Question

what is a soil horizon?

Answer 1

A soil horizon is a distinct layer of soil, roughly parallel to the ground surface, that differs in physical, chemical, and biological properties from layers above and below it. Horizons form as the result of soil-forming processes (pedogenesis) such as organic matter accumulation, leaching (eluviation), accumulation (illuviation), weathering of parent material, and biological activity.

Common horizons (from top to bottom)
- O — organic layer: mostly decomposing plant material (common in forests).
- A — topsoil: mineral soil mixed with organic matter; dark and biologically active; plant roots concentrated here.
- E — eluviation layer: lighter-colored layer where clays, iron, or organic matter have been leached out.
- B — subsoil (illuviation): zone of accumulation of clays, iron, aluminum oxides, or organic compounds; often denser and more structured.
- C — parent material: weathered pieces of rock or unconsolidated sediments with little soil development.
- R — bedrock: unweathered rock.

Other notes
- Horizons can be subdivided with suffixes (e.g., Ap = plowed A horizon; Bt = B horizon with clay accumulation).
- Boundaries between horizons may be sharp or gradual.
- Identification in the field uses color, texture, structure, root density, consistency, and chemical tests.
- Soil horizons are important for agriculture, ecology, engineering, and soil classification.