BECKETT^1,2 has discussed potassium–calcium exchange energies in soil suspensions, using a development of a thermodynamic approach suggested by Woodruff³ and Schofield and Taylor⁴. He showed that a graph of the “activity ratio” (called AR_K) in the solution plotted against the changes in exchangeable soil potassium (ΔK) is usually linear except at small values of AR_K where it curves sharply and becomes asymptotic to the ΔK axis (Fig. 2). AR_K is directly related to the change in free energy when equivalent amounts of potassium and calcium are exchanged between the soil and solution phases. Beckett² considers that the curvature is produced by a small proportion of potassium ions held on discrete exchange sites with a greater preference for potassium relative to calcium. These sites determine the exchange energy when nearly all the exchangeable potassium is removed. Very recently he has found evidence of similar behaviour for calcium and magnesium⁵.

Author notes

1. • P. B. H. TINKER

   Present address: Department of Agriculture, University of Oxford,

Affiliations

1. Brooms Barn Experimental Station, Higham, Bury St. Edmunds, Suffolk

   • P. B. H. TINKER

2. Rothamsted Experimental Station, Harpenden, Hertfordshire

   • J. BOLTON

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Publication history

DOI

https://doi.org/10.1038/212548a0

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