Agronomy Degree Practice Exam

In a soil profile, which horizon is typically found at the top?

1. O horizon

2. A horizon

3. B horizon

4. C horizon

The correct answer is: O horizon

The O horizon is typically found at the top of a soil profile and is characterized by a rich accumulation of organic material such as decomposed leaves, plant matter, and other organic debris. This layer plays a crucial role in soil fertility, as it is rich in nutrients and organic matter, which are essential for plant growth. The organic content in the O horizon not only serves as a food source for soil organisms but also enhances soil structure and water retention capabilities. The A horizon, while also rich in organic material and often referred to as topsoil, is situated just beneath the O horizon and contains both organic and mineral constituents. However, it generally has less organic matter compared to the O horizon. The B horizon, known as the subsoil, lies below the A horizon and is where mineral accumulation from above layers occurs, primarily clay, iron, and other materials leached from the A horizon. The C horizon consists of weathered parent material and is the starting point for soil formation, lying beneath the B horizon. Understanding the role of each horizon is fundamental in agronomy, as it influences soil health and planting strategies.