Assessing the quality evaluation of phosphate solubilizing bacteria from rhizospheric soil of Kathmandu Valley

In the rhizosphere, phosphate-solubilizing bacteria (PSB) play a critical role in helping to promote the solubilization of inorganic phosphorus compounds into soluble forms that are suitable for plant nourishment. The aim of this study was to investigate the phosphate solubilizing bacteria (PSB) and evaluate the qualitative and quantitative activity of phosphate solubilization. This study was carried out from March to September 2024 in the microbiology laboratory of National Soil Science Research Center (NSSRC), Nepal Agricultural Research Council (NARC), Khumaltar, Lalitpur. A total of 40 soil samples were collected from the rhizosphere of 8 different types of plants (Aloe vera, potato, wheat, banana, maize , tomato , paddy and soybean). In this study, 20 PSB isolates were obtained from different soil samples. Colony features, Gram's staining, and biochemical testing were done to identify all 20 isolates. The potential of the bacterial isolates to solubilize phosphate was assessed using both solid and liquid Pikovskaya's (PVK) media containing insoluble tricalcium phosphate. The solubilization zone surrounding colonies was visually evaluated, and the solubilized phosphates in the liquid medium were determined using the molybdenum blue technique. In agar media, the solubilization index (SI) ranged from 1.33 to 3. Based on the results of qualitative evaluation, best 6 isolates (A ₅ 2, B _4, M _5, R _1, R _3, R ₅ ) that showed the higher phosphate solubilization index was selected for the quantitative evaluation. In the liquid media, the PSB isolates showed phosphate solubilization ranging from 1.17 to 6.33 mg L−1 in the PVK medium, with highest value produced by Pseudomonas aeruginosa. Best 6 isolates were compared with commercial P fertilizer qualitatively as well as quantitatively where the commercial P fertilizer were less effective than the isolated PSB. This study suggests that these PSB isolates can solubilize phosphate and that can be exploited to produce phosphate biofertilizers.