The Soil A Horizon is defined as which layer?

• A. Decomposed organic matter/top layer
• B. Bedrock
• C. Mineral horizon with loss of silicate clay
• D. Subsoil with iron

Answer: (A)

The A horizon is the uppermost mineral layer where mineral material and organic matter are mixed. This blending gives it a darker color and makes it the zone with the most biological activity and root growth, commonly known as topsoil. It sits below the organic O layer and above deeper, less-weathered material. It is distinct from bedrock, which is unweathered rock beneath the soil; from the eluviation layer (E horizon) where minerals have been washed out; and from the subsoil (B horizon) where iron and clay have accumulated. So, describing it as the top layer of mineral soil with decomposed organic matter mixed in captures its defining characteristics.