FLORA OF NORTH AMERICA - Volume 1, Chapter 2


CHAPTER 2
The Soils of North America

Donald Steila

SOIL is a medium wherein plants are anchored and from which they draw water and mineral nutrients. Reciprocally, plants protect the soil from erosion, they increase its moisture-holding capacity via the incorporation of organic matter, and they recycle solid elements important to sustain biomass production. Soils are derived from complex interactions of geologic, biotic, and climatic factors, acting over time.

There are many kinds of soil in North America, and soil scientists group them among several categories called "orders." The geographic distribution of these soil orders is correlated to regional vegetation types and to climate, and it has been described by several biogeographers (W.E. Akin 1991; P.W. Birkeland 1984; S.W. Buol et al. 1980; S.R. Eyre 1971; D.Steila and T.E. Pond 1989).

Soil classification provides a framework whereby the relationships among the kinds of soils may be compared and patterns among them studied. As with most fields of science, a specialized nomenclatural scheme has been developed, but the details of its applications are not universally accepted. The soil classification scheme used here follows Soil Taxonomy, prepared by the U.S. Soil Survey Staff (1975). This is a hierarchical model that employs six ranks of soil taxa: order, suborder, great group, subgroup, family, and series. Soils are grouped within a given taxon on the basis of sharing certain features. The order represents the greatest degree of generalization, and 10 orders are recognized here, each with an identifying name ending in sol (Latin solum, soil). These are listed in table 2.1 with their approximate equivalents in the soils classification scheme of Canada (Canada Soil Survey Committee 1978).


TABLE 2.1. Soil Orders Name Derivation and Canadian Taxonomy Equivalents

U.S. Soil Orders Derivation of Root Word Canadian Soil Orders *
Entisols (Artificial syllable) Regosolic, some gleysolic
Vertisols Latin: verto, turn upward Some chernozemic
Inceptisols Latin: inceptum, inceptive Brunisolic, some gleysolic and podzolic
Aridisols Latin: aridus, dry Some solonetzic
Mollisols Latin: mollis, soft Chernozemic, some brunizolic, gleysolic, and solonetzic
Spodosols Greek: spodes, wood ash Podzolic
Alfisols (Artificial syllable) Luvisolic, some gleysolic and solonetzic
Ultisols Latin: ultimus, ultimate No equivalents
Oxisols Greek: oxy, sharp, acid No equivalents
Histosols Greek: histos, tissue Organic soils


*There are general equivalents, as the definitions of horizons and classes differ between the U.S. and Canadian systems.

At the order level, the position of each of the mineral soils is based largely on the degree of weathering, i.e, the degree of mineral alteration and profile development (fig.2.1). The continental distribution of the soil orders is illustrated in figure 2.2.

As a result of macroscale soil formation, or pedogenesis, regional homogeneity occurs in diagnostic soil characteristics. A detailed examination of figure 2.2 reveals a broad pattern in the distribution of soil orders that can be readily associated with relatively extensive climatic and vegetation realms (see L. Brouillet and R.D. Whetstone, chap. 1; M.G. Barbour and N.L. Christensen, chap. 5). Yet each order includes hundreds of subtypes, each of which has attributes associated with its unique environment.

Soil orders are differentiated into suborders on the basis of chemical and/or physical properties related to drainage conditions, or major parent material, because they possess genetic differences due to climate and vegetation. Suborders are separated into great groups on the basis of the kinds and array of diagnostic horizons, base status, and regimes of soil temperature and moisture. The great groups are divided into subgroup categories that indicate the extent to which the control concept of the great group is expressed. The criteria for recognition of families stress features of importance to plant growth, such as texture, mineralogy, reaction class, and soil temperature. The individual soils are called series and are distinguished on the basis of the kinds and textures of horizons and on their chemical and mineralogical properties. They are named after a natural feature or place where the soil was first recognized as distinct.

Clearly, the soils at the series level are myriad and cannot be adequately treated within the scope of this chapter. Therefore, the remainder of this essay is directed toward the soils of North America at the order level.

