What is meant by "soil horizon"?

A "soil horizon" refers to a specific layer within the soil profile that has distinct properties and characteristics, differentiating it from other layers. These properties can include variations in color, texture, structure, and organic content, which are influenced by factors such as weathering processes, biological activity, and environmental conditions.

The existence of distinct soil horizons is crucial for understanding the soil's formation and classification, as each horizon plays a specific role in the ecosystem and contributes to soil function. For example, the topmost horizon, often referred to as the "O" horizon, is rich in organic material and is critical for nutrient cycling, while the subsequent "A" horizon can contain a mix of organic and mineral materials.

The other choices do not accurately capture the definition of a soil horizon. A layer with uniform composition suggests homogeneity, which does not reflect the variability found in soil horizons. The top layer of grass pertains more to vegetation than to soil structure. Lastly, the depth to which roots can grow relates to soil depth and health but does not define the properties or characteristics of horizontal layers in the soil.