Aluminum Solubility, Calcium-Aluminum Exchange, and pH in Acid Forest Soils
1990
Reuss, J. O. | Hopper, R. W. E. | Walthall, P. M. | Roswall, E. C.
The pH-Al and Ca-Al exchange relationships are important components in several models designed to describe the effects of acid deposition on soils and surface waters. If Al solubility is controlled by Al trihydroxide minerals, the theoretical pH-Al relationship can be described by log(Al³⁺) = logKᵒ − 3pH. However, in practice it is often better described by log(Al³⁺) = logKᵒ − apH, where Kᵒ and a are empirical constants. When this relationship is combined with equations for Ca-Al exchange, a curvilinear relationship is predicted for pH as a function of Ca saturation in solutions with constant total ionic concentration. These relationships were tested using selected forest soils. The pH-Al³⁺ relationship in the mineral horizons was found to be reasonably well described by values near 2.35 for Kᵒ and 1.65 for a. When the Ca saturation is based on an effective cation exchange capacity determined at or near soil pH, the predicted curvilinear relationship between pH (measured in 0.002 M CaCl₂) and Ca saturation was observed, supporting the interpretation of this relationship in terms of Al solubility and Ca-Al exchange. Because Al solubility in Spodosol E horizons was much lower than that found in underlying horizons or in Ultisols, these E horizons did not fit the general pattern for the relationship between either pH and Al saturation or pH and Ca saturation.
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