Definition

1. Noun:
   • The top layer of a soil profile; usually contains humus: The "A horizon" is a specific, uppermost layer of soil in a vertical cross-section (soil profile). It is typically characterized by the presence of organic matter (humus) mixed with mineral particles.

Usage

• The term "A horizon" is a technical term used primarily in soil science, geology, and agriculture.
• It is often used in contrast with other soil layers, such as the B horizon or C horizon.
• It is typically preceded by the definite article "the" (e.g., A horizon).

Examples

• Noun:
  • The dark color of the A horizon is due to its high humus content.
  • Farmers aim to protect the A horizon because it is the most fertile part of the soil.
  • In the soil profile, the A horizon lies directly beneath the organic O horizon and above the B horizon.

Advanced Usage

• "Eluviated A horizon": A specific type of A horizon from which clay, iron, or aluminum have been leached (washed out), often leaving a layer enriched in quartz or other resistant materials.
  • The process of eluviation creates a distinctive, often lighter-colored, eluviated A horizon.

Variants and Related Words

• Topsoil: A common, non-technical synonym for the A horizon, referring to the upper, most cultivated layer of soil.
• Soil profile: The vertical sequence of soil layers (horizons), of which the A horizon is a part.
• B horizon: The soil layer (subsoil) typically found below the A horizon, where materials leached from above often accumulate.
• C horizon: The partially weathered parent material found beneath the B horizon.

Synonyms

• Topsoil: The surface soil, usually equivalent to the A horizon.
• Surface soil: Another general term for the uppermost layer of soil.

Related Phrases

• "Plow layer": In agriculture, the layer of soil turned over by plowing, which corresponds roughly to the A horizon.
  • The plow layer in this field is remarkably deep and rich.

Noun

1. the top layer of a soil profile; usually contains humus