Soil Genesis and Classification, Sixth Edition. S. W. Buol, R. J. Southard, R. C. Graham and P. A. McDaniel. 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc.
Ultisols: Low Base Status Soils with Finer-textured Subsoil Horizons
All Ultisols have clay content increase with depth sufficient to identify an argillic or kandic horizon. Most well-drained Ultisols with udic, ustic, or xeric soil moisture regimes have thin A horizons, distinct light-colored E horizons, and reddish-colored argillic or kandic horizons. Poorly drained Ultisols with aquic soil moisture conditions have thicker dark-colored A horizons, often lack E horizons, and have gray-colored argillic or kandic horizons. Most Ultisols are formed under forest vegetation in parentmaterials containing few basic cations. Biocycling by native vegetation has concentrated basic cations in surface horizons and base saturation percentage decreases with depth.
Historically in the United States and elsewhere most Ultisols have been classified as RedYellow Podzolic or Reddish Brown Lateritic soils and are identified as Acrisols, Alisols, and Nitosols in the world reference base (IUSS Working Group WRB 2006). A common expression for Ultisol areas in the United States and southeastern China is red clay hills.
SettingAlmost all Ultisols form in acidic parent materials in locations where precipitation exceeds potential evapotranspiration during a portion of most years. Active processes of soil formation over long periods have served to deepen soil profiles while leaching and weathering the minerals present. Ultisols with udic soil moisture regimes are extensive in the southeastern United States, and Ultisols with udic, xeric, and ustic soil moisture regimes are present in the Pacific Northwest. Krebs and Tedrow (1958) have pointed out that a significant soil boundary between Ultisols to the south and Alfisols to the north exists at the terminus of glacial material in New Jersey. In piedmont physiographic areas of the southeastern United States, typical Ultisol profiles have sola 1 meter or more thick underlain by 3 or more meters of saprolite (Cr horizons) over crystalline bedrock (Pavich 1985). On alluvial fans and coastal plain sediments, sola often extend to depths of 2 or more meters (Ogg and Baker 1999; Daniels et al. 1999).
Ultisols are extensive in southeastern Asia, the upper Amazon basin in South America, the Congo River basin in Africa, and several other areas in the humid tropics where acidic parent materials are present (Sanchez and Buol 1974; Sys 1983; Lekwa
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and Whiteside 1986). Most Ultisols were naturally forested, however, Ahmad and Jones (1969a, 1969b) have reported savanna vegetation on poorly drained Ultisols in northern Trinidad.
Several other kinds of soil are spatially associated with Ultisols. Where parent materials are sandy, Spodosols or Psamments are present. On steeper slopes, areas of Inceptisols, especially Dystrudepts and Dystustepts, are present, and in the recent floodplains, Fluvents and Aquents are present (Figure 18.1). Aquepts and Aquents are common associates of Aquults in poorly drained depressions.
Some Ultisols have previously been classified as Latosols or Laterites because of their red color and location in intertropical regions. In landscapes dominated by Oxisols, Ultisols are commonly formed on erosional surfaces, downslope from Oxisols (Moniz and Buol 1982; Anjos et al. 1998). Some kaolinite-dominated Bt (kandic) horizons have very low apparent cation exchange capacity, a paucity of weatherable minerals equivalent to oxic horizons, but because the A and E horizons are sandy or loamy texture, that is, they contain less than 40% clay in the upper 18 cm, they are excluded from the Oxisol order.
In acidic coastal plain sediments with low relief (Figure 3.12b), drainage catenae are present with poorly drained Aquults present in the centers of broad interfluves surrounded by Udults at the edges of the interflues where the water table is deeper (Daniels and Gamble 1967; Daniels et al. 1966c). Similar catenae are present in ustic soil moisture regimes of the upper Amazon basin with Paleustults present on edges of the interflues (Osher and Buol 1998). In rolling relief of acidic parent material, only a small percentage of the landscape is occupied by poorly drained Ultisols (Daniels et al. 1999).
Coastal plain sediments
Figure 18.1. Idealized block diagram showing distribution of Ultisols and associated soils in a portion of the Carolinas, USA.
