Determining the optimal inoculant rate to maximize soybean productivity in the Guinea Savanna agro-ecological zone of Ghana

Soybean productivity in sub-Saharan Africa is often constrained by low soil fertility and limited biological nitrogen fixation (BNF). This study evaluated the effects of different rhizobial inoculant rates (0, 5, 7.5, 10, and 12.5 g kg^-1 seed) on growth, yield, and biological nitrogen fixation (BNF) of three soybean varieties, Afayak, Favor, and Jenguma, using a split-plot design with variety as the main plot and inoculant rate as the subplot. Significant effects of variety and inoculant rate were observed for key agronomic traits. Compared to the uninoculated control, grain yield increased (p < 0.001) in 2023 by 27.3%, 78.8%, 45.9%, and 146.5%, and in 2024 by 47.6%, 54.2%, 87.9%, and 180.6% at 5, 7.5, 10, and 12.5 g kg^-1 seed, respectively. The 10 g kg^-1 rate yielded the highest nitrogen fixation, at 51.5 kg ha^-1 in 2023 and 56.4 kg ha^-1 in 2024. Jenguma responded best to inoculation, followed by Afayak and Favor. Economic analysis, using partial budgeting and the marginal rate of return (MRR), identified the most profitable rates as exceeding a 100% MRR threshold. Regression analysis revealed a curvilinear yield response to inoculant rate, with an optimum of about 1.9 g per plot, equivalent to 10 g kg^-1 seed for yield stability or 7.5 g kg^-1 for a more economical option. Results underscore the importance of variety-specific inoculation strategies to enhance nitrogen use efficiency and sustainable soybean production in the region.