North American Soils

Entisols

The entisols are true soils and should not be confused with geologic materials incapable of supporting plant life. An unstabilized sand dune, for example, does not constitute soil, but rather a mineral deposit. Once stabilized and supportive of vegetation, however, it represents a recent soil. The key criterion for entisols is that they lack diagnostic horizons or features that would place them in another order. In effect, soil-forming processes either have not been operative on the parent material for a sufficient period of time, or some aspect of the physical environment (e.g., wind erosion) has prevented development of traits characteristic of mature soils within the bioclimatic regime of their location. Typically they lack an E- or B-horizon and have a profile sequence of an A-horizon either overlying a C-horizon or resting on unaltered geologic material (see fig. 2.3 for soil profile and explanation of horizons). They may be found in any moisture or temperature regime, on any type of parent material, and under widely varying vegetative covers. Thus their geographic distribution is exceedingly diverse. The best means to understand the rationale for their location is by referring to figure 2.2, as each of the five suborders is described below.

Aquents (from the Latin aqua, water) are the wet entisols. They are commonly found in tidal marshes, in deltas, on margins of lakes where the soil is continuously saturated with water, in flood plains of streams where soil is saturated at some time of the year, or in very wet, sandy deposits. These soils are bluish or gray, and mottled. Temperature does not restrict the occurrence of aquents, except in areas constantly below freezing. The soil moisture regime is primarily a reducing one, virtually free of dissolved oxygen due to lengthy saturation by ground water. Aquents are generally found in recent, usually water-deposited sediments. They may support any form of vegetation that will tolerate prolonged periods of waterlogging.

Arents (from the Latin arare, to plow) are entisols that lack horizons, normally because of human interference. They have been deeply mixed by plowing, spading, or moving. Some of these soils are the result of deliberate human attempts either to modify soil or to break up and remove restrictive pans; in other instances, they result from cut and fill operations intended to reshape a surface. An uncommon few are formed from the natural effect of mass movement in earth slides. Arents are not extensively developed, and their characteristics vary from place to place.

Fluvents (from the Latin fluvius, river) are brownish to reddish entisols. They have formed in recent water-deposited sediments---primarily flood plains, fans, and deltas of rivers and small streams, but not in back swamps where drainage is poor. The age of sediments in which these soils form is usually very young, only a few years or decades. Under normal conditions fluvents are flooded frequently, and deposited materials show signs of stratification---layers of a given texture alternated with layers of other textures. Most alluvial sediments, coming from eroding surfaces or stream banks, include appreciable amounts of organic carbon that are dominantly associated with the clay fraction. These soils do not occur under any specific vegetation type and may be found in any moisture or thermal regime, except those that are subject to temperatures constantly below freezing.

Orthents (from the Greek orthos, true) are entisols occurring primarily on recently eroded surfaces created by geologic factors or by cultivation. The basic requirement for recognition of an orthent is that any former existing soil has been either completely removed or so truncated that the diagnostic horizons typical of all orders other than entisols are absent. Such formations normally support only scattered plants. If they do not support plants, they are considered to be rock rather than soil. A few orthents occur in recent loamy or fine wind-deposited sediments, in glacial deposits, in debris from recent landslides and mudflows, and in recent sandy alluvium.

Psamments (from the Greek psammos, sand) are sandy entisols that lack pedogenic horizons. They include sandy dunes, cover sands, and sandy parent material produced in an earlier geologic cycle. Others are found in water-sorted sands, like those along natural levees or beaches. Occurring under any climate or vegetation, they may be located on surfaces of virtually any age. The older psamments are usually dominated by quartz sand and cannot form subsurface diagnostic horizons.

Vertisols

By definition these soils must be at least 50 cm deep, have 30% or more smectite clay in all horizons, and have cracks at least 1 cm wide at a depth of 50 cm during part of the year. Indeed, the cracks that occur during the dry season are their most conspicuous feature. Frequently the cracks extend deeper than 1 m and are very wide.

