Ultisol

"Red clay" redirects here. For other terms, seeRed clay (disambiguation)andRed dirt (disambiguation).

Ultisol
Red Clay Soil
A Ultisol profile
Used in	USDA soil taxonomy
Key process	weathering
Climate	tropical,humid subtropical,oceanic

Ultisol,commonly known asred clay soil,is one of twelve soil orders in theUnited States Department of Agriculture soil taxonomy.The word "Ultisol" is derived from "ultimate", because Ultisols were seen as the ultimate product of continuousweatheringof minerals in a humid, temperate climate without newsoil formationviaglaciation.They are defined as mineral soils which contain nocalcareous(calcium carbonatecontaining) material anywhere within the soil, have less than 10% weatherable minerals in the extreme top layer of soil, and have less than 35%basesaturation throughout the soil. Ultisols occur in humid temperate or tropical regions. While the term is usually applied to the red clay soils of theSouthern United States,Ultisols are also found in regions of Africa, Asia, Australia and South America.

In theWorld Reference Base for Soil Resources(WRB), most Ultisols are known asAcrisolsandAlisols.Some belong to theRetisolsor to theNitisols.Aquults are typicallyStagnosolsorPlanosols.Humults may beUmbrisols.

Introduction

Ultisols vary in color from purplish-red, to a bright reddish-orange, to pale yellowish-orange and (in cooler areas such asPennsylvania) even some subdued yellowish-brown or grayish-brown tones. They are typically quiteacidic,often having apHof less than 5. The red and yellow colors result from the accumulation ofiron oxide(rust), which is highly insoluble in water. Majornutrients,such ascalciumandpotassium,are typically deficient in Ultisols,which means they generally cannot be used for sedentary agriculture without the aid oflimeand otherfertilizers,such assuperphosphate.They can be easily exhausted, and require more careful management thanAlfisolsorMollisols.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 iskaolinite.This clay has goodbearing capacityand noshrink–swell property.Consequently, well-drained kaolinitic Ultisols such as theCecil seriesare suitable for urban development.

Ultisols are the dominant soils in theSouthern United States(where the Cecil series is most famous), southeastern China, Southeast Asia, and some other subtropical and tropical areas. Their northern limit (exceptfossilsoils) is very sharply defined in North America by the limits of maximum glaciation during thePleistocene,because Ultisols typically take hundreds of thousands of years to form—far longer than the length of aninterglacialperiod today.

The oldest fossil Ultisols are known from theCarboniferousperiod when forests first developed. Though known from far north of their present range as recently as theMiocene,Ultisols are surprisingly rare as fossils overall, since they would have been expected to be very common in the warmMesozoicandTertiarypaleoclimates.

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  Red clay soil is common throughout the Southern United States, especially around thePiedmont.This photo was taken in North Carolina.
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  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.
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  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 thePiedmont Plateau,which runs as a diagonal line through the South from southeast (in Alabama) to northwest (up into parts of Maryland).

Gardening in Ultisol

The lack of organic matter in Ultisol makes it difficult for plants to grow without proper care and considerations. Soil amendments are generally required each year in order to sustain plant life in regions with primarily Ultisol soil.The use of soil tests, coupled with the corresponding provisions, can alleviate issues of nutrition and irrigation that can result from non porous Ultisol.Soil tests help indicate thepH,and red clay soil typically has a low pH.The addition oflimeis used to help to increase the pH in soil and can help increase the pH in Ultisol as well.

Clay soil is known to retain nutrients very well because its negative charge helps to attractcations.As a result, Ultisol does not often require the high amounts offertilizeradditions other types of soils often do.However, this retention of nutrients coincides with a lack of water filtration that may subject plants to highly saturated soil.

Mulchcan be used to help improve Ultisol

Possible solutions

Generally, gardeners aim to have 45% mineral, 5% organic matter and 50% pore space in their soil.The composition of Ultisol in North Carolina, for reference, is approximately 16% pore space, 2% organic matter and 82% mineral.The use of mulch is widespread in thePiedmontregion of the United States as a solution to the high temperatures and saturation of the soil.The addition of mulch helps to make the soil more porous.

Addingmanureand/orcompostcan help to boost the amount of organic material present in the soil which in turn helps to add essentialnutrients.Specifically, the addition of a 2- to 3-inch layer of compost and/or manure should be mixed into the soil to approximately match the depth of a shovel.The addition of organic material also helps to improve the drainage, while decreasing the overall weight of the soil.

A garden planted in a raised bed

However, microorganisms in the soil consume the same nutrients that plants use to grow so certain nutrients will remain unavailable to plants until the microorganisms completely break down the organic material and release nutrients.Living organisms within the soil use, and subsequently convert, organic material into usablehumus.To avoid the delay presented by this process, adding manure in the fall is advisable.

Some gardeners who live in areas with large amounts of red clay soil useraised bedsorHügelkulturto avoid having to amend the soil.By using raised beds, gardeners avoid having to deal with Ultisols altogether.

Planting in Ultisol

Plants found native to regions with high amounts of Ultisol are able to thrive. Generally these are species adapted to poorly drained, damp soils.TheMissouri Botanical Gardenrecommendstickweed,spotted jewelweed,mealycup sage,Camassia,spring starflower,ostrich fern,sideoats grama,Bouteloua curtipendula,andprairie dropseed.

Suborders

• 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

See also

• Pedogenesis
• Pedology (soil study)
• Soil classification
• Red Soil

References

• "Ultisols".United States Department of Agriculture.Retrieved2008-01-04.
• "Ultisols".University of Florida. April 2007. Archived fromthe originalon September 21, 2004.Retrieved2006-05-14.
• "Ultisols".University of Idaho.Retrieved2006-05-14.