Critical phosphorus values from the Better Fertiliser Decisions for Pastures project: early insights from validation trials

Phosphatic fertilisers have made grazing in the south-west of Western Australia (WA) viable. However, there is evidence that a large proportion of pasture paddocks exceed soil test critical values at which 95% of maximum yield is achieved as identified in the national Better Fertiliser Decisions for Pasture (BFDP) project. Of 22 000 soil samples collected between 2009 and 2020, 56% exceeded the critical value for phosphorus (P), although there were constraints to potassium (K) and sulfur (S) and from soil acidity. Soils with available P exceeding the critical value are expected to lead to excessive losses of P to waterways, resulting in eutrophication. A trial program was established to validate the critical P values from BFDP so that concerns can be addressed about the relevance of these critical P values to WA conditions and to contemporary pasture varieties. Measured relative yields for 19 trials in the first year were mostly within 10% of that predicted from BFDP for soils with a P buffering index (PBI) >10. Soils with PBI <10 had measured relative yields up to 25% greater than predicted by BFDP, suggesting response calibrations for low PBI soils may require adjustment in the BFDP dataset. Some pasture yield gaps occurred when soil pH and P were low. Application of nitrogen (N), K and S almost doubled the yield when P was limiting or sufficient. Agronomic advice and practice should seek to optimise these multiple inputs, thereby optimising P use rather than applying P to levels above the critical value.