Characterization of phosphate-solubilizing bacteria isolated from calcareous soils

Twenty phosphate-solubilizing bacteria (PSB) were isolated from calcareous rhizosphere soils. These bacterial strains were identified by sequence analysis of 16S rRNA genes as bacterial species of Bacillus megaterium (B. aryabhattai), Bacillus subtilis, Pseudomonas aeruginosa, Rhizobium sp., Acinetobacter sp., and Pseudomonas oryzihabitans. Seven of these strains were evaluated with the National Botanical Research Institute's Phosphate (NBRIP) plate culture, NBRIP liquid culture, and soil incubation. Results showed that halo zone formation by PSB on NBRIP plates was a good indicator for screening PSB, but not good enough to quantify capability of P solubilization because of poor correlation between sizes of halo zone and water soluble P (WS-P). The NBRIP liquid medium culture showed four PSB strains lowered medium pH (<4.3) and released WS-P up to 523.69 mg/l with three days incubation and Krome3 strain dissolved 95.3% tricalcium phosphate added after 35 days incubation. Incubation of PSB in a sandy soil showed that PSB increased WS-P, but not Mehlich-3 P. Therefore, each of the three culture practices has its strength and weakness for characterizing PSB. It is advisable to perform all three tests to provide acceptable indication of phosphate-solubilizing ability for PSB. (C) 2015 Elsevier B.V. All rights reserved.