What term describes the top layer of soil that is rich in organic material?

The term that describes the top layer of soil rich in organic material is topsoil. This layer is crucial for plant growth as it contains a high concentration of nutrients, organic matter, and microorganisms that support various biological processes. The organic materials, primarily from decomposed plants and animals, enhance soil fertility and structure, allowing for better water retention and aeration.

Topsoil is typically found above subsoil, which consists of minerals with less organic material, and below the surface layer of vegetation. It is also distinct from silt, which refers to a specific size of soil particles rather than a layer. The term horizon can refer to different layers of soil or soil horizons but does not specifically denote the organic-rich layer at the surface. Thus, topsoil is the most accurate term associated with the rich, organic layer essential for agriculture and ecosystems.