The arrangement of the layers that can be seen in a section of *soil
extending from the surface to the bedrock. The layers are soil horizons and there are differences in colour, texture, and composition between them. The upper layer, horizon A or the zone of eluviation, contains the most humus, is most exposed to weathering, and is the zone in which most decomposition by microorganisms takes place. The underlying horizon Β or zone of illuviation accumulates salts and colloids washed down from horizon A. Horizon C (the subsoil) consists of the mantle rock produced mainly by the mechanical weathering of the parent rock. In horticulture, the term subsoil is taken to mean the soil beneath the humus-rich horizon A (top-soil). Some soils do not have such horizons and may be more or less uniform throughout. They are generally found in mature soil in moist cool temperate climates. In arid and semiarid climates where the rate of evaporation exceeds the amount of precipitation there is a tendency for the upward movement of water by capillarity, and mineral salts, particularly calcium salts, are deposited in the upper layers resulting in calcification.