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Phosphate Sorption by Soils Evaluated by the Langmuir Adsorption Equation¹

ABSTRACT

The sorption of added inorganic phosphate (P) by three soils which varied appreciably in their ability to sorb P was evaluated using the Langmuir adsorption equation. When the sorption data were plotted according to the conventional Langmuir equation, two linear relationships were obtained, indicating the presence of two populations of sites which have a widely differing affinity for P. Previous workers have obtained a single linear Langmuir relationship over the same equilibrium P concentration range (up to 14 µg/ml). The sites in part I (straight line corresponding to lower equilibrium P concentrations) had a very much higher (between 33 and 91 times) binding energy constant (K₁), determined by regression analysis, than those in part II (straight line corresponding to higher equilibrium P concentrations). Between 40 and 52% of the total adsorption maxima was sorbed by sites in part I. A plot of the sorption data according to a rearranged form (Eadie-Hofstee plot) of the Langmuir equation gave a curve which could not be resolved satisfactorily into two straight-line components, suggesting the existence of three populations of sites. The rearranged form of the Langmuir equation was found to be preferable for evaluating P sorption at low equilibrium P concentrations. Predictions, based on the addition of high levels of added P, of the interrelationships between sorbed P and equilibrium P concentrations typical of the soil solution and soil-water systems, such as runoff, may be in error because of the failure to consider the existence of sites with appreciably higher K₁ values than those which sustain relatively high concentrations of inorganic P in solution.

¹ Contribution from the Dep. of Soil Science, Univ. of Wisconsin, Madison. Research supported by the College of Agr. and Life Sci., Univ. of Wisconsin, by the Kellogg Foundation, and by Hatch (project 1693).

² Formerly Associate Professor of Soil Science, currently Professor of Soil Science, Massey University, Palmerston North, New Zealand; Postdoctoral Fellow; formerly Research Fellow, currently Senior Lecturer of Soil Science, University College, Dublin, Ireland; and Professor, respectively.

Received for publication May 15, 1972. Accepted for publication January 4, 1973.