Introduction Soils are a complex biological, physical, mineralogical, and chemical system that have developed in response to the environment both past and present. Thus, properties of a soil at any point in the landscape can be considered the result of interactions among five factors considered by most scientists to control soil formation (Jenny, 1941). Four of these factors, climate, topography, vegetation, and parent material are active and describe the physical environment to which the soil has been exposed. The fifth, time, allows the other four factors to express themselves on the soil we observe today. Because of the range of conditions observed for these five factors, we have an almost endless diversity of soils, and local distribution of soils across the landscape often seems completely undecipherable to an untrained observer. However, a broader view of soils based on general distribution trends among the five factors, especially the four environmental factors, often yields understandable patterns that may aid in developing better understanding of patterns at the local scale. For the most part, variation in the four environmental factors and, to a lesser extent, time is reflected in the concept of Major Land Resource Areas (MLRAs)(USDA-SCS, 1981). These divisions reflect broad differences in geology, topography, climate, and vegetation across the southern region and the United States. Thus, this chapter is intended to present a general overview of topography, parent materials, and soils for each MLRA in the southern region. The MLRAs included in this chapter are those delineated in Agriculture Handbook 296 (USDA-SCS, 1981). Additional MLRAs have been recognized since this publication, and new ones will continue to be added as our understanding of soils and landscapes continues to develop. Thus, material in this chapter reflects our understanding at a fixed point in time, and will need modification as new, more detailed divisions of the landscape are created.
Coastal Plain
In the Atlantic segment and as far west as central Alabama, the Coastal Plain is bounded by metamorphic rocks of the Piedmont. Along much of this boundary, the Coastal Plain laps onto the Piedmont for short distances. This irregular and often ill-defined boundary is referred to as the fall line because the Piedmont is topographically higher than the younger Coastal Plain (Daniels et al., 1973). West and north of central Alabama, Cretaceous and Tertiary sediments of the Coastal Plain abut Paleozoic sedimentary rocks, and along the Mississippi River valley, Coastal Plain sediments are commonly loess capped. In Arkansas, Oklahoma, and east Texas, the interior boundary of the Coastal Plain is the contact between Cretaceous and older rocks. However, farther south in Texas, only Upper Cretaceous and younger sediments are included in the Coastal Plain with Lower Cretaceous rocks assigned to other geomorphic units (Thornbury, 1965).
As would be expected for such a large region, it can be subdivided into many smaller subareas. Sharp lines seldom separate one subarea from another. The section of the Atlantic Coastal Plain from Virginia to the Neuse River in North Carolina is often referred to as the "Embayed Section." This area is composed of a series of Miocene to Holocene terraces from the fall line to the coast, and rivers end in large estuaries (Thornbury, 1965; Daniels et al., 1973). South of the Neuse River, the coastwise terraces continue into the "Sea Island Section." In this section, however, the drowned river valleys of the Embayed Section disappear and a series of barrier islands are found along the coast (Daniels et al., 1973). The interior margin of this section is composed of older Eocene and Cretaceous sandy sediments forming a belt of rolling hills referred to as the Fall Line Hills or Sand Hills (MLRA 137; Carolina and Georgia Sand Hills). South of the Sea Island Section is the Florida Peninsula which consists of an upland core surrounded by irregular belts of plains and coastal lowlands (Murray, 1961).
The boundary between the Atlantic and East Gulf Coastal Plain subareas is poorly defined topographically. However, a short distance west of the Georgia-South Carolina boundary the number and thickness of Eocene and Cretaceous beds increase resulting in a widening of the Coastal Plain. Coupled with the increase in number of Eocene and Cretaceous beds is an increase in the lithologic variability of the beds producing substantial variation in the erodibility of the rocks. Therefore, the East Gulf Coastal Plain is comprised of a series of lowlands formed in erodible marls and clays separated by more resistant beds forming cuestas with in-facing escarpments (Thornbury, 1965). The alternating lowlands and areas with greater topographic relief give the region a belted topography. The coastwise stepped terraces that comprise much of the Atlantic Coastal Plain become topographically important near the Gulf, but these terraces are only about 30 to 80 km wide (Daniels et al., 1973).
