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A Universal Dimensionless Phosphate Adsorption Isotherm for Soil¹

ABSTRACT

A unique dimensionless phosphate adsorption isotherm, covering the solution concentration range 10^–6 to 10^–1M P, which has been found to fit 17 materials including 15 soils, pure kaolinite and amorphous Al(OH)₃, is derived. The soils shown to fit the isotherm include chromic luvisols, pellic vertisols, calcic luvisols, a tropeptic eutrorthox, a volcanic ash soil from New Zealand, and a number of English soils.

The necessary parameters to derive, a complete adsorption isotherm for a particular soil, from the universal isotherm, requires three adsorption experiments in the high concentration range (10^–4 to 10^–1M P) to define the linear portion of the isotherm and the critical concentration. Three other determinations in the range 10^–6 to 10^–4M P are needed to obtain the Freundlich isotherm from the data after correction for the effect of the linear region of the isotherm at high concentration. The Freundlich constant a was shown to be a capacity factor, and the constant b to be related to the chemical potential of the phosphate in solution. For the soils studied the variation in the magnitude of b was found to be smaller (0.11 to 0.37) than in a (1.60 to 9.66).

The advantage of this universal isotherm over conventional isotherms, is that it requires less experimental work to define and is applicable over a wide range of P concentrations such as might be found surrounding a fertilizer granule.

¹ Contribution from Department of Soil Science, Univ. of Sydney, N.S.W., 2006, Australia. This work was supported by research grants from The Univ. of Sydney and the Govemment of Iran.

² Postgraduate Scholar in The Univ. of Sydney supported by a scholarship awarded by the Ministry of Science and Higher Education, Government of Iran, and Senior Lecturer in Soil Science, Univ. of Sydney, respectively. Senior author is now at Arya-Mehr Univ. Of Tech., Isfahan, Iran.

Received for publication December 30, 1977. Accepted for publication June 29, 1978.