-1
) (Figures 4c and d).  
192 
 
The lower economically optimum N rate achieved using of frass fertilizer 
(Figures 4a – c) implies that one would incur less fertilizer expenses while using frass 
fertilizer for maize production compared to using urea and SAFI where higher quantities 
of fertilizers required. Although the economic optimum rate of sole frass fertilizer (7.4 t 
ha
-1
) was comparable to that of sole commercial organic fertilizer (7.5 t ha
-1
) (Figure 4c 
and 4d), it was found that using frass fertilizer would increase grain yield by 15% 
compared to commercial organic fertilizer (Table 27). The results of economic optimum 
rates are higher than the rates which produced the highest net profits, returns on 
investment and gross margins because the model used to determine optimum yields only 
considers the cost of fertilizer (Naher et al., 2011; Bachmaier & Gandorfer, 2012) and 
does not take into account the other costs of maize production such as labour.  
To determine if the BSF frass fertilizers would be preferred to commercial 
organic fertilizers, first order stochastic dominance analysis was performed on the grain 
yield obtained from each fertilizer treatment. The results of the stochastic dominance 
analysis (Figure 41) indicate that for all fertilizers, grain yield cumulative distribution 
with frass fertilizer is to the right of yield distributions of SAFI organic and urea 
fertilizers, implying that yield with frass fertilizer absolutely holds first-order stochastic 
dominance over those from commercial organic and urea treated plots. This, therefore, 
indicates that when compared to the SAFI commercial organic fertilizer and urea, 
application of frass fertilizer produced higher maize grain yields.  
 
 
193 
 
 
 
 
 
 
 
 
Figure 41: Stochastic dominance analysis of impact of BSF frass fertilizer on maize 
production. 
 
The yield distribution from plots amended with urea fertilizer dominated yield 
distributions of commercial organic fertilizer, an indication that use of urea fertilizer 
produces significantly higher grain yields than where commercial organic fertilizer is 
applied, as indicated by results of the two-sample Kolmogorov- Smirnov test (Table 29). 
Grain yield from plots treated with BSF frass fertilizer and urea fertilizer did not vary 
significantly, indicating that BSF frass fertilizer can perform equally good as urea. 
Therefore, frass fertilizer could be a complete or partial substitute to the scarce yet low-
quality organic resources used in most Sub-Saharan African countries (Ndambi et al., 
2019).  
 
 
194 
 
Table 29: Two sample Kolmogorov-Smirnov statistical test for first-order stochastic 
dominance. 
Fertilizer treatments Difference p value 
BSF frass fertilizer vs SAFI organic 0.67 0.001 
Urea vs SAFI organic  0.50 0.022 
BSF frass fertilizer vs urea 0.39 0.131 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
195 
 
CHAPTER FIVE: SUMMARY, CONCLUSIONS AND RECOMMENDATIONS 
5.1 Summary of the key findings 
i. It was established that the critical carbon to nitrogen ratio for optimal black 
soldier fly (BSF) growth and quality organic fertilizer production is 15 as 
depicted by the highest concentrations of nitrogen and phosphorus in frass 
compost, and minimal reduction in the yields of BSF larvae reared on substrates 
with C/N ratio of 15.  
ii. Amending the BSF rearing substrate with biochar and gypsum improved nutrient 
retention in frass compost compared to the unamended substrate. Whereas 
substrate amendment with biochar increased BSF larval yields (13 – 88%), 
amendment with gypsum decreased larval yields by 0.6 – 30%.  
iii. Using the BSF larvae to recycle organic substrates shortened the compost 
maturity time to 5 weeks compared to 8 – 24 weeks in conventional composting 
methods. 
iv. Soil amended with BSF frass fertilizer had more mineral N in topsoil which 
resulted in higher maize growth, chlorophyll concentration and N uptake than the 
commercial organic and mineral fertilizer treatments. However, the N release 
from both organic fertilizers during the early crop growth stages was slow, 
which led to insufficient N synchrony at periods of peak N demands.  
v. Application of sole BSF frass fertilizer at 7.5 t ha-1 produced the highest maize 
grain yield (7.2 t ha
-1
), which was 17% higher than the yield achieved when 
using equivalent rate of the commercial organic fertilizer (SAFI).  
196 
 
vi. Furthermore, maize grain yield of 6.2 t ha-1 from plots treated with 100 kg N ha-1 
of BSF frass fertilizer was 27 and 7% higher than the highest yields obtained 
using equivalent rates of commercial organic fertilizer and urea, respectively. 
vii. The agronomic nitrogen use efficiency (AEN) of maize treated with 30 kg N ha
-1
 
of BSF frass fertilizer was 27 and 116% higher than those attained using 
equivalent rates of the commercial organic fertilizer and urea, respectively. 
Likewise, the AEN of maize treated with 2.5 t ha
-1
 of BSF frass fertilizer was 2.4 
times higher than that achieved using a similar rate of the commercial organic 
fertilizer. 
viii. The nitrogen fertilizer equivalence of BSF frass fertilizer was 1.3 – 37 times 
higher than that of the commercial organic fertilizer applied at equivalent rates. 
ix. Generating frass fertilizer as a second product from BSF farming would increase 
the profitability of insect farming between 5 and 15 folds. It was estimated that 
the production of 1 tonne of dry BSF larvae would generate an income of US$ 
900. An estimated 10 – 34 tonnes of frass fertilizer would be produced from the 
frass obtained above with an additional cost of US$ 100 – 330. The income from 
the 10 – 34 tonnes of frass fertilizer would be worth US$ 3,000 – 10,200. 
x. Maize grown on plots treated with frass fertilizer yielded between 29 and 44% 
higher net income compared to plots with commercial organic fertilizer. The 
direct use of frass fertilizer by smallholder insect farmers for maize production 
would generate 30 – 259% higher net income compared to farmers purchasing 
similar frass fertilizer. 
197 
 
5.2 Conclusions  
i. This study has demonstrated that production of organic fertilizer using the BSF 
larvae is a promising technology worth exploring in the low-income countries of 
Sub-Saharan Africa where mineral fertilizer use is still limited due to high 
prices, low availability, and less effective if applied alone.  
ii. The ability of BSF larvae to develop on substrates with different C/N ratios, and 
various amendment rates of biochar and gypsum implies that with properly 
formulated feedstocks, the BSF can be successfully utilized for both larvae and 
organic fertilizer production without significantly affecting larval yields.  
iii. The ability of BSF larvae to recycle organic substrates into high-quality fertilizer 
within 5 weeks indicates the efficiency of this technology, and potential to 
benefit many farmers within a short period. 
iv. The higher maize growth performance, grain yield, N uptake, and N use 
efficiency associated with BSF frass fertilizer indicate superior agronomic 
performance compared to commercial fertilizers assessed during the study.  
v. The high nitrogen fertilizer equivalence values of BSF frass fertilizer imply that 
it can be a complete or partial substitute to existing commercial fertilizers used 
in most farming practices. In a nutshell, the advantages of BSF frass fertilizer 
such as shorter production time and less purchase and/or production costs, higher 
N mineralization rate, increased nutrient availability for plant uptake, improved 
soil moisture storage and presence of plant growth promoting organisms, make it 
a better alternative or potential complement to the conventional fertilizers. 
198 
 
vi. The higher economic returns achieved in BSF farming systems designed for both 
larval and organic fertilizer production confirm that frass fertilizer is vital for 
improving the cost-effectiveness of BSF farming.  
vii. The higher economic gains from maize production achieved while using BSF 
frass fertilizer indicate ability to improve profitability of fertilizer use when BSF 
larvae are used to produce frass fertilizer compared to commercially available 
organic and inorganic fertilizers. The additional use of frass fertilizer for crop 
production would generate much higher profits and close the nutrient cycles on 
the farm. Adoption of such production technologies would sustainably increase 
farm productivity. 
5.3 Recommendations 
1. For a farmer targeting to generate frass compost as a second product from black 
soldier fly rearing without significantly reducing larval yields, a substrate C/N 
ratio of 15 is recommended for generation of nutrient rich compost. A substrate 
with C/N ratio of 20 is recommended for farmers majorly targeting organic 
fertilizer production using the black soldier fly larvae.  
2. Amendment of black soldier fly rearing substrates with 20% biochar is 
recommended for increased yields of BSF larvae and frass compost as well as 
higher nutrient retention in frass compost and economic returns. Gypsum 
amendment (10 – 15%) is suggested for farmers targeting frass compost 
production only. 
199 
 
3. A composting period of four weeks after larval harvesting is recommended to 
convert black soldier fly frass into mature and stable compost for field 
application. 
4. To achieve high maize grain yield and net income while using purchased black 
soldier fly frass fertilizer, rates of 2.5 t ha
-1
 and 30 kg N ha
-1
 are recommended 
for sole and combined application, respectively. Higher grain yields and net 
incomes could be obtained at 7.5 t ha
-1
 and 100 kg N ha
-1
 but this is only 
economically viable for farmers who make frass fertilizer and use it directly on 
the farm. 
5. Inorganic N supplementation could be necessary to compensate for N 
immobilization observed at the earlier growth stages of maize treated BSF frass 
and SAFI organic fertilizers. 
5.4 Areas for future research 
1. The current studies only determined the amount of ammonium retained in the 
frass but did not measure gaseous N losses in form of ammonia gas emissions 
through volatilization process and other gaseous forms of nitrogen loss (N2 and 
NOx gases) through denitrification process plus carbon emissions. Future studies 
should explore this to assess effect the of BSF-mediated composting on the 
environment. Use of tagged N forms will be ideal in this case. 
2. The population and species of bacteria and fungi associated with the BSF larvae 
during the composting processes should be investigated to develop microbe-
200 
 
based strategies for optimization of the black soldier fly-assisted composting 
process. 
 
