Phosphate solubilisation in solution culture by the soil fungus Penicillium radicum

Phosphorus is a plant nutrient which is rapidly made immobile and less avai lable for plant use after addition to the soil as a soluble fertiliser. Pho sphate-solubilising microorganisms may be able to improve the P nutrition o f 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 pho sphate was studied in vitro. The fungus was grown in liquid medium cultures 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 monohydrog en phosphate (CaHPO4), calcium orthophosphate (Ca-3(PO4)(2)), crystalline f erric phosphate (FePO4. 4H(2)O), crystalline aluminium phosphate (AlPO4), c olloidal ferric phosphate or colloidal aluminium phosphate. The titratable: acidity, pH and concentrations of organic acids and soluble phosphate: wer e determined periodically during a 20 or 31 d incubation. Phosphate solubil isation was highest from CaHPO4 (475 mg P l(-1)), Ca-3(PO4)(2) (360 mg P l( -1)) and colloidal aluminium phosphate (207 mg P l(-1)). Phosphate solubili sation was generally higher when ammonium rather than nitrate was the sole source of N. Soluble phosphate concentrations in the culture medium were di rectly proportional to the titratable acidity and organic acid (principally gluconic acid) concentration and inversely related to pH. The main mechani sm for phosphate solubilisation was acid production leading to a decrease i n pH. Evidence from an abiotic study using HCl and gluconic acid to solubil ise P also indicated that chelation of Al3+ by gluconic acid may have been a factor in the solubilisation of colloidal aluminium phosphate. (C) 1999 E lsevier Science Ltd. All rights reserved.