Which horizon is primarily composed of organic matter?

The A horizon is primarily composed of organic matter, which is why it is often referred to as the topsoil. This layer contains a high concentration of decomposed plant and animal material, contributing to its rich, dark color and its essential role in supporting plant life. It is in this layer that vital nutrients are stored and made available to plants, promoting soil fertility and supporting biological activity. The presence of diverse organic materials also enhances the soil structure and moisture retention, making the A horizon crucial for ecosystem health.

In contrast, the E horizon is characterized more by the leaching of minerals and nutrients and is often lighter in color due to a lower concentration of organic matter. The C horizon consists of weathered parent material and lacks the significant organic content found in the A horizon. The R horizon refers to bedrock and is not composed of organic materials at all. Thus, the A horizon stands out as the primary layer composed of organic matter, supporting its designation as the topsoil and critical layer for plant growth and nutrient cycling.