The West Gulf Coastal Plain lies west of the Mississippi River and has many characteristics in common with the East Gulf Coastal Plain. The region is also a belted Coastal Plain composed of several topographic and geologic belts similar to those found in the East Gulf Coastal Plain but having different names. In general, the West Gulf Coastal Plain is wider than the East Gulf Coastal Plain, and consequently, major rivers have larger drainage basins and deltas that are more extensive. Low terraces along the coast are formed from fluvial-deltaic sediments derived from these major rivers (Thornbury, 1965). The Coastal Plain as a broad physiographic province includes a number of MLRAs. The remainder of this section is a brief discussion of topography, parent materials, and soils characteristic of each.
Southern Coastal Plain (MLRA 133A) This MLRA is the largest in the southern region extending from Virginia to the Mississippi River Valley. It is well dissected with nearly level to gently undulating valleys and gently sloping to steep uplands. Soil parent materials are unconsolidated sands, silts, and clays, and textural characteristics of the soils generally reflect the differences in parent material texture. The combination of parent material, climate, and age of geomorphic surface has resulted in most soils in this MLRA being well developed Ultisols that have a clay increase between A or E horizons and subjacent argillic or kandic horizons. Thickness of sand or loamy sand A and E horizons ranges from a few centimeters to more than 2 m (Quartzipsamments). Most upland soils are acidic, deep, well or moderately well drained, and have dominantly kaolinitic clays. Plinthite is a common feature in subsoils.
Soils in the Atlantic and eastern part of the Gulf portion of this MLRA (east of about Montgomery, Alabama) dominantly have kandic subsurface horizons (horizon with clay increase and low-activity clays; CEC at pH 7 <16 cmol kg-1 clay and ECEC <12 cmol kg-1 clay) in addition to argillic horizons. Thus, the dominant great groups in this part of the MLRA are Kandiudults or Kanhapludults. Kaolinite is also the major component of soils in the western part of the MLRA, but minor amounts of other, more active clay minerals increase activity of the clay fraction. Therefore, soils in this part of the MLRA have only argillic horizons and classify as Paleudults or Hapludults.
Carolina and Georgia Sand Hills (MLRA 137) This region occurs along the Coastal Plain - Piedmont boundary from the Neuse River in North Carolina to the western border of Georgia. This MLRA is underlain by Cretaceous and Tertiary deposits with abundant sand. The area is dissected and rolling to hilly. Soils are similar to those found in MLRA 133A and are deep, well drained, acidic, and have argillic and kandic horizons. Great groups are dominantly Kandiudults and Kanhapludults. The abundance of sand in this MLRA results in many of the Ultisols being in arenic and grossarenic subgroups. If sands are thicker than 2 m, the soils classify as Entisols (Quartzipsamments).
Western Coastal Plain (MLRA 133B) Similarities in climate and geology between this MLRA and the western part of the Southern Coastal Plain have resulted in similar soils and landscapes. Dominant great groups include Paleudults, Hapludults, and Fragiudults. Arenic and Grossarenic subgroups are common. Alfisols are more common than in MLRA 133A, especially in more poorly drained parts of the landscape. Rhodic subgroups occur occasionally over sediments rich in glauconite.
Texas Blackland Prairie (MLRA 86) and Texas Claypan Area (MLRA 87) South and west of MLRA 133B, the belted Coastal Plain has been subdivided into these two MLRAs. In the region encompassed by these MLRAs, rainfall amounts are less than that found in the Coastal Plain to the east, and upland soils are in ustic rather than udic moisture regimes.
The western belt of the Texas Blackland Prairie is underlain by Upper Cretaceous chalks and marls while the more eastern belts are underlain by younger clayey sediments. Landscapes in this MLRA are mostly nearly level to gently sloping, and major rivers crossing the region have broad, shallow valleys. Soils are dominantly clayey, often calcareous, and smectite is the dominant clay mineral. Thus, Vertisols are the most common order with Haplusterts being the major great group. Areas of more resistant chalk deposits occur near the inner margin of the Coastal Plain. These areas are more rolling and soils are dominantly Haplustolls and Ustorthents.
The Texas Claypan Area is a nearly level to gently sloping plain. Steeper slopes occur along entrenched creek valleys. Valleys of large streams are broad and shallow and have wide flood plains bordered by nearly level terraces. Vertisols are common in these valleys. Upland soils commonly have sandy loam surface horizons underlain by clayey or loamy argillic horizons. They have high base saturation and clays are dominantly smectitic. Thus, clayey argillic horizons have high shrink-swell and slow saturated hydraulic conductivity. Paleustalfs are the major great group.
