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Soil  Finely divided rock-derived material containing an admixture of organic matter and capable of supporting vegetation. Soils are independent natural bodies, each with a unique morphology resulting from a particular combination of climate, living plants and animals, parent rock materials, relief, the ground waters, and age. Soils support plants, occupy large portions of the Earth's surface, and have shape, area, breadth, width, and depth. (However, the term “soil” as used by engineers means unconsolidated rock material.) See also: Pedology Origin and Classification  Soil covers most of the land surface as a continuum. Each soil grades into the rock material below and into other soils at its margins, where changes occur in relief, ground water, vegetation, kinds of rock, or other factors which influence the development of soils. Soils have horizons, or layers, more or less parallel to the surface and differing from those above and below in one or more properties, such as color, texture, structure, consistency, porosity, and reaction (Fig. 1). The horizons may be thick or thin. They may be prominent or so weak that they can be detected only in the laboratory. The succession of horizons is called the soil profile. In general, the boundary of soils with the underlying rock or rock material occurs at depths ranging from 1 to 6 ft (0.3 to 1.8 m), though the extremes lie outside this range.     Fig. 1  Photograph of a soil profile showing horizons. The dark crescent-shaped spots at the soil surface are the result of plowing. The dark horizon lying 9–18 in. (23–45 cm) below the surface is the principal horizon of accumulation of organic matter that has been washed down from the surface. The thin wavy lines were formed in the same manner.     بزرگنمایی تصویر Fig. 2  Hypothetical soil profile having all principal horizons. Other symbols are used to indicate features subordinate to those indicated by capital letters and numbers. The more important of these are as follows: ca, as in Cca, accumulations of carbonates; cs, accumulations of calcium sulfate; cn, concretions; g, strong gleying (reduction of iron in presence of ground water); h, illuvial humus; ir, illuvial iron; m, strong cementation; p, plowing; sa, accumulations of very soluble salts; si, cementation by silica; t, illuvial clay; x, fragipan (a compact zone which is impenetrable by roots).       بزرگنمایی تصویر Fig. 3  Relation of the soil pattern to relief, parent material, and native vegetation on a farm in south-central Iowa. The soil slope gradient is expressed as a percentage. 1 ft = 30 cm. (After R. W. Simonson et al., Understanding Iowa Soils, Brown, 1952)         بزرگنمایی تصویر Fig. 4  General soil map of the world. Each region is identified by the name of the most extensive suborder. Other suborders are present in every region and are important in most of them. (Soil Conservation Service, USDA)   Origin  Soil formation proceeds in stages, but these stages may grade indistinctly from one into another. The first stage is the accumulation of unconsolidated rock fragments, the parent material. Parent material may be accumulated by deposition of rock fragments moved by glaciers, wind, gravity, or water, or it may accumulate more or less in place from physical and chemical weathering of hard rocks.  See also: Weathering processes The second stage is the formation of horizons. This stage may follow or go on simultaneously with the accumulation of parent material. Soil horizons are a result of dominance of one or more processes over others, producing a layer which differs from the layers above and below. Major processes The major processes in soils which promote horizon differentiation are gains, losses, transfers, and transformations of organic matter, soluble salts, carbonates, silicate clay minerals, sesquioxides, and silica. Gains consist normally of additions of organic matter, and of oxygen and water through oxidation and hydration, but in some sites slow continuous additions of new mineral materials take place at the surface or soluble materials are deposited from ground water. Losses are chiefly of materials dissolved or suspended in water percolating through the profile or running off the surface. Transfers of both mineral and organic materials are common in soils. Water moving through the soil picks up materials in solution or suspension. These materials may be deposited in another horizon if the water is withdrawn by plant roots or evaporation, or if the materials are precipitated as a result of differences in pH (degree of acidity), salt concentration, or other conditions in deeper horizons. Other processes tend to offset those that promote horizon differentiation. Mixing of the soil occurs as the result of burrowing by rodents and earthworms, overturning of trees, churning of the soil by frost, or shrinking and swelling. On steep slopes the soil may creep or slide downhill with attendant mixing. Plants may withdraw calcium or other ions from deep horizons and return them to the surface in the leaf litter. Saturation of a horizon with water for long periods makes the iron oxides soluble by reduction from ferric to ferrous forms. The soluble iron can move by diffusion to form hard concretions or splotches of red or brown in a gray matrix. Or if the iron remains, the soil will have shades of blue or green. This process is called gleying, and can be superimposed on any of the others.  See also: Diffusion The kinds of horizons present and the degree of their differentiation, both in composition and in structure, depend on the relative strengths of the processes. In turn, these relative strengths are determined by the way humans use the soil as well as by the natural factors of climate, plants and animals, relief and ground water, and the period of time during which the processes have been operating.  Composition In the drier climates where precipitation is appreciably less than the potential for evaporation and transpiration, horizons of soluble salts, including calcium carbonate and gypsum, are often found at the average depth of water penetration. In humid climates, some materials normally considered insoluble may be gradually removed from the soil or at least from the surface horizons. A part of the removal may be in suspension. The movement of silicate clay minerals is an example. The movement of iron oxides is accelerated by the formation of chelates with the soil organic matter. Silica is removed in appreciable amounts in solution or suspension, though quartz sand is relatively unaffected. In warm humid climates, free iron and aluminum oxides and low-activity silicate clays accumulate in soils, apparently because of low solubility relative to other minerals.  