SOLUBILIZATION OF ROCK PHOSPHATES BY RHIZOBIUM AND BRADYRHIZOBIUM

The ability of Rhizobium and Bradyrhizobium strains to solubilize phos phate from hydroxyapatite was determined in a medium containing NH4Cl or KNO3. The presence of NK4+ in the medium resulted in higher solubil ization of phosphate as compared to the presence of KNO3, with the exc eption of R leguminosarium bv. viceae strain TAL 1236 and 1402 which s olubilized comparable amounts of phosphate in a medium containing eith er KNO3 or NH4Cl. These results suggest that the strains employ two di fferent mechanisms for phosphate solubilization, one depending on the presence of NH4+, the other not requiring its presence. Temperature an d aeration (O-2 demand) optima were 30 degrees C and 4.2 Hz (shaking f requency), respectively. In nonsterile soil the tested strain (R melil oti TAL 1236) was very effective in solubilizing rock phosphate.