Iron (Fe) deficiency is the major constraint for soybean cultivation in calcareous soils. Its
availability affects symbiotic association of the host plant and the endosymbiont and
eventually the yield of soybean. However, the effectiveness of integrated application of
Bradyrhizobium and Fe fertilizer on nodulation and yield of soybean genotypes with
different maturity group has not been studied. Therefore, the objective of this study was
to evaluate the effect of Bradyrhizobium inoculated soybean genotypes with different
maturity group on FeSO4 and Fe-EDTA nutrient requirement supplied through the
foliage and applied directly to the soil. The experiment was conducted under greenhouse and field conditions. Six soybean genotypes, three late maturing (Wogayen, TGx-1336424 and Belsa) and the remaining three medium maturing (GIZA, Afgat and Gishame) were used for greenhouse experiment. Based on the greenhouse result, only two promising genotypes, namely GIZA and TGx-1336424, were selected for further investigation under field experiment. Eight treatments which synthesized by combining three Bradyrhizobium inoculations, with and without Fe fertilizer (0 and 4 mg kg-1 soil), including N-treated (20 kg N ha-1) and the control, were tested in pot experiment arranged in a completely randomized design (CRD). Four levels of inoculation factorially combined with three levels of Fe-EDTA application were used for field experiment. These treatments were laid out in a spilt plot in randomized complete block design (RCBD) with three replications. The results of the present study indicate significant improvement of most investigated traits of tested medium maturing genotypes, but not for late maturing soybean genotypes under greenhouse conditions. A significant increase of nodule number (NN) and nodule dry weight (NDW) with increasing rates of Fe for selected genotypes were observed in field experiment. Application of Fe, however, showed differential effect on both genotypes on other investigated traits. All investigated traits, except NN, NDW and shoot dry matter (SDW) did not improve significantly by Fe application for TGx-1336424 genotype. In contrast, significant improvement of number of pods per plant (NPP), total biomass yield (TBY), grain yield (GY) and plant total nitrogen concentration (PTC) with increasing rates of Fe were observed for GIZA genotype. The regression analysis indicates different degree of dependence of TBY, NDW, NN and GY of both genotypes with increasing rates of Fe application with different Bradyrhizobium sp. inoculation treatments. Hence, it can be concluded that the effect of Fe application is dependent on maturity groups of soybean genotypes and effectiveness of inoculated Bradyrhizobium sp.
