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Thermodynamic Study of Calcium-Sodium and Calcium-Magnesium Exchange in Calcareous Soils¹

ABSTRACT

Montmorillonite calcareous soils and a Moroccan montmorillonite clay in the Ca form were used to determine the effect of temperature and varying ionic strength on Ca-Na and Ca-Mg exchange. Ca was selectively adsorbed over Na and, to a lesser extent, over Mg. Affinity for Ca over Na or Mg ions was greatest in the calcareous soils and varied approximately in the same order as the soil organic matter content. In the Ca-Na system, the selectivity coefficient was rather independent of ionic strength. Enthalpy and entropy changes were negative for the substitution of Ca by Na. These results indicate that the preference of Ca was probably due to the overall entropy change.

The experimental results were compared to theory based on a constant selectivity coefficient. The agreement of experimental and theoretical Na or Mg loading, calculated from the composition of the soil solution, was satisfactory.

¹ Contribution from the Dep. of Soil Science, Faculty of Agronomy, University of Louvain, 1348 Louvain-la-Neuve, Belgium.

² Professor of Soil Science and Research Soil Scientist, respectively, Univ. of Louvain, 2, Place Croix du Sud, B-1348 Louvain-la-Neuve, Belgium. The junior author is now Professor, Dep. of Civil Engineering, Federal Univ. of Paraiba, Campina Grande, Brazil.

Received for publication November 5, 1979. Accepted for publication April 28, 1980.

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