DEVELOPING ALTERNATIVES TO PHOSPHATE FERTILIZERS OF HIGH WATER SOLUBILITY

Superphosphate applied to annual pastures growing on the acidic, sandy-surfaced soils of the coastal plain areas of south-western Australia is the major cause of eutrophication of waterbodies. Water-soluble phosphate, both from freshly applied and previously applied fertilizer, is leached from the profile from deep sandy soils or moved by lateral water flow on shallow sands. As part of an integrated catchment management program for the eutrophic waterbodies, an intensive research program was conducted to develop alternatives to highly water soluble phosphatic fertilizers which were argonomically and economically effective. Field, glasshouse and laboratory work identified several sources of phosphate with low initial water solubility which were at least as effective as superphosphate, and which could be formulated and commercially produced as a practical alternative to superphosphate. On the deep sandy soils, these 'slow release' sources - the most useful of which was found to be sulphur-fortified, partially-acidulated rock phosphates - had lower phosphorus leaching losses associated with their use than superphosphate on the basis of equal production of dry matter. The experimental work also investigated some of the factors controlling the agronomic effectiveness of these fertilizers, to enable identification of specific soils on which they are likely to be effective substitutes for superphosphate.