Incorporation of Legume Residues Does Not Increase Productivity of Intercropped Beans on Smallholder Farms in Trans-Nzoia District, Kenya

ABSTRACT

Smallholder farmers in Trans-Nzoia District, Kenya, observed poor productivity of intercropped beans (Phaseolus vulgaris L.) where residues of the soil-improving legume Lciblab purpureus L. had been incorporated prior to maize (Zea mays L.) and bean planting. On-farm research involving 18 replicated trials was undertaken to evaluate if lablab residues affected bean yield and the severity of farmers' pre-existing soilborne bean pest problems. The study compared incorporated lablab residues with four other maize fertilization strategies, diammonium phosphate (DAP), a DAP and farmyard manure (FYM) mixture, incorporated Mucuna pruriens L. residues, and a no-input check. In addition to yield, the abundance of root-feeding chafer grubs (Schizonycha spp.) and the mortality rates of a root rot-susceptible versus a root rot-tolerant bean variety were assessed. Beans in the lablab and mucuna residue treatments performed as poorly as in the no- input control, possibly because of legume residue-induced nutrient immobilization for the first 4–6 weeks of the growing season. During the seedling stage, the beans in the legume residue and no input treatments had lower vigour and higher damping-off rates than the DAP and DAP/FYM treatments. Later in the growing season, there was continued plant mortality in the legume residue treatments; this coincided with significantly higher populations of root-feeding chafer grubs found in those treatments. At harvest, grain yields in the legume and no input treatments were, on average, 36% lower than the DAP and DAP plus FYM treatments. Incorporating residues earlier and applying more readily available fertilizer at planting may help farmers to mitigate some of the legumes' negative effects.