3. Field experiments should be carried out in other agro-ecological zones for longer 
periods and on other crop species to further validate the results obtained in this 
study and determine the effects of BSF frass fertilizer on soil physical, chemical, 
and biological properties.  
4. Comparative analysis of performance of BSF frass fertilizer and other organic 
resources such as calliandra, senna, and lucerne should be investigated.  
5. Future studies will be necessary to determine the immobilized N both by the root 
and microbial biomasses to accurately estimate N fluxes in soils amended with 
BSF frass fertilizer.  
 
 
 
 
 
 
 
 
 
 
201 
 
 
 
 
 
REFERENCES 
Abro, Z., Kassie, M., Tanga, C., Beesigamukama, D., & Diiro, G. (2020). Socio-
economic and environmental implications of replacing conventional poultry feed 
with insect-based feed in Kenya. Journal of Cleaner Production, 265, 121871. 
https://doi.org/10.1016/j.jclepro.2020.121871 
Adamtey, N., Musyoka, M. W., Zundel, C., Cobo, J. G., Karanja, E., Fiaboe, K. K. M., 
Muriuki, A., Mucheru-Muna, M., Vanlauwe, B., Berset, E., Messmer, M. M., 
Gattinger, A., Bhullar, G. S., Cadisch, G., Fliessbach, A., Mäder, P., Niggli, U., & 
Foster, D. (2016). Productivity, profitability and partial nutrient balance in maize-
based conventional and organic farming systems in Kenya. Agriculture, 
Ecosystems and Environment, 235, 61–79. 
https://doi.org/10.1016/j.agee.2016.10.001 
Adin Yéton, B. G., Aliou, S., Noël, O., Guillaume Lucien, A., Attanda Mouinou, I., 
Victor Attuquaye, C., Christophe Achille Armand Mahussi, C., Marc, K., & Guy 
Apollinaire, M. (2019). Decomposition and nutrient release pattern of agro-
processing by-products biodegraded by fly larvae in Acrisols. Archives of 
Agronomy and Soil Science, 65(11), 1610–1621. 
https://doi.org/10.1080/03650340.2019.1572118 
Africa Fertilizer. (2019). National Fertilizer Prices. https://africafertilizer.org/national/ 
#tab-id-3. Accessed 15 June April 2020. 
Agyarko-Mintah, E., Cowie, A., Van Zwieten, L., Singh, B. P., Smillie, R., Harden, S., 
& Fornasier, F. (2017). Biochar lowers ammonia emission and improves nitrogen 
retention in poultry litter composting. Waste Management, 61(May), 129–137. 
https://doi.org/10.1016/j.wasman.2016.12.009 
Alizadeh, P., Fallah, S., & Raiesi, F. (2012). Potential N mineralization and availability 
to irrigated maize in a calcareous soil amended with organic manures and urea 
under field conditions. International Journal of Plant Production, 6(4), 493–512. 
Alltech (2016). Global feed survey. https://cdn2.hubspot.net/hubfs/ 
745395/PDFs/2016GFS-Booklet_SE3.pdf. Acessed 17
th
 April 2018 
Amoding, A. (2007). Supply potential and agronomic value of urban market crop 
202 
 
wastes. PhD Thesis, Makerere University. 
Amoding, A., Stephen Tenywa, J., Ledin, S., & Otabbong, E. (2011). Effectiveness of 
crop-waste compost on a Eutric Ferralsol. Journal of Plant Nutrition and Soil 
Science, 174(3), 430–436. https://doi.org/10.1002/jpln.200900292 
Antil, R. S., Raj, D., Narwal, R. P., & Singh, J. P. (2013). Evaluation of maturity and 
stability parameters of composts prepared from organic wastes and their response 
to wheat. Waste and Biomass Valorization, 4(1), 95–104. 
https://doi.org/10.1007/s12649-012-9141-7 
 
Aparna, C., Saritha, P., Himabindu, V., & Anjaneyulu, Y. (2008). Techniques for the 
evaluation of maturity for composts of industrially contaminated lake sediments. 
Waste Management, 28(10), 1773–1784. 
https://doi.org/10.1016/j.wasman.2007.07.008 
Awasthi, M. K., Wang, M., Chen, H., Wang, Q., Zhao, J., Ren, X., Li, D., Awasthi, S. 
K., Shen, F., Li, R., & Zhang, Z. (2017a). Heterogeneity of biochar amendment to 
improve the carbon and nitrogen sequestration through reduce the greenhouse gases 
emissions during sewage sludge composting. Bioresource Technology, 224, 428–
438.  
 https://doi.org/10.1016/j.biortech.2016.11.014 
Awasthi, M. K., Wang, Q., Chen, H., Wang, M., Ren, X., Zhao, J., Li, J., Guo, D., Li, 
D., Awasthi, S. K., Sun, X., & Zhang, Z. (2017b). Evaluation of biochar amended 
biosolids co-composting to improve the nutrient transformation and its correlation 
as a function for the production of nutrient-rich compost. Bioresource Technology, 
237, 156–166. https://doi.org/10.1016/j.biortech.2017.01.044 
Awasthi, M. K., Wang, Q., & Wang, M. (2016). Effect of biochar on nitrogen loss 
control and the change of bacterial populations in sewage sludge composting. 
Azim, K., Soudi, B., Boukhari, S., Perissol, C., Roussos, S., & Thami Alami, I. (2018). 
Composting parameters and compost quality: a literature review. Organic 
Agriculture, 8(2), 141–158. https://doi.org/10.1007/s13165-017-0180-z 
Bachmaier, M., & Gandorfer, M. (2012). Estimating Uncertainty of Economically 
Optimum N Fertilizer Rates. International Journal of Agronomy, volume 2012, 1–
10. https://doi.org/10.1155/2012/580294 
Baligar, V. C., Fageria, N. K., & He, Z. L. (2001). Nutrient Use Efficiency in Plants. 
Commun. Soil sci. Plant anal., 32(7&8), 921–950. https://doi.org/10.1007/978-3-
319-10635-9 
Banks, I. J., Gibson, W. T., & Cameron, M. M. (2014). Growth rates of black soldier fly 
larvae fed on fresh human faeces and their implication for improving sanitation. 
Tropical Medicine and International Health, 19(1), 14–22. 
203 
 
https://doi.org/10.1111/tmi.12228 
Barrington, S., Choinière, D., Trigui, M., & Knight, W. (2002). Effect of carbon source 
on compost nitrogen and carbon losses. Bioresource Technology, 83(3), 189–194. 
https://doi.org/10.1016/S0960-8524(01)00229-2 
Bernal, M. P., Alburquerque, J. A., & Moral, R. (2009). Composting of animal manures 
and chemical criteria for compost maturity assessment. A review. Bioresource 
Technology, 100(22), 5444–5453. https://doi.org/10.1016/j.biortech.2008.11.027 
 
