Solubilization of rock phosphate by Azotobacter spp

Abstract

To find out a potent phosphate solubilizing strain of diazotrophs, fifteen Azotobacter strains were isolated from local sources. They have been tested for the ability to solubilize rock phosphate in vitro and production of acid in the medium. Almost all the strains solubilized rock phosphate which ranged up to 45.5%. Azotobacter sp. R12 was found to be a potent one, which solubilized 30 to 46% phosphate from different rock phosphates. Almost maximum solubilization of P and acid production was achieved at four days of growth of the organism. The strain produced ascorbic acid in the culture medium, the primary cause of rock phosphate solubilization. Addition of (NH4)2S04 in the culture medium reduced the solubilization of phosphate with increasing concentration beyond 0.25 mg/ml for Jordon rock phosphate. Increasing concentration of CaCl2 and CaCO3 also reduced the P solubilization from Jordon rock phosphate by the strain. Kinetics of solubilization and acid production showed a linear relationship till the 5th day of incubation, then P level becomes static. Adding EDTA at 5 mg/ml as chelating agent of calcium ions to 5 days old culture increased the P solubilization to 81%. Increasing EDTA concentration in 3 days old culture showed linear relationship between concentration of EDTA and P solubilization.