ABSTRACT
We shall now examine the soils that rest on carbonate-bearing parent materials: CaCO3 and MgCO3. It will also be fruitful to say a few words on soils over gypsum (CaSO4.2H2O) as well. Carbonate-bearing rocks are rather rare in the world: about 20 per cent of exposed land (Pédro 1972). They pertain mostly to Europe, North Africa, the Middle East, China…
Decarbonatation ƒ Calcium carbonate, CaCO3, is a relatively soluble salt. It is thus slowly evacuated from the profiles in humid climate. In these conditions, three cases can be distinguished on calcareous rocks (Fig. 7.1): The calcaric soils that contain CaCO3 in their matrix. The Ca-saturated (eutric) soils from which the CaCO3 has disappeared but which are still rich in Ca++ retained by the organic matter and the clay. The dystric soils that have all the features of their equivalents formed on acid rocks, but for the possible presence of more or less calcareous horizons at depth. The pedogenesis in humid climate thus consists of elimination of bases, that is, the evolution calcaric soil Æ Ca-saturated soil Æ dystric soil.
