Soybean yield variability in Northern Ghana: Effects of rhizobia inoculation, P application, and soil exchangeable Mg content

Seed inoculation with effective rhizobia (Rh) strains and phosphorus (P) application are sustainable agricultural practices that enhance soybean grain yield. However, adoption of these practices in northern Ghana is hindered by unpredictable yield responses and poor understanding of the factors that cause yield variation. We compared soybean yield from inputs managed trials such as Rh inoculants and P sources.

Soybean trials were conducted on-station and on-farm with one of three Rh inoculants [Bradyrhizobium japonicum (Bj), B. elkanii (Be), and B. diazoefficiens (Bd)], and with one of three P sources [no-P, Triple Superphosphate (TSP), and a mixture of rock-P (low solubility) and TSP (water-soluble P)]. Yield was compared at harvest. The random forest (RF) model was used to predict yield, and factors of yield variation were determined using forward redundancy analysis (RDA).

The soybean yield was consistently higher in the Rh × P treatments compared to the control in on-station and on-farm experiments. The treatments that received P fertilizer (RP and TSP) showed a higher grain yield than the control. The on-station experiment yielded more accurate results than the on-farm dataset, with a trained coefficient of determination (R²) of 0.95 and 0.87, respectively. The yield variation was explained by the Rh × P fertilizer, P sources, Rh strains, and exchangeable soil Mg.

The exchangeable magnesium in the soil contributed significantly to the variation in yield. Further research is recommended to study the mechanisms that underline the impact of magnesium on soybean growth.