Rio Grande Plains and Valley (MLRAs 83A, 83B, 83C, and 83D) To the south and west of MLRAs 86 and 87 lies this broad region influenced by the current and past Rio Grande River and the Rio Grande Embayment. Sediments include marine and deltaic deposits, eolian sands, and Rio Grande River fluvial deposits. Landscapes in this region are mostly nearly level to gently undulating. There is a strong rainfall gradient from the coast to the western limit of these MLRAs. Thus, soils in both ustic and aridic moisture regimes are found in the region. Soils are commonly calcareous, and the abundance of smectitic clays results in high cation exchange capacities.
In the northern section of the region (MLRA 83A), soils are deep and moderately coarse to coarse textured. Major great groups include Paleustalfs, Argiustolls, Calciustolls, and Haplustepts. Petrocalcic horizons are common in certain landscapes. MLRA 83B (Western Rio Grande Plain) contains deep, fine-textured soils that are commonly saline. Great groups common in the region include Haplusterts, Haplustolls, Calciustolls, and Haplocalcids. Soils in MLRA 83C (Central Rio Grande Plain) are deep and moderately coarse to coarse textured. Common great groups include Paleustalfs, Haplustalfs, and Calciustolls. In parts of the MLRA with a thick capping of eolian sand, Arenic and Grossarenic subgroups of Paleustalfs are common as are Ustipsamments (sands > 2 m deep). The Lower Rio Grande Valley (MLRA 83D) is comprised of the floodplain and terraces of the Rio Grande River. Much of the area is nearly level, and drainageways are shallow with low gradient. Saline soils are common. Dominant great groups include Paleustalfs, Haplustalfs, Calciustolls, and Haplusterts.
Alabama, Mississippi, and Arkansas Blackland Prairie (MLRA 135) Outcroppings of Cretaceous clays, marls, and chalks similar to those found in the Texas Blackland Prairies are found in the Coastal Plain of Arkansas, Mississippi, Alabama, and western Georgia. These outcroppings have resulted in a narrow discontinuous belt of clayey soils comprising this MLRA. Relief is generally low. Surface horizons are often acidic, but pH and base saturation increase with depth, and many soils in the region are calcareous in lower horizons. In contrast to surrounding areas on the Coastal Plain, soils in this MLRA have clay fractions dominated by smectite that result in high shrink-swell and very slow saturated hydraulic conductivity. Vertisols, Mollisols, and Alfisols are common. Major great groups include Dystruderts, Epiaquerts, Dystraquerts, Hapluderts, Paleudalfs, and Eutrudepts.
Lower Atlantic and Gulf Coastal Plains
Atlantic Coast Flatwoods (MLRA 153A), Tidewater Area (MLRA 153B), and Eastern Gulf Coast Flatwoods (MLRA 152A) Soils in these MLRAs are mostly acidic with low base saturation. Subsoil textures range from sand to clay and many soils have a clay increase between sandy A and E horizons and underlying argillic horizons. Many of the sandy soils have subsoil accumulations of Al and organic C (spodic horizons). Major great groups include Alorthods, Alaquods, Paleaquults, Quartzipsamments, Umbraquults, and Paleudults. Histosols are common in large swampy areas and closed depressions.
Western Gulf Coast Flatwoods (MLRA 152B) Soils in this region are deep and have high base saturation. Subsoil textures are loamy and clayey and most soils have an argillic horizon. Many areas have seasonal water tables at or near the surface. Common great groups include Glossaqualfs, Epiaqualfs, Albaqualfs, Paleudalfs, and Paleudults.
Gulf Coast Marsh (MLRA 151) This MLRA is found only in Louisiana. Elevations are commonly less than 2 m except for beach ridges and salt dome islands. The low areas are subject to frequent flooding either by fresh water from adjoining uplands or by salt water from the Gulf of Mexico. Soils are poorly or very poorly drained. Histosols are common, as are Haplaquolls and Fluvaquents.
