Effectiveness of Indigenous Bradyrhizobium Strains on Promiscuous Soybean Genotypes

Abstract

Small-holder fanners frequently cannot cope with the high and ever increasing prices of inorganic fertilizers. Consequently, low yields, chronic food and nutritional insecurity are frequent phenomena. Cheaper and sustainable alternatives to improving soil fertility especially for N are being sought. The use of indigenous strains of Bradyrhizobium inoculants offers a potential economic and sustainable option in meeting soybean nitrogen needs. Inoculation of soybean in Kenya has often relied on introduced rhizobia. This is because elite indigenous strains have not been isolated, tested and selected for use under local conditions. This study was conducted to assess the nodulation and nitrogen fixation of nine indigenous Bradyrhizobium strains inoculated on three promiscuous soybean varieties grown in sand under greenhouse conditions. Three soybean and nine indigenous Bradyrhizobium strains genotypes were evaluated in a randomized complete block design. There was a significant (P<O.Ol) indigenous strain effect on dry shoot biomass and nodulation (P<0.05). Inoculation with TSBF 442 resulted in 66.9% and 154.7% more shoot biomass than inoculation with USDA 110 and negative control respectively; while inoculation with TSBF 531 elicited 59.4% and 143.2% more shoot biomass than USDA 110 and negative control respectively. All tested indigenous strains nodulated significantly better than the introduced strain (USDA 110) on all promiscuous soybean varieties. Inoculation with TSBF 442 resulted in 110% more nodules than USDA 110 while inoculation with TSBF 531 resulted in 850% more nodules than USDA 110. There was a significant interaction between soybean genotypes and indigenous rhizobia strains for nodule dry weight. Inoculation of SB 8, SB 9 and SB 19 with TSBF 531, TSBF 101 and TSBF 344, respectively, resulted in high nodule dry weight. This study indicates that potential exists to deploy indigenous rhizobia strains to replace/supplement introduced USDA 110 currently used in soybean inoculants in Kenya. Competitiveness of these strains, however, needs to be tested before deployment.