Protecting water from phosphorus run-off

Soil phosphorus and runoff concentrations take a long time to decline, once they build up from application of phosphorus fertiliser and are sensitive to application of more.

NSW Department of Primary Industries (DPI) has found that on a soil with optimal soil phosphorus levels, even though no fertiliser was applied for four years, runoff concentration did not decline, according to DPI soil scientist, Dr Warwick Dougherty.

"This highlights the importance of getting soil phosphorus management right," Dr Dougherty said.

"Within the constraints of dairying systems in most of NSW, farmers can only influence some of the factors affecting phosphorus concentration in runoff."

Research conducted over several years and recently completed at Elizabeth Macarthur Agricultural Institute (EMAI), Camden highlighted two factors that farmers can manage - the rate and timing of fertiliser application.

"Within pasture systems, it is essential not to apply more than the minimum amount of fertiliser required to maintain optimal production," Dr Dougherty said.

"Excessive rates of application can substantially increase runoff concentrations," he said.

The time lapse between fertiliser application and runoff occurring also has a major effect on runoff concentrations.

If runoff occurs within the week or so after fertiliser is applied, concentrations in runoff can be very high.

Avoiding application in high risk runoff periods is a useful strategy for reducing runoff losses to waterways in regions with distinctly seasonal rain.

Whenever possible, farmers should consider the possibility of rain and subsequent runoff, although Dr Dougherty acknowledged that predicting when both will occur is difficult in the environments in which most NSW dairy farmers operate.

Research also revealed that three other key factors - the time between grazing and a runoff event, pasture biomass, and phosphorus in dung, influenced runoff.

"Further, the majority of the phosphorus in runoff from pastures is dissolved, not particulate," Dr Dougherty said.

"Hence traditional strategies to reduce phosphorus in runoff such as buffer or filter strips are unlikely to be effective."

The research was jointly funded by Dairy Australia and Land and Water Australia.