Chemical mechanisms underlying soil phosphorus tests – Why they can be misused?

Y. Audette, L. Evans and R. P. Voroney


The current accredited soil phosphorus (P) test in Ontario, an index of plant available forms of soil P, does not measure total soil P, but rather measures the dissolved ionic forms (H2PO4- and HPO42-) and the labile P, which includes adsorbed inorganic P onto the surfaces of clays and secondary minerals, and mineral phosphates. Various soil P testing methods have been proposed since the early 1950s, each method designed to reflect the distribution of various forms of P for specific soil types. However, the chemistry of the testing methods is not well understood, and therefore they can be easily misused.

The objective of this study was to clarify the chemical mechanisms underlying the different soil P testing methods. Using the Olsen soil P test (Olsen et al 1956) as reference, which was designed for calcareous soils, and the Bray soil P test (Bray and Kurts 1945), which was designed for acidic soils, the extracted soil P forms from different soil types (acidic or alkaline) were examined. In addition, factors which have an effect on the analysis, including the soil to extractant ratio, the preparation of soil, shaking time and the analytical techniques (Autoanalyzer and ICP-OES), will be explained based on the chemical mechanisms of these two soil P tests.


Proceedings of the 13th International Symposium for Soil and Plant Analysis. Queenstown, New Zealand. 9-12 April 2013

Author Locations and Affiliations

School of Environmental Sciences, University of Guelph, Ontario, Canada

Posted May 2013