Soil Genesis and Classification (Buol/Soil Genesis and Classification) || Ultisols: Low Base Status Soils with Finer-Textured Subsoil Horizons

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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 parent materials 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 Red–Yellow 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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376 Soil Genesis and Classification

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).

Dystrudepts(steep slopes)

Udifluvents(flood plains)

Fall line

Piedmont

Coastal plain sediments

Hapludalfs

Granite-Gneiss

Kandiudults

and

Paleudults

(oldest terra

ces)

Kanhapludults

and

Hapludults

Hapludults

(younger terra

ces)

Psamments

(outer banks)

Ocean

Coastal plain

Aquults

Spodosols(in sand)

Quartzipsamments(in sand)

Mafic volcanics

Figure 18.1. Idealized block diagram showing distribution of Ultisols and associated soils in a portion of the Carolinas, USA.


18 / Ultisols: Low Base Status Soils with Finer-textured Subsoil Horizons 377

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



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 cm−3 to as low as 1 g cm−3 by isovolumetric weathering (Calvert et al. 1980a; O’Brien 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



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.



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 common when rolling to hilly Ultisol landscapes are cultivated. On the steeper slopes, pedons have naturally thinner A and E horizons than on more level areas (McCracken et al. 1989), and cultivation often incorporates finer-textured material from the Bt horizon into the Ap horizon. The resulting finer-textured Ap horizons have lower infiltration rates, higher run-off rates, and lower crop yields than less sloping areas (Stone et al. 1985).

Timber production is extensive on many Ultisols. Compared to cultivated food crops, forests have an extremely low rate of nutrient requirement that, combined with deeper root proliferation, allows trees to grow on Ultisols too nutrient poor to sustain cultivated food crops (Buol 2008). Harvesting of native forests often leads to degradation in soil fertility, and it has been postulated that savanna vegetation has replaced forests following timber harvest in some tropical areas. Well-managed timber operations often find it profitable to fertilize Ultisols when replanting commercial timber.

Most Ultisols have relatively high contents of quartz sand and 1:1 clay that provides stable materials for earthen construction. However, in nearly level landscapes with aquic soil moisture conditions (Aquults and associated soils) in a high proportion of the area, road construction can be expensive.

Classification of UltisolsThe major diagnostic features of Ultisols are the presence of an argillic or kandic horizon and a base saturation (by sum of cations method, CEC

8.2) of less than 35% at

1.25 m (50 in.) below the upper boundary of the argillic or kandic horizon or at 1.8 m (72 in.) below the surface, whichever is deeper. If CEC

8.2 base saturation data are not

available, base saturation of 50% of CEC7 has been found to closely approximate

35% base saturation at CEC8.2

in most kaolinite-dominated Bt horizons. If a lithic or paralithic contact is shallower than either of the above depths, the base saturation (CEC

8.2) at that contact must be less than 35%. These base saturation criteria that

separate Ultisols from Alfisols are at best an arbitrary limit of doubtful genetic or agronomic significance. However, some distinction needs to be made between soils



where the base saturation percentage decreases with depth (Ultisols) and soils where the base saturation percentage increases with depth (Alfisols), and no other criterion has been found to work as well as a specific limit of base saturation percentage at specified depths in the pedon. Base saturation criteria were placed deep enough to avoid changes in classification due to additions of lime to the Ap horizon. However, some Ultisols have been converted to Alfisols by rising sea levels and the encroachment of bases around Chesapeake Bay (Stolt and Rabenhorst 1991).

The Ultisol order is subdivided into five suborders using soil moisture regime and organic carbon content criteria (Figure 18.3).

