| 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 continuousweathering of minerals in a humid, temperate climate without newsoil formation viaglaciation. They are defined as mineral soils which contain nocalcareous (calcium carbonate containing) material anywhere within the soil, have less than 10% weatherable minerals in the extreme top layer of soil, and have less than 35%base saturation 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 asAcrisols andAlisols. Some belong to theRetisols or to theNitisols. Aquults are typicallyStagnosols orPlanosols. Humults may beUmbrisols.[1]
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 apH of less than 5. The red and yellow colors result from the accumulation ofiron oxide (rust), which is highly insoluble in water. Majornutrients, such ascalcium andpotassium, are typically deficient in Ultisols,[2] which means they generally cannot be used for sedentary agriculture without the aid oflime and otherfertilizers, such assuperphosphate. They can be easily exhausted, and require more careful management thanAlfisols orMollisols. However, they can be cultivated over a relatively wide range of moisture conditions. Where the organic matter content is high, as in Humults like theOlympic series, the soil is relatively fertile.
Ultisols can have a variety of clay minerals, but in many cases the dominant mineral iskaolinite. This clay has goodbearing capacity and noshrink–swell property. Consequently, well-drained kaolinitic Ultisols such as theCecil series are 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 (exceptfossil soils) 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 aninterglacial period today.
The oldest fossil Ultisols are known from theCarboniferous period 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 warmMesozoic andTertiary paleoclimates.
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 flourishing plant life in regions with primarily Ultisol soil.[3] The use of soil tests, coupled with the corresponding provisions, can alleviate issues of nutrition and irrigation that can result from non porous Ultisol.[4] Soil tests help indicate thepH, and red clay soil typically has a low pH.[5] The addition oflime is used to help to increase the pH in soil and can help increase the pH in Ultisol as well.[6]
Generally, gardeners aim to have 45% mineral, 5% organic matter and 50% pore space in their soil.[7] The composition of Ultisol in North Carolina, for reference, is approximately 16% pore space, 2% organic matter and 82% mineral.[8] The use of mulch is widespread in thePiedmont region of the United States as a solution to the high temperatures and saturation of the soil.[9] The addition of mulch helps to make the soil more porous.[10]
Addingmanure andcompost can help boost the amount of organic material present in the soil, which in turn helps add essentialnutrients. Specifically, adding a 2- to 3-inch layer of compost and manure should be mixed into the soil to match a shovel's depth.[11] The addition of organic material also helps to improve the drainage, while decreasing the overall weight of the soil.[12]
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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.[13] Living organisms within the soil use, and subsequently convert, organic material into usablehumus.[14] To avoid the delay presented by this process, adding manure in the fall is advisable.[15]
Some gardeners who live in areas with large amounts of red clay soil useraised beds orHügelkultur to avoid having to amend the soil.[16] By using raised beds, gardeners avoid having to deal with Ultisols altogether.
Plants found native to regions with high amounts of Ultisol can thrive. Generally, these species adapt to poorly drained, damp soils.[17] TheMissouri Botanical Garden recommendstickweed,spotted jewelweed,mealycup sage,Camassia,spring starflower,ostrich fern,sideoats grama,Bouteloua curtipendula, andprairie dropseed.[18]
