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TECHNICAL REPORT
Landscape and Watershed Processes

Timing of Phosphorus Fertilizer Application within an Irrigation Cycle for Perennial Pasture

^a Dep. of Natural Resources and Environment, Institute of Sustainable Irrigated Agriculture, Ferguson Rd., Tatura, Victoria, 3616, Australia
^b Water-Use Efficiency Unit, NSW Agriculture, P.O. Box 865, Dubbo, NSW, 2830, Australia

Corresponding author (garnc{at}bigpond.com)

Received for publication January 5, 2000. Irrigated pastures are significant contributors of phosphorus (P) to inland watercourses, with much of the P coming from applied fertilizer. It was hypothesized that the timing of P fertilizer application relative to irrigation regulates P concentrations in runoff and infiltrating water. To test this hypothesis, a two-by-two factorial experiment was conducted on twelve 8- x 30-m border-irrigated bays growing perennial pasture. Phosphorus fertilizer in the form of single superphosphate (44 kg P ha^-1) was surface-broadcast onto the bays when the nominal change in soil water deficit reached 0 or 50 mm (U.S. Class A pan evaporation minus rainfall). Following fertilizer application, the bays were again irrigated when the nominal soil water deficit between fertilizing and the subsequent irrigation reached either 0 or 50 mm. The volume of water applied, runoff volume, and changes in soil water content were recorded for the three irrigations following fertilizer application. Total phosphorus (TP) and filtrable reactive phosphorus (FRP, <0.45 µm) concentrations in runoff and at depths of 0.1, 0.3, and 0.6 m in the soil were also measured. Soil water content at fertilizer application had less effect on P concentrations in runoff and soil water than the additional time between fertilizing and irrigating. By allowing a deficit of 50 mm between fertilizer application and irrigation, the average concentration of P in runoff and moving below a soil depth of 0.1 m was approximately halved. To maximize fertilizer use efficiency and minimize environmental effects, a delay should occur between applying P fertilizer and irrigating perennial pasture.

Abbreviations: E_p, U.S. Class A pan evaporation • F, fertilizer application • _nF, where _n represents the nominal soil water deficit at fertilizer application • F_m, where _m represents the nominal change in soil water content between fertilizer application and the subsequent irrigation • FRP, filtrable reactive phosphorus (<0.45 µm) • I₁, first irrigation following fertilizer application • I₂, second irrigation following fertilizer application • I₃, third irrigation following fertilizer application • I_p, irrigation prior to experimental treatments being imposed (prior to fertilizer application) • R, rainfall • SE, standard error • TP, total phosphorus

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