Integrated Effect of Rhizobium Inoculation and Phosphorus Application on Soybean Growth and Yield

Abstract

There has been a decline in soybean production in many parts of Kenya due to limited soil fertility, especially nitrogen and phosphorus contents. A field experiment was conducted at Chuka University Demonstration farm to determine the effect of integration of rhizobium inoculation (R) and phosphorus (P) on growth, nodulation and yields of soybeans in Meru South Sub County, Tharaka Nithi County. The experiment was laid out in a randomized complete block design (RCBD) in a split-split plot arrangement with each treatment replicated three times. The first cultivation (Trial I) was done and repeated in second cultivation (Trial II) in 2018. The aim of the experiment was to assess integration effect of R and P for sustainable soybean production in the study area. Treatments included; three rates of P (0, 20 and 30 Kg ha-1), three rates of rhizobia (0, 100 and 200 g ha-1) either applied alone or integrated and two soybean genotypes (SB19 and SB24). Triple superphosphate (0:46:0) was used as the source of the phosphorus. The soybean genotypes were assigned to the main plot with rhizobia strain in the sub-plot and phosphorus in the sub-subplots which was repeated once. Data was collected on plant height, number of pods, nodules and branches, fresh and dry nodule weight, fresh and dry shoot weight, length of root and seed yield. The data collected was subjected to analysis of variance (ANOVA) using the statistical analysis software (SAS) and significantly different means separated using Tukeys test at (p≤0.05). The results showed statistically significant difference in growth, nodulation, yield components and yields 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-1 showed highest growth characteristics compared to other treatments applied. For instance, soybean plant height increased by 9.82 cm and 9.81 cm, and 10.99 cm and 11 cm at integration of 200 g and 30 Kg ha-1 compared to control for SB19 and SB24, in Trial I and II, respectively. Similarly, at integration of R and P at the rate of 200 g and 30 Kg ha-1, the dry nodule weight increased from the control treatment (0 g R and 0 Kg P per ha) to 0.81 g and 2.54 g, and 0.81 g and 2.59 g per plant for SB19 and SB24 genotypes in Trial I and II, respectively. Integration of R and P at the rate of 200 g and 30 Kg ha-1 increased grain yield by 101% and 98%, and 158% and 138%. 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, respectively. Both soybean genotypes performed well in all evaluated parameters. Adoption of the integration of R and P at 200 g and 30 kg P per ha and use of either of the genotypes by farmers show greater potential of enhancing soybean productivity in Meru South Sub County.