PHOSPHORUS LOSSES IN IRRIGATION RUNOFF FROM FERTILIZED PASTURE

Runoff from flood-irrigated perennial pastures contains significant lo ads of P. Factors affecting the P load in runoff are unknown, and ther e are no guidelines currently available for management of phosphatic f ertilizers to reduce runoff P loads, This study was conducted to deter mine the relationship between single superphosphate application rate a nd runoff P concentration, Four rates of single superphosphate (250, 5 00, 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 linea rly with application rate, with initial concentrations (5 min) being s ignificantly 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) wa s in direct proportion to single superphosphate application rate (P = 0.03). Normalized runoff losses of single superphosphate in consecutiv e irrigations followed a single exponential decay, The primary loss me chanism for P following single superphosphate application was through dissolution, rather than sediment transport.