Vertisols are mostly found in regions with distinct wet-dry seasons. They are of minor extent in North America, being found mainly in east central and southeastern Texas, west central Alabama, and east central Mississippi.

Their development takes place on alkaline parent materials wherein clays form that have high shrink-swell properties. When the soil dries, clay shrinkage produces the characteristic deep cracks, which are frequently arranged in polygonal patterns. During this time, surface soil material collects in the cracks by several processes: deposition of wind-borne materials, surface-washing and dislodgment by rainfall, and grazing of animals. When wetted, the clays expand and the cracks close, trapping the displaced particles below the surface. This extra volume at lower levels prevents the expanding clay from occupying its former space, thus exerting pressure on adjacent soil particles that forces them to move laterally and upward. Consequently, lower soil material moves upward, toward the surface. This continuous cycle of overturning has earned these soils the reputation of being self-plowing or inverted, hence the name vertisols. The entire landscape of a vertisol habitat may become mottled with a mosaic of micromounds and depressions, referred to as "gilgai topography."

Inceptisols

Similar to the entisols, inceptisols may be found in almost any bioclimatic regime. Some are weathered sufficiently to form slightly altered horizons, yet they normally lack a well-developed B- or E-horizon. In brief, inceptisols exhibit traits associated with their soil-forming environment, but the features are too weakly developed for them to be considered mature soils. Generally they bear close relationship to their parent material.

The primary difference between entisols and inceptisols is that the direction of soil development is evident in the latter, suggesting that it is possible to predict their development into a mature form, such as an alfisol or ultisol. Typically they have altered horizons that have been somewhat leached of either bases or iron and aluminum. At the same time, they usually lack subsurface horizons that have been enriched with silicate clays. The diversity of inceptisols creates problems with attempts to generalize their geographic distribution. It is safe to state only that they may be found almost anywhere and simply represent an intergradational state between the immature soil of regionally dominant entisols and the mature soil of the dominant bioclimatic regime.

Aridisols

North America's aridisols are found primarily west of longitude 100° W in areas of desert where water is lacking for long periods. During the time when these soils are warm enough for plant growth, the climate is moisture-deficient. Aridisols have surface horizons that normally are light in color and low in organic matter. They also may contain pedogenic horizons derived from the translocation and accumulation of carbonates, salts, or silicate clays, or that have formed by carbonate or silica cementation.

Aridisols are associated with climatic regimes that have meager precipitation and relatively high potential evapotranspiration, meaning that certain types of chemical weathering are limited. Hence excessive leaching of soluble minerals is prevented. Although many sets of soil-forming processes predominate in arid regimes, two dominate: calcification and salinization. The former is extensive within moisture-deficient climates, and the latter occurs in more localized sites.

Calcification is the process by which calcium carbonate or calcium and magnesium carbonate accumulate in the soil. The primary genetic factor has been associated with limited rainfall (S.W. Buol et al. 1980; D.Steila and T.E. Pond 1989). In some desert areas, these processes are sufficiently active to cement the carbonates into a solid rocklike mass, commonly called "caliche" in the Southwest.

Salinization leads to the accumulation of mineral salts in the soil in sufficient concentration to limit plant production to halophytes. This occurs in localized areas of entrapped surface drainage. Such conditions are common in valley basins with no exterior drainage (bolsons), where rainfall can become imponded. Water descending from surrounding valley slopes assimilates soluble minerals and transports them into the basin, where an intermittent lake may be formed during precipitation events. Between periods of precipitation, the entrapped water evaporates, leaving behind a residue of mineral salts encrusted on the surface and/or precipitated in the soil. The principal salts are cations of calcium, magnesium, and sodium, and the chloride and sulfate anions. Also occurring, in smaller quantities, are potassium, bicarbonates, and nitrate ions, and, even less commonly, borates. Borates have received considerable attention because of their high toxicity to plants.

The aridisols have a sparse growth of plants. The organic matter within the surface soil is low, as are carbon/nitrogen ratios and microbial populations. Barren areas are not uncommon, and plant life is dominated by annuals and by xerophytic perennials.

