The Apparent Solubility of Calcium Carbonate in Soils

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Abstract
Soils in the laboratory were reacted with agricultural drainage‐ and distilled waters at various carbon dioxide (CO2) levels. The soil water solutions yielded different ion activity product (IAP) values than that of calcite, depending on experimental technique and whether calcite “equilibrium” is approached from supersaturation or undersaturation. Sample storage conditions affected IAP values as did partial pressure of CO2 and soil/water ratio. With sufficient reaction time all soil waters became supersaturated with respect to calcite—but to different degrees. The only calcium carbonate (CaCO3) phase detected in these soils was low Mg calcite. Calcium carbonate particles isolated from the soils gave IAP values expected for calcite (10−8.45 to 10−8.49). The calcite supersaturation in soil appears to be due to the presence of silicates in the soil more soluble than calcite, and is not the result of unstable CaCO3 phases. It thus appears inappropriate to try to determine the solubility of soil CaCO3 by reacting soil containing CaCO3 and water in the laboratory. Since soil‐ and drainage waters are not in equilibrium with calcite (or any other CaCO3 phase), calcite solubility is inappropriate for arid land soil‐water models. It is likely that the kinetic factors controlling soil solution composition under field conditions are not simulated by conventional types of laboratory experiments.

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