Soil phosphorus tests II: A comparison of soil test–crop response relationships for different soil tests and wheat
Simon D. Speirs A E , Brendan J. Scott B , Philip W. Moody C and Sean D. Mason DA Graham Centre for Agricultural Innovation (NSW Department of Primary Industries and Charles Sturt University), Private Bag 4008, Narellan, New South Wales 2567, Australia.
B Graham Centre for Agricultural Innovation (NSW Department of Primary Industries and Charles Sturt University), Private Mail Bag, Pine Gully Road, Wagga Wagga, NSW 2650, Australia.
C Science Delivery, Department of Science, Information Technology, Innovation and the Arts, Dutton Park, Qld 4102, Australia.
D School of Agriculture, Food and Wine, University of Adelaide, PMB 1, Glen Osmond, SA 5064, Australia.
E Corresponding author. Email: simon.speirs@dpi.nsw.gov.au
Crop and Pasture Science 64(5) 469-479 https://doi.org/10.1071/CP13111
Submitted: 5 April 2013 Accepted: 26 May 2013 Published: 22 August 2013
Abstract
The performance of a wide range of soil phosphorus (P) testing methods that included established (Colwell-P, Olsen-P, BSES-P, and CaCl2-P) and more recently introduced methods (DGT-P and Mehlich 3-P) was evaluated on 164 archived soil samples corresponding to P fertiliser response experiments with wheat (Triticum aestivum) conducted in south-eastern Australia between 1968 and 2008. Soil test calibration relationships were developed for relative grain yield v. soil test using (i) all soils, (ii) Calcarosols, and (iii) all ‘soils other than Calcarosols’. Colwell-P and DGT-P calibration relationships were also derived for Calcarosols and Vertosols containing measureable CaCO3. The effect of soil P buffer capacity (measured as the single-point P buffer index corrected for Colwell-P, PBICol) on critical Colwell-P values was assessed by segregating field sites based on their PBICol class: very very low (15–35), very low (36–70), low (71–140), and moderate (141–280). All soil P tests, except Mehlich 3-P, showed moderate correlations with relative grain yield (R-value ≥0.43, P < 0.001) and DGT-P exhibited the largest R-value (0.55). Where soil test calibrations were derived for Calcarosols, Colwell-P had the smallest R-value (0.36), whereas DGT-P had an R-value of 0.66. For ‘soils other than Calcarosols’, R-values >0.45 decreased in the order: DGT-P (r = 0.55), Colwell-P (r = 0.49), CaCl2-P (r = 0.48), and BSES-P (r = 0.46). These results support the potential of DGT-P as a predictive soil P test, but indicate that Mehlich 3-P has little predictive use in these soils. Colwell-P had tighter critical confidence intervals than any other soil test for all calibrations except for soils classified as Calcarosols. Critical Colwell-P values, and confidence intervals, for the very very low, very low, and low P buffer capacity categories were within the range of other published data that indicate critical Colwell-P value increases as PBICol increases. Colwell-P is the current benchmark soil P test used in Australia and for the field trials in this study. With the exception of Calcarosols, no alternative soil P testing method was shown to provide a statistically superior prediction of response by wheat. Although having slightly lower R-values (i.e. <0.1 difference) for some calibration relationships, Colwell-P yielded tighter confidence intervals than did any of the other soil tests. The apparent advantage of DGT-P over Colwell-P on soils classified as Calcarosols was not due to the effects of calcium carbonate content of the analysed surface soils.
Additional keywords: Better Fertiliser Decisions for Cropping, BSES-P, CaCl2-P, Colwell-P, DGT-P, Mehlich 3-P, phosphorus buffer index, Olsen-P.
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