Mollisols

Mollisols are relatively fertile, dark-colored, humus-rich soils with a soft surface horizon. They are thought to be formed primarily by the underground decomposition of organic matter, related to the rooting and life cycle of grasses. They occur in regions transitional between the arid and humid climates of North America. Natural vegetation ranges from sparse shortgrass prairie along the arid margins to a rich, luxuriant tallgrass prairie along the humid boundaries.

Grasslands differ from forests in both the total amount of organic matter incorporated within the soil and in its distribution throughout the profile. In forested areas, the major source of humus is leaf-fall that accumulates on the surface. Grasses also produce an organic mat as their decaying aerial parts accumulate. In contrast to the roots of trees, however, the dense, fibrous masses of grass roots permeate the soil profile. As the plants die and decay continuously, a large quantity of humus is incorporated within the soil.

In general, grasses require greater quantities of basic mineral nutrients, particularly calcium, than do trees. As a result, their organic remains are richer in base nutrients, which upon decay are returned to the soil. The released bases are available for use by the plants in a continuous cycle that under natural conditions maintains a relatively high degree of soil fertility. The less soluble calcium salts have a tendency to accumulate in lower horizons because normal rainfall, which carries them down in solution, reaches only a few feet into the subsoil; they may later become solidified into hard nodules.

Spodosols

The spodosols of North America are sporadically found in scattered parts of the Atlantic Coastal Plain and in high mountainous areas, both east and west, but they are most extensively developed around the Great Lakes region, eastern Canada, and the northeastern United States. Typically they are associated with coniferous vegetation or heath plants. The dominant climatic regime in which they occur is cool and humid (humid continental and subarctic realms).

The classic spodosol profile is an ashy gray sand overlying a dark sandy loam; an abrupt boundary separates the horizons. The soil reaction is acidic, the parent material is quartzose sand, and a spodic horizon is present. Spodic horizons are illuvial subsurface horizons in which amorphous materials composed of organic matter, aluminum, and/or iron have accumulated.

In regions of spodosol occurrence, surplus precipitation infiltrates organic litter to form organic acids. The acid solution percolates through the soil, leaching lime and/or salts to allow clays to disperse. The dispersion (called peptization) speeds the movement of clays from the surface horizon. As a consequence, in time, the soil develops stratas of different texture and degree of mineral alteration. The surface horizon is depleted of fine-sized particles, leaving a coarse texture through which organic colloids may be transported downward. Calcium and magnesium are leached from the topsoil, resulting in a horizon dominated by acidic, silicate minerals. Simultaneously, aluminum and/or iron may be liberated from their mineral bonds and migrate downward from the surface horizon. Subsequently these precipitate in the subsoil to form the spodic horizon.

Alfisols

Alfisols contain significantly more clay in the B-horizon than in the A-horizon and have a base-saturation greater than 35%, indicating that they are relatively fertile. In addition, most have a pale-colored surface horizon. They form in many bioclimatic regimes, but they are most extensive in humid and subhumid temperate climates. They occur widely on lands where deciduous forest is present or where it formerly occurred, on some prairie lands, on calcareous glacial drift, and within loess deposits.

The high clay content of the alfisol's B-horizon is attributed to substantial translocation of clay downward from the soil's upper A- and E-horizons. This soil feature is referred to as an argillic horizon. Clays migrate downward rather than being synthesized in situ.

Ultisols

Ultisols are deeply weathered, acidic soils that are confined largely to the southeast and eastern seaboard of the United States. They are found on old surfaces that lie south of the advance of the last glaciation. The climatic regime is one of both relatively high precipitation and high summertime potential evapotranspiration. In many respects, ultisols are morphologically similar to alfisols. Processes leading to the formation of a clay-enriched subsurface horizon are common to both. And both can be found in adjacent landscapes within a uniform climatic regime, but on different parent materials. The distinct difference between the two is that the alfisols have moderate to high fertility, whereas the ultisols are less fertile and are acidic for a considerable depth, particularly when the parent materials are deficient in bases, as are siliceous crystalline rocks, for example.

