Phosphorus losses in irrigation runoff from fertilized pasture

Runoff from flood-irrigated perennial pastures contains significant loads of P. Factors affecting the P load in runoff are unknown, and there are no guidelines currently available for management of phosphatic fertilizers to reduce runoff P loads, This study was conducted to determine the relationship between single superphosphate application rate and runoff P concentration, Four rates of single superphosphate (250, 500, 750, and 1000 kg/ha) were applied to 12 30 by 8-m flood irrigated bays in a randomized design, on a Lemnos loam (Natric Xeralf), in the Shepparton Irrigation Region, Victoria, Australia. In runoff, total P (TP), filtrable reactive P (FRP) concentrations and EC increased linearly with application rate, with initial concentrations (5 min) being significantly higher than those later in a runoff event (P < 0.025). In the irrigation directly following fertilizer application the depth of irrigation water that infiltrated (calculated from volume balance) was in direct proportion to single superphosphate application rate (P = 0.03). Normalized runoff losses of single superphosphate in consecutive irrigations followed a single exponential decay, The primary loss mechanism for P following single superphosphate application was through dissolution, rather than sediment transport.