Isolation, characterization, and evaluation of the phosphate-solubilizing capacity of rhizospheric bacteria associated with Pterocarpus indicus

Phosphorus (P) deficiency remains a critical factor limiting crop productivity, particularly in tropical soils where most phosphorus occurs in insoluble complexes with iron (Fe³⁺) and calcium (Ca²⁺) ions. This study aimed to isolate, characterize, and evaluate the phosphate-solubilizing capacity of rhizospheric bacteria associated with Pterocarpus indicus. Soil samples were analyzed for moisture content, bacterial abundance, and phosphate-solubilizing bacteria (PSB) diversity, followed by the isolation of PSB on selective media. Each isolate was characterized morphologically and microscopically, and its phosphate-solubilizing ability was quantified using the solubilization index (SI) on Pikovskaya agar. The results showed that soil moisture strongly influenced bacterial abundance, with the highest PSB populations recorded in samples A1 (41 ± 2.16 × 10⁵ CFU g⁻¹ at 65.84% moisture) and A3 (41 ± 2.83 × 10⁵ CFU g⁻¹ at 58.74%). In contrast, the lowest population was observed in sample A2 (15 × 10⁵ CFU g⁻¹ at 46.84%), confirming that moderate soil moisture (58–65%) supports optimal PSB growth. Ten distinct PSB isolates (PSB-L1 to PSB-L10) were successfully purified and exhibited wide phenotypic diversity, including bacillus- and coccus-shaped forms with both Gram-positive and Gram-negative reactions. Phosphate-solubilization indices ranged from 0.80 to 4.00, with PSB-L3 and PSB-L8 showing the highest solubilization capacity. These findings demonstrate that native PSB associated with P. indicus possess strong phosphate-solubilizing potential and represent promising candidates for the development of locally adapted biofertilizers aimed at enhancing soil phosphorus availability and supporting sustainable crop production.