Ultisols are considered the most weathered of all midlatitude soils, hence their name. The key to understanding their traits lies in the extremely long period they have been subjected to chemical and physical weathering and their occurrence in a climatic regime propitious to rapid disintegration and decay of parent material. The soils are deep and leached of soluble bases and frequently have colors associated with oxidation of their iron and magnesium compounds. Consequently, reddish, orange, and mahogany colors indicate free drainage, ample oxidation, and the presence of residual iron (red soil) and magnesium (mahogany soil). Other ultisols have yellow to gray colors that reflect poorer drainage conditions. The yellow results from hydration of iron whereas the gray colors occur under reducing conditions in water-saturated soils.

Oxisols

Oxisols are soils found on ancient landscapes within the humid tropics and consequently are not represented within the flora area. These are deep, thoroughly weathered soils, with few remaining, weatherable minerals and containing a horizon of hydrated oxides of iron and/or aluminum along with kaolinite clays. They are mentioned here solely to provide a complete discussion of all soil orders.

Histosols

Unlike the other soil orders, the histosols are not considered primarily mineral but organic. These soils are commonly called bog, moor, peat, or muck. They are rather extensively developed along the southern margins of Hudson Bay, in the Northwest Territory, and occur in smaller areas in Minnesota within the Gulf and Eastern coastal plains. Isolated pockets can, however, occur within any bioclimatic regime, as long as water is available. They occur in the tundra and on the equator, and their vegetation consists of a wide variety of water-tolerant plants.

The histosols contain organic materials that are either more than 12% to 18% organic carbon by weight or well over half organic matter by volume. The presence of water is the common denominator in all histosols, regardless of location. Unless drained, most histosols are saturated or nearly saturated most of the year.

In addition to water, the controlling factors that regulate the accumulation of organic matter include the temperature regime, the character of the organic debris, the degree of microbial activity, and the length of time in which organic accumulation has taken place. The deposition of organic material within a water-saturated environment rapidly leads to the depletion of oxygen in the wet zone through decomposition by aerobic microorganisms. This eventually results in an anaerobic milieu, in which the rate of organic matter mineralization is considerably reduced. As a consequence, organic materials accumulate. The development of these soils is, therefore, characterized by a deepening of the organic layer.

The major distinction among the suborders of histosols relates to the degree of decomposition of organic remains and the soil's moisture. There are four suborders: fibrists, folists, hemists, and saprists.

Fibrists (from the Latin fibra, fiber) are histosols of relatively unaltered plant remains. The reasons for the preservation of plant remains vary, but the absence of oxygen is probably the most important factor. The rate of mineralization varies according to the thermal regime and the nature of the vegetative debris.

Folists (from the Latin folia, leaf) are rather freely drained histosols consisting of an organic horizon derived from leaf litter, twigs, and branches (but not sphagnum), resting on rock or on fragmental materials made of gravel, stones, and boulders, with the interstices filled or partially filled with organic materials. There is very little evidence of mineral soil development, and plant roots are restricted to the organic materials.

Hemists (from the Greek hemi, half) are histosols in which the organic material is strongly, but not completely, decomposed. Enough, however, has been broken down to the point that its biologic origin cannot be determined. These soils are normally found where the ground water is at or very near the surface most of the year. The groundwater levels can fluctuate, but they seldom drop more than a few centimeters below the surface tier. These soils were once classified as bog. They are usually found in closed depressions and in broad, very poorly drained flat areas such as coastal plains.

Saprists (from the Greek sapros, rotten) consist almost entirely of decomposed plant remains. Their color is usually black. The botanic origin of the materials is, for the most part, obscure. They occur in areas where the groundwater level fluctuates within the soil, and they are subject to aerobic decomposition.

ACKNOWLEDGMENTS

I am indebted to Drs. Tyrel G. Moore and James F. Matthews of the University of North Carolina at Charlotte for their helpful comments on this manuscript, and to Andrew C. Nunnally for his aid in drafting the Soils Map of North America.