Horizon A is formed primarily through the addition and incorporation of organic matter from plants into the upper soil layers by the action of various organisms.
Understanding Horizon A Formation
Based on the reference provided, the formation of a thin, dark A horizon begins over an existing C horizon through a specific process involving plants and soil-dwelling organisms.
Here’s how this process unfolds:
- Plant Material Input: Plant growth is robust, with roots spreading below ground. In the fall, senesced (dead or dying) plant material, particularly leaves, falls to the surface of the soil.
- Decomposition and Mixing: Organisms such as worms, ants, beetles, and others play a crucial role. They actively break these leaves into smaller pieces.
- Incorporation: These organisms also mix the broken-down organic material into the upper layers of the soil.
This entire process is referred to as addition, where new organic material is added to the mineral soil layers and blended in. The result is the development of a thin, dark layer known as the A horizon, situated above the underlying C horizon.
This biological activity and the consistent input of organic matter are key to creating the fertile, dark characteristics typical of the A horizon, also often called topsoil.
