Editing Ultisol (section)

== Introduction ==

Ultisols vary in color from purplish-red, to a bright reddish-orange, to pale yellowish-orange and (in cooler areas such as [[Pennsylvania]]) even some subdued yellowish-brown or grayish-brown tones. They are typically quite [[acid]]ic, often having a [[Soil pH|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 [[nutrient]]s, such as [[calcium]] and [[potassium]], are typically deficient in Ultisols,<ref name="VitousekSanford1986">{{Cite journal |last1=Vitousek |first1=P. M. |last2=Sanford |first2=R. L. |date=1986 |title=Nutrient Cycling in Moist Tropical Forest |url=https://www.jstor.org/stable/2096992 |journal=Annual Review of Ecology and Systematics |volume=17 |issue=1 |pages=137–167 |doi=10.1146/annurev.es.17.110186.001033 |jstor=2096992 |bibcode=1986AnRES..17..137V |issn=0066-4162|url-access=subscription }}</ref> which means they generally cannot be used for sedentary agriculture without the aid of [[agricultural lime|lime]] and other [[fertilizer]]s, 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. Where the organic matter content is high, as in Humults like the [[Olympic (soil)|Olympic series]], the soil is relatively fertile.

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 capacity|shrink–swell property]]. Consequently, well-drained kaolinitic Ultisols such as the [[Cecil (soil)|Cecil series]] are suitable for urban development.

Ultisols are the dominant soils in the [[Southern 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.

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File:Nc-red-clay-soil-2.jpg|Red clay soil is common throughout the Southern United States, especially around the [[Piedmont (United States)|Piedmont]]. This photo was taken in North Carolina.
File:Ultisol-map-1.jpg|Map showing distribution and types of Ultisols throughout the United States; there is no Ultisol on the Ohio River flood plains, as the river has historically deposited other soil types there during its regular natural flooding.
File:Ultisol-map-2.gif|Map of the United States showing what percentage of the soil in a given area is classified as an Ultisol-type soil. The great majority of the land area classified in the highest category (75%-or-greater Ultisol) lies in the South and overlays with the [[Piedmont (United States)|Piedmont Plateau]], which runs as a diagonal line through the South from southeast (in Alabama) to northwest (up into parts of Maryland).
UltisolWorld.jpg|Utlisols of the world
</gallery>