Which of the following best describes the A horizon of soil?

The A horizon of soil, commonly referred to as the topsoil, is best described as a layer that contains a significant amount of organic material along with minerals. This layer is crucial for supporting plant life because it serves as the primary area for root growth and is enriched with nutrients from decomposed organic matter, making it fertile. This fertility is essential for agriculture and natural vegetation, as it provides necessary nutrients for plants to thrive.

In contrast, the other descriptions do not accurately reflect the characteristics of the A horizon. The A horizon is situated above the B horizon, rather than below it, which dismisses the idea of it being located underneath another layer. It is not composed mostly of parent material; parent material is typically found in deeper layers like the C horizon. While the A horizon does contain minerals, it is primarily defined by its rich organic content, which distinguishes it from horizons that may have higher mineral content without the same level of organic matter. Thus, identifying it as the layer that contains organic material and is fertile aligns best with its recognized characteristics in soil science.