Climate: Ultisols are formed in climatic regions, where precipitation exceeds potential evapotranspiration during some periods each year. Also, the precipitation amount has to exceed the water storage capacity of the soil for some time of the year to allow water to percolate through the soil. This is essential to maintaining the low base status. Ultisols are found in tropical areas, where they tend to have somewhat finer textured E horizons, containing more organic matter and iron, than do the majority of Ultisols formed in temperate climate. Ultisols also may form in frigid soil temperature regimes. Xeric, perudic, udic, ustic, and aquic soil moisture regimes are present in various Ultisols.
Vegetation: Many Ultisols are formed under forest vegetation (e.g. mixed hardwood, pine, oak, hickory forest) although savannah or even swamp vegetation is possible. Because of their low base status most Ultisols are used for timber production but they are also used in agriculture, where liming and fertilization is important to decrease acidity and incease soil fertility. Where adequate agricultural management is applied these Ultisols are quite productive.
Relief: There are no limitations for relief where Ultisols might form. They may occupy hillslopes or level upland areas. The position they occupy is controlled by the relationship between geomorphology and other factors of formation and the resulting rates and degree of expression of pedogenic processes.
Parent Material: Common parent materials for the development of Ultisols contain few basic cations such as siliceous crystalline rocks (e.g. granite) or sedimentary material that is relatively poor in bases (e.g. highly weathered coastal plain sediment). Most of geologically old landscapes are covered by parent material rich in silica but poor in bases. There are some Ultisols formed in parent material with higher base status and less weathered material (e.g. volcanic ash, basic ignenous or metamorphic rocks). Rapid leaching of bases can occur where precipitation amounts are high to form Ultisols.
Time: Time periods involved in development of Ultisols depend on other factors of soil formation and the rate of specific pedogenic processes. A Pleistocene or older age is assigned most parent materials where Ultisols occur. The geologic age of parent materials, however, serves only to fix an absolute maximum on possible periods of time involved in soil formation. The actual time periods involved may be, and generally are, much less.
Many Ultisols develop from parent material that initially contain appreciable quantities of weatherable minerals. Mineral components released through weathering of these materials are susceptible to leaching.
Eluviation (translocation or leaching of clays) and illuviation (precipitation of clays) are major processes which form Ultisols. The upper soil profile is depleted by clays and lower soil horizons enriched in clays, i.e., an argillic or kandic diagnostic horizon is formed. Fine clays are more likely to be translocated compared to coarse clays. Also newly formed clay is more likely to move in percolating water than is clay coated with humic substances. In Ultisols the accumulation, decomposition and humification of organic matter in the topsoil is limited thus less organo-mineral complexes are formed, which increase the probability of eluviation and illuviation. The specific mechanisms of mobilization, translocation and deposition of clays can be explained by the following model: Clays are suspended by dispersion and moved downward with percolating water. Redeposition results from the effect as water is withdrawn by capillarity into the soil leaving the suspended clays as coatings on the surface of peds. Other particles, such as sesquioxides and organic matter may also be translocated in this manner. Only limited leaching is required to form Ultisols in acid parent materials containing few weatherable minerals. If the parent material is rich in bases extensive leaching over a long time is necessary to form Ultisols. Most of the Ultisols share the common characteristic of complete alteration of weatherable primary minerals into secondary minerals and oxides. Granite and other siliceous parent materials, slowly permeable materials, fluctuating water tables, and low-lying landscape positions favor kaolinite formation. Clay minerals found in Ultisols are mainly kaolinite and gibbsite as well as some 2:1 clay minerals.
It has been postulated that many Ultisols formed on geologically old landscapes do not have clay skins in the argillic or kandic horizon because the lessivage process is not active in soils with low weatherable mineral content, although it may have been more active during earlier stages of pedogenesis. The accumulation of clays in the B horizon is probably also a result of in situ weathering . With increasing depth below the soil surface, clay content is probably influenced less by processes of translocation and more by parent material and weathering. Argillic horizons increase with time and with increasing contents of silt and san-size resistant minerals in the parent materials. Argillic horizons may develop upward or downward in the solum and can be either constructive, destructive, or possibly 'equilibrium' develpment stages.The zone of illuviation is normally an argillic horizon but may be a fragipan that meets the requirements of an argillic horizon or has thick argillans.