Gulf Coast Prairies (MLRA 150A) This region is nearly level with low local relief. Clayey sediments with smectitic mineralogy are common resulting in soils with slow or very slow hydraulic conductivity. Because of low conductivity and low relief, most soils are somewhat poorly or poorly drained. Base saturation is high, as is cation exchange capacity (CEC). Common great groups include Hapluderts, Dystraquerts, Endoaquolls, Epiaqualfs, and Albaqualfs.
Gulf Coast Saline Prairies (MLRA 150B) Elevations in this MLRA are commonly less than 3 m. The area is composed of nearly level to gently sloping lowlands and island flats. Soils on barrier islands are commonly Psammaquents and Udipsamments. Great groups common on the mainland include Endoaquolls, Haplaquolls, and Albaqualfs. Hapluderts occur over alkaline clayey sediments.
Peninsular Florida
South-central Florida Ridge (MLRA 154) This region is nearly level to gently rolling coastal plain with a sandy mantle overlying limestone. The land surface is irregular with many sinkholes forming closed depressions. Thickness of the sand mantle ranges from less than 50 cm to more than 2 m. Sandy surface horizons overlie loamy argillic horizons. Soils are acidic, kaolinitic, and have low base saturation. Upland soils are well or moderately well drained. Soils in depressions and on low landscape positions are poorly or very-poorly drained. Great groups include Paleudults, Paleaquults, Endoaquults, and Quartzipsamments.
Southern Florida Flatwoods (MLRA 155) This MLRA is a nearly level coastal plain with a sand mantle overlying limestone. Local relief is generally less than 1 m, and swamps, marshes, and lakes are common. Sand thickness ranges from less than 50 cm to more than 2 m, and most soils are somewhat poorly or poorly drained. Most soils are acidic and kaolinitic, and many have spodic or argillic horizons. A few poorly drained soils with limestone parent materials have dark surface horizons, high base saturation, and are Mollisols. Common great groups in the area are Alaquods, Psammaquents, Paleaquults, and Argiaquolls.
Florida Everglades and Associated Areas (MLRA 156A) As the name implies, this region is a level low coastal plain. It is mostly flat with many swamps, marshes, and poorly defined broad streams. Most of the soils are poorly or very-poorly drained. Histosols are common, as are wet sandy soils. Common great groups include Haplosaprists, Haplofibrists, Psammaquents, and Endoaqualfs. Sulfihemists, Sulfisaprists, and Sulfaquents are found along the coast.
Southern Florida Lowlands (MLRA 156B) This MLRA consists of broad flat lowlands with a sand mantle over loamy sediments. Surface horizons are sandy and overlie sandy C horizons or loamy argillic horizons. Most soils are poorly or very poorly drained. Soils in the region include Endoaqualfs, Argiaquolls, Psammaquents, Haplaquolls, and Alaquods.
Piedmont
There are two minor areas within the Piedmont with geology and thus, soils that are considerably different than the rest of the region. The first is a series of down-faulted basins containing sedimentary sandstone, siltstone, mudstone, and shale of Triassic and Jurassic age (Hack, 1989). The second is generally referred to as the Carolina Slate Belt which is an area of low relief containing Cambrian and Precambrian low-grade metamorphic rocks of volcanic origin (slate, acidic and basic tuff, breccia, and flows) (Hack, 1989).
The Piedmont is generally well dissected with commonly accordant summits constituting a broad plateau-like surface. Maximum relief in the region is generally <350 m though higher isolated peaks occur (Murray, 1961). The region slopes to the east and south with general surface slope being about 4 m km -1 . The lack of topographic expression on the interfluves has been used as evidence that the Piedmont is a peneplain (Thornbury, 1965; Holmes, 1964). An alternate hypothesis is that this surface is the result of long-term weathering and uplift in the region (Pavich, 1985; 1986).
Most soils are well drained and at least moderately permeable. Surface horizons are commonly sandy loam with clayey or loamy subsoils. Soils developed over felsic saprolite are acidic, have low base saturation, have kandic horizons (low activity clays), and are dominantly Ultisols. Dominant great groups include Kanhapludults and Hapludults. Soils developed from intermediate and mafic saprolite have high base saturation and clays with higher activity. Thus, many of these soils are Alfisols with mixed or smectitic mineralogy although Ultisols are also common. Common great groups are Hapludalfs and Hapludults. High iron content in the mafic parent material has resulted in many soils being in Rhodic subgroups.
