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On the Chemical Foundation of the Sodium Adsorption Ratio¹

ABSTRACT

The theoretical chemical basis of the sodium adsorption ratio (SAR) was examined in order to clarify two important issues that arise in connection with the problem of estimating potential sodium hazard: (i) the relation between SAR and the theory of cation exchange, and (ii) the implications of the universal practice of replacing SAR by what may be termed the practical SAR, which employs total concentrations of Na, Ca, and Mg instead of their free cationic concentrations. In regard to (i), it was confirmed that SAR can be given a chemical basis in terms of the Gapon cation exchange equilibrium constant which is provided in this paper with a rigorous derivation for the first time. The Gapon constant is shown to differ from the Vanselow exchange constant only because of the (arbitrary) convention adopted as to what entity is termed the ligand in the cation exchange reaction. In regard to (ii), it was shown through a direct calculation of SAR and the practical SAR for 161 different soil solutions or water extracts, utilizing a comprehensive chemical equilibrium computer program, that an exact functional relationship between the two parameters does not exist. Only a statistical correlation between SAR and practical SAR could be established because of the random variation in the composition of metal-complexing anions in the soil solutions or water extracts. These anions reduce the free cationic concentrations of Na, Ca, and Mg, but their amounts are not chemically related to the presence of the metals and, therefore, to the value of the practical SAR. It follows that the practical SAR has no exact chemical relation to the theory of cation exchange, although its use in strictly empirical studies of Na hazard may be justified, in a statistical sense, by its acknowledged practical value.

¹ Contribution from the Dep. of Soil and Environ. Sciences, Univ. of California, Riverside, CA 92521.

² Associate Professor of Soil Science and Research Soil Scientist, respectively.

Received for publication July 21, 1976. Accepted for publication October 20, 1976.

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