There is a coexistence between lessivage and podzolization in Ultisols. Podzolization is the downward movement of sequioxides and organic components from the A and E horizons to the argillic or kandic horizon. The soluble ferrous iron forms (Fe2+) at the sites of eluviation, and the insoluble ferric iron forms (Fe 3+ ) at the sites of illuviation. Translocation of Fe may occur as independent finely divided Fe-oxides, Fe-oxides attached to clays, clay-Fe-organic complexes, and soluble Fe-organic complexes.
The major process for the formation of plinthite requires appreciable periods of time in soils having adequate supplies of iron together with alternating oxidizing and reducing conditions associated with water tables that fluctuate through a limited segment of the solum for long periods during the year.
Accumulation, decomposition and humification are minor processes to form Ultisols. Most Ultisols exhibit a thin organic matter darkened surface layer.
Generally, an ochric epipedon and an argillic or kandic diagnostic horizon is found in Ultisols. In some Ultisols there are umbric or mollic epipedons. Most Ultisols are formed in weathered parent rock thus the subsurface horizons are underlain by a saprolite zone. A major characteristic of Ultisols is low base saturation throughout the soil profile with slightly higher base contents in the upper soil horizon due to biocycling. The low base saturation status is mainly due to formation in parent material high in silica but low in bases. In some soils, the low base status results from intense leaching of parent material initially high in content of weatherable minerals, while in others, a low base status and small quantities of weatherable minerals were initial parent material characteristics. Typically, the cation exchange capacity (CEC) is low with slightly higer CEC in the upper horizon due to biocycling of nutrients. In many Ultisols there are continuous losses of bases through leaching and erosion, therefore, the CEC remains low. In poorly drained Ultisols, such as the Umbraquults, the base content is slightly higher than in typical Ultisols. Abrupt decreases in base saturation are frequently associated with plinthite, fragipans, or other zones that are saturated for prolonged periods.
Associated with low base (low nutrient) content is a high soil acidity . Surface horizons rarely have pH values less than 5.0 or greater 5.8. In general, the pH values decrease with depth to a minimum of 4.0 to 5.5 in the argillic horizon. In highly weathered and leached Ultisols a decreasing pH is evident throughout the solum.
In most Ultisols organic matter is restricted to the light-colored ochric epipedon. This can be attributed to high decomposition rates by aerobic micro-organisms under warm climates and free soil-drainage. Most of the annual increments of added organic residues are on the surface, where the oxygen and nutrient status of Ultisols are most suitable for high populations of micro-organisms. Organic matter content and thickness of the surface horizon increase in most Ultisols with decreasing internal soil drainage and aeration and umbric epipedons can form under these conditions. Ultisols with high organic matter are typified by the Humic taxa. The organic matter content found in many Ultisols is low compared to other soil orders such as Mollisols or Alfisols.
Clays in Ultisols are usually of the 1:1 type (kaolinite) or gibbsite - there are less clays of the 2:1 type. Therefore, the cation exchange capacity and water holding capacity is relatively low in most Ultisols. These limitations can be overcome by the application of lime to decrease acidity and fertilizers to add bases to the soil but Ultisols are commonly not as productive as Mollsiols or Alfisols. Clay content increases regularly from A, E or upper B horizons to a maximum in the upper part of the argillic horizon, then decreases regularly with depth into the C horizon.
Iron oxides, released from other minerals through weathering or inherited as such from the parent material, are important pedogenic and taxonomic indicators in Ultisols. Goethite is the dominant crystalline form in most Ultisols and commonly associated with lesser quantities of hematite, maghemite, and magnetite. The amounts of hematite are generally greater in Ultisols developed from basic rocks and are more abundant in tropical then temperate regions. This accounts for the red color in well-drained tropical Ultisols compared to other rgions. The red or yellow colors found in the argillic horizon and underlying materials in many Ultisols are due to iron oxides. In most Ultisols, various proportions of the soil are comprised of reddish and grayish or light-colored mottles. This condition is normally associated with segregation of Fe-oxides by alternating oxidation and reduction. Reduction forms relatively soluble Fe2+ which may migrate to more oxidizing locations before reoxidization or reoxidize and precipitate in situ on existing Fe3+ compounds. Repetitions of the cycle result in development of zones with high and low free iron contents corresponding to the reddish and grayish colors. The behavior of Mn in oxidizing and reducing environments is analogous to that of Fe. Through continued segregation and concentration of oxides by alternating oxidizing and reducing conditions plinthite or fragipans are formed. Plinthite are humus-poor but sequioxide-rich horizons that hardens irreversibly to ironstone hardpans or aggregates with repeated wetting and drying. When sesquioxide-rich features are found on the soil surface or exposed in a cut bank, they are commonly called 'laterite'. It is assumed that the formation of plinthite is associated with a seasonally fluctuating water table and the translocation of sesquioxides. The consequence of plinthite is impeded drainage and waterlogging. In Soil Taxonomy 'plinthite' is used for characterization of Ultisols when > 5 % of the volume of a soil horizon is occupied by plinthite. Fragipans form in similar environmental settings and fragipans and plinthite can occur in the same soil. Fragipans are layers of high bulk density, brittle when moist, and hard when dry. Many fragipans in Ultisols are associated with either, or both, lithologic or chronological discontinuities in the parent material. It has been postulated that many fragipans in Ultisols are a result of pedogenic processes, i.e., the precipitation of silica, clays, and/or sequioxides, which result in high bulk densities. The brittlesness is attributed to binding of amorphous material and the formation of aluminosilicate binding agents.
