Validation of critical soil-test phosphorus values from the Better Fertiliser Decisions for Pastures meta-analysis

Abstract

Context Historical application of phosphorus (P) fertilisers has enabled grazing agriculture in south-west Western Australia but has led to excessive soil P levels that pose a eutrophication risk. However, adoption of critical soil-test P values for pastures determined from the Better Fertiliser Decisions for Pastures (BFDP) meta-analysis of historical Australian trials has been poor because of perceived lack of relevance to the soils and contemporary pasture species in the region. Aims We aimed to validate critical Colwell P values from the BFDP project for soils and contemporary pasture species of south-west Western Australia. Methods Fifty P-rate response trials were performed with contemporary pasture species, and the results compared with BFDP. Key results Trial results were consistent with BFDP and fell within 95% prediction intervals of response calibrations of Colwell P and relative yield pairs for different P buffering index (PBI) ranges. Soils with PBI <15 required lower critical Colwell P values. Responsiveness to P application was 8% (±4%), consistent with a 5% response expected for Colwell P levels supporting 95% relative yield. Application of nitrogen, potassium and sulfur significantly increased yield while increasing basal cover of annual ryegrass (Lolium multiflorum) and reducing that of clover (Trifolium spp.) in the sward. Response calibration coefficients and critical Colwell P values before and after addition of the trial data to BFDP showed a strong correlation (r2 >0.99), although critical values were slightly lower for lower soil PBI. Conclusions Critical Colwell P values from BFDP are relevant and applicable to soils and contemporary pasture species of south-west Western Australia. Implications Agronomic advice and application of P should be based on interpretation of Colwell P values, with P applied only when levels are below the critical value. Pasture composition should also be considered when interpreting Colwell P values. This approach minimises risks of P loss and improves economic outcomes for growers.