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.
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Garrison Sposito (1984) studied this question.