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

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

Answer: (B)

Humus that is partly decomposed organic material mixed with mineral soil sits in the topsoil layer, where organic matter and minerals are blended. This is the A horizon, the zone of greatest biological activity and soil mixing, which gives the soil a darker color and higher fertility. The surface layer called the O horizon is mainly organic material on top of the mineral soil and isn’t mixed with minerals. The B horizon lies below the topsoil and is where materials like clays and oxides accumulate from above, with less organic matter mixed in. The C horizon is largely weathered parent material with little soil development. So, the horizon that contains humus mixed with mineral soil is the A horizon.