Abstract

L. B. Taiwo1* and M. Ogundiya2

Biological solubilization of rock phosphate is more environmentally friendly than acidulation. There is a need therefore to develop a microbial process that will make phosphorus available for plant use with minimum pollution to the environment. Solubilization of phosphorus (P) from Ogun rock phosphate (ORP) in a defined growth medium and in soil was carried out to evaluate the potential of a mixed culture of soybean and cowpea rhizobia (IRj 284 and IRc 252, respectively), as well as a mixed culture of Bacillus subtilis MS10 and Aspergillus niger Tiegh for use in such a process. In broth medium, the amount of P released increased as the rhizobial inoculum size increased. An increase in microbial population correlated positively (r2 = 0.72) with P release in the broth at day 1. In the soil, however, the P release peaked at 10 days after inoculation. In broth medium, a mixed culture of B. subtilis and A. niger increased P release earlier in the solubilization period compared to that obtained with cowpea and soybean rhizobia. There was also a positive correlation (r2 = 0.58) between an increase in the microbial population and P release a day after inoculation. In contrast, the microbial population was poorly correlated with P release a day after inoculation in soil. However, a stronger correlation was observed at day 5 and 10 with r2 values of 0.309 and 0.420, respectively, compared to the initial r2 value of 0.009 at day 1. Nevertheless, efficient ORP solubilization occurred in the soil between the 5th and 15th day after microbial inoculation. It is expected that legumes may benefit from this process, because this period marks the beginning of nodule initiation, nodule formation and nitrogen fixation.