Here's why:

* Topsoil: This is the uppermost layer of soil, rich in decomposed plant and animal matter (humus). This organic matter is crucial for soil fertility, providing nutrients and improving soil structure.

* Subsoil: This layer lies beneath the topsoil and is typically less rich in organic matter. It mainly consists of minerals and weathered rock.

* Bedrock: This is the solid, unweathered rock beneath the subsoil. It doesn't contain organic matter.

While some organic matter can be found in the subsoil, the majority is concentrated in the topsoil.