The layer of soil that contains humus is the topsoil, also known as the A horizon.

Here's why:

* Humus is the decomposed organic matter from plants and animals.

* Topsoil is the uppermost layer of soil, rich in nutrients and organic matter. It's where most plant roots grow and where the majority of biological activity occurs.

While other layers of soil (like the subsoil or bedrock) might contain some organic matter, the topsoil is where the highest concentration of humus is found.