What term describes the dark-colored layer rich in organic material just below surface vegetation?

The term that describes the dark-colored layer rich in organic material just below surface vegetation is the O Horizon. This layer is specifically recognized for its accumulation of organic matter, primarily from decomposed plant material such as leaves and roots. It plays a crucial role in supporting soil fertility and overall ecosystem health, as it is rich in nutrients and provides a habitat for various microorganisms.

In contrast, the other horizons have different characteristics: the A Horizon, often referred to as topsoil, contains both organic material and minerals but is typically lighter in color compared to the O Horizon. The B Horizon, or subsoil, primarily accumulates minerals and nutrients leached from above, while the C Horizon consists of parent material and is less influenced by organic matter. This distinct differentiation in composition and function helps categorize these layers within soil profiles, making the O Horizon unique in its specific focus on organic richness.