Influence of native rhizobacteria co-inoculation and formulation of bacterial inoculants on the growth and yield of common bean (Phaseolus vulgaris L.)

Introduction: Incorporation of inoculum in different carrier materials may increase the efficacy of bacterial inocula. Materials and Methods: Field experiments were conducted using two strains of rhizobium and phosphate-solubilizing bacteria (PSB) and their respective combinations using different carrier materials in common bean-growing regions in three soil types in Kenya. The field experiment was laid out in a split-plot arrangement with the strain inoculations as the main plot while the subplots consisted of the carrier materials (filter mud, peat moss and yeast extract mannitol broth [YEMB]). Each main plot included two controls: uninoculated negative control and uninoculated controls that received N and P fertilizer. The experiment was conducted for two cropping seasons. Data were collected on the nodulation, shoot and root biomass and yield. Results: Co-inoculation of the common bean with Rhizobium phaseoli + Bacillus aryabhattai strains had significantly higher number of nodules (55 nodules per plant) compared to single R. phaseoli inoculation (38 nodules). The co-inoculation of the rhizobia and the PSB yielded statistically at par with the application of diammonium phosphate (18:46:0) across the soil types and seasons. The use of filter mud as a carrier material led to a higher number of nodules for most of the rhizobia strains inoculation and their respective co-inoculation with the bacillus strains. Significantly higher yield was obtained with the filter mud (1.64 Mg ha-1) while there was no significant difference in the yield of common bean between peat moss and YEMB as carrier materials for the bacterial strains. Conclusion: The solid carrier material, specifically filter mud, showed potential for use in the formulation of inoculants. Specific co-inoculation of rhizobia (R. phaseoli and Rhizobium pusense strains) and PSB (Paenibacillus polymyxa and B. aryabhattai) increased the growth, nodulation and yield of common bean more efficiently than the control.