Identifying potassium-deficient soils in Kenya by mapping and analysis of selected sites

Potassium (K) deficiency is emerging in intensively cropped soils of Kenya, but the magnitude of the problem has not been determined. This study was conducted to ascertain the possibilities of mapping K-deficient soils by the use of existing soil and climatic databases and validate the developed map. Mapping was done in Geographical Information Systems, using moisture data, in the Almanac Characterization Tool. Six study sites were selected: three (Itare, Keumbu, and Ndanai) in the southern geomorphic area (SGA) and three (Ebukanga, Yala, and Kabula) in the northern geomorphic area. Soil samples were analyzed for texture, clay mineralogy, exchangeable K, and other fertility attributes. Maize response to increasing levels of K (0, 25, 50, and 75 kg ha−1, as KCl) was studied in five of the six sites. Mapped area occupied 13% of the land area in Kenya. Exchangeable K ranged from 0.2 to 1.6 cmol (p+) kg−1 and increased significantly with increase in organic C (P = 0.015), pH (P = 0.023), Ca + Mg content (P = 0.031), and K saturation of exchange complex (P = 0.013). Maize response to KCl was not significant in all soils possibly because the three-season testing period was too short, but the probability of response and the profitability of K use in maize increased with seasons in SGA because of increased K depletion. Potassium fertilizer was recommended in soils from the SGA at 25 kg ha−1. Application of nitrogen-phosphate-K fertilizers was not economical in the northern geomorphic area because of attack by witchweed, Striga hermonthica.