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Soil Solution Composition and Stability of Clay Minerals¹

ABSTRACT

Common soil minerals, such as the silicates, usually have very low solubilities and slow rates of dissolution and precipitation in the soil solution. This has tended to obscure the fact that their equilibrium reactions with the soil solution can actually be described by elementary thermodynamics. Ion analyses for soil solution-mineral equilibria involve the same procedures and problems as do most other chemical analyses of the soil solution. The same is true for calculation of ionic strength, ion activities, equilibrium constants, and G values. However, in determining mineral stabilities by the solubility method, special precautions are necessary to ensure that the activity of the desired ion species can be obtained from the measurements made and that these measurements represent true equilibrium conditions. The process involved in determining mineral stabilities is illustrated using Belle Fourche montmorillonite as the example.

The usefulness of G values for common soil minerals is greatly enhanced when suitable stability equations are derived from them. These stability equations can be graphed to reveal the control of weathering relationships by the composition of the soil solution. The graphed stability equations are not merely an exercise in algebra, but are shown to represent precipitation and dissolution phenomena of real minerals. This is illustrated for the mineral group consisting of amorphous silica, montmorillonite, kaolinite, and gibbsite. The graphs help clarify field relationships that are incompletely understood and help uncover relationships previously unsuspected. The stability equations can also be used to construct equilibrium composition models for elements in the soil solution. This is a necessary first step to work on the kinetics of approach to composition limits by the soil solution.

¹ This work was supported in part by grant A-042 from the Department of the Interior, Office of Water Resources Research, and the State of Washington Water Research Center. Scientific paper no. 3583, College of Agr., Washington State Univ., Pullman 99163, proj. 1885. Presented before a joint meeting of Div. S-2, S-1, S-4, and S-9 of the Soil Science Society of America as part of a symposium, "The Soil Solution," Aug. 24, 1970, at Tucson, Ariz.

² Professor of Soils.

Received for publication December 16, 1970. Accepted for publication January 14, 1971.