Within the Carolina Slate Belt, interfluves are irregular, and sharp topographic breaks are common. Deep soils generally occupy more gently sloping parts of the region, and shallow soils occur on convex parts of the landscape (Daniels et al., 1984). The fine grain size of the rocks in this region results in soils with higher silt and very fine sand contents than the rest of the Piedmont. Surface textures are generally silt loam and Bt horizon textures range from silty clay loam to clay. As in other Piedmont soils, the soils are acidic, have low base saturation, and the dominant clay mineral in most of these soils is kaolinite. Soils in the Carolina Slate Belt are dominantly Hapludults.
The Triassic Basin is topographically lower than the Piedmont landscapes that surround it, and local relief is generally less than most of the Piedmont. The Triassic rocks include shale, dark and light colored sandstone, mudstone, siltstone, and conglomerate. These rock types are easier to erode than the surrounding crystalline rocks, which accounts for the topographic low nature of the basin (Daniels et al., 1984). Most soils in this region are moderately permeable, well drained, and have dominantly kaolinitic clays. The dominant great group in the region is Hapludults.
Blue Ridge (MLRA 130)
Mountains in this MLRA are often described as subdued as compared with those in the western United States (Thornbury, 1965). Peaks are generally rounded, and a mantle of saprolite commonly occurs over harder rocks forming the core of the mountains (Daniels et al., 1973). Bare cliffs and peaks are rare. Soil creep is common on steep slopes, and colluvium is a common parent material on lower hillslope segments and in narrow valleys (Stolt et al., 1993).
Parent materials in this MLRA are dominantly acid igneous and metamorphic rocks. Soils are commonly deep, well drained, and acidic. Base saturation is generally low, and kaolinite commonly is the major clay mineral in these soils. Because of free movement of water, soils in the Blue Ridge are often strongly desilicated, and gibbsite contents are often high even in moderately developed soils (Calvert et al., 1980; Norfleet and Smith, 1989).
Ultisols (Kanhapludults and Hapludults) are common on gentle slopes at lower elevations. Inceptisols (Dystrudepts) are the most common soils on steep slopes. At high elevations (>900 to 1000 m) especially on north and northeast facing slopes, Humic Dystrudepts with thick, dark surface horizons are common. Alfisols and Inceptisols with high base saturation are commonly found overlying isolated areas of mafic rocks in the region.
Southern Appalachian Ridges and Valleys (MLRA 128)
Most soils in this MLRA are well drained, acidic, and have low base saturation. Soils on stable positions on ridges and in valleys have argillic horizons and are dominantly Hapludults and Paleudults. Soils on steep slopes are commonly Dystrudepts. Soils over cherty limestone are often gravelly or stony.
Cumberland Plateau and Mountains (MLRA 125) and Sand Mountain (MLRA 129)
Interior Low Plateaus
The interior part of the region is an undulating to rolling plateau with an extensively dissected rim surrounding a less dissected interior (Schoeneberger, 1996). Sinkholes are common in many areas underlain by limestone. The western and northern parts of the area are capped by loess deposited from the Mississippi and Ohio Rivers.
Kentucky and Indiana Sandstone and Shale Hills and Valleys (MLRA 120) This MLRA is hilly with broad undulating ridgetops and broad, nearly-level terraces along the Ohio and other major rivers. Parent materials are loess and loess over residuum from sandstone, shale, and siltstone. Alfisols are abundant. Fragiudalfs and Hapludalfs are dominant on upland ridges and sideslopes. Hapludolls, Eutrudepts, Fluvaquents, and Endoaquepts are common on floodplains of major streams.
Kentucky Bluegrass (MLRA 121) Topography ranges from highly dissected hills to broad undulating upland plains. Upland parent materials are Ordovician and Devonian limestone. Sinkholes occur but are less common than in the Highland Rim and Pennyroyal (Thornbury, 1965). Soils are dominantly Alfisols (Paleudalfs and Hapludalfs) formed from limestone from thinly interbedded limestone, shale, and siltstone, or from loess over residuum.
