What defines a soil horizon?

A soil horizon is defined as a distinct layer of soil that has different physical and chemical properties compared to the layers above and below it. The correct answer describes a soil horizon as a layer that lies below another layer and parallel to the surface, which captures the essential aspect of soil horizons being organized into distinct, horizontal layers based on composition, texture, and other characteristics. This layered structure is crucial for soil classification and understanding soil profiles.

Other options present characteristics that may describe certain aspects or types of soil, but they do not adequately define what a soil horizon is. For example, referring to a layer of soil as less fertile is a specific characteristic that does not encompass the general definition of a horizon. The notion of an organic top layer pertains to the uppermost horizon, known as the O or organic horizon, but it does not represent horizons as a whole. Lastly, a layer defined by high moisture levels does not specifically encapsulate the broader definition of a soil horizon, which is focused on distinct layers rather than moisture content alone.