Here's why:
* Humus: Humus is decomposed organic matter, formed from the breakdown of leaves, twigs, and other plant and animal material. This decomposition happens primarily in the topsoil.
* Smaller Rock: The topsoil is formed from the weathering of bedrock, which breaks down into smaller particles over time.
Other soil layers:
* O horizon: This is the uppermost layer of soil, consisting of freshly fallen leaves, twigs, and other organic debris.
* B horizon: This is the subsoil layer, often rich in clay and minerals leached from the topsoil.
* C horizon: This is the layer directly above bedrock, containing weathered rock fragments.
While the topsoil is the richest in humus and smaller rock, other layers can also contain some of these elements. However, the A horizon is the most fertile and supports the majority of plant life.