Highland Rim and Pennyroyal (MLRA 122) This area is topographically diverse with low rolling hills, upland flats, and narrow valleys. Steep slopes are common along borders with the Nashville Basin (MLRA 123) and Southern Coastal Plain (MLRA 133A). Parent material is dominantly Pennsylvanian limestone, and many areas are pitted with limestone sinks. The northern part of the MLRA has thin loess over residuum. Soils are commonly deep and many are acidic with low base saturation, especially in middle and southern parts of the MLRA. Common great groups in uplands include Paleudults, Hapludults, and Fragiudults. Fragiaquults are common in shallow depressions. Paleudalfs are common on broad smooth loess-capped areas in the northern part of the MLRA.
Nashville Basin (MLRA 123) Ordovician, Silurian, and Devonian limestone is the dominant soil parent material. The outer part of the basin is deeply dissected and consists of steep slopes from narrow valleys to narrow rolling ridgetops. The inner part of the basin is undulating to rolling hills with common limestone sinks and limestone outcrops. Limestone in the inner part of the basin is low in P. However, limestone in the outer part has up to 23% P2 O5, and soils are generally higher in P than other soils in the southeast (Thornbury, 1965; Edwards et al., 1974). Paleudalfs and Hapludalfs are dominant great groups in both the inner and outer basins. Skeletal Hapludults occur on steep slopes on the rim of the basin.
Southern Mississippi Alluvial Valley
Much of the valley is occupied by alluvium, but upland ridges break the continuity of the valley floor. Crowley’s Ridge is the most prominent of these. It extends for about 300 km from southeast Missouri to east central Arkansas and rises as much as 70 m above the valley floor. This ridge is composed of Eocene Coastal Plain deposits capped by Pliocene gravel which is capped by up to 20 m of loess (Thornbury, 1965; West et al., 1980). South of Crowley’s Ridge in extreme southeast Arkansas and northeast Louisiana is Macon Ridge. This ridge is about 7 to 13 m higher than the valley floor and is composed of Pleistocene alluvium capped by loess (Daniels et al., 1973).
On the west side of the Mississippi River valley in the lowlands between Crowley’s Ridge and the Ozark Highlands is a series of Pleistocene and Holocene alluvial terraces formed by the ancestral Mississippi River before its diversion to the lowlands east of Crowley’s Ridge (Saucier, 1974). The oldest and highest terraces are capped by loess deposits while the lower terraces are primarily alluvium (Rutledge et al., 1985; West and Rutledge, 1987). The Grand Prairie of Arkansas also occurs west of the Mississippi River and is composed of Mississippi and Arkansas River sediments capped by loess.
Southern Mississippi Valley Alluvium (MLRA 131) Characteristics of alluvial soils in this MLRA depend on age, depositional environment within the river floodplain, and natural drainage. Natural levees in the modern Mississippi River floodplain are up to 2 km wide with correspondingly wide backswamps and abandoned meander loops. Surface texture of soils on natural levees and low terraces are silt loam to sandy loam, and most soils have an argillic horizon and associated subsoil clay increase. Soil drainage is related to its position on the levee and ranges from well to poorly drained. Dominant great groups are Hapludalfs, Hapludolls, Endoaqualfs, and Natraqualfs. Soils in backswamp areas are normally clayey and poorly drained. Common great groups include Haplaquerts, Dystraquerts, and Endoaquepts although Histosols occur in abandoned meander loops, especially in the southern part of the valley.
Southern Mississippi Valley Silty Uplands (MLRA 134) Soils in this MLRA vary depending on loess thickness, physiography, landscape position, and natural drainage. In areas of thick loess near the Mississippi River, soils are commonly Hapludalfs and Fragiudalfs. On the east side of the valley at a distance from the Mississippi River, soils with thin loess over Coastal Plain sediments are common and classify in Hapludalfs, Fragiudalfs, and Hapludults great groups. Loess capped terraces west of the Mississippi River generally have low relief, and soils are Hapludalfs, Fragiudalfs, Glossaqualfs, Albaqualfs, and Endoaqualfs.
Ozark Plateau (MLRAs 116A and 117)
Soils in the Ozark Highland (MLRA 116A) are primarily developed from limestone. Most are deep and well or moderately- well drained, and many have abundant chert (loamy-skeletal and clayey-skeletal particle size classes) inherited from the underlying rock. Textures are silty, and fragipans are common even in soils with high chert contents. Dominant great groups include Paleudults, Paleudalfs, and Fragiudults.