Aquults either are saturated with water at some period of time during a normal year or are artificially drained. Because it is not practical to observe a soil throughout the year in order to classify it, certain other morphological features associated with wetness are used as criteria for defining Aquults. The criteria used are presence of redoximorphic features in all layers below 25 cm or any Ap horizon if present and above a 40-cm depth and either, (1) color chroma of 2 or less on 50% or more of ped faces, (2) chroma of 1 or less on ped faces and in the ped matrix, or (3) if in a thermic or isothermic or warmer soil temperature a hue of 2.5Y or 5Y in ped matrix within the upper 12.5 cm of the kandic or argillic horizon. Also if enough ferrous iron is present to give a positive reaction to alpha, alpha-dipyridyl within 50 cm of the surface in normal years at a time when the soil is not being irrigated identifies Aquults.

Humults have high organic carbon contents but do not have characteristics of wetness specified in the Aquults suborder. By definition, they contain more than 0.9% organic carbon in the upper 15 cm (6 in.) of the argillic or kandic horizon or 12 kg or more organic carbon in a cubic meter of the upper 1 m of the pedon, excluding O horizons. In the United States, Humults are present mainly in northwestern states where some have received surface deposits of volcanic ash and have andic-like soil

Figure 18.3. Diagram showing some relationships among suborders of Ultisols.

Wet

Aquults

UdultsH

umul

ts

Ustults

Xerults

DryHumic(not wet)



properties in surface horizons. Humults may have ustic, xeric, or udic soil moisture regimes that are identified as subgroups.

Udults have a perudic or udic soil moisture regime, dry periods are of short duration, and organic carbon contents are lower than in the Humults. The water table may be in the solum for short periods each year, but aquic conditions of the Aquults are not present within 50 cm of the surface.

Ustults have an ustic soil moisture regime. Although moisture is limiting, it is seasonally available in adequate amounts for at least one crop per year.

Xerults are in areas of extremely dry summers and moist winters (xeric soil moisture regimes).

Ultisol great groups are listed in Table 18.1. Several great groups identify pedon features common to several Ultisol suborders. “Pale” great groups identify Ultisols with thick argillic horizons while “Hapl” great groups identify thinner argillic horizons. A “thick” argillic horizon is identified as having a clay content decrease of less than 20% of the maximum clay content within 150 cm of the soil surface whereas maximum clay content decreases more rapidly with depth within “thinner” argillic horizons. This criterion was selected because “thick” Bt horizons differentiates most Ultisols on the oldest geomorphic surfaces on Atlantic coastal plain sediments from “thinner” Bt horizons in most Ultisols formed on younger geomorphic coastal plain surfaces and residual rock in the piedmont physiographic region of the southeastern USA.

Ultisols with Bt horizons that have apparent CEC7 values of 16 cmol kg−1 clay or

less have kandic horizons. Ultisols with kandic horizon thickness equivalent to the argillic horizon thickness of “Pale” great groups were identified as “Kandi” great groups. Thinner kandic horizons are identified as “Kanhapl” (combination of “kandi” and “hapl”) great groups.

“Plinth” great groups are recognized when plinthite forms a continuous layer in 50% or more of some horizon within 150 m of the surface. Plinthic subgroups are recognized when plinthite is present in more than 5% of the volume of a soil horizon within 150 cm of the soil surface. (See Figure 18.2.)

“Rhod” great groups identify Ultisols with dark red colors (hue of 2.5YR or redder and moist value of 3 or less) that have 1 unit or less value change when dry in all horizons to 100 cm. “Rhod” great groups are viewed by soil scientists as “better” than most other Ultisols. No satisfactory explanation for the distinctive color characteristics has been found, but higher than normal contents of manganese oxide are a common feature of such soils. Most have a somewhat higher base saturation percentage than other Ultisols and appear to have formed in parent materials that contain more bases.

