The Apparent Solubility of Calcium Carbonate in Soils·

was dissolved from the soil; Le., equilibrium was ap­ Soils in the laboratory were reacted with agricultural drainage­ and distilled waters at various carbon dioxide (COl) levels. The soil water solutions yielded different Ion activity product (lAP) values than that of calcite, depending on experimental technique and whether calcite "equilibrium" is approached from supersaturation or under­ saturation. Sample storage conditions affected lAP values as did par­ tial pressure of CO, 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 (CaCO,) phase detected In these soils was low Mg calcite. Calcium carbonate particles isolated from the soils gave lAP values expected for calcite (10-0•45 to 10-··..). The calcite supersaturation in soil appears to he due to the presence of silicates in the soil more soluble than calcite, and is not the result of unstable CaCOJ phases. It thus appears inappropriate to try to determine the solubility of soil CaCOJ by reacting soil containing CaCO, and water in the laboratory. Since soiland 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