The Boston Mountains (MLRA 117) lie south of the Ozark Highland. Parent materials are Pennsylvanian sandstone and shale. Ridgetops are more narrow and rolling than those in the Ozark Highland. Soils shallow to sandstone or shale are common as are those with abundant rock fragments. Dominant great groups include Hapludults and Paleudults.
Arkansas Valley and Ridges (MLRA 118)
The Arkansas valley forms an east to west strip 30 to 60 km wide along the Arkansas River. This region is underlain by Pennsylvanian sandstone and shale and has less intense folding and lower relief than the Ouachita Mountains. Ridges are underlain by resistant sandstone, and valleys are underlain by less resistant sandstone and shale. Ridge crests are narrow with steep sideslopes into broad valleys. Hapludults and Paleudults are common on ridgetops and sideslopes while Fragiudults are often found in valleys. The floodplain and terraces of the Arkansas River are composed of Quaternary alluvial deposits, and soils are dominantly Hapludalfs and Argiudolls.
The Ouachita Mountains are composed of intensely folded, structurally complex, east-west trending mountain groups separated by basins. Rocks comprising these mountains are Ordovician to Pennsylvanian sandstone, quartzite, chert, shale, and slate. Topography is generally controlled by rock folding (synclines and anticlines), and consequently, the area has a trellis drainage pattern. Local relief ranges up to 350 m. In general, ridges are underlain by sandstone, quartzite, or chert, and valleys are underlain by shale or slate. Soils on ridgetops and sideslopes are mostly well drained moderately deep Hapludults. Shallow Dystrudepts occur on steep slopes.
Central Lowlands
Central Rolling Red Plains (MLRA 78) This area is underlain predominately by gently dipping Permian-aged, weakly-consolidated sandstone, siltstone, and shale. The area has broad gently rolling interfluves with steep slopes into narrow valleys. In places, eolian sands form a rolling dunal topography bordering valleys. Soils in the region are Argiustolls, Paleustolls, Haplustalfs, Paleustalfs, Haplustepts, and Haplusterts with mixed mineralogy.
Central Rolling Red Prairies (MLRA 80A) Rocks in this region are Pennsylvanian and Permian limestone, sandstone, and shale. Topography in the area is dominantly broad undulating divides and broad valleys partially filled with local alluvium. Soils in the area include Argiustolls, Paleustolls, and Haplustolls.
Texas North Central Prairies (MLRA 80B) This region is topographically and stratigraphically similar to the Central Rolling Red Prairies. However, isolated mesas capped by consolidated limestone or sandstone are scattered through the region. Soils on broad divides and valleys are Haplusterts, Haplustolls, Paleustalfs, and Calciustolls. Soils on mesa tops are generally shallow to rock and include Calciustolls, Haplustepts, and Argiustolls.
Cross Timbers (MLRA 84A) This MLRA is underlain by Pennsylvanian sandstone and sandy shale. The topography is dominantly rolling to hilly uplands with narrow stream valleys. Soils in the region have loamy sand or sandy loam surface horizons underlain by loamy or clayey subsoils. Dominant great groups are Haplustalfs, Paleustalfs, and Paleustults.
West Cross Timbers (MLRA 84B) Rocks in this region are chiefly Cretaceous and Permian weakly-consolidated sandstone. Topography in this area is nearly level to rolling with narrow stream valleys. Soils have loamy sand and sandy loam surface horizons, loamy or clayey argillic horizons, and are mostly Paleustalfs and Haplustalfs. A few areas of Quartzipsamments are found in areas of thick sand.
East Cross Timbers (MLRA 84C) This area is composed of gently sloping to rolling uplands with narrow stream valleys. Rocks are mostly Cretaceous and Tertiary weakly-consolidated sandstone. Soils have sandy loam surfaces underlain by loamy subsoils. Common great groups include Paleustalfs, Haplustalfs, Haplustults, and Quartzipsamments.
Grand Prairie (MLRA 85) The Grand Prairie is dominated by flat-topped mesas with steep sideslopes into broad valleys. Rocks are Cretaceous-aged weakly and strongly consolidated limestone. Mesas are capped by strongly consolidated limestone and variable consolidation of deeper limestone strata gives the mesa sideslopes a characteristic benched topography with varying soil depth (West et al., 1988). The broad valleys are gently undulating to rolling, and soils include Haplusterts, Haplustolls, Calciustolls, and Argiustolls. Soils on mesas tops and sideslopes are commonly shallow to limestone and many are stony. Great groups include Haplustolls, Calciustolls, Argiustolls, and Haplustepts.
