Evaluation of Bradyrhizobium formulations on performance of soybean grown on soil without a long-term history of the crop

Farmers have raised concerns about the inability of some commercial soybean inoculants to elicit effective nodulation. Nodulation failure has been attributed to, among others, high temperatures, soil acidity and type of inoculant used. This work examined the influence of two Bradyrhizobium japonicum inoculant formulations on soybean grown on soil without a soybean cropping history for about eight years. The experiment was conducted in a controlled environment facility at the University of KwaZulu-Natal. The treatments were two levels of a liquid formulation of the Bradyrhizobium WB 74 initially consisting of 2.6×109 colony forming units ml-1 and one level of a powder formulation containing a minimum of 6.5×108 live cells g-1. The results showed that applying the two formulations of B. japonicum to seed, enhanced soybean nodule number, leaf number, nutrient concentration and uptake compared to the control treatment, and both powder and liquid formulations enhanced yield components to the same extent. Although the low concentration of the liquid formulation was less effective in increasing nodule number, other responses it elicited were comparable to the high concentration of the liquid and powder formulations. We conclude that small differences may exist between commonly available commercial inoculants of Bradyrhizobium WB 74 but they are effective in promoting nodulation and growth of soybean, and that reported major nodulation failures may be attributed to management factors other than inoculant formulation per se.