Effect of Phosphorus Fertilizer Application on Yields of Cowpea (Vigna unguiculata) Varieties across Sites of Differing Soil Fertility in western Kenya

Phosphorus deficiency remains a key constraint to cowpea productivity in Kenya. Continuous cultivation of smallholder farms confounded with inadequate use of fertilizer and use of poor local cowpea (Vigna unguiculata) seed varieties, has led to decreased cowpea yields in western Kenya. Earlier studies have indicated that application of phosphorus fertilizer at recommended rates as well as growing of improved cowpea varieties may result to increased grain yield. Against this background, improved cowpea varieties were screened under different phosphorus (Triple Super Phosphate) fertilizer rates to investigate their response on grain yield production, at sites of differing soil fertility status (Bonjoge and Koibem) in western Kenya. Field experiments were conducted during the short rain (SR) seasons in 2019. The experiments were factorially arranged and laid out in a RCBD with three replications at both sites. P was applied at three levels of 0, 15, and 30 kg P/ha. Six cowpea varieties; one local check (Khaki) and five improved varieties (ICV1, 1CV12, CB46, IT92K-282-2 and IT83D-442) were tested. Grain yield data collected was subjected to analysis of variance using SAS statistical software, release 8.2. Least Significant Difference test separated means of parameters whenever significant difference was detected at ≥ 95% confidence level. Phosphorus application resulted to insignificant (p<0.05) increase in cowpea grain yield across varieties. Highest and least average yields were from variety CB 46 (325 kg/ha) and variety Khaki (216 kg/ha) and IT 90K-284-2 (216 kg/ha), respectively. At Bonjoge, cowpea grain yields varied from 206 to 434 kg/ha and were significantly (p<0.05) higher than the yields at Koibem which ranged from 127 to 342 kg/ha. The results of this study show that while cowpea productivity is influenced by soil fertility status, application of P was not essential for grain yield production. Interaction of fertilizer and varieties in Bonjoge resulted to a positive grain yield response, hence suitable site for cowpea.