Dairy farms within the Goulburn-Broken catchment contribute 26% of Australia’s milk production. The nutrient management strategy for the catchment aims to achieve a 65% reduction in phosphorus loads to freshwater ecosystems by 2016. To achieve this and maintain a competitive position in the global market, dairy farmers have focused on increasing productivity through ecologically sustainable practices.

In intensive irrigated pastures, the build up of soil phosphorus and potential for offsite eutrophication is directly related to continuous inputs of P fertilizer. Phosphorus budgets indicate only a quarter of added phosphorus fertiliser is removed in products, the remainder is either retained in soil or is lost by surface runoff and drainage water. These losses can be minimised by careful timing of application, and use of “slow release P fertilisers”. Partially acidulated rock phosphate contains both water soluble and slow release P and is less costly per unit P, it may be used a P source on acid, irrigated pastures. There are, however, limited studies comparing slow release P fertilisers to superphosphate from both agronomic and environmental standpoint.

These field studies focus on appropriate P sources for conventionally run dairy farm under maintenance conditions. Twelve irrigation bays were chosen for the experiment. Olsen soil tests currently exceed the optimum level and must be regarded as a high-risk situation for P fertiliser use. Surface soils are slightly acidic and the pastures consist of a mixture of white clover, paspalum and perennial ryegrass. Phosphorus loss was measured during flood irrigation from microplots within the bays.