Which soil type is characterized by a horizon of clay accumulation?

The soil type characterized by a horizon of clay accumulation is Ultisols. These soils are typically found in humid subtropical and tropical regions and have a distinct clay-rich horizon known as the illuvial horizon, where clay has leached down from the upper layers and accumulates below. This clay accumulation affects the soil's texture, drainage, and nutrient availability, often leading to lower fertility compared to other soil types. Ultisols are usually highly weathered, with a color that can range from yellow to red due to the presence of iron oxides, indicating significant weathering processes.

Mollisols, on the other hand, are noted for their rich organic content and are typically found in grassland regions. They support high biological productivity and contain a dark, fertile topsoil but do not specifically feature a horizon solely defined by clay accumulation. Andisols are soils formed from volcanic ash and are rich in minerals and nutrients but do not have a distinct clay accumulation horizon typical of Ultisols. Aridisols, found in arid environments, generally have a poorly developed horizon structure and may contain calcium carbonate but lack the illuvial horizon indicative of clay accumulation associated with Ultisols.