This report is based on two studies:
- Annual soil testing over 10 years of 48 dairy paddocks at Vasse Research Centre in the south-west of Western Australia.
- Phosphorus experiments on commercial dairy farms in the south-west of Western Australia.
The main messages for farmers from this part of the study were:
- Soil testing for phosphorus (P) provides a reliable indication of when the level of P in a soil is likely to be adequate for pasture production in the next growing season.
- It is a waste of money to apply P fertiliser when soil testing indicates none is required, or to acidified soils, or to undergrazed pastures or to pastures dominated by poorly productive species.
- Ryegrass-dominant pastures have a lower requirement for P than clover-dominant pastures.
- Critical soil test P values are determined from the Phosphorus Buffering Index (PBI) for that soil type.
- No fertiliser P is required when soil test P is above the critical value for that soil. When soil test P is above the critical value for that soil, adding fertiliser P will have no effect on pasture production. It will also unnecessarily increase the already high P status of the soil, increasing the risk of P loss from the farm, contributing to eutrophication of adjacent waterbodies.
- When soil test P is above the critical value for that soil and no fertiliser P is applied, soil test P drops gradually. The rate of decline decreases as the PBI value of the soil increases. For most dairy soils in WA with relatively high PBI values, the rate of decline is slow and may take years to approach the critical soil test P value.
- Critical soil test P values for each pasture species do not change when nitrogen fertiliser is applied.