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Pedogenic ProcessesSeveral individual reactions and processes are involved in the formation of Ultisols. Some seasonal leaching is present in all Ultisols. Only limited leaching is required to form Ultisols in naturally acid parent materials containing no carbonates and few weatherable minerals. Extensive leaching over a long period is characteristic of Ultisols formed in more basic parent materials. Base saturation percentage decreases with depth in Ultisols. The relative concentration of bases in A horizons suggests that biocycling by perennial tree vegetation is responsible for translocation of bases from the subsoil and depositing them as vegetative litter, this is, O horizons, on the soil surface that decomposes and is mixed into shallow A horizons. Although trees extend roots deeply into Ultisols, it is common to find the most intense proliferation of roots in the more nutrient-rich A horizons.
Extensive alteration of weatherable minerals into secondary clay minerals and oxides has taken place in many Ultisols. The clay mineral suite in Ultisols is most often dominated by kaolinite, associated with gibbsite and hydroxy-interlayered 2:1 minerals (Southern Regional Project S-14 1959). Lower apparent cation exchange capacity of the clay and thus kandic horizons are present on older, more stable geomorphic surfaces (Kleiss 1994). Muscovite mica is commonly present, probably as remnants of incompletely weathered primary minerals and tending to be more prominent in the coarse clay and silt fractions than in the finer clay fractions. Greater mica contents are most often present in Ultisols formed from mica gneiss and schist parent material (Rebertus et al. 1986). A few Ultisols formed in montmorillonite-rich sediment have montmorillonitic mineralogy (Karathanasis et al. 1986).
Lessivage, leading to the formation of argillic and kandic horizons, is very pronounced. Inability to reconstruct enough A horizon thickness to account for the large amount of clay in the argillic and kandic horizons induced Simonson (1949) to discount lessivage in Ultisols and place more emphasis on clay formation in situ in the Bt horizons. Clay formation via in situ weathering is significant in Ultisols, but the often sandy A and E horizons strongly indicate that clay eluviation has also taken place. The clay in Bt horizons clay films appears to be poorly crystalline kaolinite eluviated from the E horizon (Khalifa and Buol 1968). McCaleb (1959) postulated that clay film development in the Bt horizon was limited by the supply of weatherable minerals from which clay could form in the overlying A and E horizons. Micromorphological studies indicate that clay films in the upper Bt horizon may be destroyed and the clay released transported to form clay films in the lower Bt and upper C horizons (Brook and Van Schuylenburgh 1975; Vepraskas et al. 1996). Many Ultisols, most often those on stable upland and thus apparently the oldest profiles in a given area, do not have identifiable clay films in their argillic and kandic horizons (Gamble et al. 1970b). Lack of identifiable clay films in the kandic horizons of some Ultisol subsoils indicates that the lessivage process is relatively inactive in soils with low weatherable mineral content, although it may have been more active during
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earlier stages of pedogenesis (Rebertus and Buol 1985b). Pedoturbation processes appear to destroy clay films more rapidly than they form in Kandiudults and Paleudults on stable land surfaces.
Thick, sandy, arenic, and grossarenic surfaces are common in Ultisols formed from coarser-textured parent materials. In some soils, these thick sandy E horizons are the locus of spodic horizon formation. The resulting bisequal profiles with a spodic horizon overlying an argillic or kandic horizon are classified as Ultic Haplorthods and Ultic Alorthods, the overlying spodic horizon having precedence in Soil Taxonomy.
