ABSTRACT
Inorganic carbon occurs in soils commonly as the carbonate minerals calcite (CaCO3),
dolomite (CaMg(CO3)2), and magnesian calcites (Ca1xMgxCO3). Other less common forms are aragonite (CaCO3) and siderite (FeCO3). Carbonate in soils can be of primary
(inherited from parent material) or secondary (pedogenic) origin. Secondary carbonates are
usually aggregates of silt-and clay-sized calcite crystals that are easily identified in grain
mounts. Larger crystals of calcite or dolomite are of primary origin (Doner and Lynn 1989).
Once routinely reported by sedimentologists, the qualitative and quantitative determination,
especially of Ca and Mg carbonates, is useful in studies of soil genesis and classification, and
micronutrient and phosphorus sorption. Furthermore, soil carbonates affect root and water
movement, soil pH (Nelson 1982), and the nature of the exchange complex (St. Arnaud and
Herbillon 1973). The variability in topsoil carbonate content due to incorporation of subsoil
calcite and dolomite has been used successfully to explain differences in crop yield in eroded
landscapes (Papiernik et al. 2005).