PerspectiveThe most significant Ultisols features are abrupt or a clear clay content increase from the A or E horizons to kandic or argillic Bt horizons and a low base saturation percentage that decreases with depth. Ultisols are almost exclusively formed from


383

Table 18.1. Suborders and great groups in the Ultisols order

Suborder Great Groups

Aquults Plinthaquults—plinthite forms over one-half of a horizon within 150 cm of the surface Fragiaquults—fragipan surface within 100 cmAlbaquults—abrupt textural change between an ochric epipedon or albic horizon and

an argillic or kandic horizon that has a hydraulic conductivity of 0.4 cm hr−1 or less Kandiaquults—kandic horizon in which relative clay content decreases less than 20%

within 150 cm of the surfaceKanhaplaquults—kandic horizon in which relative clay content decreases more than

20% within 150 cm of the surfacePaleaquults—relative clay content in argillic horizon decreases less than 20% within

150 cm of the surfaceUmbraquults—have an umbric or mollic epipedonEpiaquults—have episaturation Endoaquults—other Aquults

Humults Sombrihumults—sombric horizon within 100 cm of the surfacePlinthohumults—have continuous-phase plinthite in over 50% of some horizon within

150 cm of the surfaceKandihumults—kandic horizon in which relative clay content decreases less than 20%

within 150 cm of the surfaceKanhaplohumults—kandic horizon in which relative clay content decreases more than

20% within 150 cm of the surfacePalehumults—relative clay content in argillic horizon decreases less than 20% within

150 cm of the surfaceHaplohumults—other Humults

Udults Plinthudults—have continuous phase plinthite in over 50% of some horizon within 150 cm of the surface

Fragiudults—fragipan surface within 100 cm of soil surfaceKandiudults—kandic horizon in which relative clay content decreases less than 20%

within 150 cm of the surfaceKanhapludults—kandic horizon in which relative clay content decreases more than

20% within 150 cm of the surfacePaleudults—relative clay content in argillic horizon decreases less than 20% within

150 cm of the surfaceRhodudults—moist color value of epipedon is 3 or less; argillic horizon has a color hue

of 2.5 YR or redder with moist value of 3 or less that changes by no more than 1 unit of color value when dry

Hapludults—other UdultsUstults Plinthustults—have continuous-phase plinthite in over 50% of some horizon within

150 cm of the surface Kandiustults—kandic horizon in which relative clay content decreases less than 20%

within 150 cm of the surface Kanhaplustults—kandic horizon in which relative clay content decreases more than

20% within 150 cm of the surface Paleustults—relative clay content in argillic horizon decreases less than 20% within

150 cm of the surfaceRhodustults—moist color value of epipedon is 3 or less; argillic horizon has a color

hue of 2.5 YR or redder with moist value of 3 or less that changes by no more than 1 unit of color value when dry

Haplustults—other UstultsXerults

Palexerults—relative clay content decreases less than 20% within 150 cm of the surface Haploxerults—other Xerults



acid (felsic) crystalline rock or acidic sediments. Ustic, xeric, udic, and perudic soil moisture regimes are present in various Ultisols depending on rainfall distribution. Aquic conditions related to the presence of shallow groundwater or perched water tables are present in the suborder Aquults.

Most Ultisols support lush forest growth, but low base saturation percentage, acidic conditions, and low contents of phosphorus limit the number of agronomic crops that can be harvested. Continuous agricultural harvests are sustainable only when lime, to overcome acidity and aluminum toxicity, and fertilizer to supply plant essential elements are routinely applied. Slash and burn (shifting cultivation) agricultural systems are common practice where lime and fertilizer are not available. Slash-and-burn agriculture, wherein one to three agronomic crops can be successfully grown immediately after nutrients accumulated during several years by slow-growing forest vegetation, are rapidly released by cutting, drying, and burning the forest vegetation. After agronomic crops are abandoned, slow-growing natural forests usually regenerate, and it takes from 15 to 30 or more years before the natural forest vegetation accumulates enough essential nutrients that it can be cut and burned to again contribute enough ash to fertilize one to three agronomic crops.


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Soil Genesis and Classification (Buol/Soil Genesis and Classification) || Ultisols: Low Base Status Soils with Finer-Textured Subsoil Horizons