A typical sequence in an Ultisol profile could be characterized by a distinct E horizon that thickens upward into the overlying argillic and downward into the fragipan, such as A, E, BE, Bt, BC, and C horizons.The A horizons are commonly less than 15 cm thick with (dark) grayish-brown colors and weak or moderate granular structure. E horizons are comparable in thickness and have a weak structure or are structureless and may meet the criteria set for albic horizons. Chromas of 3 to 5 and values of 4 to 6 are common in most E horizons. Colors of the B horizon are generally 10YR or redder hue with values of 4 to 6 and chromas 6 to 8. The structure of the B horizon is typically moderate, medium subangular blocky and becomes weaker and coarser with increasing depth. The underlying C horizon has weaker and coarser structure or is structureless. Colors are less red and clay contents lower.
The requirements to qualify as an Ultisols are:
The major requirements, an argillic horizon and low base status, may develop simultaneously or sequentially with either preceding the other.
A distinct E horizon is not required in Ultisols. Ultisols occupy extensive areas in the south-eastern United States, east central Africa, northeast India, southwest China, and northeastern Australia. There are 5 suborders in the Ultisol order, whereas soil moisture regime and organic matter are used to distinguish the suborders.
Aquults: They are saturated with water at some period of the year or are artificially drained. Aquic conditions form redoximorphic features.
Ustults: Ultisols formed in ustic soil moisture regime are classified as Ustults. Although moisture is limiting, it is seasonally available in adequate amounts for at least one crop per year.
Humults: They have high organic matter contents but do not have other characteristics of wetness. Humults are found in Hawaii, eastern California, and Washington.
Udults:Ultisols formed in humid regions, where dry periods are short are classifed as Udults. Their organic matter content is low. For short periods of time there might be a high water table in the solum but Udults do not show distinct redoximorphic features. For example, Udults extend from the east coast (Maryland to Florida) and beyond the Mississippi River Valley and are the most extensive soils in the humid southeast.
Xerults: Ultisols formed in xeric soil moisture regimes.
In Soil Taxonomy, the content and distribution of organic matter together with soil-drainage characteristics are definitive criteria for Humic, Umbric, and Sombric taxa. A sombric horizon is a subsurface horizon of illuvial accumulation of organic matter, which is not found under an albic horizon (e.g. Sombrihumults, Sombric Kandiudults). They are not known to occur in the U.S. and have been reported only in cool, moist, high plateau and mountain areas in intertropical regions. Organic matter in sombric horizons is not associated with large quantities of Al to the extent it is in spodic horizons. Umbric, i.e., the presence of an umbric epipedon is considered at subgroup level (e.g. Umbric Fragiaquults). Humic Ultisols show either an Ap horizon, or an A horizon 15 cm or more thick, that has a color value, moist, of 3 or less and a color value, dry, of 5 or less, which indirectly describes the presence of humus (e.g. Humic Hapludults).
Several soil moisture regimes are considered at sugroup level ranging from dry to wet conditons: Xeric (e.g. Xeric Kandihumults), aridic (e.g. Aridic Aridic Kandiustults), udic (e.g Udic Kandiustults), ustic (e.g. Ustic Kandihumults), and aquic (e.g. Aquic Paleudults).
The fragipan horizon is diagnostic for fragic great groups and subgroups in Ultisols (e.g. Fragiaquults, Fragic Paleudults, Fragic Hapludults). Soils that meet the fragipan criteria are common in the eastern part of the United States. Ultisols with a plinthic diagnostic horizon are, for example, Plinthquults, Plinthic Paleaquults.
