Many sandy soils used for grazing agriculture in coastal regions of southern Western Australia have inherently very low capacity to retain phosphate-P and are oversaturated with this nutrient. During winter, these soils are major contributors of nutrients to waterways via surface runoff and leaching. Applications of high P sorbing materials such as Iron Man Gypsum (IMG) or natural clays are a means to mitigate P loss but depend on understanding soil indicators of P leaching and runoff risk to guide their use.
We carried out paddock-scale experiments to assess runoff and leaching of P following a single broad-cast application of IMG at 5 t/ha, 10 t/ha or 20 t/ha onto sandy soil under annual pasture. Runoff water quality was assessed in winter on a series of quadrants using a portable rainfall simulator. Leachates from the topsoil (0-10 cm) were measured in drier areas using pan lysimeters.
Untreated soils contained significant surface (0-2 cm) concentrations of plant available and free-P (measured as CaCl2-extractable P) with very low P adsorption capacity (phosphate buffering index or PBI < 20). Slight increases in PBI (from <15 to >31) of the surface following application of IMG immediately reduced free-P by up to 93%. Concentrations of phosphate-P in runoff were reduced by 77% and were strongly correlated with free-P in the surface soil (r2=0.78) but less so with the whole topsoil or other P risk indicators. In areas with no runoff, leached P concentrations were reduced by >75%, sustained over 5 years. Concentrations were correlated with free-P in the topsoil (r2=0.41) despite the effect of IMG being concentrated in the surface. Our research shows that P loss from sandy soils is a function of free-P concentrations that can be reduced with a targeted application of high P sorbing material to the surface.