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.

磷肥使西澳大利亚 (WA) 西南部的放牧变得可行。然而，有证据表明，大部分牧场围场超过了土壤测试临界值，达到了国家牧场更好肥料决策 (BFDP) 项目中确定的最大产量的 95%。在 2009 年至 2020 年期间收集的 22 000 个土壤样本中，56% 的磷 (P) 超过了临界值，尽管钾 (K) 和硫 (S) 以及土壤酸度受到限制。可利用磷超过临界值的土壤预计会导致磷过度流失到水道，导致富营养化。建立了一个试验计划来验证来自 BFDP 的关键 P 值，以便可以解决这些关键 P 值与 WA 条件和当代牧草品种的相关性的问题。对于第一年进行的19个试验，测得的相对产量大多在BFDP预测的P缓冲指数（PBI）> 10的土壤的相对产量的10％以内。PBI <10 的土壤测量的相对产量比 BFDP 预测的高 25%，这表明低 PBI 土壤的响应校准可能需要在 BFDP 数据集中进行调整。当土壤 pH 值和 P 较低时，会出现一些牧草产量差距。当磷有限或充足时，氮 (N)、钾和硫的施用几乎使产量翻了一番。农艺建议和实践应寻求优化这些多种投入，