What characterizes the B horizon?

The B horizon, commonly referred to as the subsoil, is characterized by the accumulation of materials that have been leached from the upper layers of soil, primarily the A horizon, which is the topsoil. In this layer, minerals such as iron, clay, and other nutrients that have washed down from the surface layers through the process of eluviation are deposited. This process results in a higher concentration of these materials, which can influence the soil's physical and chemical properties significantly.

The enrichment of the B horizon with these leached materials contributes to various soil features, including color, texture, and structure. This horizon typically has a denser, more compact structure compared to the overlying horizons, due to the accumulation of minerals and clay particles.

Other options highlight properties associated with different horizons or improper functions of the B horizon. For example, the accumulation of organic matter is characteristic of the A horizon, and the leaching layer primarily refers to the E horizon, where leaching occurs. Finally, the topsoil layer, which supports a high level of biological activity, is identified as the A horizon as well. Therefore, the correct choice clearly reflects the defining characteristics of the B horizon regarding the accumulation of leached materials and minerals.