Calcite supersaturation in soil solutions has been attributed to errors in measuring Ca2+and CO32-activities, metastable forms of CaCO3or soluble silicates, and organic matter mineralization. Laboratory batch suspensions of soils under controlled CO2and temperature were studied to clarify the mechanism producing calcite supersaturation. Treatments included the addition and removal of organic matter prior to reaction and the use of toluene to suppress microbial activity. The soil solutions were sampled over time and analyzed for Ca2+and CO32- activity. Carbonate activity was determined by two methods: 1) a double titration procedure that separates non-carbonate alkalinity from carbonate alkalinity and 2) acidification and outgassing of dissolved inorganic carbon species as CO2. Calcium activity was determined by two methods: 1) specific ion electrode and 2) total Ca analysis plus ion speciation modeling. Calcium-organic matter complexation was taken into account in the speciation calculations. It was found that calcite supersaturation in soil suspensions was not an artifact of inaccurate Ca2+or CO32-activity values. Rather, calcite supersaturation occurred because rapid organic matter mineralization increased the Ca and HCO3concentrations. These elevated Ca and HCO3concentrations persisted because of the slow precipitation kinetics of calcite. Attempts to block organic matter mineralization with toluene only slowed down the reaction in the soils. Removal of organic matter with H2O2resulted in Caoxalate formation, and oxidation of the oxalate to HCO3-produced elevated CO32-activities.