Browsing by Author "Onyuka, Enos"

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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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    Cadmium Uptake and Distribution to Edible Organs in African Indigenous Vegetables
    (Tropentag, 2015) Onyuka, Enos; Husen, Wilhelm van; Nambafu, Godfrey N.; Bessler, Holger; Andika, Darius O.; Gweyi-Onyango, Joseph P.; Mwonga, Samuel; Ulrichs, Christian; Engels, Christof
    Cadmium (Cd) is toxic to humans. The most important source for dietary intake is consumption of plant products, whereby Cd concentrations in leafy vegetables are particularly high. In periurban areas, soils are often contaminated with Cd. Thus, recommendations to increase vegetable consumption should consider species-specific uptake and allocation to edible plant organs. Cadmium uptake and distribution within plant was compared in various species to derive recommendations for vegetable production on contaminated soils. Four African indigenous vegetable species (African Nightshade Solanum scabrum, amaranth Amaranthus cruentus, cowpea Vigna unguiculata, spider plant Cleome gynandra), and a standard species commonly grown in Asia (Pak Choi Brassica rapa chinensis) were grown in nutrient solution at three Cd-concentrations: 0.1 (low), 1.0 (moderate) and 2.5 μM Cd (high contamination). Biomass and mineral nutrient concentrations were measured in roots, leaves, stems and lateral branches. Growth was little affected by Cd supply with the exception of amaranth and cowpea. In these species, growth of shoot organs was substantially reduced at moderate and high supply. Growth depression was associated with low leaf iron concentrations indicating Cd-induced iron deficiency. In all species Cd concentrations were higher in roots than in shoot organs, which is in accordance with xylem loading being an important barrier for Cd distribution within plants. At moderate and high supply, Cd concentrations in shoot organs strongly differed among species. Concentrations were particularly low in cowpea, and high in amaranth and African Nightshade. Low Cd concentrations in shoot organs of cowpea were associated with low apparent uptake and translocation rates of Cd from roots to shoots. At all rates of Cd supply, Cd density in edible organs was significantly influenced by species. Cadmium density was particularly low in cowpea and Pak Choi. Based on a vegetable dish of 250 g fresh mass, best choice of species reduced Cd intake by 0.1 mg at low supply (amaranth versus African Nightshade), 0.7 mg at moderate (cowpea versus African Nightshade) and 1.7 mg at high Cd supply (cowpea versus
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    Iron Concentrations in Roots and Edible Organs of African Indigenous Vegetable Species
    (Tropentag, 2015) Eckhardt, Ulrich; Beer, Julius; Bessler, Holger; Nambafu, Godfrey N.; Onyuka, Enos; Mwonga, Samuel; Andika, Darius O.; Gweyi-Onyango, Joseph P.; Engels, Christof
    Iron (Fe) deficiency is among the 10 leading human health risks causing “hidden hunger” in sub- Saharan Africa. Iron concentrations and bioavailability are often higher in leafy vegetables than in staple crops like maize and sorghum. Therefore, increasing food diversity through vegetable production and consumption is an option to mitigate Fe deficiency. The aim was to quantify Fe uptake and distribution between roots, stems and leaves in selected African indigenous vegetable (AIV) species, and to compare species responses to different rates of Fe supply. Plants of five leafy AIV species (African night shade Solanum scabrum, amaranth Amaranthus cruentus, cowpea Vigna unguiculata, spider plant Cleome gynandra, Ethiopian kale Brassica carinata) and a standard species commonly grown in Kenya (kale Brassica oleracea acephala group) were grown in nutrient solution at three rates of Fe supply (sub-optimal to induce Fe deficiency responses, optimal for growth, supra-optimal to test if Fe density in edible organs can be enhanced by additional Fe fertiliser application). Leaf chlorophyll content (SPAD-meter) and pH of the nutrient solution were measured at regular intervals to quantify shoot and root responses to Fe deficiency. At harvest, biomass and mineral nutrient concentrations were measured separately for roots, stems and leaves. The species markedly differed in their early root responses to Fe deficiency. Whereas in amaranth, proton extrusion from roots was significantly increased at low Fe supply compared to medium and high Fe supply, in spider plant and kale proton extrusion was not influenced by Fe supply. Fe concentrations in the plant dry mass strongly varied among plant organs and decreased in the order roots >> leaves > stems. In all organs, Fe concentrations were significantly affected by species. Concentrations in shoots varied from 17 (Ethiopian kale) to 39 mg Fe kg−1 dry mass (amaranth) at low Fe supply, and from 42 (Ethiopian kale) to 127 mg Fe kg−1 dry mass (cowpea) at high supply respectively Responses of Fe density in edible plant organs indicate that the potential for increasing the nutritional value of leafy vegetables by Fe fertilisation (biofortification) is largest in cowpea and spider plant.
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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.