Effect of aerial topdressing with superphosphate on the loss of phosphate from a pasture catchment

Abstract

The aerial application of single superpho.sphate (30 kg P/ha) to a pasture catchment in June 1976 resulted in an immediate increase in particulate P (PP) and dissolved inorganic P (DIP) co.ncentratio.ns in a stream under low-flow conditions; maximum values of 1.56 and 9.83 mg/I, respectively, were attained 5 h after application. The increased concentrations were not sustained, returning to the pre-fertiliser application concentrations (0.01 and 0.03 mg/l for PP and DIP, respectively) 24 h after application. The data indicated that 74% of the fertiliser P falling directly into the stream channel was transported in stream flow in the 24 h of low flow after application. The mean concentrations of PP and DIP were approximately 30 times higher immediately after fertiliser P application but these decreased exponentially with time. If it is assumed that the mean concentrations of P forms in flow events would have remained constant if no fertiliser P had been applied, the increased transport of total P, PP, and DIP between June and October 1976 accounted for 5.3, 2.9, and 2.0% of the fertiliser P applied. The importance of surface runoff in the transport of fertiliser P is indicated.