a-horizon
Noun The A-horizon is the uppermost layer of a soil profile in geology and soil science. It is typically characterized by the presence of humus (decomposed organic matter) and minerals, and is the primary zone where biological activity and leaching occur.
The term A-horizon is used specifically to describe a distinct, scientific layer of soil. It is a technical term from pedology (the study of soils).
Examples * In a typical soil profile, the A-horizon lies beneath the organic O-horizon and above the B-horizon. * The dark color of the A-horizon is often due to the accumulation of humus. * Farmers aim to protect the A-horizon because it contains most of the nutrients essential for plant growth.
- Eluviation: The A-horizon is sometimes called the zone of eluviation because water percolating through it can leach (wash out) soluble minerals and fine clay particles, transporting them to lower layers.
- Topsoil: In non-technical agricultural contexts, the A-horizon is often synonymous with topsoil, though technically topsoil may include some of the organic O-horizon as well.
- Topsoil: The upper, outermost layer of soil, usually corresponding to the A-horizon.
- Soil horizon: A distinct layer of soil, parallel to the surface, with unique properties. The A-horizon is one type.
- B-horizon: The soil layer below the A-horizon, often called the subsoil, where materials leached from above accumulate.
- O-horizon: The organic layer of soil, above the A-horizon, consisting of fresh and decomposing plant matter.
- Zone of eluviation
- Topsoil (in a general sense)
The term A-horizon has a single, specific meaning within soil science. It is not used in other general contexts. The hyphen is a standard part of the term when referring to this soil layer.
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the top layer of a soil profile; usually contains humus
