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This article in SSSAJ

  1. Vol. 37 No. 3, p. 358-363
     
    Received: May 15, 1972


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doi:10.2136/sssaj1973.03615995003700030015x

Phosphate Sorption by Soils Evaluated by the Langmuir Adsorption Equation1

  1. J. K. Syers,
  2. M. G. Browman,
  3. G. W. Smillie and
  4. R. B. Corey2

Abstract

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.

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