ABSTRACT. The ability of Rhizobium and Bradyrhizobium strains to solubilize phosphate from hydroxyapatite was determined
in a medium containing NH4CI or KNO3. The presence of NH4 + in the medium resulted in higher solubilization of phosphate
as compared to the presence of KNO3, with the exception of R. leguminosarium by. viceae strain TAL 1236 and 1402 which
soluhilized comparable amounts of phosphate in a medium containing either KNO3 or NH4Ci. These results suggest that the
strains employ two different mechanisms for phosphate solubilization, one depending on the presence of NH4 +, the other not
requiring its presence. Temperature and aeration (02 demand) optima were 30 *C and 4.2 Hz (shaking frequency), respectively.
In nonsterile soil the tested strain (R. meliloti TAL 1236) was very effective in solubilizing rock phosphate.