See also: Chelation In cool humid climates, solution losses are evident in such minerals as feldspars. Free sesquioxides tend to be removed from the surface horizons and to accumulate in a lower horizon, but mixing by animals and falling trees may counterbalance the downward movement. Structure  Concurrently with the other processes, distinctive structures are formed in the different horizons. In the surface horizons, where there is a maximum of biotic activity, small animals, roots, and frost action keep mixing the soil material. Aggregates of varying sizes are formed and bound by organic matter, microorganisms, and colloidal material. The aggregates in the immediate surface tend to be loosely packed with many large pores among them. Below this horizon of high biotic activity, the structure is formed chiefly by volume changes due to wetting, drying, freezing, thawing, or shaking of the soil by roots of trees swaying with the wind. Consequently, the sides of any one aggregate, or ped, conform in shape to the sides of adjacent peds. Water moving through the soil usually follows root channels, wormholes, and ped surfaces. Accordingly, materials that are deposited in a horizon commonly coat the peds. In the horizons that have received clay from an overlying horizon, the peds usually have a coating or varnish of clay making the exterior unlike the interior in appearance. Peds formed by moisture or temperature changes normally have the shapes of plates, prisms, or blocks.  Horizons Pedologists have developed sets of symbols to identify the various kinds of horizons commonly found in soils. The nomenclature originated in Russia, where the letters A, B, and C were applied to the main horizons of the black soils of the steppes. The letter A designated the dark surface horizon of maximum organic matter accumulation, C the unaltered parent material, and B the intermediate horizon. The usage of the letters A, B, and C spread to western Europe, where the intermediate or B horizon was a horizon of accumulation of free sesquioxides or silicate clays or both. Thus the idea developed that a B horizon is a horizon of accumulation. Some, however, define a B horizon by position between A and C. Subdivisions of the major horizons have been shown by either numbers or letters, for example, Bt or B2. No internationally accepted set of horizon symbols has been developed. In the United States the designations (Fig. 2) have been widely used since about 1935, with minor modifications made in 1962. Lowercase letters were added to numbers in B horizons to indicate the nature of the material that had accumulated. Generally, “h” is used to indicate translocated humus, “t” for translocated clay, and “ir” for translocated iron oxides. Thus, B2t indicates the main horizon of clay accumulation.  Classification  Systems of soil classification are influenced by concepts prevalent at the time a system is developed. Since ancient times, soil has been considered as the natural medium for plant growth. Under this concept, the earliest classifications were based on relative suitability for different crops, such as rice soils, wheat soils, and vineyard soils. Early American agriculturists thought of soil chiefly as disintegrated rock, and the first comprehensive American classification was based primarily on the nature of the underlying rock. In the latter part of the nineteenth century, some Russian students noted relations between the steppe and black soils and the forest and gray soils. They developed the concept of soils as independent natural bodies formed by the influence of environmental factors operating on parent materials over time. The early Russian classifications grouped soils at the highest level, according to the degree to which they reflected the climate and vegetation. They had classes of Normal, Abnormal, and Transitional soils, which later became known as Zonal, Intrazonal, and Azonal. Within the Normal or Zonal soils, the Russians distinguished climatic and vegetative zones in which the soils had distinctive colors and other properties in common. These formed classes that were called soil types. Because some soils with similar colors had very different properties that were associated with differences in the vegetation, the nature of the vegetation was sometimes considered in addition to the color to form the soil type name, for example, Gray Forest soil and Gray Desert soil. The Russian concepts of soil types were accepted in other countries as quickly as they became known. In the United States, however, the term “soil type” had been used for some decades to indicate differences in soil texture, chiefly texture of the surface horizons; so the Russian soil type was called a Great Soil Group. Many systems of classification have been attempted but none has been found markedly superior; most systems have been modifications of those used in Russia. Two bases for classification have been tried. One basis has been the presumed genesis of the soil; climate and native vegetation were given major emphasis. The other basis has been the observable or measurable properties of the soil. To a considerable extent, of course, these are used in the genetic system to define the great soil groups. The morphologic systems, however, have not used soil genesis as such, but have attempted to use properties that are acquired through soil development. The principal problem in the morphologic systems has been the selection of the properties to be used. Grouping by color, tried in the earliest systems, produces soil groups of unlike genesis. The Soil Survey staff of the U.S. Department of Agriculture (USDA) and the land-grant colleges adopted a different classification scheme in 1965. The system differs from earlier systems in that it may be applied to either cultivated or virgin soils. Previous systems have been based on virgin profiles, and cultivated soils were classified on the presumed characteristics or genesis of the virgin soils. The system has six categories, based on both physical and chemical properties. These categories are the order, suborder, great group, subgroup, family, and series, in decreasing rank. Nomenclature  The names of the taxa or classes in each category are derived from the classic languages in such a manner that the name itself indicates the place of the taxa in the system and usually indicates something of the differentiating properties. The names of the highest category, the order, end in the suffix “sol,” preceded by formative elements that suggest the nature of the order. Thus, Aridisol is the name of an order of soils that is characterized by being dry (Latin arudys, dry, plus sol, soil). A formative element is taken from each order name as the final syllable in the names of all taxa of suborders, great groups, and subgroups in the order. This is the syllable beginning with the vowel that precedes the connecting vowel with “sol.” Thus, for Aridisols, the names of the taxa of lower classes end with the syllable “id,” as in Argid and Orthid (Table 1). Suborder names have two syllables, the first suggesting something of the nature of the suborder and the last identifying the order. The formative element “arg” in Argid (Latin argillus, clay) suggests the horizon of accumulation of clay that defines the suborder. Great group names have one or more syllables to suggest the nature of the horizons and have the suborder name as an ending. Thus great group names have three or more syllables but can be distinguished from order names because they do not end in “sol.” Among the Argids, great groups are Natrargids (Latin natrium, sodium) for soils that have high contents of sodium, and Durargids (Latin durus, hard) for Argids with a hardpan cemented by silica and called a duripan. Subgroup names are binomial. The great group name is preceded by an adjective such as “typic,” which suggests the type or central concept of the great group, or the name of another great group, suborder, or order converted to an adjective to suggest that the soils are transitional between the two taxa. Family names consist of several adjectives that describe the texture (sandy, silty, clayey, and so on), the mineralogy (siliceous, carbonatic, and so on), the temperature regime of the soil (thermic, mesic, frigid, and so on), and occasional other properties that are relevant to the use of the soil. Series names are abstract names, taken from towns or places near where the soil was first identified. Cecil, Tama, and Walla Walla are names of soil series.  Order  In the highest category, 10 orders are recognized. These are distinguished chiefly by differences in kinds and amount of organic matter in the surface horizons, kinds of B horizons resulting from the dominance of various specific processes, evidences of churning through shrinking and swelling, base saturation, and lengths of periods during which the soil is without available moisture. The properties selected to distinguish the orders are reflections of the degree of horizon development and the kinds of horizons present. Suborder  This category narrows the ranges in soil moisture and temperature regimes, kinds of horizons, and composition, according to which of these is most important. Moisture or temperature or soil properties associated with them are used to define suborders of Alfisols, Mollisols, Oxisols, Ultisols, and Vertisols. Kinds of horizons are used for Aridisols, compositions for Histosols and Spodosols, and combinations for Entisols and Inceptisols. Great group  The taxa (classes) in this category group soils that have the same kinds of horizons in the same sequence and have similar moisture and temperature regimes. Exceptions to horizon sequences are made for horizons so near the surface that they are apt to be mixed by plowing or lost rapidly by erosion if plowed.  Subgroup  The great groups are subdivided into subgroups that show the central properties of the great group, intergrade subgroups that show properties of more than one great group, and other subgroups for soils with atypical properties that are not characteristic of any great group.  Family  The families are defined largely on the basis of physical and mineralogic properties of importance to plant growth.  Series  The soil series is a group of soils having horizons similar in differentiating characteristics and arrangement in the soil profile, except for texture of the surface portion, and developed in a particular type of parent material.  Type  This category of earlier systems of classification has been dropped but is mentioned here because it was used for almost 70 years. The soil types within a series differed primarily in the texture of the plow layer or equivalent horizons in unplowed soils. Cecil clay and Cecil fine sandy loam were types within the Cecil series. The texture of the plow layer is still indicated in published soil surveys if it is relevant to the use of the soil, but it is now considered as one kind of soil phase. Classifications of soils have been developed in several countries based on other differentia. The principal classifications have been those of Russia, Germany, France, Canada, Australia, New Zealand, and the United States. Other countries have modified one or the other of these to fit their own conditions. Soil classifications have usually been developed to fit the needs of a government that is concerned with the use of its soils. In this respect soil classification has differed from classifications of other natural objects, such as plants and animals, and there is no international agreement on the subject. Many practical classifications have been developed on the basis of interpretations of the usefulness of soils for specific purposes. An example is the capability classification, which groups soils according to the number of safe alternative uses, risks of damage, and kinds of problems that are encountered under use.  Surveys  Soil surveys include those researches necessary (1) to determine the important characteristics of soils, (2) to classify them into defined series and other units, (3) to establish and map the boundaries between kinds of soil, and (4) to correlate and predict adaptability of soils to various crops, grasses, and trees; behavior and productivity of soils under different management systems; and yields of adapted crops on soils under defined sets of management practices. Although the primary purpose of soil surveys has been to aid in agricultural interpretations, many other purposes have become important, ranging from suburban planning, rural zoning, and highway location, to tax assessment and location of pipelines and radio transmitters. This has happened because the soil properties important to the growth of plants are also important to its engineering uses. Soil surveys were first used in the United States in 1898. Over the years the scale of soil maps has been increased from ½ or 1 in. to the mile (8 or 16 mm to the kilometer) to 3 or 4 in. to the mile (47 to 63 mm to the kilometer) for mapping humid farming regions, and up to 8 in. to the mile (126 mm to the kilometer) for maps in irrigated areas. After the advent of aerial photography, planimetric maps were largely discontinued in favor of aerial photographic mosaics. The United States system has been used, with modifications, in many other countries.  See also: Aerial photography Two kinds of soil maps are made. The common map is a detailed soil map, on which soil boundaries are plotted from direct observations throughout the surveyed area. Reconnaissance soil maps are made by plotting soil boundaries from observations which are made at intervals. The maps show soil and other differences that are of significance for present or foreseeable uses. The units shown on soil maps usually are phases of soil series. The phase is not a category of the classification system. It may be a subdivision of any class of the system according to some feature that is of significance for use and management of the soil, but not in relation to the natural landscape. The presence of loose boulders on the surface of the soil makes little difference in the growth of a forest, but is highly significant if the soil is to be plowed. Phases are most commonly based on slope, erosion, presence of stone or rock, or differences in the rock material below the soil itself. If a legend identifies a phase of a soil series, the soils so designated on a soil map are presumed to lie within the defined range of that phase in the major part of the area involved. Thus, the inclusion of lesser areas of soils having other characteristics is tolerated in the mapping if their presence does not appreciably affect the use of the soil. If there are other soils that do affect the use, inclusions up to 15% of the area are tolerated without being indicated in the name of the soil. If the pattern of occurrence of two or more series is so intricate that it is impossible to show them separately, a soil complex is mapped, and the legend includes the word “complex,” or the names of the series are connected by a hyphen and followed by a textural class name. Thus the phrase Fayette-Dubuque silt loam indicates that the two series occur in one area and that each represents more than 15% of the total area. In places the significance of the difference between series is so slight that the expense of separating them is unwarranted. In such a case the names of the series are connected by a conjunction, for example, Fayette and Downs silt loam. In this kind of mapping unit, the soils may or may not be associated geographically. It is possible to make accurate soil maps only because the nature of the soil changes with alterations in climatic and biotic factors, in relief, and in ground waters, all acting on parent materials over long periods of time. Boundaries between kinds of soil are made where such changes become apparent. On a given farm the kinds of soil usually form a repeating pattern related to the relief (Fig. 3). Because concepts of soil have changed over the years, maps made 50 or more years ago may use the same soil type names as modern maps, but with different meanings.  Soil Suborders  Soil suborders are broad classes at one level in the soil classification system adopted in the United States in 1965. A total of 47 suborders form the full set of classes in the second highest category (each category is a set of classes of parallel rank) in the system. The number of local kinds of soils in a large country is also large. For example, 11,500 soil series have been recognized in soil surveys made in the United States through 1979. On the average, a series consists of six phases, which are the local kinds of soils. This means that approximately 66,000 local kinds have been defined up to the present time in a single large country, though all parts of that country have not been studied.   Despite the myriads of local kinds over the land surface of the Earth, all soils share some characteristics. They can all be related to one another in some way. The relationships are close for some pairs of local kinds and distant for others. The similarities and differences among the thousands of local kinds permit their grouping into sets of progressively broader classes in order to show degrees of kinship. Though it is impossible for a single mind to retain concepts of 11,500 series or 66,000 phases, the salient features of a few dozen broad classes can be remembered. Consequently, the nature of the 47 suborders is described in this article. The purpose is to provide a general picture of the kinds of soils in the United States and the world. The 47 suborders grouped into the 10 orders are:   The brief individual descriptions of the suborders are arranged in the same sequence as the list. Broad regional distribution of soils for the world has been recognized (Fig. 4). Each region outlined on the map is identified by the name of the most extensive suborder among the component soils. In every region, suborders other than the most extensive one are important. Alfisols  These soils have A and E horizons that are mostly pale in color and that have lost silicate clay, sesquioxides, and bases such as calcium and magnesium. The soils have B horizons with accumulations of silicate clay and with moderate to high levels of exchangeable calcium and magnesium. The C horizons are usually lighter in color and lower in clay than the B horizons. Alfisols are most extensive in humid, temperate regions but range from the edges of the tundra and the desert into the tropics. Mostly, the soils were under forest or savanna vegetation, though some were under prairie. All have been formed on land sufaces that are neither old nor yet among the youngest in the world. Occurring as they do in many parts of the world, these soils are used for a wide variety of crops. Some remain in forest, and those under drier climates are used chiefly for grazing. Aqualfs These are the seasonally wet Alfisols. They generally occur in depressions or on rather wide flats in local landscapes. In addition to the general morphology and composition shared with other soils of the order, Aqualfs are marked by gray or mottled colors reflecting their wetness (Fig. 5).     Fig. 5  Profile of an Aqualf with pale A and E horizons about 18 in. (45 cm) thick resting on B horizon high in clay which grades into C horizon at a depth of about 4 ft (1.2 m); numbers on scale indicate feet. 1 ft = 30 cm. (Photograph by R. W. Simonson) Boralfs These are the well-drained Alfisols of cool or cold regions, such as west-central Canada and Russia. The soils occur either at high altitudes or in high latitudes, including some frigid zones. In their morphology and composition, the soils are much like the Udalfs, though colors are more dull on the whole and the surplus of calcium and magnesium a little higher. Udalfs These are the well-drained Alfisols of humid, temperate climates. The soils are important in the north-central part of the United States, in western Europe, and in eastern Asia. Udalfs differ from Aqualfs in that B horizons are characteristically brown or yellowish brown and lack marks of wetness. These have higher mean annual temperatures than do the Boralfs and are moist for higher proportions of the year than the Ustalfs and Xeralfs (Fig. 6). Fig. 6  Profile of a Udalf with A and E horizons 12 in. (30 cm) thick over darker B horizon with blocky structure grading into C horizon at a depth of about 4 ft (1.2 m); larger numbers on scale indicate feet. 1 ft = 30 cm. (Photograph by R. W. Simonson)  Ustalfs These are well-drained Alfisols occurring in somewhat drier and mostly warmer regions than Udalfs. On the whole, the soils have more reddish B horizons and are a little higher in calcium and magnesium than Udalfs. These soils are intermittently dry during the growing season. Xeralfs These are well-drained Alfisols found in regions with rainy winters and dry summers, in what are called mediterranean climates. Like the Ustalfs in nature of B horizons, the soils have A horizons that tend to become massive and hard during the dry season. Some of the soils have duripans [cemented layers at depth of 2 or 3 ft (0.6 or 0.9 m)] that interfere with root growth. Aridisols These are major soils of the world's deserts, which form about one-fourth of the land surface. Soils of other orders, especially the Entisols, are also present but less extensive in the deserts. Formed under low rainfall, Aridisols have been leached little and are therefore high in calcium, magnesium, and other more soluble elements. The low rainfall has also limited growth of plants, mostly shrubs and similar species, so that the soils are low in organic matter and nitrogen. The combined A and B horizons are rarely more and usually less than 1 ft (30 cm) thick. The A horizons are light-colored and usually calcareous. All horizons are neutral or mildly alkaline in reaction. Most Aridisols in use provide some grazing for nomadic herds. On the other hand, if water and other resources, including adequate skills, are available and climate is favorable, some Aridisols will support a large variety and produce high yields of crops. Argids    These well-drained Aridisols have B horizons of silicate clay accumulation. The B horizons are characteristically brown or reddish in color and grade into lighter colored C horizons marked by carbonate accumulation. On the whole, these soils occupy the older land surfaces in desert regions (Fig. 7).     Fig. 7  Profile of an Argid with pale silty A and E horizons, darker B horizon higher in clay, and calcareous C horizon; profile shown in 20 in. (50 cm) deep; numbers on scale indicate inches. 1 in. = 2.5 cm. (Photograph by R. W. Simonson)  Orthids These Aridisols lack B horizons of clay accumulation. Many are free of carbonates in the A and upper B horizons; most are well drained. Common colors are gray or brownish gray with little change from top to bottom of the profile. A few Orthids are fairly high in soluble salts such as sodium sulfate and sodium chloride, whereas others are high in calcium carbonate throughout. More extensive than the Argids, generally, Orthids occupy younger but not the youngest land surfaces in deserts. Entisols These soils have few and faint horizons, with reasons for the limited horizonation differing among suborders. Reasons for the practical absence of horizons are indicated for individual suborders. Entisols occur in all parts of the world and may be found under a wide variety of vegetation. Most, though not all, are on young land surfaces, distributed from the tundra through the tropics and from the deserts to the rainiest climates. Entisols have a wide range in usefulness. Some are highly productive and others are not. Aquents These Entisols have been under water until very recent times at the margins of oceans, lakes, or seas. The wetness is reflected in the bluish-gray or greenish-gray colors. Examples are the soils in recently reclaimed polders of the Netherlands. The total extent of Aquents in the world is very small. Arents These are Entisols because of severe disturbance of soils formerly classifiable in other orders. The sequence of horizons has been disrupted completely, and remnants of those horizons can be found randomly distributed in the profiles of Arents.  Fluvents  These well-drained Entisols are in recently deposited alluvium. They occur along streams or in fans where the rate of sediment deposition is high. Marks of sedimentation are still evident, and identifiable horizons are lacking, except for slightly darkened surface layers or A horizons. Small bodies of these soils are scattered over all parts of the world.  Orthents  These well-drained Entisols are of medium or fine texture, mostly on strong slopes. The grass strips serve to remove sediment from overland flow. The soils may have A horizons or slightly darkened surface layers an inch or so thick but otherwise lack evidence of horizonation. Many of the soils are shallow to bedrock.  Psamments  These Entisols are of sandy texture. Like the Orthents, these may have thin A horizons, which grade into thick C horizons. The sandiness is the distinctive character of the suborder (Fig. 8).  Fig. 8  Profile of a Psamment lacking evident horizons and consisting of sand throughout; numbers on scale indicate feet. 1 ft = 30 cm. (Photograph by R. W. Simonson) Histosols These are wet soils consisting mostly of organic matter, popularly known as peats and mucks. Most have restricted drainage and are saturated with water much of the time. A few are wet but not fully saturated. Widely distributed over the world, these soils may occur in small or large bodies, with the latter occurring chiefly at high latitudes. A large proportion of the total area is idle. Where the climate is favorable, some of the soils have been drained and are producing vegetables and other crops.  See also: Bog; Peat Fibrists  These Histosols consist mainly of recognizable plant residues or sphagnum moss. They are saturated with water most of the year unless drained (Fig. 9).   Fig. 9  Profile of a Fibrist with little or no change from the surface to a depth of 5 ft (1.5 m); soil consists of partly decayed plant residues; numbers on scale indicate feet. 1 ft = 30 cm. (Photograph by R. W. Simonson)  Folists These Histosols consist of forest litter resting on rock or rubble. Drainage is not restricted, but a combination of rainfall, fog, and low temperatures keeps the litter wet.  Hemists  These Histosols consist of partially decayed plant residues. Plant structures have largely been destroyed but an appreciable share of the mass remains as fibers when rubbed vigorously. The soils are saturated with water much of the time unless drained. Saprists  These Histosols consist of residues in which plant structures have been largely obliterated by decay. A very small part of the mass remains as fibers after vigorous rubbing. The soils are saturated with water much of the time unless drained. Most Saprists in the United States are known as muck.  Inceptisols These soils have faint to moderate horizonation but lack horizons of accumulation of translocated substances other than carbonates and silica. Two of the suborders have distinct dark A horizons, and most have B horizons formed by losses and transformations without corresponding gains in substances. Thus, the Inceptisols are in some ways intermediate in horizonation between the Entisols and Vertisols on the one hand, and the Alfisols, Mollisols, Spodosols, and Ultisols on the other. Inceptisols are widely distributed, ranging from the arctic through the tropics and from the margins of the desert into regions of heavy rainfall. They may consequently be found under a wide variety of vegetation. Usefulness of the soils has as wide a range as does their distribution. Some are highly productive and others are of little or no value. Andepts These Inceptisols are formed chiefly in volcanic ash or in regoliths with high components of ash. Mostly, the soils tend to be fluffy. They have thick dark A horizons, rather high levels of acidity, and poorly crystalline clay minerals. The soils are widely distributed but seem to be restricted to regions of fairly recent volcanic activity (Fig. 10).   Fig. 10  Profile of an Andept with thick, dark A horizon, faint B horizon, and lighter C horizon; fine plant roots are numerous; numbers on scale indicate feet. 1 ft = 30 cm. (Photograph by R. W. Simonson)  Aquepts  These Inceptisols are wet or have been drained. Like the Aqualfs, the soils have gray or mottled B and C horizons, but they lack silicate clay accumulation in their profiles. The A horizons may be dark and fairly thick or they may be thin, as they are in many of the soils (Fig. 11).   Fig. 11  Profile of an Aquept with thin, dark A horizons, fairly thick, light-gray B horizon, and stone in C horizon beside tape; numbers on scale indicate feet. 1 ft = 30 cm. (Photograph by R. W. Simonson)  Ochrepts These Inceptisols have pale A horizons, darker B horizons, and lighter colored C horizons. The B horizons lack accumulations of translocated clay, sesquioxides, or humus. The soils are widely distributed, occurring from the margins of the tundra region through the temperate zone but not in the tropics. Ochrepts also occur in the fairly dry regions though not in deserts (Fig. 12).    Fig. 12  Profile of an Ochrept with litter on the surface, dark A horizon 4 in. (10 cm) thick, thin B horizon, and pale C horizon; deeper profile is marked by plant roots and traces of former roots; numbers on scale indicate inches. 1 in. = 2.5 cm. (Photograph by R. W. Simonson) Plaggepts  These Inceptisols have very thick surface horizons of mixed mineral and organic materials added as manure or as human wastes over long periods of time. For the world as a whole, such soils are of negligible extent, but they are conspicuous where found.  Tropepts  These Inceptisols have moderately dark A horizons with modest additions of organic matter, B horizons with brown or reddish colors, and slightly paler C horizons. The soils are less strongly weathered than the geographically associated Ultisols and Oxisols. In general appearance, the profiles are much like that of the Orthox (Fig. 13). Tropepts are restricted to tropical regions, largely to those of moderate and high rainfall.   Fig. 13  Profile of an Orthox with slightly darkened A horizon about 1 ft (30 cm) thick, little further change with depth, and deep penetration by fine plant roots; scale in feet. 1 ft = 30 cm. (Photograph by R. W. Simonson)   Umbrepts These Inceptisols have dark A horizons more than 10 in. (25 cm) thick, brown B horizons, and slightly paler C horizons. The soils are strongly acid, and the silicate clay minerals are crystalline rather than amorphous as in the Andepts. The Umbrepts occur under cool or temperate climates, are widely distributed, and are of modest extent.  Mollisols These soils have dark or very dark, friable, thick A horizons high in humus and bases such as calcium and magnesium. Most have lighter colored or browner B horizons that are less friable and about as thick as the A horizons. All but a few have paler C horizons, many of which are calcareous. Major areas of Mollisols occur in subhumid or semiarid cool and temperate regions. They meet the desert along their drier margins and meet soils such as the Alfisols at their more humid margins. Mollisols were formed under vegetation consisting chiefly of grasses and are thus the major ones of former prairies and steppes. The soils occupy rather young land surfaces. Though they produce a variety of crops, Mollisols are largely used for cereals. These soils now produce a major share of the world's output of corn and wheat. Topography is generally favorable for the operation of large machinery, and many Mollisols are therefore in large farms. Yields have a wide range, depending on climatic conditions. Wide fluctuations in yield with wet and dry years are normal for the Mollisols marginal to arid regions. On the other hand, yields are consistently high for those under more humid climates. Albolls These are Mollisols with dark A horizons, pale E horizons, distinct B horizons marked by clay accumulation, and paler C horizons. The soils are set, especially in the upper part, for some part of the year. Mostly, the soils occur on upland flats and in shallow depressions.  Aquolls These are wet Mollisols unless they have been drained. Because they were formed under wet conditions, the soils have thick or very thick, nearly black A horizons over gray or mottled B and C horizons. If they have not been drained, the soils may be under water for part of the year, but they are seasonally rather than continually wet (Fig. 14).   Fig. 14  Profile of an Aquoll with very thick, dark A horizon, signs of mixing and burrowing by animals at depths between 3 and 3.5 ft (0.9 and 1.1 m), and lighter C horizon at the bottom; the numbers on the scale to the left indicate feet. 1 ft = 30 cm. (Photograph by R. W. Simonson)  Borolls  These are Mollisols of cool and cold regions. Most areas are in moderately high latitudes or at high altitudes. The soils have fairly thick, nearly black A horizons, dark grayish-brown B horizons, and paler C horizons that are commonly calcareous. The B horizons of some soils have accumulations of clay. These soils are extensive in western Canada and Russia.  Rendolls  These are the Mollisols formed in highly calcareous parent materials, regoliths with more than 40% calcium carbonate. The soils may be calcareous to the surface and must have high levels of carbonates within a depth of 20 in. (50 cm). Rendolls do not have horizons of carbonate accumulation. The profiles consist of dark or very dark A horizons grading into pale C horizons. For the most part, Rendolls are restricted to humid, temperate regions.  Udolls  These are Mollisols of humid, temperate and warm regions where maximum rainfall comes during the growing season. The soils have thick, very dark A horizons, brown B horizons, and paler C horizons. Throughout the profile these soils are browner than the Borolls and are not as cold. Udolls lack horizons of accumulation of powdery carbonates. Some of the soils have B horizons of clay accumulation and others do not. These soils are major ones of the Corn Belt of the United States (Fig. 15).    Fig. 15  Profile of a Udoll with thick, dark A horizon, B horizon gradational in color, and rather pale C horizon; filled former animal burrows in B horizon; numbers on scale indicate feet. 1 ft = 30 cm. (Photograph by R. W. Simonson)  Ustolls  These are the Mollisols of temperate and warm climates with lower rainfall than the Udolls. The soils are therefore dry for an appreciable part of each year, usually more than 90 cumulative days. Horizons and their sequence are much the same as for Udolls except that many Ustolls have accumulations of powdery carbonates at depths of 40 in. (100 cm) or less.  Xerolls  These are Mollisols of regions with rainy winters and dry summers. The nature and sequence of horizons are much like those of the Ustolls. The soils are completely dry for a long period during the summer of each year.  Oxisols These soils have faint horizonation, though formed in strongly weathered regoliths. The surface layers or A horizons are usually darkened and moderately thick, but there is little evidence of change in the remainder of the profile. Because of the intense or long weathering, the soils consist of resistant minerals such as kaolinite, forms of sesquioxides, and quartz. Weatherable minerals such as feldspars have largely disappeared. Moreover, the clay fraction has limited capacity to retain bases such as calcium and magnesium. The soils are porous and readily penetrated by water and plant roots. A distinctive feature of Oxisols is the common occurrence of tubular pores about the diameter of ordinary pins extending to depths of 6 ft (1.8 m) or more. Oxisols are largely restricted to low altitudes in humid portions of the tropics. Any occurring elsewhere seem to be relicts of earlier geologic ages. All occupy old land surfaces. Most were formed under forest, with some under savanna vegetation. Regions with Oxisols as major soils are extensive, ranking second in total area only to the Aridisols of deserts. Most Oxisols remain in forest or savanna and produce little food and fiber. Many regions are sparsely inhabited, with natives depending on shifting cultivation for much of their food. A small proportion of the total area is cultivated with modern technology and is highly productive. Even so, management to ensure sustained high yields is still to be developed for the more strongly weathered Oxisols. Aquox These are seasonally wet Oxisols found chiefly in shallow depressions. Because of their wetness, deeper profiles are dominantly gray, with or without mottles and nodules or sheets of iron and aluminum oxides. Total extent is extremely small.  Humox  These are well-drained Oxisols high in organic matter and moist all or nearly all year. Profiles have dark A horizons 1 ft (0.3 m) or so thick over generally reddish B and C horizons. The high amounts of organic matter distinguish Humox from other suborders of Oxisols, and the soils are also moist much more of the year than are Torrox and Ustox. Humox are believed to be of limited extent, restricted to relatively cool climates and high altitudes for Oxisols. Orthox These are well-drained Oxisols moderate to low in organic matter and moist all or nearly all year. Orthox are much like Humox in general appearance, but their profiles are lower in organic matter. They are moist more of each year than are Torrox and Ustox. Although good data on extent are lacking, Orthox are believed to be extensive at low altitudes in the heart of the humid tropics (Fig. 13). Torrox These are well-drained Oxisols low in organic matter and dry most of the year. Profiles resemble those of Orthox except that A horizons are more poorly expressed. The soils are believed to have been formed under more rainy climates of past eras. Total extent of the Torrox seems to be extremely small. Ustox These are well-drained Oxisols low to moderate in organic matter and dry for periods of at least 90 cumulative days each year. Profiles resemble those of Orthox, on the whole. Ustox are lower in organic matter than Humox, dry for longer periods each year than Humox and Orthox, and moist for longer periods than Torrox. Good data on extent are lacking, but Ustox are believed to be extensive.  Spodosols  These soils have B horizons with accumulations of one or both of organic matter and compounds of aluminum and iron. The accumulated substances are amorphous in nature. They impart red, brown, or black colors to the B horizons, which may have irregular lower boundaries with tongues extending downward a foot or more. If the soil has not been disturbed, the surface layer consists of both fresh and partly decayed litter. This rests on a very pale, leached E horizon overlying a highly contrasting B horizon. The Spodosols formed from sands under boreal coniferous forests have some of the most striking profiles in the world. Mostly, the soils are strongly acid because of the small supplies of bases. Spodosols are most extensive in humid, cool climates, but some occur at low elevations under tropical and subtropical climates. The soils were largely under forest. The bulk of the Spodosols have been formed in sandy regoliths, with others in loamy regoliths. Land surfaces are fairly young. Most Spodosols remain in forest, but some are cultivated in both cool and tropical regions. The variety of crops produced is large because of the wide climatic range under which the soils occur. The range in yields is also wide, being dependent on the combination of climatic conditions and prevailing level of technology. Production is modest for most Spodosols under cool climates and simple management. Production is high from some soils cultivated with complex management in tropical climates. Aquods These are seasonally wet Spodosols. The soils may be wet most of each year but not all of the time. The B horizons are black or dark brown in color and some are cemented. Aquods occupy depressional areas or wide flats from which water cannot escape easily (Fig. 16). Fig. 16  Profile of an Aquod with distinct E horizon, dark B horizon at depth of 18 in. (45 cm), pale C horizon below, and part of buried profile below 4 ft (1.2 m); soil consists of sand; numbers on scale indicate feet. 1 ft = 30 cm. (Photograph by R. W. Simonson)  Ferrods  These are well-drained Spodosols having B horizons of iron accumulation with little organic matter. Appearance of the profile is much like that of Orthods.  Humods  These are well-drained Spodosols having B horizons of humus accumulation, usually black or dark brown in color. Aluminum usually accumulates with the humus but iron is lacking, especially from the upper part of B horizons. Where formed in white or nearly white sands, the soils have striking profiles, as in parts of western Europe.  Orthods  These are well-drained Spodosols having B horizons of humus, aluminum, and iron accumulation. The B horizons are mostly red or reddish in color and are friable. They grade downward into lighter-colored C horizons which are commonly less friable and may be very firm. Orthods form the most extensive suborder among the Spodosols, being widespread in Canada and Russia.   Ultisols  Like Alfisols, these soils have A and E horizons that have lost silicate clays, sesquioxides, and bases. Most A and E horizons are pale, though not all are. The B horizons have accumulations of silicate clays and low levels of exchangeable calcium and magnesium. The C horizons are usually lighter in color and lower in clay than the B horizons. Combined thickness of the A, E, and B horizons is greater, on the average, for Ultisols than for Alfisols. Ultisols are strongly acid throughout their profiles, reflecting the low levels of exchangeable bases. Ultisols are most extensive under humid, warm-temperate climates but extend through the tropics. They are not found in cold regions. The largest bodies of the soils are in southeastern Asia, nearby islands, and the southeastern United States. The soils were usually under forest but some were covered by savanna vegetation. All were formed in strongly weathered regoliths on old land surfaces. Many Ultisols remain in forest. Among those producing crops, a majority are used under some method of shifting cultivation. Production is limited in such circumstances. On the other hand, the variety of crops and yields obtained can be large if cultivators are in a position to apply complex technology to Ultisols. Aquults  These are seasonally wet Ultisols, saturated with water for an important part of the year unless drained. Usually the soils have thin, dark A horizons, but they may be as thick as 20 in. (50 cm). Deeper profiles are gray, with or without red mottles. Aquults occur in depressions or on wide upland flats from which water moves very slowly.  Humults  These are well-drained Ultisols formed under rather high rainfall distributed evenly over the year. The soils are high in organic matter throughout their profiles, and most have darkened A horizons of moderate thickness. Deeper profiles tend to be brown, reddish-brown, or yellowish-brown in color. Humults are common in southeastern Brazil.  Udults  These are well-drained Ultisols of humid, warm-temperate and tropical regions. The soils are low or relatively low in humus and typically have thin, darkened A horizons. The B horizons are yellowish-red, red, brown, or yellowish-brown in color and are fairly thick. Rainfall is high enough and distributed evenly enough over the year so that soils are dry for only short periods. Udults are major soils in the southeastern parts of the United States and Asia (Fig. 17), and their total extent is large. Fig. 17  Profile of a Udult with pale A horizon 16 in. (40 cm) thick, darker B horizon higher in clay and iron oxides, and C horizon near bottom; numbers on scale indicate feet. 1 ft = 30 cm. (Photograph by R. W. Simonson)  Ustults  These are well-drained Ultisols of warm-temperate and tropical climates with moderate or low rainfall. The soils are like the Udults in general appearance but are dry for appreciable periods each year. Examples of Ustults may be found in northeastern Australia. Total extent of the suborder is appreciably less than that of Udults. Xerults  These are well-drained Ultisols of regions with warm, dry summers and cool, rainy winters. The soils are like Ustults in appearance but become and remain dry for longer periods in summers. Total extent is small.  Vertisols  These soils have faint horizonation for two main reasons. In the first place, Vertisols are formed in regoliths that are high in clay and therefore resistant to change. In the second place, the clay fraction in the soils has high levels of activity. The soils are therefore subject to marked swelling and shrinking as they wet and dry. Cracks formed as the soils become dry may extend to depths of several feet. Because of the shrinking and swelling of the soils, materials from deeper profiles are forced upward in places so that entire soils are slowly but continually overturned and mixed. Vertisols have therefore been called “self-swallowing” and “soils that plow themselves.” Some Vertisols have darkened A horizons, whereas others do not. All are low in organic matter and high in bases. Many are calcareous in deeper profiles. Most are neutral or mildly alkaline in reaction because of goodly supplies of bases. Vertisols occur in warm-temperate and tropical climates with one or more dry seasons. The soils were under savanna vegetation for the most part with a few in forest. Land surfaces are old or fairly old. Large bodies of Vertisols are found on the Deccan Plateau of India, in the Gezira of Sudan, and in Australia. Because they are high in active clays, Vertisols are hard to cultivate. The soils therefore remain in savanna in many places, and the savannas are used for grazing or left alone. Large areas of the soils are cultivated, some with simple, bullock-drawn implements and others with large machinery. A wide variety of crops are produced, but yields are generally modest, especially for cultivators dependent on simple technology. The soils will, however, produce crops indefinitely under simple management. Torrerts  These are Vertisols of arid regions, the driest soils of the order. Because the soils are dry most of the time, cracks that form tend to remain open. The soils do become wet enough at rare intervals to permit cracks to close. Torrerts are of limited extent.  Uderts  These are Vertisols of humid regions; each profile is moist in some part most of each year. The soils do dry out enough to permit formation of cracks once every year, as a rule. Uderts are moist more of each year than other Vertisols. The soils are of moderate extent.  Usterts  These are Vertisols of subhumid and semiarid regions, chiefly under climates with two rainy and two dry seasons each year. Cracks formed during dry seasons are open for at least 90 cumulative days per year. The Usterts are thus intermediate in moisture regimes between Uderts on the wet side and Torrets on the dry side. Usterts are dry for shorter periods during summer than Xererts. Usterts are extensive, represented by large bodies in Australia and India (Fig. 18).    Fig. 18  Profile of a Ustert with thick, dark A horizon, which is due in part to mixing and churning of soil mass and which grades into the lighter-colored C horizon; scale in feet. 1 ft = 30 cm. (Photograph by E. H. Templin)  Xererts  These are Vertisols of regions with warm, dry summers and cool, rainy winters—the mediterranean climates. Cracks formed as the soils dry out each summer remain open for at least 60 consecutive days. Xererts are higher in moisture than Torrerts and lower than Uderts. They have longer dry periods during warm seasons than Usterts. The soils are of limited extent. Zonality  Many soils that are geographically associated on plains have common properties that are the result of formation in similar climates with similar vegetation. Because climate determines the natural vegetation to a large extent and because climate changes gradually with distance on plains, there are vast zones of uplands on which most soils have many common properties. This was first observed in Russia toward the end of the nineteenth century by V. V. Dokuchaev, the father of modern soil science. He also observed that on floodplains and steep slopes and in wet places the soils commonly lacked some or most of the properties of the upland soils. In mountainous areas, climate and vegetation tend to vary with altitude, and here the Russian students observed that many soils at the same altitude had many common properties. This they called vertical zonality in contrast with the lateral zonality of the soils of plains. