Soybean (<em>Glycine max</em>) Grain Yield Response to Inoculation with Novel Bradyrhizobia Strains Across Different Soil Fertility Conditions in Zimbabwe

Agronomic effectiveness of biofertilizers is influenced by strain origin, genetic identity, crop genotype, soil type and environmental conditions. For best results both the plant and rhizobia strain must be adapted to the common harsh soil conditions. While plant varieties have changed over the years, complementary research on new strains effectiveness under varying soil fertility conditions has lagged in southern Africa. Seven field experiments were established in the main soybean producing areas of Zimbabwe in the north, central and north-east regions to evaluate agronomic benefits of new rhizobia strains against the current exotic commercial strain (MAR 1491). One site was irrigated (site 3) and the other six sites were rainfed (site 1, 2, 4, 5, 6, and 7). While trends in inoculation response varied from site to site due to site conditions, inoculation with the strains NAZ15, NAZ 25, and NAK128 consistently yielded high grain yields which were similar to the current commercial strain MAR 1491 and to application of mineral fertilizer (51.75 and 100 kg N ha-1). Grain yield levels were generally below 2 t ha-1 for site 2, 3, and 5, above 2t ha-1 for sites 1, 4, and 6, while for the irrigated site 3 they ranged upwards of 3 t ha-1. When irrigated all strains except NAK9 performed similarly in terms of grain yields and aboveground N uptake. We recommend further testing on the inclusion of the indigenous strains NAZ15, NAZ 25 and NAK128 in multi-strain commercial inoculant production targeting application in regions and soils where they excel beyond the current exotic strain MAR 1491.