Introduction

Phosphorus is an essential element for all cells, being present in nucleic acids and phospholipids. As well as being an important constituent of organic molecules, very often in fungi, as in other living organisms, phosphorus can be present not only as orthophosphate but also as condensed inorganic phosphates (Kulaev, 1979; Kulaev & Vagabov, 1983). Phosphate groups in organic compounds and often those of inorganic phosphates can provide a significant contribution to the negative charge within the protoplasm.

Phosphorus is frequently not readily available in nature (Larsen, 1967; Sutton & Gunary, 1969). In soil, the pool of soluble phosphate in solution is small, much phosphate is adsorbed onto surfaces and there can be a great deal of phosphate in insoluble form as salts of calcium, aluminium and iron. There can also be a significant fraction of phosphorus in organic compounds. Fungi possess a number of mechanisms for releasing phosphorus and these are considered first.

Phosphorus solubilisation

Inorganic phosphate

The ability of microorganisms to bring mineral phosphates into solution is agriculturally important as well as having more general significance in phosphorus cycling in the natural environment. Microbial mechanisms for phosphate solubilisation include production of (a) inorganic acids, (b) organic acids, (c) chelators and (d) hydrogen sulphide, which reacts with iron phosphate to liberate the phosphate while precipitating the iron as the sulphide (Ehrlich, 1981). As far as fungi are concerned, it has been assumed frequently that organic acids are the major means of solubilisation (Beever & Burns, 1980).