 
Bernal, M. P., Paredes, C., Sánchez-Monedero, M. A., & Cegarra, J. (1998). Maturity 
and stability parameters of composts prepared with a wide range of organic wastes. 
Bioresource Technology, 63(1), 91–99. https://doi.org/10.1016/S0960-
8524(97)00084-9 
Bowden, C., Spargo, J., & Evanylo, G. (2007). Mineralization and N fertilizer 
equivalent value of composts as assessed by tall fescue (festuca arundinacea). 
Compost Science and Utilization, 15(2), 111–118. 
https://doi.org/10.1080/1065657X.2007.10702320 
Brinton, W. (2000). Compost quality standards and guidelines: an international view. In 
Woods End Research Laboratory, Inc., USA. http://solvita.com/pdf-files/nysar-
ne.pdf 
Cammack, J. A., & Tomberlin, J. K. (2017). The impact of diet protein and carbohydrate 
on select life-history traits of the black soldier fly Hermetia illucens (L.) (Diptera: 
Stratiomyidae). Insects, 8(2). https://doi.org/10.3390/insects8020056 
Cabrera, M. L., Kissel, D. E., & Vigil, M. F. (2005). Nitrogen Mineralization from 
Organic Residues. Journal of Environmental Quality, 34(1), 75–79. 
 https://doi.org/10.2134/jeq2005.0075 
Cai, A., Xu, H., Shao, X., Zhu, P., Zhang, W., Xu, M., & Murphy, D. V. (2016). Carbon 
and nitrogen mineralization in relation to soil particle-size fractions after 32 years 
of chemical and manure application in a continuous maize cropping system. PLoS 
ONE, 11(3), 1–14. https://doi.org/10.1371/journal.pone.0152521 
Caruso, D., Devic, E., Subamia, I. W., Talamond, P., & Baras, E. (2014). Technical 
handbook of domestication and production of diptera Black Soldier Fly (BSF) 
Hermetia illucens, Stratiomyidae. (D. N. Murti (ed.); First, Issue 3). Percetakan 
IPB. 
Cavalli, D., Cabassi, G., Borrelli, L., Geromel, G., Bechini, L., Degano, L., & Marino 
Gallina, P. (2016). Nitrogen fertilizer replacement value of undigested liquid cattle 
manure and digestates. European Journal of Agronomy, 73, 34–41. 
 https://doi.org/10.1016/j.eja.2015.10.007 
204 
 
Ch’Ng, H. Y., Ahmed, O. H., & Majid, N. M. A. (2016). Improving phosphorus 
availability, nutrient uptake and dry matter production of Zea mays L. on a tropical 
acid soil using poultry manure biochar and pineapple leaves compost. Experimental 
Agriculture, 52(3), 447–465. https://doi.org/10.1017/S0014479715000204 
Chefetz, B., Chen, Y., Hadar, Y., & Hatcher, P. G. (2010). Characterization of 
Dissolved Organic Matter Extracted from Composted Municipal Solid Waste. Soil 
Science Society of America Journal, 62(2), 326. 
https://doi.org/10.2136/sssaj1998.03615995006200020005x 
Cheng, J. Y. K., Chiu, S. L. H., & Lo, I. M. C. (2017). Effects of moisture content of 
food waste on residue separation, larval growth and larval survival in black soldier 
fly bioconversion. Waste Management, 67, 315–323. 
https://doi.org/10.1016/j.wasman.2017.05.046 
Chia, S. Y., Macharia, J., Diiro, G. M., Kassie, M., Ekesi, S., van Loon, J. J. A., Dicke, 
M., & Tanga, C. M. (2020). Smallholder farmers’ knowledge and willingness to 
pay for insect-based feeds in Kenya. PLoS ONE, 15(3), 1–25. 
 https://doi.org/10.1371/journal.pone.0230552 
Chia, Shaphan Y., Tanga, C. M., van Loon, J. J., & Dicke, M. (2019a). Insects for 
sustainable animal feed: inclusive business models involving smallholder farmers. 
Current Opinion in Environmental Sustainability, 41, 23–30. 
https://doi.org/10.1016/j.cosust.2019.09.003 
Chia, Shaphan Y., Tanga, C. M., Osuga, I. M., Alaru, A. O., Mwangi, D. M., Githinji, 
M., Subramanian, S., Fiaboe, K. K. M., Ekesi, S., van Loon, J. J. A., & Dicke, M. 
(2019b). Effect of dietary replacement of fishmeal by insect meal on growth 
performance, blood profiles and economics of growing pigs in Kenya. Animals, 
9(705), 1–19. 
 https://doi.org/10.3390/ani9100705 
Chia, Shaphan Y., Tanga, C. M., Osuga, I. M., Mohamed, S. A., Khamis, F. M., Salifu, 
D., Sevgan, S., Fiaboe, K. K. M., Niassy, S., van Loon, J. J. A., Dicke, M., & 
Ekesi, S. (2018).  Effects of waste stream combinations from brewing industry on 
performance of Black Soldier Fly, Hermetia illucens (Diptera: Stratiomyidae). 
PeerJ, 6, e5885. 
 https://doi.org/10.7717/peerj.5885 
Chia, Shaphan Yong, Tanga, C. M., Khamis, F. M., Mohamed, S. A., Salifu, D., 
Sevgan, S., Fiaboe, K. K. M., Niassy, S., van Loon, J. J. A., Dicke, M., & Ekesi, S. 
(2018). Threshold temperatures and thermal requirements of black soldier fly 
Hermetia illucens: Implications for mass production. PloS One, 13(11), e0206097. 
 https://doi.org/10.1371/journal.pone.0206097 
Choi, I. H., & Moore, P. A. (2008). Effect of various litter amendments on ammonia 
volatilization and nitrogen content of poultry litter. Journal of Applied Poultry 
Research, 17(4), 454–462. https://doi.org/10.3382/japr.2008-00012 
205 
 
Choi, S., & Hassanzadeh, N. (2019). BSFL Frass : A Novel Biofertilizer for Improving 
Plant Health While Minimizing Environmental Impact. The Candian Science Fair 
Journal, 2(2), 41–46. https://doi.org/10.18192/csfj.v2i220194146 
Clark, T. M., Flis, B. J., & Remold, S. K. (2004). Differences in the effects of salinity on 
larval growth and developmental programs of a freshwater and a euryhaline 
mosquito species (Insecta: Diptera, Culicidae). Journal of Experimental Biology, 
207(13), 2289–2295. https://doi.org/10.1242/jeb.01018 
Cobo, J. G., Dercon, G., & Cadisch, G. (2010). Nutrient balances in African land use 
systems across different spatial scales: A review of approaches, challenges and 
progress. Agriculture, Ecosystems and Environment, 136(1–2), 1–15. 
 https://doi.org/10.1016/j.agee.2009.11.006 
 
de Bartoldi. (1983). The biology of composting: A review. Waste Management & 
Research, 1(2), 157–176. https://doi.org/10.1016/0734-242X(83)90055-1 
De Notaris, C., Sørensen, P., Møller, H. B., Wahid, R., & Eriksen, J. (2018). Nitrogen 
fertilizer replacement value of digestates from three green manures. Nutrient 
Cycling in Agroecosystems, 112(3), 355–368. https://doi.org/10.1007/s10705-018-
9951-5 
De Tender, C., Mesuere, B., Van der Jeugt, F., Haegeman, A., Ruttink, T., 
Vandecasteele, B., Dawyndt, P., Debode, J., & Kuramae, E. E. (2019). Peat 
substrate amended with chitin modulates the N-cycle, siderophore and chitinase 
responses in the lettuce rhizobiome. Scientific Reports, 9(1), 1–11. 
https://doi.org/10.1038/s41598-019-46106-x 
Debode, J., De Tender, C., Soltaninejad, S., Van Malderghem, C., Haegeman, A., Van 
der Linden, I., Cottyn, B., Heyndrickx, M., & Maes, M. (2016). Chitin mixed in 
potting soil alters lettuce growth, the survival of zoonotic bacteria on the leaves and 
associated rhizosphere microbiology. Frontiers in Microbiology, 7, 1–15. 
 https://doi.org/10.3389/fmicb.2016.00565 
Dias, B. O., Silva, C. A., Higashikawa, F. S., Roig, A., & Sánchez-Monedero, M. A. 
(2010). Use of biochar as bulking agent for the composting of poultry manure: 
Effect on organic matter degradation and humification. Bioresource Technology, 
101(4), 1239–1246. https://doi.org/10.1016/j.biortech.2009.09.024 
Diclaro, J. W., & Kaufman, P. E. (2012). Black soldier fly Hermetia illucens Linnaeus ( 
Insecta : Diptera : Stratiomyidae). University of Florida, IFAS extension EENY 
461. http://entomology.ifas.ufl.edu/creatures. Acessed 16
th
 April 2018  
Diener, S., Zurbrügg, C., & Tockner, K. (2015). Bioaccumulation of heavy metals in the 
black soldier fly, Hermetia illucens and effects on its life cycle. Journal of Insects 
as Food and Feed, 1(4), 261–270. https://doi.org/10.3920/JIFF2015.0030 
206 
 
