Soil horizon

Definition

A soil horizon is a distinct layer in a soil profile, set apart from the layers above and below by colour, texture, structure or composition. Horizons form over very long periods as rock weathers and organic material is added, which is one reason soil is so slow to replace. Typical examples are a dark organic-rich surface layer, a lighter layer below it and a layer of weathered rock at the base. Together the horizons make up the soil profile. See also: Soil profile, Topsoil, Subsoil, Parent material, and the guide "How to Observe and Assess Your Soil, and Where to Find Public Data".