Agronomic soil tests can be used to estimate dissolved reactive phosphorus loss
David Weaver A * , Robert Summers B and Andreas Neuhaus CA Department of Primary Industries and Regional Development, Western Australia, 444 Albany Highway, Albany, WA 6330, Australia.
B Department of Primary Industries and Regional Development, Western Australia, 45 Mandurah Terrace, Mandurah, WA 6210, Australia.
C CSBP Limited, Kwinana Beach Road, Kwinana, WA 6966, Australia.
Soil Research 61(7) 627-646 https://doi.org/10.1071/SR22167
Submitted: 18 July 2022 Accepted: 26 May 2023 Published: 15 June 2023
© 2023 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)
Abstract
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 r2 > 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.
Keywords: calcium chloride extractable P, Colwell P, dissolved reactive P, P environmental risk index, P fertility index, PBI, phosphorus, stratification.
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