Diener, Stefan. (2010). Valorisation of Organic Solid Waste using the Black Soldier Fly 
, Hermetia illucens , in Low and Middle ‐ Income Countries. PhD Thesis ETH 
ZURICH, 80 pp. 
Diener, Stefan, Studt Solano, N. M., Roa Gutiérrez, F., Zurbrügg, C., & Tockner, K. 
(2011). Biological treatment of municipal organic waste using black soldier fly 
larvae. Waste and Biomass Valorization, 2(4), 357–363. 
https://doi.org/10.1007/s12649-011-9079-1 
Diener, S., Zurbrügg, C., & Tockner, K. (2009). Conversion of organic material by 
black soldier fly larvae: establishing optimal feeding rates. Waste Management & 
Research, 27(6), 603–610. https://doi.org/10.1177/0734242X09103838 
Dobermann, D., Swift, J. A., & Field, L. M. (2017). Opportunities and hurdles of edible 
insects for food and feed. Nutrition Bulletin, 42(4), 293–308. 
https://doi.org/10.1111/nbu.12291 
Dortmans, B. (2015). Valorisation of Organic Waste – Effect of the Feeding Regime on 
Process Parameters in a Continuous Black Soldier Fly Larvae Composting System. 
Master’s Thesis; Swedish University of Agricultural Sciences Faculty, 38 pp 
Dortmans, B.M.A., Diener, S., Verstappen, B. M., & Zurbrügg C. (2017). Black Soldier 
Fly Biowaste Processing-A Step-by-Step Guide. Dübendorf: Eawag, Swiss Federal 
Institute ofAquatic Science and Technology, 1–87. 
Ebanyat, P. (2009). A road to food? Efficacy of nutrient management options targeted to 
heterogeneous soilscapes in the Teso farming system, Uganda [Wageningen, 
Netherlands]. In English. https://doi.org/ISBN 9789085854227 
Ebanyat, P., Ridder, N., de Jager, A., Delve, R. J., Bekunda, M. A., GIller, K. E., (2010). 
Drivers of land use change and household determinants of sustainability in 
smallholder farming systems of Eastern Uganda. Population and Environment, 31, 
474–506. 
 https://doi.org/10.1007/s11111-010-0104-2 
Ekinci, K., Keener H.M., E. D. L. (2000). Composting short paper fiber with broiler 
litter and additives. Compost Science and Utilization, 64, 1332–1334. 
Emino, E. R., & Warman, P. R. (2004). Biological assay for compost quality. Compost 
Science and Utilization, 12(4), 342–348. 
https://doi.org/10.1080/1065657X.2004.10702203 
Epstein, E. (1997). The science of composting. In The Science of Composting. 
Technomic Pub. Co. https://doi.org/10.1201/9780203736005 
Ermolaev, E., Lalander, C., & Vinnerås, B. (2019). Greenhouse gas emissions from 
small-scale fly larvae composting with Hermetia illucens. Waste Management, 96, 
65–74. https://doi.org/10.1016/j.wasman.2019.07.011 
FAO. (2017). The State of Food and Agriculture. Leveraging food systems for inclusive 
207 
 
rural transformation. FAO pulbishing group, Rome. 
https://doi.org/10.2307/2938399 
Fornara, D. A., Bardgett, R., Steinbeiss, S., Zak, D. R., Gleixner, G., & Tilman, D. 
(2011). Plant effects on soil N mineralization are mediated by the composition of 
multiple soil organic fractions. Ecological Research, 26(1), 201–208. 
https://doi.org/10.1007/s11284-010-0777-0 
Friedel, J. K., Herrmann, A., & Kleber, M. (2000). Ion exchange resin-soil mixtures as a 
tool in net nitrogen mineralisation studies. Soil Biology and Biochemistry, 32(11–
12), 1529–1536. https://doi.org/10.1016/S0038-0717(00)00064-X 
Friedrich, E., & Trois, C. (2016). Current and future greenhouse gas (GHG) emissions 
from the management of municipal solid waste in the eThekwini Municipality - 
South Africa. Journal of Cleaner Production, 112, 4071–4083. 
 https://doi.org/10.1016/j.jclepro.2015.05.118 
 
Gachene, C., & Kimaru, G. (2003). Soil fertility and land productivity: A guide for 
extension workers in the eastern Africa region. English Press Limited, Nairobi, 
Kenya. ISBN 9966896651 
Gachimbi, L. N., van Keulen, H., Thuranira, E. G., Karuku, A. M., de Jager, A., Nguluu, 
S., Ikombo, B. M., Kinama, J. M., Itabari, J. K., & Nandwa, S. M. (2005). Nutrient 
balances at farm level in Machakos (Kenya), using a participatory nutrient 
monitoring (NUTMON) approach. Land Use Policy, 22(1), 13–22. 
https://doi.org/10.1016/j.landusepol.2003.07.002 
Gómez-Muñoz, B., Hinojosa, M. B., & García-Ruiz, R. (2015). In situ net N 
mineralisation and nitrification under organic and conventionally managed olive oil 
orchards. Nutrient Cycling in Agroecosystems, 101(2), 223–239. 
https://doi.org/10.1007/s10705-015-9672-y 
Goyal, S., Dhull, S. K., & Kapoor, K. K. (2005). Chemical and biological changes 
during composting of different organic wastes and assessment of compost maturity. 
Bioresource Technology, 96(14), 1584–1591. 
https://doi.org/10.1016/j.biortech.2004.12.012 
Grigatti, M., Boanini, E., Cavani, L., Ciavatta, C., & Marzadori, C. (2015). Phosphorus 
in Digestate-Based Compost: Chemical Speciation and Plant-Availability. Waste 
and Biomass Valorization, 6(4), 481–493. https://doi.org/10.1007/s12649-015-
9383-2 
Guo, R., Li, G., Jiang, T., Schuchardt, F., Chen, T., Zhao, Y., & Shen, Y. (2012). Effect 
of aeration rate, C/N ratio and moisture content on the stability and maturity of 
compost. Bioresource Technology, 112, 171–178. 
https://doi.org/10.1016/j.biortech.2012.02.099 
208 
 
Hao, X., & Benke, M. B. (2008). Nitrogen Transformation and Losses during 
Composting and Mitigation Strategies. Dynamic Soil, Dynamic Plant, 2(1), 10–18. 
Hashemimajd, K., Farani, T. M., & Jamaati-e-Somarin, S. (2012). Effect of elemental 
sulphur and compost on pH, electrical conductivity and phosphorus availability of 
one clay soil. African Journal of Biotechnology, 11(6), 1425–1432. 
 https://doi.org/http://dx.doi.org/10.5897/AJB11.2800 
Hijbeek, R., ten Berge, H. F. M., Whitmore, A. P., Barkusky, D., Schröder, J. J., & van 
Ittersum, M. K. (2018). Nitrogen fertiliser replacement values for organic 
amendments appear to increase with N application rates. Nutrient Cycling in 
Agroecosystems, 110(1), 105–115. https://doi.org/10.1007/s10705-017-9875-5 
Houben, D., Daoulas, G., Faucon, M. P., & Dulaurent, A. M. (2020). Potential use of 
mealworm frass as a fertilizer: Impact on crop growth and soil properties. Scientific 
Reports, 10(1), 1–9. https://doi.org/10.1038/s41598-020-61765-x 
 
Huang, G. F., Wong, J. W. C., Wu, Q. T., & Nagar, B. B. (2004). Effect of C/N on 
composting of pig manure with sawdust. Waste Management, 24(8), 805–813. 
 https://doi.org/10.1016/j.wasman.2004.03.011 
Humanitarian data exchange. (2020). Kenya maize prices 2019. 
 https://data.humdata.org/dataset/wfp-food-prices-for-kenya. Accessed 15 May 2020 
Isibika, A., Vinnerås, B., Kibazohi, O., Zurbrügg, C., & Lalander, C. (2019). Pre-
treatment of banana peel to improve composting by black soldier fly (Hermetia 
illucens (L.), Diptera: Stratiomyidae) larvae. Waste Management, 100, 151–160. 
 https://doi.org/10.1016/j.wasman.2019.09.017 
Jackson, M. L. (1973). Soil Chemical Analysis. Prentice Hall of India Pvt. Ltd., New 
Delhi, 498. 
Jambeck, J., Hardesty, B. D., Brooks, A. L., Friend, T., Teleki, K., Fabres, J., Beaudoin, 
Y., Bamba, A., Francis, J., Ribbink, A. J., Baleta, T., Bouwman, H., Knox, J., & 
Wilcox, C. (2018). Challenges and emerging solutions to the land-based plastic 
waste issue in Africa. Marine Policy, 96, 256–263. 
https://doi.org/10.1016/j.marpol.2017.10.041 
Janczak, D., Malińska, K., Czekała, W., Cáceres, R., Lewicki, A., & Dach, J. (2017). 
Biochar to reduce ammonia emissions in gaseous and liquid phase during 
composting of poultry manure with wheat straw. Waste Management, 66, 36–45. 
 https://doi.org/10.1016/j.wasman.2017.04.033 
Johnson, H. J., Colquhoun, J. B., Bussan, A. J., & Laboski, C. A. M. (2012). Estimating 
nitrogen mineralization of composted poultry manure, organic fertilizers, and green 
manure crops for organic sweet corn production on a sandy soil under laboratory 
conditions. HortTechnology, 22(1), 37–43. 
209 
 
https://doi.org/10.21273/HORTTECH.22.1.37 
Jucker, C., Erba, D., Leonardi, M. G., Lupi, D., & Savoldelli, S. (2017). Assessment of 
Vegetable and Fruit Substrates as Potential Rearing Media for Hermetia illucens 
(Diptera: Stratiomyidae) Larvae. Environmental Entomology, 46(October 2017), 
1415–1423. https://doi.org/10.1093/ee/nvx154 
Kagata, H., & Ohgushi, T. (2012). Positive and negative impacts of insect frass quality 
on soil nitrogen availability and plant growth. Population Ecology, 54(1), 75–82. 
https://doi.org/10.1007/s10144-011-0281-6 
Kammann, C. I., Schmidt, H. P., Messerschmidt, N., Linsel, S., Steffens, D., Müller, C., 
Koyro, H. W., Conte, P., & Stephen, J. (2015). Plant growth improvement 
mediated by nitrate capture in co-composted biochar. Scientific Reports, 5, 1–13. 
https://doi.org/10.1038/srep11080 
 
