Abstract

Most agricultural soils contain large reserves of phosphorous, a considerable part of which accumulates as a consequence of regular applications of phosphate fertilizers. Nearly 95 - 99% is present in the insoluble form and hence cannot be utilized by plants. Fungi have been reported to possess greater ability to solubilize insoluble phosphates than bacteria. In the present study, fungal strains isolated from agriculture soil, having potential to solubilize insoluble inorganic phosphates on Pikovskya’s (PVK) medium with 0.5% (w/v) tricalcium phosphate (TCP) were characterized. Aspergillus niger (F7), A. niger (F4), A. niger and Penicillium sp. showed 107.7, 108.3, 112.7 and 110.3% phosphate solubilisation efficiency on PVK medium with 0.5% (w/v) TCP and 285, 187.5, 258 and 70.5 mg/ml phosphate, respectively from 0.5% (w/v) TCP in liquid broth in 5 days of growth. A. niger (F7), showed 107.7% phosphate solubilization efficiency on PVK agar medium and 285mg/ml phosphate, in solid and liquid medium respectively from 0.5% (w/v) TCP in 5 days of growth and hence was selected for further studies. F7 showed diverse levels of phosphate solubilization activity in both solid and liquid broth culture in presence of various carbon and nitrogen sources and different media. Presence of soluble phosphates, in terms of different concentrations of KH₂PO₄ supplemented in PVK agar media, suppressed TCP solubilization activity by F7. F7 showed different levels of phosphate solubilization under different saline conditions tolerating maximum salinity up to 2% NaCl concentrations. The strain Aspergillus sp. F7 can thus be of great benefit in maintaining the available phosphate level for crops in saline alkaline soils. Phosphate solubilizing microorganisms convert insoluble phosphates into soluble forms generally through the process of acidification, chelation and exchange reactions. Thus such microorganisms may not only compensate for higher cost of manufacturing fertilizers in industry but also mobilize the fertilizers added to soil.

Key words: Aspergillus sp. F7, phosphate solubilization, environmental factors, salinity.