Bioactivation of Poorly Soluble Phosphate Rocks with a Phosphorus‐Solubilizing Fungus

Many studies have demonstrated ineffectiveness of finely ground phosphate rock (PR) use due to the low solubility of its P contents. This study was conducted to develop a simple, effective, and environmentally sound process to improve P availability of PR to crops by using a phosphate‐solubilizing fungus (PSF), Aspergillus niger BCC F.194, isolated from tropical acid soils. The optimum incubation period and the optimum level of PR were determined. The P‐solubilizing effect of the supernatant of 9‐d‐old liquid culture supernatant (LCS) of the fungus was also determined by reacting it at various concentrations with Morrocan phosphate rock (MPR). The inoculation of the growth media with the PSF A. niger resulted in the highest P solubility of the rock after 9 d of culturing at 2.5 g L −1 MPR. Up to a certain degree, direct inoculation of fungal biomass and its LCS onto MPR caused a remarkable increase in 2% citric acid–soluble P, but not in water‐soluble P content. The possibilities of using the LCS instead of H 2 SO 4 in superphosphate (SP) production and using both with lower H 3 PO 4 concentrations were investigated with MPR as raw materials. Replacement of H 2 SO 4 by the LCS in the SP production process yielded a comparable 2% citric acid–soluble P content. Combining the LCS and H 2 SO 4 reduced the consumption of H 3 PO 4 that occurs in standard SP production. This LCS technique provides a practical means for an effective bioactivation of PR intended for both as a P fertilizer and a raw material of the SP.