Ultisols are an order in USDA 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 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".

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.

Typically Ultisols are red to yellow in color and are quite acidic, often having a pH of less than 5. The red and yellow colors result from the accumulation of iron oxide 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.

Ultisols can have a variety of clay minerals, but in many cases the dominant mineral is kaolinite. This clay has good bearing capacity and no shrink-swell property. Consequently, well-drained kaolinitic Ultisols such as the Cecil series are suitable for urban development.

Ultisols are the dominant soils in the South of the United States (where the Cecil series is most famous), southeastern China, southeast Asia and some other subtropical and tropical areas. Their northern limit (except fossil soils) is "very sharply" defined in North America by the limits of maximum glaciation during the Pleistocene because Ultisols typically take "hundreds of thousands of years" to form - far longer than the length of an interglacial period today.

The oldest fossil Ultisols are known from the Carboniferous period when forests first developed. Though known from far north of their present range as recently as the Miocene, Ultisols are surprisingly rare as fossils overall, since they would have been expected to be very common in the warm Mesozoic and Tertiary paleoclimates.

In USDA soil taxonomy, Ultisols are divided into:

Aquults - Ultisols with a water table at or near the surface for much of the year

Humults - well-drained Ultisols that have high organic matter content

Udults - Ultisols of humid climates

Ustults - Ultisols of semiarid and subhumid climates

Xerults - temperate Ultisols with very dry summers and moist winters


* cite web | url =http://soils.ag.uidaho.edu/soilorders/ultisols.htm | title =Ultisols| publisher =University of Idaho
accessdate =2006-05-14

ee also

* Pedogenesis
* Pedology (soil study)
* Soil classification

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