A horizon in the context of Fluvent

Entisols are soils, as defined under USDA soil taxonomy, that do not show any profile development other than an A-horizon (or “A” horizon). Entisols have no diagnostic horizons, and are unaltered from their parent material, which could be unconsolidated sediment, or rock. Entisols are the most common soils, occupying about 16% of the global ice-free land area.

Because of the diversity of their properties, suborders of entisols form individual Reference Soil Groups in the World Reference Base for Soil Resources (WRB): psamments correlate with arenosols, and fluvents with fluvisols. Many orthents belong to regosols or leptosols. Most wassents and aquic subgroups of other suborders belong to the gleysols.

Mollisol is a soil type which has deep, high organic matter, nutrient-enriched surface soil (a horizon), typically between 60 and 80 cm (24–31 in) in depth. This fertile surface horizon, called a mollic epipedon, is the defining diagnostic feature of Mollisols. Mollic epipedons are created by long-term addition of organic materials derived from plant roots and typically have soft, granular soil structure.

Mollisols typically occur in savannahs and mountain valleys (such as Central Asia, and the North American Great Plains). These environments have historically been strongly influenced by fire and abundant pedoturbation from organisms such as ants and earthworms. It was estimated that in 2003, only 14 to 26 percent of grassland ecosystems remained in a relatively natural state (that is, they were not used for agriculture due to the fertility of the horizon). Globally, they represent ~7% of ice-free land area. As the world's agriculturally most productive soil order, the Mollisols represent one of the most economically important soil orders.