 
Keino, L., Baijukya, F., Ng’etich, W., Otinga, A. N., Okalebo, J. R., Njoroge, R., & 
Mukalama, J. (2015). Nutrients limiting soybean (Glycine max L.) growth in 
acrisols and ferralsols of Western Kenya. PLoS ONE, 10(12), 1–20. 
https://doi.org/10.1371/journal.pone.0145202 
Kenya Bureau of Standards. (2017). Organic fertilizer-Specification. Kenya Standard 
KS 2290:2017. 
Kenya Government. 2019. The regulation of wages (general) (amendment) Order, 2018. 
Kenya gazette supplement No. 1.  
Khan, N., Clark, I., Sánchez-Monedero, M. A., Shea, S., Meier, S., & Bolan, N. (2014). 
Maturity indices in co-composting of chicken manure and sawdust with biochar. 
Bioresource Technology, 168, 245–251. 
https://doi.org/10.1016/j.biortech.2014.02.123 
Kihara, J., Nziguheba, G., Zingore, S., Coulibaly, A., Esilaba, A., Kabambe, V., 
Njoroge, S., Palm, C., & Huising, J. (2016). Understanding variability in crop 
response to fertilizer and amendments in sub-Saharan Africa. Agriculture, 
Ecosystems and Environment, 229, 1–12. 
https://doi.org/10.1016/j.agee.2016.05.012 
Kim, W., Bae, S., Park, K., Lee, S., Choi, Y., Han, S., & Koh, Y. (2011). Biochemical 
characterization of digestive enzymes in the black soldier fly, Hermetia illucens 
(Diptera: Stratiomyidae). Journal of Asia-Pacific Entomology, 14(1), 11–14. 
 https://doi.org/10.1016/j.aspen.2010.11.003 
Kimani, N. G., De Jong, R.,  & Akumu, J. (2007). Environmental pollution and impact 
to public health; implication of the dandora municipal dumping site in Nairobi, 
210 
 
Kenya. United Nations Environment Programme (UNEP), 40 pp. 
Kimetu, J. M., Mugendi, D. N., Palm, C. A., Mutuo, P. K., Gachengo, C. N., Bationo, 
A., Nandwa, S., & Kungu, J. B. (2004). Nitrogen fertilizer equivalencies of 
organics of differing quality and optimum combination with inorganic nitrogen 
source in Central Kenya. Nutrient Cycling in Agroecosystems, 68(2), 127–135. 
https://doi.org/10.1023/B:FRES.0000019043.33580.a6 
Kleber, M. (2010). What is recalcitrant soil organic matter? Environmental Chemistry, 
7(4), 320–332. https://doi.org/10.1071/EN10006 
Koenig, R. T., Palmer, M. D., Miner, F. D., Miller, B. E., & Harrison, J. D. (2005). 
Chemical amendments and process controls to reduce ammonia volatilization 
during in-house composting. Compost Science and Utilization, 13(2), 141–149. 
 https://doi.org/10.1080/1065657X.2005.10702231 
Komakech, A. J., Zurbrügg, C., Miito, G. J., Wanyama, J., & Vinnerås, B. (2016). 
Environmental impact from vermicomposting of organic waste in Kampala, 
Uganda. Journal of Environmental Management, 181, 395–402. 
 https://doi.org/10.1016/j.jenvman.2016.06.028 
Komakech, A. J. (2014). Urban waste management and the environmental impact of 
organic waste treatment systems in Kampala, Uganda. PhD thesis. Swedish 
University of Agricultural Sciences, Uppsala and Makerere University, Kampala, 
93. 
Lalander, C., Diener, S., Zurbrügg, C., & Vinnerås, B. (2019). Effects of feedstock on 
larval development and process efficiency in waste treatment with black soldier fly 
(Hermetia illucens). Journal of Cleaner Production, 208, 211–219. 
 https://doi.org/10.1016/j.jclepro.2018.10.017 
Lalander, C. H., Fidjeland, J., Diener, S., Eriksson, S., & Vinnerås, B. (2015). High 
waste-to-biomass conversion and efficient Salmonella spp. reduction using black 
soldier fly for waste recycling. Agronomy for Sustainable Development, 35(1), 
261–271. 
 https://doi.org/10.1007/s13593-014-0235-4 
Lalander, C., Senecal, J., Gros Calvo, M., Ahrens, L., Josefsson, S., Wiberg, K., & 
Vinnerås, B. (2016). Fate of pharmaceuticals and pesticides in fly larvae 
composting. Science of the Total Environment, 565, 279–286. 
https://doi.org/10.1016/j.scitotenv.2016.04.147 
Lalander, Cecilia, Nordberg, Å., & Vinnerås, B. (2017). A comparison in product-value 
potential in four treatment strategies for food waste and faeces - assessing 
composting, fly larvae composting and anaerobic digestion. GCB Bioenergy, 1–8. 
 https://doi.org/10.1111/gcbb.12470 
Lalor, S. T. J., Schröder, J. J., Lantinga, E. A., Oenema, O., Kirwan, L., & Schulte, R. P. 
O. (2011). Nitrogen Fertilizer Replacement Value of Cattle Slurry in Grassland as 
211 
 
Affected by Method and Timing of Application. Journal of Environmental Quality, 
40(2), 362–373. https://doi.org/10.2134/jeq2010.0038 
Lefcourt, A. M., & Meisinger, J. J. (2010). Effect of Adding Alum or Zeolite to Dairy 
Slurry on Ammonia Volatilization and Chemical Composition. Journal of Dairy 
Science, 84(8), 1814–1821. https://doi.org/10.3168/jds.s0022-0302(01)74620-6 
Li, H., Dong, X., da Silva, E. B., de Oliveira, L. M., Chen, Y., & Ma, L. Q. (2017). 
Mechanisms of metal sorption by biochars: Biochar characteristics and 
modifications. Chemosphere, 178(2017), 466–478. 
https://doi.org/10.1016/j.chemosphere.2017.03.072 
Li, L., & Li, S. (2014). Nitrogen mineralization from animal manures and its relation to 
organic N fractions. Journal of Integrative Agriculture, 13(9), 2040–2048. 
 https://doi.org/10.1016/S2095-3119(14)60769-3 
Li, Y., Luo, W., Li, G., Wang, K., & Gong, X. (2018). Performance of phosphogypsum 
and calcium magnesium phosphate fertilizer for nitrogen conservation in pig 
manure composting. Bioresource Technology, 250, 53–59. 
 https://doi.org/10.1016/j.biortech.2017.07.172 
 
Liang, Y., Leonard, J. J., Feddes, J. J. R., & McGill, W. B. (2006). Influence of carbon 
and buffer amendment on ammonia volatilization in composting. Bioresource 
Technology, 97(5), 748–761. https://doi.org/10.1016/j.biortech.2005.03.041 
Lim, S.S., Park, H.J., Lee, S.I., Lee, D.S., Kwak, J.H., Choi, W. J. (2009). The role of 
organic amendments with different biodegradability in ammonia volatilisation 
during composting of cattle manure. Korean Journal of Environemental 
Agriculture, 28(1), 20–24. 
Lim, S. S., Park, H. J., Hao, X., Lee, S. Il, Jeon, B. J., Kwak, J. H., & Choi, W. J. 
(2017). Nitrogen, carbon, and dry matter losses during composting of livestock 
manure with two bulking agents as affected by co-amendments of phosphogypsum 
and zeolite. Ecological Engineering, 102, 280–290. 
https://doi.org/10.1016/j.ecoleng.2017.02.031 
Liverpool-Tasie, L. S. O., Omonona, B. T., Sanou, A., & Ogunleye, W. O. (2017). Is 
increasing inorganic fertilizer use for maize production in SSA a profitable 
proposition? Evidence from Nigeria. Food Policy, 67, 41–51. 
https://doi.org/10.1016/j.foodpol.2016.09.011 
Loecke, T. D., Cambardella, C. A., & Liebman, M. (2012). Synchrony of net nitrogen 
mineralization and maize nitrogen uptake following applications of composted and 
fresh swine manure in the Midwest U.S. Nutrient Cycling in Agroecosystems, 
93(1), 65–74. https://doi.org/10.1007/s10705-012-9500-6 
Luo, Y., Liang, J., Zeng, G., Chen, M., Mo, D., Li, G., & Zhang, D. (2018). Seed 
212 
 
