Contributions of Rhizobium Inoculants and Phosphorus Fertilizer to Biological Nitrogen Fixation, Growth and Grain Yield of Three Soybean Varieties on a Fluvic Luvisol

An experiment to identify an efficient strategy to optimize biological nitrogen fixation (BNF) in three soybean varieties {Jenguma (TGx1448-2E), Anidaso and Quarshie (TGx1445-2E)} was conducted in the experimental field of the CSIR-Savanna Agricultural Research Institute, Nyankpala, Northern Region, Ghana. The experiment had a split-split plot design with three replications and interactions tested were inoculation rate (0, 50 and 100% inoculation) (main plot), soybean variety (sub-plot) and phosphorus rate (0, 22.5 and 45.0 kg P2O5/ha) (sub-sub plot) using maize as a reference crop. The soybean was sown at two seeds per hill at a spacing of 50 cm x 10 cm. The results showed that inoculating soybean with the commercial inoculant Legumefix (Bradyrhizobium strain 532c) had no effect on plant height, nodule number, nodule dry weight, shoot dry weight, pod number, grain yield and 100-seed weight. However, the soybean varieties showed significant differences in pod number, 100-seed weight and phosphorus uptake efficiency (PUE). The amount of N2 fixed (measured by Total Nitrogen Difference method) ranged between 52.3-71.0 kg N/ha. Phosphorus applications of 22.5 and 45.0 kg P2O5/ha increased grain yield by 35.4 and 33.9%, respectively and also increased N2 fixation by 49.39 and 69.82%, respectively over the unfertilized control. The interactions among these treatments did not significantly influence the parameters measured except PUE for which there were significant differences among the soybean varieties and the phosphorus rates. Inoculation did not therefore increase nodulation and BNF of the three soybean varieties but phosphorus application increased the growth and grain yield of the soybeans.