Production of Organic Fertilizer from Black Soldier Fly Frass for Improved Soil Health and Maize Productivity in Nairobi City County, Kenya
Dennis, Beesiga Mukama
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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.