Assessment of the quality and effectiveness of plant growth promoting rhizobacteria on soybean (Glycine max) and common bean (Phaseolus vulgaris) growth

Abstract

The use of plant growth promoting rhizobacteria offers an alternative to the use of chemical fertilizers. The quality and efficacy of inoculants is critical to realize the benefits ofinoculation. The objectives of this study were; 1) to test the quality of 3 inoculants (Legumefix,Biofix for soybean and Biofix for common bean) and evaluate the response of soybean andcommon bean to inoculation; 2) to test the effect of soil fertility on inoculation response ofsoybean and common bean; 3) to isolate and characterize phosphate solubilizing bacteria fromcommon bean nodules and; 4) to assess the effect of co-inoculation of rhizobium and phosphate solubilizing bacteria on growth of common bean in a low phosphorous soil. For objective 1 the inoculants were tested for the purity and number of viable cells. For the efficacy test, seeds were inoculated at the recommended rate at planting. At mid podding, nodule number and weight, shoot and root dry biomass, biologically fixed nitrogen and the symbiotic efficiency of the rhizobia were determined. For objective 2 the legumes were planted and inoculated with the rhizobia inoculants using two soils (Andosol and Nitisol) and data taken as described in objective 1. For objective 3, soils were collected from different parts of Nakuru County and used to trap the nodules and the strains isolated and characterized in the laboratory. For objective 4, common bean were inoculated with Rhizobium strains singly or in a combination with two PSB to evaluate their effect on growth. The inoculants showed variable quality in terms of the number of viable cell and the number of microorganisms. Inoculation of common beans with Biofix led to significantly higher number of nodules per plant and nodule weight (117 nodules and 2.09 g respectively) than the control (75 nodules and 0.84 g respectively). Nodulation was higher in high fertility soil in both legumes. Co-inoculation of IITA-PAU987 + B. megaterium recorded the highest nodule and shoot dry weight (405.2 mg and 6.84 g respectively) compared to IITAPAU987 alone (324.8 mg and 5.32 g respectively). CIAT 899 + B. megaterium recorded a highershoot weight (401.2 mg) compared to CIAT 899 alone (337.2 mg) but no significant differencewas observed when co-inoculated with P. polymyxa. The results on quality testing of inoculantsshow a need for manufacturing companies to have a quality control system in the productionprocess. These results indicate that co-inoculation of PSB and rhibozia has a synergistic effect ongrowth of common bean. Testing of the plant growth promoting rhizobacteria under field conditions will further elucidate their effectiveness on grain yield of common bean.