Which horizon is the organic surface layer?

• A. O
• B. A
• C. B
• D. C

Answer: (A)

The main concept here is recognizing how soil horizons are organized by what they’re made of. The organic surface layer is the layer on top that is rich in organic matter like leaf litter and decomposed material, often called humus. This layer sits above the mineral soil and is identified as the organic horizon, labeled with the symbol O. It differs from the topsoil, which is a mineral-rich layer mixed with some organic matter; the layer beneath is the subsoil (B horizon) where materials like clay and iron accumulate, and the deepest weathered material (C horizon) contains little organic matter. So, the organic surface layer is the O horizon.