EFFECT OF PHOSPHORUS FERTILIZER AND CROPPING SYSTEMS ON RICE BEAN (Vigna umbellata, L.) PRODUCTION IN WESTERN KENYA

Abstract

Low soil phosphorus (P) remains a critical constraint to sustainable legume production in Western Kenya. Rice bean (Vigna umbellata L.), a resilient and nutrient-rich yet underutilized legume, offers opportunities for food security enhancement and soil fertility restoration. The crop productivity has been hindered by nutrient deficiencies and suboptimal cropping systems. This study evaluated the effect of phosphorus fertilizer and cropping systems on rice bean soil fertility dynamics, phenology and yields across three agroecological sites; Siaya, Rongo and Kaimosi over two consecutive seasons (September– December 2020 and March–August 2021). A split-plot design was used, with cropping system (monocrop, conventional intercrop and MBILI) as the main factor and P amendments (control at 0 kg P ha−1, FYM at 5 kg P ha−1, combined FYM + inorganic at 15.5 kg P ha−1, and sole inorganic at 26 kg P ha−1) as the subplot factor, replicated three times per site. Data on soil available P, phenological stages, pods per plant, grain yields, phosphorus agronomic efficiency (PAE) and harvest index (HI) were collected and analyzed using Analysis of Variance (ANOVA). Results showed that soil available P was highly responsive to P amendments, with 26 kg P ha−1application producing the highest levels (up to 8.88 mg kg−1 at Rongo monocrop), while 15.5 kg P ha−1 application maintained moderately high levels that balanced immediate nutrient supply with long-term fertility. Cropping system effects on soil P were less pronounced, but monocropping and MBILI showed slightly greater retention than conventional intercropping. Phenological responses were notable: 0 kg P ha−1 plots matured 12–17 days later than 15.5 kg P ha−1 or 26 kg P ha−1 treatments, with delayed flowering and extended vegetative growth under P deficiency. Across sites and seasons, grain yields ranged from 0.24 t ha−1 in conventional controls at Siaya to 1.72 t ha−1 under monocrop with 15.5 kg P ha−1 P in Rongo, highlighting the clear yield advantage of combined nutrient management in low-P soils. PAE varied widely, peaking at 63.42 kg kg−1 under monocrop with fertilizer treatment of 15.5 kg P ha−1 at Rongo. The PAE was higher under the fertilizer treatments of 5 kg P ha−1 and 15.5 kg P ha−1 treatments than under 26 kg P ha−1, indicating superior nutrient use efficiency with organic amendments. Harvest index values reflected biomass partitioning, highest in conventional and MBILI systems (up to 11.10%) compared to 3–4% in monocrops. The study concludes that combining organic and inorganic phosphorus sources within site-specific cropping systems accelerates phenological development, improves nutrient use efficiency and maximizes yield in rice bean production. Extension programs should prioritize farmer training on integrated soil fertility management to promote organic–inorganic nutrient combinations. These findings provide a framework for sustainable intensification of underutilized legumes in phosphorus-deficient soils.