germination test for toxicity evaluation of compost: Its roles, problems and 
prospects. Waste Management, 71, 109–114. 
https://doi.org/10.1016/j.wasman.2017.09.023 
Ma, J., Lei, Y., Rehman, K. U., Yu, Z., Zhang, J., Li, W., Li, Q., Tomberlin, J. K., & 
Zheng, L. (2018). Dynamic Effects of Initial pH of Substrate on Biological Growth 
and Metamorphosis of Black Soldier Fly (Diptera: Stratiomyidae). Environmental 
Entomology, 47(1), 159–165. https://doi.org/10.1093/ee/nvx186 
Makkar, H. P. S., Tran, G., Heuzé, V., & Ankers, P. (2014). State-of-the-art on use of 
insects as animal feed. Animal Feed Science and Technology, 197, 1–33. 
 https://doi.org/10.1016/j.anifeedsci.2014.07.008 
Manurung, R., Supriatna, A., & Esyanthi, R. R. (2016). Bioconversion of Rice straw 
waste by black soldier fly larvae ( Hermetia illucens L .) : Optimal feed rate for 
biomass production. Journal of Entomology and Zoology Studies, 4(4), 1036–1041. 
Muchena, F. N., Onduru, D. D., Gachini, G. N., & de Jager, A. (2005). Turning the tides 
of soil degradation in Africa: Capturing the reality and exploring opportunities. 
Land Use Policy, 22(1), 23–31. https://doi.org/10.1016/j.landusepol.2003.07.001 
 
Mucheru-Muna, M., Mugendi, D., Kung’u, J., Mugwe, J., & Bationo, A. (2007). Effects 
of organic and mineral fertilizer inputs on maize yield and soil chemical properties 
in a maize cropping system in Meru South District, Kenya. Agroforestry Systems, 
69(3), 189–197. https://doi.org/10.1007/s10457-006-9027-4 
Mucheru-Muna, M., Mugendi, D., Pypers, P., Mugwe, J., Kung’U, J., Vanlauwe, B., & 
Merckx, R. (2014). Enhancing maize productivity and profitability using organic 
inputs and mineral fertilizer in central Kenya small-hold farms. Experimental 
Agriculture, 50(2), 250–269. https://doi.org/10.1017/S0014479713000525 
Muniafu, M. and Otiato, E. (2010). Solid Waste Management in Nairobi, Kenya. A case 
for emerging economies. The Journal of Language, Technology & 
Entrepreneurship in Africa, 2(1), 342–350. 
Musinguzi, P., Ebanyat, P., Tenywa, J. S., Basamba, T. A., Tenywa, M. M., & Mubiru, 
D. N. (2016). Critical soil organic carbon range for optimal crop response to 
mineral Fertiliser nitrogen on a Ferralsol. Experimental Agriculture, 52(4), 635–
653. 
 https://doi.org/10.1017/S0014479715000307 
Musyoka, M. W., Adamtey, N., Bünemann, E. K., Muriuki, A. W., Karanja, E. N., 
Mucheru-Muna, M., Fiaboe, K. K. M., & Cadisch, G. (2019a). Nitrogen release 
and synchrony in organic and conventional farming systems of the Central 
Highlands of Kenya. Nutrient Cycling in Agroecosystems, 113(3), 283–305. 
https://doi.org/10.1007/s10705-019-09978-z 
213 
 
Musyoka, M. W., Adamtey, N., Muriuki, A. W., Bautze, D., Karanja, E. N., Mucheru-
Muna, M., Fiaboe, K. K. M., & Cadisch, G. (2019b). Nitrogen leaching losses and 
balances in conventional and organic farming systems in Kenya. Nutrient Cycling 
in Agroecosystems, 114(3), 237–260. https://doi.org/10.1007/s10705-019-10002-7 
Musyoka, M. W., Adamtey, N., Muriuki, A. W., & Cadisch, G. (2017). Effect of organic 
and conventional farming systems on nitrogen use efficiency of potato, maize and 
vegetables in the Central highlands of Kenya. European Journal of Agronomy, 86, 
24–36. https://doi.org/10.1016/j.eja.2017.02.005 
Naher, U. A., Saleque, M. A., Panhwar, Q. A., Radziah, O., & Jusop, S. (2011). 
Techniques of efficient fertilizer management for wetland rice- a review. 
Australian Journal of Crop Science, 5(12), 1661–1669. 
Ndambi, O. A., Pelster, D. E., Owino, J. O., de Buisonjé, F., & Vellinga, T. (2019). 
Manure Management Practices and Policies in Sub-Saharan Africa: Implications on 
Manure Quality as a Fertilizer. Frontiers in Sustainable Food Systems, 3, 1–14. 
 https://doi.org/10.3389/fsufs.2019.00029 
Nelson, D. W., & Sommers, L. E. (1982). Total carbon, organic carbon, and organic 
matter. Methods of soil analysis Part 2: Chemical and microbiological properties. 
American Society of Agronomy, 9, 539–579. doi: 10.2134/agronmonogr9.2.2ed.c29 
 
Nkonya, E., Pender, J., Kaizzi, K. ., Kato, E., Mugarura, S., Ssali, H., & Muwonge, J. 
(2008). Linkages between land management, land degradation, and Poverty in Sub-
Saharan Africa: the Case of Uganda. In IFPRI research report No. 159 (Vol. 159). 
 https://doi.org/10.2499/9780896291683RR159 
Nweke, O. C., & Sanders, W. H. (2009). Modern environmental health hazards: A 
public health issue of increasing significance in Africa. Environmental Health 
Perspectives, 117(6), 863–870. https://doi.org/10.1289/ehp.0800126 
Ogunwande, G., Ogunjimi, L. O., & Fafiyebi, J. O. (2008). Effects of turning frequency 
on composting of chicken litter in turned windrow piles. Int. Agrophysics, 22, 159–
165. http://www.cigrjournal.org/index.php/Ejounral/article/view/1208 
Okalebo, J. R., Gathua, W., Woomer, L. (2002). Laboratory Methods of Soil and Plant 
Analysis. A working manual (2
nd
 ed.). TSBF-CIAT and SACRED Africa 
Okello, A. O., Nzuma, J. M., Otieno, D. J., Kidoido, M., & Tanga, C. M. (2021). 
Farmers’ Perceptions of Commercial Insect-Based Feed for Sustainable Livestock 
Production in Kenya. Sustainability, 13(5359), 1–13. https://doi.org/https:// 
doi.org/10.3390/su13105359 
Okot-Okumu, J. (2012). Solid Waste Management in African Cities – East Africa. In 
Waste Management - An Integrated Vision (Vols. 57–72, Issue Liyala 2011, pp. 3–
20). https://doi.org/10.5772/50241 
214 
 
Onyango, C. M., Harbinson, J., Imungi, J. K., Shibairo, S. S., & van Kooten, O. (2012). 
Influence of organic and mineral fertilization on germination, leaf nitrogen, nitrate 
accumulation and yield of vegetable amaranth. Journal of Plant Nutrition, 35(3), 
342–365. https://doi.org/10.1080/01904167.2012.639917 
Oonincx, D. G. A. B., van Huis, A., & van Loon, J. J. A. (2015). Nutrient utilisation by 
black soldier flies fed with chicken, pig, or cow manure. Journal of Insects as Food 
and Feed, 1(2), 131–139. https://doi.org/10.3920/JIFF2014.0023 
Osterholz, W. R., Rinot, O., Liebman, M., & Castellano, M. J. (2017). Can 
mineralization of soil organic nitrogen meet maize nitrogen demand? Plant and 
Soil, 415(1–2), 73–84. https://doi.org/10.1007/s11104-016-3137-1 
Palashikar, G., Ranade, A., & Veerappapillai, S. (2016). A brief review on emission of 
gaseous ammonia from composting of various waste materials. In International 
Journal of Pharmaceutical Sciences Review and Research, 38(2), 97–101. 
Park, S. I., Chang, B. S., & Yoe, S. M. (2014). Detection of antimicrobial substances 
from larvae of the black soldier fly, Hermetia illucens (Diptera: Stratiomyidae). 
Entomological Research, 44(2), 58–64. https://doi.org/10.1111/1748-5967.12050 
 
