Soils and Decomposition

Soil properties change with depth. Layers of soil that are nearly parallel with the soil surface and that are distinguishable from adjacent layers are called soil horizons. Horizons are visible on the vertical wall of a pit dug into the soil; the entire series of horizons visible on this wall is known as a soil profile. An idealized soil profile is illustrated in Figure 1. Soil classification requires an examination of the soil profile. Some of the major soil classes are identified on the basis of particular soil horizons known as diagnostic horizons. Diagnostic horizons that occur near the surface include mollic and umbric. Mollic horizons are dark in colour, have more than 1% organic matter and greater than 50% base cation saturation. Umbric horizons are similar to mollic, but have less than 50% base cation saturation. Diagnostic subsurface (B) horizons include argillic horizons that are created by the accumulation of clay that has moved into the horizon. Other subsurface diagnostic horizons are the cambic, which is weakly coloured, and the spodic, which forms due to an accumulation of humus or iron and aluminium hydrous oxides.