In which layer of the soil profile does most biological activity occur?

In this question, we are being asked to identify the layer of the soil profile where most biological activity occurs.

The soil profile is a vertical section of the soil that shows the different layers or horizons. Each horizon has distinct characteristics and properties.

The correct answer to this question is Option E: A-horizon. The A-horizon, also known as the topsoil, is the uppermost layer of the soil profile. It is rich in organic matter, such as decomposed plant and animal material, and has a high concentration of microorganisms like bacteria, fungi, and earthworms. These microorganisms break down organic matter and release nutrients that are essential for plant growth.

The biological activity in the A-horizon is crucial for soil fertility because it helps to cycle nutrients, improve soil structure, and enhance water-holding capacity. This layer is where most plant roots are found, as it provides the necessary nutrients and support for plant growth.

In contrast, the other options are not correct. The parent material (Option A) refers to the unweathered rock or sediment from which the soil is formed. The D-horizon (Option B) is the layer beneath the A-horizon and is often composed of partially weathered rock fragments. The C-horizon (Option C) is primarily composed of weathered rock material and has less biological activity compared to the A-horizon. The B-horizon (Option D) is the layer beneath the A and C horizons, and it contains accumulations of clay, minerals, and nutrients leached from the upper horizons.

So, in summary, the A-horizon is the layer of the soil profile where most biological activity occurs, making it vital for plant growth and soil fertility.

Note: The answer provided is based on the information available in the question. However, it's important to consult the recommended textbooks or additional resources for a more comprehensive understanding of soil profiles and their characteristics.