Which soil order typically shows clay accumulation and a base saturation below 35%?

Ultisols are a soil order that is characterized by clay accumulation in the subsoil, known as argillic horizons, and typically exhibit low base saturation, often below 35%. This low base saturation indicates a higher presence of acidic cations and a lower capacity for holding essential nutrients, reflecting the leaching processes that occur in humid environments where Ultisols are commonly found, such as the southeastern United States.

The clay accumulation in Ultisols often leads to distinct soil horizons, where nutrient-poor layers form as a result of the weathering of primary minerals. This weathering process contributes to the formation of secondary minerals like kaolinite, which further influences the soil's physical and chemical properties.

In contrast, the other soil types listed have different characteristics. Vertisols are known for their high clay content and shrink-swell capacity but do not necessarily maintain low base saturation. Aridisols typically develop in arid regions and can have variable clay content along with higher base saturation due to limited leaching. Mollisols, characterized by a dark, organic-rich upper layer, usually have higher nutrient content and base saturation, making them highly fertile and not fitting the criteria of clay accumulation with low base saturation.