An Evaluation of Phosphorus Fertilisers of Differing Solubility On An Irrigated Dairy Farm in South-Eastern Australia.

Responding to increasing concern about environmental quality, water authorities are imposing legislation limiting phosphorus concentrations in water, which is having an impact on farming practice. This experiment investigated the agronomic effects and runoff losses associated with different forms of phosphatic (P) fertiliser applied to an irrigated dairy pasture (Olsen P average, 59 mg P kg-1 soil) in north-central Victoria, Australia. Single superphosphate (SSP), Diammonium phosphate (DAP), or partially acidulated phosphate rock (PAPR) was surface-applied at 50 kg P ha-1 in March 2005 to a border-check, flood-irrigated dairy pasture (ryegrass-white clover) ten-days prior to a scheduled irrigation. Dissolved reactive P (DRP) and total P (TP) were measured in runoff from whole bays on one replicate and from microplots on all 3 replicates for a period of nine weeks. In all runoff events, concentrations of DRP and TP in runoff greatly exceeded water quality guidelines for acceptable limits (0.045 mg L-1 P). Superphosphate resulted in higher concentrations of P in runoff than the less water-soluble fertilisers. By irrigation five, runoff from the water-soluble fertilisers still exceeded the control (approximately 1.5 times) whereas PAPR was similar to the control. These results suggest that (a) there are significant environmental savings to be made in maintenance fertiliser situations from using partially acidulated phosphate rock and (b) the current recommendation of withholding irrigation for three days after fertiliser application is insufficient to prevent potentially significant losses of P occurring.