Cherokee Prairies (MLRA 112) This MLRA is gently rolling to hilly with local relief mostly less than 30 m. Topographic highs are commonly east facing cuestas formed by the regional westward dip to the strata and a few low sandstone capped buttes. Rocks are Pennsylvanian limestone, shale, and limy clay. Soils over most of the region are Albaqualfs, Argiaquolls, Argiudolls, Paleudolls, and Hapludolls.
Within this MLRA lie the Wichita and Arbuckle Mountains. The Wichita Mountains rise about 150 to 400 m above the surrounding plains and are composed of Precambrian and Cambrian igneous rocks (Thornbury, 1965). Soils in these mountains are generally stony and many are shallow to rock. Great groups include Argiustolls and Haplustolls. The Arbuckle Mountains are generally lower than the Wichita Mountains and are composed of a core of Precambrian granite surrounded by Cambrian to Pennsylvanian limestone and shale (Thornbury, 1965). Soils are commonly shallow to rock and include Haplustalfs and Haplustolls.
Great Plains
Covering a large portion of the Southern High Plains are Pleistocene eolian sands deposited from the southwest which server as the parent material for most soils in the MLRA. Only in the southwest portion of the region is a dunal topography present, and thus, the eolian sand capping is generally referred to as "cover sands" (Daniels et al., 1973).
A distinctive feature of the region is numerous shallow closed depressions or playas. Larger playas contain water most of the time, but smaller ones are only filled with water after heavy rains. The origin of the playas is unclear, but wind deflation appears to be a contributing mechanism as dunes are commonly present on the east sides of the depressions (Daniels et al., 1973).
Soils on the broad interfluves in this MLRA are well drained and include Paleustolls, Paleustalfs, Haplustalfs, and Calciustolls. In playas, soils are often poorly drained and clayey. They include Epiaquerts and Haplusterts with Haplustolls and Calciustepts occurring on the rim of the playa. Soils on steep breaks into stream valleys are commonly shallow and include Haplocalcids and Torriorthents.
Edwards Plateau (MLRA 81) This region is an extensive tableland capped by resistant level-bedded Cretaceous limestone. Valleys are broad and partially filled with local alluvium. Mesa tops are nearly level, and valleys are gently undulating to rolling. Mesa sideslopes have a characteristic benched topography with varying soil depth due to differential consolidation of limestone strata out-cropping on the sideslopes. Soils on mesa tops are generally shallow to limestone, often stony, and include Calciustolls, Argiustolls, Haplustolls, and Haplustepts. In the western part of the region, rainfall is low, and soils on mesas are Aridisols (Haplocambids and Petrocalcids). Soils in valleys are deeper and are commonly Haplusterts, Haplustolls, or Calciustolls.
Central Texas Basin (MLRA 82) This area was formed as a dome of intruded igneous rock that, because overlying sediments have been eroded away, is now a topographic low area. The central part of the region is composed of Precambrian granite and schist that is circled by a ring of Cambrian sandstone (Thornbury, 1965). Most of the area is gently rolling, but steeper slopes occur. Soils are commonly Paleustalfs, Haplustalfs, and Haplustepts.
Summary The diversity of soils in the Southern Region is great with nine of the 12 soil orders found in the region. This diversity, however, only reflects the diversity in the five factors that control formation and properties of the soil at any point on the landscape. Climate ranges from warm humid in Florida to cool semi-arid in the Texas High Plains. Local relief ranges from more than 300 m in the Blue Ridge Mountains to less than 1 m in the Texas High Plains and in coastal areas. Both forest and grassland native vegetation occurs in the region with considerable variation in species within each. Soil parent materials cover the full range from peat to thick eolian sands to strongly consolidated rocks with corresponding differences in texture and composition. Age of soils ranges from very young along active floodplains to very old in the Piedmont and other geomorphically stable parts of the region.
Soils vary across a large region such as the South, but they also vary at the local scale. This local variability is a function of the same factors that created the regional variability, and this variability can be understood if the environment under which the soil developed can be understood. Only with this understanding can we hope to preserve the non-renewable soil resource for future generations.
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