Some peatland authorities consider soils with surface organic layer less than 30% also to be peat but this is debateable. This approach gives rise to larger estimates of the global area of peatland although it does not increase the carbon store appreciably.
Organic soil is differentiated according to the degree of decomposition of organic matter. There are four Suborders of Histosol, namely, Fibrists, Hemists, Saprists and Folists. The first three represent increasing degree of decomposition and decreasing evidence of plant remains while the last is tree litter overlying rock or rubble.
The surface organic layer of those that are most decomposed and humified (Saprists) tends to be thin (<30cm) with plant remains difficult or impossible to identify. In contrast, thicker peat (>30cm) contains slightly decomposed and undecomposed organic matter in which the remains of the different kinds of plants can be identified (Fibrists and Hemists).