Which horizon in soil profiles is characterized primarily by organic material?

The O-Horizon is primarily characterized by the accumulation of organic material, including decomposing leaves, plant matter, and other organic debris. This layer represents the uppermost part of the soil profile and plays a crucial role in supporting a variety of life forms, particularly in forested ecosystems. It is rich in nutrients that are essential for plant growth and is the first layer where organic matter begins to decompose to create humus, which improves soil fertility and structure.

In contrast, other horizons such as the A-Horizon contain a mix of organic material and minerals but are not exclusively composed of organic material. The B-Horizon is primarily a zone of accumulation where minerals and nutrients leached from upper layers are deposited. The C-Horizon consists of unweathered parent material that has not undergone significant soil formation processes, hence lacking considerable organic content. This distinction makes the O-Horizon unique in its high concentration of organic matter, setting it apart from the other soil horizons.