Ultisol

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.

The word "ultisol" is derived from "ultimate", because ultisols were seen as the ultimate product of continuous weathering of minerals in a humid, temperate climate without new soil formation via glaciation.

Ultisols vary in color from purplish-red, to a bright reddish-orange, to pale yellowish-orange and even some subdued yellowish-brown tones. They are typically quite acidic, often having a pH of less than 5. The red and yellow colors result from the accumulation of iron oxide (rust), which is highly insoluble in water. Major nutrients, such as calcium and potassium, are typically deficient in ultisols, which means they generally cannot be used for sedentary agriculture without the aid of lime and other fertilizers, such as superphosphate. They can be easily exhausted, and require more careful management than alfisols or mollisols. However, they can be cultivated over a relatively wide range of moisture conditions.