Yield Responses of Cowpea (Vigna unguiculata) Varieties to Phosphorus Fertilizer Application Across a Soil Fertility Gradient in Western Kenyan Highlands

Abstract

Declining soil fertility resulting from continuous cultivation, confounded with inadequate fertilizer use and the growing of poor local cowpea (Vigna unguiculata) seed varieties has led to decreased yields in smallholder farms in the highlands of western Kenya. However, applying phosphorus (P) and planting improved varieties could result in increased yields of biomass and grain. This study screened the performance of local and improved cowpea varieties under different application rates of P – fertilizer (Triple Super Phosphate). The study was conducted in Nandi South District and considered four sites where the soil fertility was classified as low (Kapkerer), medium-low (Kiptaruswo), medium-high (Bonjoge) and high (Koibem). Field experiments were conducted in 2009 during the long rains (LR) and short rains (SR) seasons. The experiments were factorially arranged and laid out in a randomized complete block design (RCBD) with three replications at all the sites. Phosphorus was applied at three levels: 0, 15, and 30 kg P/ha. During LR 2009, three local cowpea varieties were tested; Enzegu, Khaki and Ilanda. During SR 2009, six cowpea varieties were tested; one local check (Khaki) and five improved varieties, ICV1, 1CV12, CB46, IT92K-282-2 and IT83D-442. The resultant data on dry matter (DM) and grain yields were collected and subjected to statistical analysis to determine differences in the means. Across varieties, during LR and SR, application of P resulted in insignificant (p < 0.05) increases in mean cowpea DM within sites. During SR, application of P resulted in insignificant (p < 0.05) increases in the yield of cowpea grain across varieties. Within sites, however, the 15 kg P/ha rate resulted in a 33 % increase in grain yield relative to the control in Bonjoge and to a 62 % increase in Koibem. The results of this study show that, despite the high fertility status of the Bonjoge and Koibem sites, they still require the application of P – fertilizer for increased production of cowpea grain. Kapkerer and Bonjoge sites had the highest DM and grain yields. Across sites, the most suitable cowpea varieties for the production of DM are Enzegu during LR and Khaki and ICV 6 during SR. The most suitable cowpea varieties for the production of grain are CB 46 in Bonjoge and ICV 12 in Koibem. Although cowpea productivity is influenced by the soil’s fertility status, the application of P is essential for enhancing DM accumulation and grain yield production.