<p>Lowland irrigated schemes contribute the most rice produced in Kenya. However, production is low and highly variable due to management problems. Production could be increased with appropriate soil management which requires that baseline fertility status of the soils and how they vary be known. This study examined the variability of selected soil chemical properties in the Mwea Irrigation Scheme in Central Kenya. Soil samples were collected from the top 0-15 cm depth in August 2013 and 2014 and analysed for pH, electrical conductivity (EC) and the exchangeable cations potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺) and sodium (Na⁺). Significant variability in soil EC as well as soil cation concentration was observed among units. Overall results showed soil pH ranged from 4.56 (very strongly acidic) to 8.05 (moderately alkaline). Soil EC varied from 0.17 to 1.52 dS m^-1 with higher elevation areas recording lower values (&lt; 0.50 dS m^-1) and lower elevation areas recording higher EC values (&gt; 0.50 dS m^-1). On average, exchangeable Ca²⁺ was 38.17 cmol_c kg^-1, Mg²⁺ 23.80 cmol_c kg^-1, Na⁺ 1.24 cmol_c kg^-1 and K⁺ 0.35 cmol_c kg^-1. The soil exchange complex was mainly dominated by Ca²⁺ and Mg²⁺ and cation concentration in the soil was in the order Ca²⁺ &gt; Mg²⁺ &gt; Na⁺ &gt; K⁺. Soil K is low and severe cation imbalances exist with regard to K⁺ and other cations thus making K⁺ deficient for plant uptake. Management practices and farming systems which enhance soil K status should be encouraged to help boost and sustain rice yield.</p>

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Soil degradation reduces agricultural productivity and poses a great threat on food security status of households. In Kenya, farmers have for a long time been using only nitrogen and phosphorous based fertilizers oblivious of the soil fertility status. In most cases, there has been lack of plant response to these fertilizers, which could be due to a limitation of nutrients other than nitrogen and phosphorous. Soils are considered as variable natural bodies because of combined intrinsic and extrinsic factors of different intensities at a field or a larger region scale therefore an understanding of such variability is imperative to provide insights needed in their management. This study was thus initiated to assess the availability of soil micronutrients from rice growing Vertisols in the Mwea irrigation scheme. Top (0-15 cm) soil samples were collected across paddy fields in the irrigation scheme and analyzed for iron (Fe), zinc (Zn), copper (Cu) and manganese (Mn) by the 0.1 N HCl extraction method. Soil pH (water and 1 M KCl) was also measured using the glass electrode pH meter. Soil solution pH ranged from 4.56 to 8.05 and 3.33 to 6.63 for water and 1 M KCl respectively. Soil Fe, Zn, Cu and Mn concentration varied greatly and ranged from undetected to 1360.6, 0.12 to 8.00, undetected to 9.29 and 1.50 to 849.2 mg/kg respectively. Coefficient of variations (CVs) for soil micronutrients ranged from 64% to 154% indicating very high variability. Soil pH was least variable with CVs 12% and 15% for water and KCl respectively. These results imply that the paddy soils in Mwea region are highly heterogeneous and soil micronutrients are enriched in some areas and depleted in others due to farm management practices and soil properties. Averagely, soil available Zn was deficient across the scheme and as such Zn fertilization can be effective in increasing soil Zn concentration and availability in the soil-root interface further enhancing soil productivity and yield quality. Attention should also be paid to appropriate farm management practices to avoid accumulation or depletion of nutrients.