(from Latin ultimus, "last") are 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. Intense weathering of primary
minerals has occurred, and much Ca, Mg, and K has been leached from
these soils. Ultisols have a subsurface horizon in which clays have
accumulated, often with strong yellowish or reddish colors resulting
from the presence of Fe oxides. The 'red clay' soils of the southeastern
United States are examples of Ultisols.
occupy ~8.1% of the global ice-free land area and support 18% of
the world's population. They are the dominant soils of much of the
southeastern US and occupy ~9.2% of the total US land area.
of the favorable climate regimes in which they are typically found,
Ultisols often support productive forests. The high acidity and
relatively low quantities of plant-available Ca, Mg, and K associated
with most Ultisols make them poorly suited for continuous agriculture
without the use of fertilizer and lime. With these inputs, however,
Ultisols can be very productive.
are divided into 5 suborders: Aquults, Humults,
Udults, Ustults, and Xerults. Click
here for more information about these suborders. Click
here to view a map of their distribution in the US.