Ultisols, commonly known as red clay soils, are one of twelve soil orders in the United States Department of Agriculture soil taxonomy. They are defined as mineral soils which contain no calcareous material anywhere within the soil, have less than 10% weatherable minerals in the extreme top layer of soil, and have less than 35% base saturation throughout the soil. Ultisols occur in humid temperate or tropical regions. While the term is usually applied to the red clay soils of the Southern United States, ultisols are also found in regions of Africa, Asia, and South America. In the World Reference Base for Soil Resources system, most ultisols are known as acrisols. Others with higher-activity clays are classed as alisols or nitisols.