 
Poveda, J., Jiménez-Gómez, A., Saati-Santamaría, Z., Usategui-Martín, R., Rivas, R., & 
García-Fraile, P. (2019). Mealworm frass as a potential biofertilizer and abiotic 
stress tolerance-inductor in plants. Applied Soil Ecology, 142(April), 110–122. 
 https://doi.org/10.1016/j.apsoil.2019.04.016 
Quilliam, R. S., Nuku-Adeku, C., Maquart, P., Little, D., Newton, R., & Murray, F. 
(2020). Integrating insect frass biofertilisers into sustainable peri-urban agro-food 
systems. Journal of Insects as Food and Feed, 6(3), 315–322. 
https://doi.org/10.3920/jiff2019.0049 
R Core Team (2019). R: A language and environment for statistical computing. R 
Foundation for Statistical Computing. Vienna, Austria. 
Rana, K. M. S., Salam, M. A., Hashem, S., & Islam, A. (2015). Development of Black 
Soldier Fly Larvae Production Technique as an Alternate Fish Feed. International 
Journal of Research in Fisheries and Aquaculture, 5(1), 41–47. 
Rehman, K., Cai, M., Xiao, X., Zheng, L., Wang, H., Soomro, A. A., Zhou, Y., Li, W., 
Yu, Z., & Zhang, J. (2017). Cellulose decomposition and larval biomass production 
from the co-digestion of dairy manure and chicken manure by mini-livestock 
(Hermetia illucens L.). Journal of Environmental Management, 196, 458–465. 
 https://doi.org/10.1016/j.jenvman.2017.03.047 
Ren, L. M., Li, G. X., Shen, Y. J., Schuchardt, F., & Lu Peng. (2010). Chemical 
precipitation for controlling nitrogen loss during composting. Waste Management 
215 
 
and Research, 28(5), 385–394. https://doi.org/10.1177/0734242X09336546 
Roig, A., Cayuela, M. L., & Sánchez-Monedero, M. A. (2004). The use of elemental 
sulphur as organic alternative to control pH during composting of olive mill wastes. 
Chemosphere, 57(9), 1099–1105. 
https://doi.org/10.1016/j.chemosphere.2004.08.024 
Rovira, P., & Vallejo, V. R. (2002). Labile and recalcitrant pools of carbon and nitrogen 
in organic matter decomposing at different depths in soil: An acid hydrolysis 
approach. Geoderma, 107(1–2), 109–141. https://doi.org/10.1016/S0016-
7061(01)00143-4 
Rufino, M. C., Dury, J., Tittonell, P., van Wijk, M. T., Herrero, M., Zingore, S., 
Mapfumo, P., & Giller, K. E. (2011). Competing use of organic resources, village-
level interactions between farm types and climate variability in a communal area of 
NE Zimbabwe. Agricultural Systems, 104(2), 175–190. 
https://doi.org/10.1016/j.agsy.2010.06.001 
Rufino, M. C., Tittonell, P., van Wijk, M. T., Castellanos-Navarrete, A., Delve, R. J., de 
Ridder, N., & Giller, K. E. (2007). Manure as a key resource within smallholder 
farming systems: Analysing farm-scale nutrient cycling efficiencies with the 
NUANCES framework. Livestock Science, 112(3), 273–287. 
https://doi.org/10.1016/j.livsci.2007.09.011 
 
 
Rusinamhodzi, L., Corbeels, M., Zingore, S., Nyamangara, J., & Giller, K. E. (2013). 
Pushing the envelope? Maize production intensification and the role of cattle 
manure in recovery of degraded soils in smallholder farming areas of Zimbabwe. 
Field Crops Research, 147, 40–53. https://doi.org/10.1016/j.fcr.2013.03.014 
Rusinamhodzi, L., Dahlin, S., & Corbeels, M. (2016). Living within their means: 
Reallocation of farm resources can help smallholder farmers improve crop yields 
and soil fertility. Agriculture, Ecosystems and Environment, 216, 125–136. 
 https://doi.org/10.1016/j.agee.2015.09.033 
Said-Pullicino, D., Kaiser, K., Guggenberger, G., & Gigliotti, G. (2007). Changes in the 
chemical composition of water-extractable organic matter during composting: 
Distribution between stable and labile organic matter pools. Chemosphere, 66(11), 
2166–2176. https://doi.org/10.1016/j.chemosphere.2006.09.010 
Sánchez-García, M., Alburquerque, J. A., Sánchez-Monedero, M. A., Roig, A., & 
Cayuela, M. L. (2015). Biochar accelerates organic matter degradation and 
enhances N mineralisation during composting of poultry manure without a relevant 
impact on gas emissions. Bioresource Technology, 192, 272–279. 
https://doi.org/10.1016/j.biortech.2015.05.003 
216 
 
Sanchez-Monedero, M. A., Cayuela, M. L., Roig, A., Jindo, K., Mondini, C., & Bolan, 
N. (2018). Role of biochar as an additive in organic waste composting. Bioresource 
Technology, 247, 1155–1164. https://doi.org/10.1016/j.biortech.2017.09.193 
Sánchez-Monedero, M. A., Roig, A., Paredes, C., & Bernal, M. P. (2001). Nitrogen 
transformation during organic waste composting by the Rutgers system and its 
effects on pH, EC and maturity of the composting mixtures. Bioresource 
Technology, 78(3), 301–308. https://doi.org/10.1016/S0960-8524(01)00031-1 
Sánchez, Ó. J., Ospina, D. A., & Montoya, S. (2017). Compost supplementation with 
nutrients and microorganisms in composting process. In Waste Management (Vol. 
69, Issue 26, pp. 136–153). https://doi.org/10.1016/j.wasman.2017.08.012 
Sheng, J., Adeli, A., & Miles, D. M. (2015). Effects of N and P Immobilizing Agents on 
Ammonia Emissions and Nutrient Contents of Broiler Litter. JSM Environmental 
Science & Ecology, 3(2), 1017. 
Shumo, M., Osuga, I. M., Khamis, F. M., Tanga, C. M., Fiaboe, K. K. M., Subramanian, 
S., Ekesi, S., van Huis, A., & Borgemeister, C. (2019). The nutritive value of black 
soldier fly larvae reared on common organic waste streams in Kenya. Scientific 
Reports, 9(1), 1–13. https://doi.org/10.1038/s41598-019-46603-z 
 
 
Singh, A., & Kumari, K. (2019). An inclusive approach for organic waste treatment and 
valorisation using Black Soldier Fly larvae: A review. Journal of Environmental 
Management, 251(April), 109569. https://doi.org/10.1016/j.jenvman.2019.109569 
Somorin, T. O., Adesola, S., & Kolawole, A. (2017). State-level assessment of the 
waste-to-energy potential (via incineration) of municipal solid wastes in Nigeria. 
Journal of Cleaner Production, 164, 804–815. 
https://doi.org/10.1016/j.jclepro.2017.06.228 
Spranghers, T., Ottoboni, M., Klootwijk, C., Ovyn, A., Deboosere, S., De Meulenaer, 
B., Michiels, J., Eeckhout, M., De Clercq, P., & De Smet, S. (2017). Nutritional 
composition of black soldier fly (Hermetia illucens) prepupae reared on different 
organic waste substrates. Journal of the Science of Food and Agriculture, 97(8), 
2594–2600. 
 https://doi.org/10.1002/jsfa.8081 
Supriyatna, A., Manurung, R., & Esyanti, R. R. (2016). Growth of black soldier larvae 
fed on cassava peel wastes , An agriculture waste. Journal of Entomology and 
Zoology Studies, 4(6), 161–165. 
Sylvia, D. M., Hartel, P. G., Fuhrmann, J. F., Zuberer, D. A. (2005). Principles and 
applications of soil microbiology (2
nd
 ed.). Pearson Prentice Hall, Upper Saddle 
River, New Jersey, USA 
217 
 
Teresa, M., & Remigio, B. (2011). A Review on the Use of Phytotoxicity as a Compost 
Quality Indicator. Dynamic Soil, Dynamic Plant. Global Science Books, 5(2), 36–
44. 
Tilman, D., Cassman, K. G., Matson, P. A., Naylor, R., & Polasky, S. (2002). 
Agricultural sustainability and intensive production practices. Nature Publishing 
Group, 418, 671–677. 
Tittonell, P., Corbeels, M., Van Wijk, M. T., Vanlauwe, B., & Giller, K. E. (2008a). 
Combining organic and mineral fertilizers for integrated soil fertility management 
in smallholder farming systems of Kenya: Explorations using the crop-soil model 
FIELD. Agronomy Journal, 100(5), 1511–1526. 
https://doi.org/10.2134/agronj2007.0355 
Tittonell, P., Vanlauwe, B., Corbeels, M., & Giller, K. E. (2008b). Yield gaps, nutrient 
use efficiencies and response to fertilisers by maize across heterogeneous 
smallholder farms of western Kenya. Plant and Soil, 313(1–2), 19–37. 
https://doi.org/10.1007/s11104-008-9676-3 
Tom Richard, & Nancy Trautmann. (1996). C/N ratio - Cornell Composting. 
 http://compost.css.cornell.edu/calc/cn_ratio.html. Accessed 16
th
 April 2018. 
Tomberlin, J. K., Sheppard, D. C., & Joyce, J. A. (2002). Selected life-history traits of 
black soldier flies (Diptera: Stratiomyidae) reared on three artificial diets. Annals 
of the Entomological Society of America, 95(3), 379–386. 
https://doi.org/10.1603/0013-8746(2002)095[0379:slhtob]2.0.co;2 
Tubail, K., Chen, L., Kost, D., Dick, W. A., Michel, F. C., Keener, H. M., Rigot, J. F., & 
Klingman, M. (2008). Gypsum Additions Reduce Ammonia Nitrogen Losses 
During Composting of Dairy Manure and Biosolids. Compost Science and 
Utilization, 16(4), 285–293. https://doi.org/10.1080/1065657X.2008.10702390 
Tully, K., Sullivan, C., Weil, R., & Sanchez, P. (2015). The state of soil degradation in 
Sub-Saharan Africa: baselines, trajectories, and solutions. Sustainability, 7, 6523–
6552. https://doi.org/10.3390/su7066523 
Tumuhairwe, J. B., Tenywa, J. S., Otabbong, E., & Ledin, S. (2009). Comparison of 
four low-technology composting methods for market crop wastes. Waste 
Management, 29(8), 2274–2281. https://doi.org/10.1016/j.wasman.2009.03.015 
van Huis, A. (2013). Potential of Insects as Food and Feed in Assuring Food Security. 
Annual Review of Entomology, 58(1), 563–583. https://doi.org/10.1146/annurev-
ento-120811-153704 
van Huis, A., & Oonincx, D. G. A. B. (2017). The environmental sustainability of 
insects as food and feed. A review. Agronomy for Sustainable Development, 37(5), 
1–14. https://doi.org/10.1007/s13593-017-0452-8 
van Middelkoop, J. C., & Holshof, G. (2017). Nitrogen Fertilizer Replacement Value of 
218 
 
