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The Future of an Illusion: Ion Activities in Soil Solutions¹

ABSTRACT

The thermodynamic significance of the single-ion activity is limited to its use, through a self-consistent set of conventions, to predict accurately the activities of electrolytes. Despite this fact, single-ion activities appear to be of increasing importance in the study of soil-plant relationships and in the surface chemistry of soils. Thus it is worthwhile to suggest a practical algorithm for the estimation of single-ion activities in soil solutions: (i) Extraction and determination of the composition of the soil solution. (ii) Speciation of the extracted solution with an ion-association computer model. (iii) Calculation of ion activities based on species concentrations and the Davies equation. This self-consistent approach can be tested successfully with the performance criteria of: (i) adherence to Debye-Hückel theory and Young's rules and (ii) accurate prediction of both conditional equilibrium constants and electrolyte activities that are important in soil chemistry.

¹ Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521. Presented at the Div. S-2 Symposium "Chemistry in the Soil Environment: Activities," Soil Sci. Soc. Am., 29 Nov. 1982, Anaheim, CA.

² Professor of Soil Science.

Received for publication March 31, 1983. Accepted for publication May 20, 1983.