Potential water quality impact and agronomic effectiveness of different phosphorus fertilisers under grazed dairying in Southland

Abstract

Phosphorus (P) loss from land is a central factor in poor surface water quality in Southland. Much loss of P can occur if surface runoff occurs soon after the application of highly water soluble P fertilisers (e.g. superphosphate). Three P fertilisers (superphosphate, serpentine super, and a Ca-phosphate) of different water solubilities were applied (30 kg P/ha in spring) to a grazed dairy pasture, and the relative agronomic effectiveness and P losses determined. Across all 3 years, there were no differences in annual pasture production among the different types of P fertilisers. For 2 years out of 3, significantly more P was lost via surface runoff from the superphosphate-treated plots than from plots treated with either serpentine super or the Ca-phosphate. On average, the use of low water soluble Ca-P fertiliser decreased P losses by an average of 47% over the 3 years. It is currently recommended that to decrease P losses associated with fertiliser, applications should be timed when runoff events are unlikely for at least 3 weeks following application. If this runoff cannot be avoided, or to ensure P losses are as low as possible, the use of a low water soluble P product may be of benefit. Key words: Surface runoff, phosphate fertilisers, phosphorus loss, superphosphate.