Ultisols are the dominant soils formed on the piedmont of the southeastern United States where a steady-state system of weathering, erosion, and isostatic uplift has oper-ated for several million years. A typical Ultisol profile on the Piedmont consists of a 1 m or thicker solum with a sandy loam ochric epipedon and a reddish-colored, clay- textured, argillic or kandic horizon. The solum is underlain by a 1- to 10-meter-thick saprolite zone (Cr horizon) in which much of the rock structure is preserved but the den-sity of the rock has been reduced from 2.5 g cm3 to as low as 1 g cm3 by isovolumetric weathering (Calvert et al. 1980a; OBrien and Buol 1984; Buol et al. 2000). The transi-tion from the saprolite to the argillic or kandic horizon is gradual and contains both relict rock structure and illuvial clay (Stolt et al. 1991). There are few continuous pores, and hydraulic conductive is lower in this B/C transitional horizon than in either the more clayey argillic or kandic horizon above or the underlying saprolite (Buol and Weed 1991; Schoeneberger et al. 1995; Vepraskas et al. 1996). Thinner E horizons are observed to develop in Ultisols formed from slightly basic parent rocks, such as diorite gneiss and hornblende schist saprolite, than in soils formed from granite saprolite (England and Perkins 1959). The surface soil of most well-drained Ultisols is light colored (ochric epipedon). There is usually a slight darkening of the upper 10 cm or so of most Ultisols through melanization. Solum thickness is less on slopes than on foot slope positions (McCracken et al. 1989). In tropical areas, Ultisols tend to have thinner and somewhat less distinct E horizons, containing more organic carbon and iron than do the majority of Ultisols in the southeastern United States.
Relatively high organic carbon content umbric epipdons are commonly observed in the poorly drained members of the Ultisol order, namely the Umbraquults. Under natural conditions, base saturation (CEC
7) is normally less than 50%, but many areas
Umbraquults have been drained to facilitate cultivation and have for many years received applications of lime and fertilizer. The naturally occurring umbric epipedons now have a higher base saturation percentage and classify as mollic epipedons. Mollic epipedons are allowed in the Ultisols order if the underlying material has a sufficiently low base saturation status.
Two other features common to, but not definitive for, Ultisols are plinthite and fragipans. A precursor of plinthite appears to be a mottled pattern of reddish and gray colors that forms at a depth in the soil subjected to a seasonal fluctuation of the water table and often referred to as redoximorphic features (Vepraskas 1994). However, not all reddish-colored and iron-rich mottles harden irreversibly upon repeated wetting
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and drying, and thus, many horizons with redoximorphic features are not plinthite (Daniels et al. 1978a). Although incipient redoximorphic features are observed in many Ultisols, only in those cases where the plinthite acts to impede drainage is it recognized in Soil Taxonomy. Plinthite is most often present in the subsoil of Ultisols on the most stable and hence oldest parts of the landscape (Gamble et al. 1970a, 1970b). (See Figure 18.2.)
Fragipans are found in some Ultisols, especially those with some indication of poor drainage. Fragipans, like plinthite layers, act to restrict water movement in the soil. In Ultisols, fragipans have often been confused with plinthite when gray mottles occur in a horizon of reticulate red plinthite. Both fragipans and plinthite perch water. Peds from a fragipan readily slake when dried and then submerged in water and gently agitated. Dried peds of plinthite do not slake when subjected to similar treatment (Smith and Callahan 1987). The occurrence of fragipans in Ultisols has been described by several authors (Daniels et al. 1966c; Nettleton et al. 1968; Porter et al. 1963; Soil Survey Staff 1960; Steele et al. 1969; Ogg and Baker 1999), but the genesis of fragipans remains obscure.
Uses of UltisolsHistorically, mature natural forests present on Ultisols have invited agricultural development. When native forests are cut and burned, it is usually possible to produce a few good crop yields fertilized by plant essential nutrients contained in the ashes.
Figure 18.2. Photo of a Dothan (fine-loamy, kaolinitic, thermic Plinthic Kandiudults) profile formed in coastal plain sediments in Johnston County, North Carolina, USA.
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As the meager supply of nutrients is removed in crop harvest, farmers either move to another location or restore nutrients with manure and/or mineral fertilizer. The low nutrient content and low base status, that is, high subsoil acidity and extractable aluminum content, of Ultisols has been, and in many areas continues to be, a major limitation to agricultural use. This limitation can be overcome by modern agricultural practices of liming and fertilization. It is necessary, however, to have adequate quantities of lime, fertilizer, and management talent available for sustained crop production. Although the immediate effect of fertilizer and lime is in the Ap horizon, increased contents of exchangeable bases and decreased acidity have been identified to depths of more than 1.5 m in Ultisols after many years of lime and fertilizer application (Buol and Stokes 1997). This results in increased rooting depth of agronomic crops (Hardy et al. 1990).
Where sustainable farming is successful via fertilization, spatial heterogeneity is co...