Concentrated Liquid Fraction of Separated Pig Slurry Applied to Grassland. 
Communications in Soil Science and Plant Analysis, 48(10), 1132–1144. 
https://doi.org/10.1080/00103624.2017.1323101 
Vanlauwe, B., Coyne, D., Gockowski, J., Hauser, S., Huising, J., Masso, C., Nziguheba, 
G., Schut, M., & Van Asten, P. (2014). Sustainable intensification and the African 
smallholder farmer. Current Opinion in Environmental Sustainability, 8, 15–22. 
https://doi.org/10.1016/j.cosust.2014.06.001 
Vanlauwe, B., Descheemaeker, K., Giller, K. E., Huising, J., Merckx, R., Nziguheba, 
G., & Wendt, J. (2015). Integrated soil fertility management in sub-Saharan 
Africa : unravelling local adaptation. Soil Journal, 1, 491–508. 
https://doi.org/10.5194/soil-1-491-2015 
von Blottnitz, H., Kasozi, A., & Cohen, B. (2010). Integrated Solid Waste Management 
Plan for the City of Nairobi (Issue February). 
www.unep.or.jp/.../IS_6_4_Nairobi_ISWMpl 
Wang, Q., Awasthi, M. K., Ren, X., Zhao, J., Li, R., Wang, Z., Wang, M., Chen, H., & 
Zhang, Z. (2018). Combining biochar, zeolite and wood vinegar for composting of 
pig manure: The effect on greenhouse gas emission and nitrogen conservation. 
Waste Management, 74, 221–230. https://doi.org/10.1016/j.wasman.2018.01.015 
Wang, X., Selvam, A., Chan, M., & Wong, J. W. C. (2013). Nitrogen conservation and 
acidity control during food wastes composting through struvite formation. 
Bioresource Technology, 147, 17–22. 
https://doi.org/10.1016/j.biortech.2013.07.060 
Wang, X., Selvam, A., & Wong, J. W. C. (2016). Influence of lime on struvite formation 
and nitrogen conservation during food waste composting. Bioresource Technology, 
217, 227–232. https://doi.org/10.1016/j.biortech.2016.02.117 
Wang, Y. Q., & Schuchardt, F. (2010). Effect of C/N ratio on the composting of 
vineyard pruning residues. Landbauforschung, 60(3), 131–138. 
Wichuk, K. M., & McCartney, D. (2007). A review of the effectiveness of current time-
temperature regulations on pathogen inactivation during composting. In Journal of 
Environmental Engineering and Science (Vol. 6, Issue 5, pp. 573–586). 
https://doi.org/10.1139/S07-011 
Wortmann, C. S., Kaizzi, K. C., Maman, N., Cyamweshi, A., Dicko, M., Garba, M., 
Milner, M., Senkoro, C., Tarfa, B., Tettah, F., Kibunja, C., Munthali, M., Nalivata, 
P., Nkonde, D., Nabahungu, L., Ouattara, K., & Serme, I. (2019). Diagnosis of crop 
secondary and micro-nutrient deficiencies in sub-Saharan Africa. Nutrient Cycling 
in Agroecosystems, 113(2), 127–140. https://doi.org/10.1007/s10705-018-09968-7 
Wu, D., Liu, P., Luo, Y., Tian, G., & Mahmood, Q. (2010). Nitrogen transformations 
during co-composting of herbal residues, spent mushrooms, and sludge. Journal of 
Zhejiang University, 11(7), 497–505. 
219 
 
https://doi.org/http://dx.doi.org/10.1631/jzus.B0900271 
Xiao, R., Awasthi, M. K., Li, R., Park, J., Pensky, S. M., Wang, Q., Wang, J. J., & 
Zhang, Z. (2017). Recent developments in biochar utilization as an additive in 
organic solid waste composting: A review. Bioresource Technology, 246, 203–213. 
 https://doi.org/10.1016/j.biortech.2017.07.090 
Yang, F., Li, G., Shi, H., & Wang, Y. (2015). Effects of phosphogypsum and 
superphosphate on compost maturity and gaseous emissions during kitchen waste 
composting. Waste Management, 36, 70–76. 
https://doi.org/10.1016/j.wasman.2014.11.012 
Yuan, Z., Cao, Q., Zhang, K., Ata-Ul-Karim, S. T., Tan, Y., Zhu, Y., Cao, W., & Liu, X. 
(2016). Optimal leaf positions for SPAD meter measurement in rice. Frontiers in 
Plant Science, 7, 1–10. https://doi.org/10.3389/fpls.2016.00719 
Zdybicka-Barabas, A., Bulak, P., Polakowski, C., Bieganowski, A., Wasko, A., & 
Cytrynska, M. (2017). Immune response in the larvae of the black soldier fly 
Hermetia illucens. Isj-Invertebrate Survival Journal, 14, 9–17. 
Zingore, S., Murwira, H. K., Delve, R. J., & Giller, K. E. (2007). Influence of nutrient 
management strategies on variability of soil fertility, crop yields and nutrient 
balances on smallholder farms in Zimbabwe. Agriculture, Ecosystems and 
Environment, 119(1–2), 112–126. https://doi.org/10.1016/j.agee.2006.06.019 
 
 
APPENDICES 
Appendix I: Papers emanating from the thesis 
1. Beesigamukama, D., Mochoge, B., Korir, N. K., Fiaboe, K. K. M., Nakimbugwe, D., 
Khamis, F. M., Dubois, T., Subramanian, S., Wangu, M. M., Ekesi, S., & Tanga, C. M. 
(2020). Biochar and gypsum amendment of agro-industrial waste for enhanced black 
soldier fly larval biomass and quality frass fertilizer. PloS One, 15(8), e0238154. 
https://doi.org/10.1371/journal.pone.0238154  
2. Beesigamukama, D., Mochoge, B., Korir, N. K., Fiaboe, K. K. M., Nakimbugwe, D., 
Khamis, F. M., Subramanian, S., Dubois, T., Ekesi, S., Kelemu, S., & Tanga, C. M. 
(2020). Exploring Black Soldier Fly Frass as Novel Fertilizer for Improved Growth, 
Yield, and Nitrogen Use Efficiency of Maize Under Field Conditions. Frontiers in Plant 
Science, 11(September), 1-17. https://doi.org/10.3389/fpls.2020.574592  
220 
 
3. Beesigamukama, D., Mochoge, B., Korir, N., Musyoka, M. W., Fiaboe, K. K. M., 
Nakimbugwe, D., Khamis, F. M., Subramanian, S., Dubois, T., Ekesi, S., & Tanga, C. 
M. (2020). Nitrogen Fertilizer Equivalence of Black Soldier Fly Frass Fertilizer and 
Synchrony of Nitrogen Mineralization for Maize Production. Agronomy, 10(1395), 1–9. 
4. Beesigamukama, D., Mochoge, B., Korir, N. K., K.M. Fiaboe, K., Nakimbugwe, D., 
Khamis, F. M., Subramanian, S., Wangu, M. M., Dubois, T., Ekesi, S., & Tanga, C. M. 
(2021). Low-cost technology for recycling agro-industrial waste into nutrient-rich 
organic fertilizer using black soldier fly. Waste Management, 119, 183–194. 
https://doi.org/10.1016/j.wasman.2020.09.043 
 
 
 
 
  
 
 
 
 
 
Appendix II: NACOSTI research permit 
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