Ultisols (some varieties, such as those prevalent in the American South, colloquially known as "red clay soil") are an order in USDA soil taxonomy. They're 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 the 35% base saturation throughout the soil. In the FAO soil classification system, most Ultisols are known as Acrisols. Others are classed as Lixisols or Nitosols. Th… (More on Ultisols)