What do Ultisols primarily form from?

Ultisols primarily form from the weathering of rocks and accumulation of clay particles over time. This soil order is characterized by a subsurface horizon rich in clay, which develops due to the leaching of base cations and the overall weathering process. As the parent material weathers, clay minerals are created and the horizon becomes more enriched with these materials, leading to the distinctive clay-rich profile.

Additionally, Ultisols typically develop in warm temperate to tropical climates, where high rainfall promotes leaching and clay accumulation. This clay content is crucial for the soil's nutrient-holding capacity and overall structure. The presence of clay also affects water retention and soil permeability, making it an important aspect of Ultisol formation.

The other options relate to soil characteristics that do not specifically define the formation process of Ultisols. For instance, high organic matter content pertains more to soil types like Mollisols, while sand and silt particles are more relevant to Entisols or Inceptisols. Frozen organic materials relate to soils found in colder climates, such as gelisols, and are not a characteristic of Ultisol formation.