In a well-developed soil profile, which horizon is the uppermost layer?

In a well-developed soil profile, the A horizon, also known as the topsoil, is the uppermost layer. This layer is critical for plant growth as it contains high levels of organic material, nutrients, and microorganisms that support life. The A horizon is dark in color compared to deeper layers because of the accumulation of decomposed organic matter, which improves soil fertility and structure.

In contrast, other horizons like the B horizon, located beneath the A horizon, is where minerals leached from the upper layers accumulate but is not the top layer. The C horizon consists of weathered rock material and is also located below the A and B horizons, serving as the parent material from which the upper soils develop. The parent horizon refers to the bedrock or undecomposed rock from which soil forms but is not part of the active soil profile itself. Thus, the A horizon's position as the topmost layer and its vital functions in ecology and agriculture make it the correct response.