The A Horizon is best described as which of the following?

• 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 upper part of the mineral soil, commonly called the topsoil. It’s where mineral material from the parent rocks is mixed with decomposed organic matter (humus) from surface residues. That blend gives it a darker color and makes it rich in nutrients, which is where most plant roots actively explore and feed. It sits just below any surface organic layer and above deeper horizons where leaching and illuviation occur more. So, describing it as the upper mineral layer with decomposed organic matter aligns with its role as the main root zone and the zone of integrated mineral matter and humus. The other horizons are different: bedrock is the R horizon, the subsoil with iron is the B horizon, and the mineral horizon with loss of silicate clay is the E horizon.