Which soil horizon is humus, partly decomposed organic material with mixed mineral soil?

• A. O Horizon
• B. C Horizon
• C. B Horizon
• D. A Horizon

Answer: (D)

Identifying soil horizons by their content of organic material versus minerals. The A horizon is mineral soil that has organic matter mixed in, including humus formed from partly decomposed material, which gives this layer its darker color and higher fertility. The O horizon sits on the surface and is mostly organic material—litter and partly decomposed matter—with little mineral soil mixed in. The C horizon is largely weathered parent material with little organic content, and the B horizon is a subsoil where minerals and organic matter accumulate. So the description “humus, partly decomposed organic material with mixed mineral soil” best fits the A horizon, the topsoil where organic material is integrated with mineral grains.