Agronomic soil tests can be used to estimate dissolved reactive phosphorus loss

Context. Phosphorus (P) use in agriculture can lead to eutrophication. Agronomic soil tests such as Colwell P and P buffering index (PBI) define critical soil P levels for pasture production. These tests have potential for re-use as environmental risk indicators of dissolved reactive P (DRP) loss from paddocks but are constrained because a 0-10 cm sample does not necessarily align with the dominant hydrological loss pathways of runoff or leaching.Aims. To identify influences on the benchmark environmental measure of DRP (CaCl2-extractable P or CaCl2-P) by agronomic-based measures such as PBI, Colwell P and depth, and Colwell P to PBI ratio (P environmental risk index; PERI). To estimate CaCl2-P at any depth from a 0-10 cm sample, and the potential for change in DRP loss risk through the adoption of evidence-based fertiliser management based on soil testing.Methods. Archives of 692 0-10-cm soil samples, along with 88 sites sampled at 0-10 cm and 0-1, 1-2, 2-5, 5-10, 10-20, and 20-30 cm were analysed for Colwell P, PBI, CaCl2-P, PERI, and P fertility index (PFI). Derived relationships between CaCl2-P and Colwell P for different PBI were applied to 30 981 0-10-cm samples to estimate the potential for DRP reduction resulting from the adoption of evidence-based fertiliser management.Key results. CaCl2-P, Colwell P, PERI, and PFI decreased with depth, with an associated increase in DRP loss risk from surface soil. The CaCl2-P decreased with increasing PBI. The CaCl2-P, Colwell P, PERI, and PFI could be estimated at any depth from a 0-10 cm sample, with r(2) > 0.77. The CaCl2-P was estimable from PERI, and soils with low PBI or with high PFI had high DRP loss risk. The CaCl2-P was positively correlated with Colwell P, with the slope decreasing with increasing PBI and becoming invariant when PBI > 100. When applied to the current soil Colwell P and estimated current CaCl2-P and compared to CaCl2-P at the critical Colwell P for different relative yields (RYs), DRP loss risk could be reduced by 24% for a RY target of 95%, and 59% for a RY target of 80%.Conclusions. Because current Colwell P levels in soils exceed critical values, DRP loss risk can be substantially reduced by adopting evidence-based fertiliser management with little or no loss of utilised pasture.Implications. Fertiliser management based on evidence of P requirements determined from soil testing has a significant role in reducing DRP loss risk.