Browsing by Author "Nambafu, Godfrey"

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    Biophysical and farm management drivers of soil fertility under African Leafy Vegetable production small-scale systems in Kenya
    (Elsevier B.V., 2025-06-09) Onyuka, Enos; Nambafu, Godfrey; Darius Andika; Mwonga, Samwel; Joseph Gweyi -Onyango; Gitari, Harun; Mairura, Franklin S.
    While several studies have examined soil fertility management systems under key food and cash crops in the Sub-Saharan Africa (SSA) region, there is a knowledge gap on soil fertility management under African Leafy Vegetables (ALV) small-holder farming practices particularly in Kenya. The objective of this study was to evaluate the influence of agronomic management and socio-economic factors on soil chemical fertility in the major ALV production zones in Kenya. Farm surveys were used to gather socio-economic data, while soil sampling was undertaken in all fields to collect soil chemical data. The findings indicated that Kisii County was characterized by higher proportions of wealthy and more educated farmers, with higher frequencies of input applications relative to Kakamega County. The soil pH was highest in Kenyenya, followed by Gucha, Butere, Mumias, and Khwisero. Gucha and Kenyenya had significantly the highest record of soil carbon concentrations of 3.2 and 3.1 %, respectively relative to other sites, such as Butere (2.0 %) and Khwisero (1.8 %), which had similar soil carbon concentrations, with Khwisero having the least (1.5 %) soil C. The highest mean Zn concentration was recorded in Gucha (6.5 mg kg−1), followed by Kenyenya (6.4), Mumias (4.3), Butere (3.1) and Khwisero (3.0). The study showed that soil fertility under ALV farms in western Kenya was influenced by socio-economic factors and agronomic management factors. In general, the soil fertility status in Kisii County was better, compared to Kakamega County. Soil carbon was significantly influenced by site, soil texture, plot distance, fertilizer sources, and organic and inorganic fertilizer application frequency. Gucha and Kenyenya Sub-Counties had higher SOC levels, averaging at 1.2 and 1.1 % higher, relative to Butere Sub-County, while Mumias and Khwisero Sub-Counties had lower average SOC levels relative to Butere Sub-County. Site differences can differentiate soil nutrient distribution due to inherent differences in soil characteristics and indirect effects arising from localized soil management practices.
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    Plant Residue-Derived Organic Carbon Input into Soil in African Indigenous Vegetable Production Systems
    (Tropentag, 2016) Onyuka, Enos; Nambafu, Godfrey; Bessler, Holger; Adam, Anna; Andika, Darius O.; Gweyi-Onyango, Joseph P.; Mwonga, Samuel; Engels, Christof
    Food production in sub Saharan Africa is constrained by low soil fertility, whereby soil organic matter (SOM) is key factor regulating many soil functions that determine the yielding ability of soils. SOM content is influenced by the mass and quality of organic carbon input into soil. In smallholder farming systems manure and composts are scarce, and only small fractions are allocated to soil amendment due to alternative use as feed and fuel. Therefore, organic matter input into soil is often restricted to plant residues remaining in the field. In this study, we quantified the effects of species and harvesting method on the mass and quality of plant-derived carbon input into soil with the aim to improve soil fertility management in African indigenous vegetable (AIV) production systems. Five AIV species (amaranthus - Amaranthus cruentus, cowpea - Vigna unguiculata, African kale - Brassica carinata, African nightshade - Solanum scabrum, spider plant - Cleome gynandra) and common kale (Brassica oleracea acephala) were grown in a field experiment. Plants were harvested by two different methods, which are both commonly used in Kenya: Plants were either pulled out with some coarse roots adhering to the stems or cut about 5 cm above the soil surface. Leaf litter, above-ground plant residues and below-ground residues (coarse root, fine roots in 0–0.3m and 0.3– 0.6m soil depth) were quantified and analysed for C content. The humification efficiency of plant residues was determined in incubation studies under controlled conditions. Input of plant residue-derived organic carbon into the soil significantly differed among species with minimum of 0.3 kg C m−2 for cowpea and maximum of 0.8 kg C m−2 for amaranthus. In all species, input with leaf litter was negligible, while the contribution of fine roots to C input varied between about 40% in spider plant and 80% in cowpea. Pulling out instead of cutting plants reduced carbon input by 14% in cowpea and nearly 60% in African kale and spider plant. It is concluded that in AIV production systems with low availability of organic fertilisers species selection and harvesting method are important determinants for soil C dynamics and fertility.