Phosphate solubilisation in solution culture by the soil fungus Penicillium radicum

Phosphorus is a plant nutrient which is rapidly made immobile and less available for plant use after addition to the soil as a soluble fertiliser. Phosphate-solubilising microorganisms may be able to improve the P nutrition of plants and thus stimulate plant growth. Penicillium radicum, a phosphate-solubilising fungus isolated from the rhizosphere of wheat roots, has shown promise in plant growth promotion. Its ability to solubilise inorganic phosphate was studied in vitro. The fungus was grown in liquid medium culture containing either ammonium or nitrate as the sole source of N. Insoluble, or sparingly-soluble P (1000 mg P l−1) was supplied as calcium monohydrogen phosphate (CaHPO4), calcium orthophosphate (Ca3(PO4)2), crystalline ferric phosphate (FePO4·4H2O), crystalline aluminium phosphate (AlPO4), colloidal ferric phosphate or colloidal aluminium phosphate. The titratable acidity, pH and concentrations of organic acids and soluble phosphate were determined periodically during a 20 or 31 d incubation. Phosphate solubilisation was highest from CaHPO4 (475 mg P l−1), Ca3(PO4)2 (360 mg P l−1) and colloidal aluminium phosphate (207 mg P l−1). Phosphate solubilisation was generally higher when ammonium rather than nitrate was the sole source of N. Soluble phosphate concentrations in the culture medium were directly proportional to the titratable acidity and organic acid (principally gluconic acid) concentration and inversely related to pH. The main mechanism for phosphate solubilisation was acid production leading to a decrease in pH. Evidence from an abiotic study using HCl and gluconic acid to solubilise P also indicated that chelation of Al3+ by gluconic acid may have been a factor in the solubilisation of colloidal aluminium phosphate.