Influence of Soil Chemical Properties on the Distribution and Efficiency of Phosphate Solubilizing Bacteria in Common Bean (Phaseolus vulgaris L.) Rhizosphere Soils of Southern Ethiopia

Soil phosphorus deficiency is a major constraint to crop production in Ethiopian highlands. Phosphate solubilizing bacteria (PSB) can enhance phosphorus availability, but their distribution and efficiency are influenced by soil chemical properties. This study investigated the relationship between soil chemical properties and the distribution of PSB in common bean (Phaseolus vulgaris L.) rhizosphere soils across six kebeles (Tore, Tore Badiya, Kersa, Bariti, Wachu, and Jirme) in Galana District, southern Ethiopia. Thirty composite rhizosphere soil samples were collected in March 2021. Soil pH, electrical conductivity (EC), organic carbon (OC), available phosphorus (P), and cation exchange capacity (CEC) were analyzed using standard procedures. PSB were isolated on Pikovskaya agar, and solubilization efficiency was determined by the solubilization index (SI). A total of 54 bacterial isolates were obtained, with 10 strains showing high solubilization efficiency (SI 2.27–3.88). Soil properties varied considerably across sites: pH ranged from 5.42 to 6.99, available P from 1.12 to 27.52 ppm, OC from 2.06% to 4.51%, and CEC from 7.10 to 32.45 meq/100g. Pearson correlation analysis revealed a significant negative relationship between soil available P and PSB efficiency (r = -0.68, p < 0.05), indicating that phosphorus-deficient soils harbor more efficient PSB strains. The most efficient isolate (GPSB04, SI 3.88) originated from Wachu, a site with high OC (4.51%) and near-neutral pH (6.99). These findings suggest that targeted isolation of PSB from low-P, high-OC soils can yield superior strains for biofertilizer development. The identified strains hold promise for improving phosphorus availability and sustainable crop production in Ethiopian smallholder farming systems.