Risk of phosphorus runoff following wastewater application for two Tasmanian pasture soils used for dairying

Native soil phosphorus (P) levels are regularly augmented to provide the nutrient levels required by pastures to support milk and livestock production. Food processing industries produce large quantities of wastewater that can contain P in concentrations useful for optimising pasture growth. Wastewaters that do not contain harmful contaminants can provide viable alternatives to supplement traditional fertilisers for properties located near industries. As with all fertiliser use, wastewater irrigation must be managed to prevent runoff and leaching of nutrients that could be detrimental to surface and ground water quality. In this study the P sorption properties of two Tasmanian pasture soils commonly used for dairying were investigated - a high P sorbing Ferrosol (clay loam) and low P sorbing Hydrosol (sandy loam). A series of runoff experiments were conducted on miniswards of perennial ryegrass to determine temporal trends in P runoff concentration with the timing of a runoff event after P application from wastewater and other sources. The sorption properties of soils, the timing of runoff events and soil P concentration are all important variables in determining the risk of P being transported in runoff from pastures. High P sorbing soils have a lower risk of environmentally detrimental P concentration occurring in runoff compared to low P sorbing soils. The risk of environmentally detrimental P concentrations occurring in runoff increases after application of P and at higher P application rates.