Globally, soil carbonate contains at least 800 x 10¹⁵ grams of carbon, which exceeds the amount of carbon in the atmosphere (ca. 750 x 10¹⁵ grams) or the amount in land plants (ca. 560 x 10¹⁵ grams). Although the dissolution/precipitation reactions of soil carbonate have been understood for over a century, the role of soil carbonate in carbon sequestration is deceptively complex. For example, if soil carbonate results from the accumulation of calcareous dust, a lateral transfer has occurred with no net carbon sequestration. Similarly, if calcareous dust dissolves, releases Ca, and re-precipitates as pedogenic carbonate, again no net carbon sequestration has taken place. However, if soil carbonate results from Ca released from in situ weathering of silicate minerals, combined with bicarbonate generated by root and microbial respiration, then carbon sequestration has occurred. We present a classification system for carbonate-C sequestration that takes into account three factors: the environment and era of carbonate precipitation, and the source of Ca. The major classification categories of this system include pedogenic, non-pedogenic, calcic-pedogenic, and silicatic-pedogenic carbonate. Because the factors of carbonate formation operate at several temporal and spatial scales, the degree to which a calcic soil has sequestered atmospheric carbon dioxide is scale-dependent. A calcic soil containing carbonate only as detrital limestone sand, for instance, contains geogenically-sequestered carbon from the era in which the limestone precipitated. These distinctions are important for developing inventories of the amounts and types of carbonate-C sequestered by soils.