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RESEARCH ARTICLE
Contents Vol 47(7)

Comparison of bicarbonate-extractable soil phosphorus measured by ICP-AES and colourimetry in soils of south-eastern New South Wales

M. R. Hart A B and P. S. Cornish A
+ Author Affiliations
- Author Affiliations

A School of Natural Sciences, University of Western Sydney, Bldg J4 Hawkesbury Campus, Locked Bag 1797 Penrith South DC, NSW 1797, Australia.

B Corresponding author. Present address: Department for Environment, Food and Rural Affairs, Area 5c, 9 Millbank House, c/o Nobel House, 17 Smith Square, London SWIP3JR, UK. Email: Murray.Hart@defra.gsi.gov.uk

Australian Journal of Soil Research 47(7) 742-746 https://doi.org/10.1071/SR09034
Submitted: 24 February 2009  Accepted: 24 July 2009   Published: 6 November 2009

Abstract

Soil testing for plant-available phosphorus (P) in Australia is most commonly conducted using alkaline sodium bicarbonate extraction (Olsen or Colwell tests), followed by a colourimetric assay to measure the concentration of P in solution. Analysis by inductively coupled plasma (ICP) spectroscopy has become increasingly popular internationally for other soil P tests, especially Mehlich 3, due to its efficiency and ability to measure multiple elements in the one extract. The use of ICP in place of colourimetry has been used in some Australian laboratories for bicarbonate-extractable P. However, the method is known to measure forms of P (organic) that are not measured by the colourimetric assay. This study presents data comparing soil Colwell P measured by the 2 methods for 714 soil samples from pastoral sites in south-east New South Wales. Measurement by ICP consistently yielded significantly higher P concentrations than the colourimetric method (ICP-P = 1.122Col-P + 57.0, r2 = 0.95, P < 0.001). Differences between the 2 techniques were more marked in 0–20 mm than 0–100 mm depth soil samples, and in soils with greater clay contents, suggesting that the difference was related to soil organic matter, and thereby organic P contents. Relative differences were greatest in soils with lower P concentrations, i.e. within the agronomic optimum range of most interest to farmers. ICP analysis cannot be directly correlated with colourimetrically measured P in bicarbonate extracts, and would need to be developed and calibrated as a separate, new soil P test.

Additional keywords: phosphorus, ICP, colourimetry, soil organic matter, soil testing.


Acknowledgments

This work was funded by SCA/UWS Collaborative Research Project 91001274 and ARC Linkage grant LP0561858. Technical assistance was provided by Linda Allanson and Peter Paterson. Soil P analyses were conducted by the Environmental and Analytical Laboratories, Charles Sturt University, Wagga Wagga, NSW. Soil C and N analyses were conducted by the Environmental Analysis Laboratory, Southern Cross University, Lismore, NSW.


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