SOLUBILIZATION OF ROCK PHOSPHATE BY RHIZOBIUM AND BRADYRHIZOBIUM

Abstract

The potential of strains of Rhizobium and Bradyrhizobium to solubilize rock phosphate was evaluated in vitro. Almost all organisms tested effectively solubilized rock phosphate and lowered the medium pH. The presence or absence of (NH₄)₂SO₄ made little difference in the solubilization of rock phosphate. Among the strains, Rhizobium leguminosarum biovar viceae BICC635 was the most effective solubilizer. Maximum solubilization of phosphate and acid production was achieved after 3 days of incubation. The strain produced 2-ketogluconic acid in the culture medium, the primary cause of rock phosphate solubilization. Increasing the phosphate status of the medium had little effect on the extent of dissolution of Purulia rock phosphate. Adding calcium as CaCl₂, CaCO₃ and Ca(OH)₂ reduced the phosphate solubilization from phosphate rocks. The results indicated that pH, per se, is of less importance in phosphate solubilization. EDTA increased the extent of rock phosphate solubilization, possibly by chelation of the calcium ions that the solubilization produced.