Grain Yield and Economic Benefit of Soybean as Affected by Integration of Rhizobium Inoculation and Phosphorus Application in Chuka Sub-County, Kenya

Soybean (Glycine max [L.] Merrill) yields are far below the potential yield on most smallholder farms in Kenya. This necessitates suitable interventions to bridge this yield gap and result to profitable soybean production. An experiment was conducted in Chuka University demonstration farm, Chuka Sub County to determine the effect of integration of rhizobium inoculation (R) and phosphorus (P) on yields and net economic returns in soybean cultivation. The experiment was laid out in a randomized complete block design in a split-split plot arrangement with each treatment replicated three times. The experiment which was conducted between 2018 and 2019 was repeated once and the treatments included; three P rates (0, 20 and 30 kg/ha), three rhizobium rates (0, 100 and 200 g/ha) and two soybean genotypes (SB19 and SB24). Triple superphosphate (0:46:0) was used as the source of the phosphorus. The soybean genotypes, rhizobium and phosphorus rates were assigned to the main plot, sub-plot and the sub-sub plots respectively. Data collected included soybean yields (kg/ha) and economic analysis was calculated. The data collected was subjected to analysis of variance using Statistical Analysis System® 9.4 and significantly different means separated using Tukeys test at (p≤0.05). The results showed statistically significant difference in soybean yields and net economic benefit within SB19 and SB24 genotypes in both trials at (p≤0.05). Integration of R and P at the rate of 200 g and 30 kg/ha increased soybean yield by 101% and 98%, and 158% and 138% for SB19 and SB24 in trial I and II respectively. This earned a net economic benefit of ksh. 239,496 and 192,730, and ksh. 297,930 and 239,330 for SB19 and SB24 in both trials I and II, respectively. Both genotypes performed well in yield and net economic benefit and application of R and P at the rate of 200 g/ha and 30 kg/ha promoted yield and net economic benefit of soybean.