TeXes Science Practice Test

What is the top layer of soil called in a soil profile?

1. A-Horizon

2. B-Horizon

3. O-Horizon

4. C-Horizon

The correct answer is: O-Horizon

The top layer of soil in a soil profile is known as the O-Horizon. This layer is composed primarily of organic matter, including decomposed leaves, plants, and other organic debris. It plays a crucial role in supporting plant life because it is rich in nutrients and microorganisms that are essential for soil health and fertility. The organic material in the O-Horizon contributes to soil structure and water retention, making it vital for ecosystems. In contrast, the other horizons serve different functions. The A-Horizon, or topsoil, is located just below the O-Horizon and consists of a mix of organic matter with minerals. The B-Horizon, often referred to as subsoil, accumulates minerals leached down from the layers above it but contains less organic material. The C-Horizon consists of weathered parent material, which has not yet fully developed into soil and is made up of larger rock fragments. Understanding the composition and function of the O-Horizon helps illustrate the importance of maintaining healthy topsoil for agriculture and ecological balance.