In some Ultisols spodic characteristics are present (e.g. Spodic Paleudults), i.e., an illuvial accumulation of sequioxides and/or organic matter. It is suggested that the spodic horizon developed in a thick, sandy eluvial horizon of an existing Ultisol. Simultaneous formation and expression of argillic and spodic horizon characteristics are essentially mutually exclusive phenomena. Continued development of the spodic horizon should eventually result in either destruction of the argillic horizon or its translocation to a greater depth. Typically spodic horizons are found in the Spodosol order.
Ultisols in Vertic subgroups (e.g. Vertic Paleudults, Vertic Albaquults) do have appreciable shrink-swell capacities and extensive cracks can be observed in the B horizon during dry season. They develope in clayey sediment, for example, in Puerto Rico and the southeastern United States. A low weatherable mineral content in the non-clay fraction is considered essential to their development. Bases lost through leaching are not replenished by weathering and a low base saturation can develop in relatively short time periods.
Ultisols developed in volcanic ash or other pryoclastics are classified as 'Andic'. They have significant quantities of highly reactive allophane or similar amorphous aluminosilicate materials. Their bulk density is low (<= 1.0 g/cm3) but they show high water-retention capabilities (e.g. Andic Kandihumults, Andic Kanhaplustults). Such soils can be found in Hawaii.
Soil texture is used to define 'Arenic' (soils that have a sandy or sandy-skeletal particle-size class throughout a layer extending from the mineral soil surface to the top of an argillic horizon at a depth of 50 to 100 cm) and 'Psammentic' (soils that have a sandy particle-size class throughout the upper 75 cm of the argillic horizon, or throughout the entire argillic horizon if it is less than 75 cm thick) (e.g. Arenic Paleaquults, Psammentic Rhodudults).
Ultisols with a soil color that have 50 percent or more chroma of 3 or more in one or more horizons between either the A or Ap horizon or a depth of 25 cm from the mineral soil surface, whichever is deeper, and a depth of 75 cm are defined as 'Aeric' (e.g. Aeric Paleaquults). Ultisols which show a red color are defined by 'Rhodic' (a hue of 2.5YR or redder; and a value moist of 3 or less; and a value dry no more than 1 unit higher than the value moist) (e.g. Rhodic Kandiudults).
Shallow Ultisols are defined as 'Lithic' (e.g. Lithic Kanhaplohumults, Lithic Haplustults).
Ultisols which are grouped as 'old soils' are denoted by 'Pale' (e.g. Palehumults, Palexerults). They are not allowed to have a densic, lithic, paralithic, or petroferric contact within 150 cm of the mineral soil surface. Other limitations to qualify for a Pale subgroup within the Ultisol order are texture changes or skeletans on the faces of peds.
Ultisols and Alfisols share the presence of argillic diagnostic horizons but the low base status of Ultisols is the primary characteristic differentiating them from most Alfisols. Most Ultisols are more highly weathered and acidic than Alfisols but generally Ultisols are not as acid as Spodosols.
The absence of an argillic horizon and the absence of an argillic horizon above an oxic horizon in Inceptisols and Oxisols, resprectively, are the criteria used to distinguish them from Ultisols. To distinguish between Ultisols and Oxisols - there are still some weatherable minerals found in Ultisols compared to Oxisols. If base saturation < 35 %, a kandic horizon is present, and less than 40 % clay is found in the surface 18 cm the soil is classified as an Ultisol. In contrast, similar soils with more than 40 % clay in the surface are recognized as distinctive great groups of Oxisols.
Mollisols may occupy drier less leached positions, wetter positions, where leaching has been retarded and/or secondary enrichment with bases has occured. In areas with coarse-textured parent material, Spodosols may develop in low, poorly drained landscape positions with Ultisols on the better-drained sites. Histosols may develop in flat, depressional or poorly drained areas surrounded by Ultisols. Entisols can develop in association with Ultisols in very poorly drained areas or on steep rapidly eroding areas. Aridisols and Vertisols can occur in close proximity to Ultisols in areas where Ultisols adjoin arid climates. Inceptisols form on less stable landscape positions (steep slopes) and at higher elevations in mountainous areas or on floodplains (e.g. Fluvaquents). Soils associated with Ultisols are Psamments in areas of extremely sandy material.