Abstract
Physico-chemical properties of Vertisols in the Mwea area, the major irrigated rice area of Kenya, were investigated to develop recommendations for soil and nutrient management for intensive rice cultivation. Soil depth ranged from 0.8 to 1.8 m with dark clay layers with a heavy texture predominated by montmorillonite. Only a weak horizon differentiation was recognized in the clay layers except for a slight change in the structure with depth. Carbonates as nodules and concretions were commonly detected in deep to shallow layers. The soil chemical fertility was estimated to be medium to high in terms of major essential elements in general, except for K whose amount was found to be rather low in some areas. Available N level was medium to high for Vertisols, but it was obvious that efficient N application was essential to achieve a high yield standard considering the range of soil available N content.
Low yield was often associated with a low topographic position of the fields. In the fields with poor drainage mostly found in depressed areas, strong reduction of soil and high base concentration might disturb the root activity for nutrient uptake.
From the physical viewpoint, difficulty in plowing was a major constraint. Dry tillage could be an option to minimize water percolation into deep layers along cracks to maintain the trafficability. Incorporation of organic matter such as rice straw or upland crop residues is being tested to improve the friability of soil for better workability in dry tillage and drainage, which may also contribute to the recycling of nutrient sources like K.
