EFFECT OF PHOSPHATE FERTILIZER ON GROWTH AND YIELD OF COWPEA (Vigna unguiculata) IN NANDI SOUTH

Abstract

Phosphorus deficiency remains a key constraint to cowpea productivity in Kenya. In addition, there exist conflicting results about the effect of leaf harvest on cowpea yield. Therefore, an on-farm and greenhouse study was conducted to establish which local cowpea varieties can positively responds to P fertilizer and produce vegetable without compromising on the dry matter yield. The on farm study was conducted in Bonjoge and Koibem regions of Nandi South during the long rain season in 2009. Three cowpea varieties (Enzegu, Ilanda and Khaki) were assessed against the application of three P rates (0, 15 and 30 kg ha-1) and two leaf management practices (with and without leaf harvests). The on-farm trials were laid out in a Randomized Complete Block Design (RCBD) with three replications. Data collected consisted of seedling survival, above ground biomass (with or without leaf harvests) and P uptake. Thereafter, a greenhouse experiment was conducted for 3 months at the University of Eldoret using the two soils from earlier study. A Complete Randomized Design (CRD) with three replications was adopted with data collected on soil available P (before and after the experiment), plant height, number of pods, and seed weight. Results from this study indicated that Ilanda and Enzegu recorded the highest seedling survival rate at 46% and 64% in Bonjoge and Koibem, respectively. Ilanda (452 kg ha-1) in Bonjoge and Enzegu (712 kg ha-1) in Koibem, produced the largest biomass when fertilizer was applied at 30 kg P ha-1 and leaf harvesting was not done. Khaki (with leaf harvesting) in Bonjoge and Enzegu (without leaf harvesting) in Koibem produced the largest P uptake of 1.0 kg ha-1 and 1.8 kg ha-1 respectively, when fertilizer was applied at 30 kg P ha-1. At 30 kg P ha-1, Ilanda obtained the largest grain weight at 7.7g per plant for Bonjoge soil and 7.0g for Koibem soil. Final soil available P values were significantly (P<0.05) highest with Khaki (8.2 kg ha-1) in Bonjoge and Enzegu (8.3 kg ha-1) in Koibem at 30 kg P ha-1 fertilizer application rate. This study, therefore, shows that external addition of P fertilizer is essential for increased cowpea yields. In both sites, Ilanda and Enzegu (without leaf harvesting) are best for dry matter production with highest P fertilizer addition. Ilanda is best for grain yield production. Variations in performance of cowpea varieties provide a basis for selecting cowpea lines that respond to positively to phosphate fertilizers in soils poor in phosphorus.