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

Here's why:

* Humus: The A horizon is rich in humus, which is the dark, decomposed organic matter from dead plants and animals.

* Biological Activity: This layer is teeming with microorganisms (like bacteria and fungi) that break down organic matter.

* Nutrient-Rich: The decomposition process releases nutrients that are essential for plant growth.

While other soil layers can have some decayed material, the A horizon is where the concentration of decomposed organic matter is the highest.