Critical phosphorus values from the Better Fertiliser Decisions for Pastures project: early insights from validation trials
David Rogers
A H , David Weaver A , Robert Summers B , Eric Dobbe A , Ronald Master A , Robert McFerran C , Graham Mussell D , Luke Dawson E , Justin Mercy E , Peta Richards F and Deborah Holtham G
+ Author Affiliations
- Author Affiliations
A Department of Primary Industries and Regional Development, 444 Albany Highway, Albany, WA 6330, Australia.
B Department of Primary Industries and Regional Development, 45 Mandurah Terrace, Mandurah, WA 6210, Australia.
C Department of Water and Environmental Regulation, 72 Duchess Street, Busselton, WA, Australia.
D Graham Mussell Consulting, 52 Whitemoss Drive, Vasse, WA 6280, Australia.
E CSBP Limited, Kwinana Beach Road, Kwinana, WA 6966, Australia.
F Department of Primary Industries and Regional Development, 28527 South West Highway, Manjimup, WA 6258, Australia.
G Department of Water and Environmental Regulation, Prime House, 8 Davidson Terrace, Joondalup, WA 6027, Australia.
H Corresponding author. Email: david.rogers@dpird.wa.gov.au
Crop and Pasture Science - https://doi.org/10.1071/CP20236
Submitted: 6 July 2020 Accepted: 1 February 2021 Published online: 27 May 2021
Journal Compilation © CSIRO 2021 Open Access CC BY
Abstract
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.
Keywords: Colwell P, critical values, pasture, phosphorus, phosphorus buffering index, soil testing.
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