Ultisols are an order in
USDA soil taxonomy. They are defined as mineral soils which contain "no calcareousmaterial 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.
FAO soil classificationsystem, 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
weatheringof 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 pHof 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 calciumand potassium, are typically deficient in Ultisols, which means they generally cannot be used for sedentary agriculture without the aid of lime and other fertilizerssuch as superphosphate. They can be easily exhausted, and require more careful management than Alfisolsor 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 capacityand 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 Asiaand some other subtropical and tropical areas. Their northern limit (except fossilsoils) is "very sharply" defined in North Americaby the limits of maximum glaciationduring the Pleistocenebecause Ultisols typically take "hundreds of thousands of years" to form - far longer than the length of an interglacialperiod today.
The oldest fossil Ultisols are known from the
Carboniferousperiod 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 Mesozoicand Tertiarypaleoclimates.
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
Pedology (soil study)
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