1. Strongly leached, acid forest soils with relatively low native fertility.

They are found primarily in humid temperate and tropical areas of the world, typically on older, stable landscapes.

Ultisols have a subsurface horizon in which clays have accumulated, often with strong yellowish or reddish colors that result from the presence of Fe (metal) oxides. The "red clay" soils of the southeastern United States are examples of Ultisols.

With the use of fertilizer and lime, these Ultisols can be very productive for agriculture.

Ultisols are divided into five suborders: Aquults, Humults, Udults, Ustults, and Xerults; all of which are defined in this unit.

2. From the U.S. Soil Taxonomy soil-order classification system.
3. Etymology: from Latin ultimus, "last".