Desorption of phosphate from sugarcane soils into simulated natural waters

A laboratory-based study of the behaviour of phosphorus (P) was carried out on the soils of the lower Herbert River catchment, Queensland, Australia. The aim was to explore the potential for P sorption or desorption by Herbert soils in associated river and estuary waters, so that the extent of problems associated with sugarcane production and soil-derived inputs to streamwater could be defined. Anion exchange resin was used as a sink for P. The equilibrium phosphate concentration (EPC) measured in simulated soil pore water (0.01 M CaCl2), and the EPC in the simulated river and estuary waters were strongly correlated. Based on this, and the close relationship between P sorption and selected soil properties, it was possible to estimate P desorption using commonly measured properties. Much less desorption of P took place in simulated estuary waters than in simulated river water of much lower ionic strength. This suggests that environmental degradation arising from the downstream export of soil-borne P from Herbert cane lands is likely to be concentrated in freshwater areas. Sorption properties of P in soils of the lower Herbert appear to be closely associated with aluminium-rich minerals, rather than with iron (hydr)oxides.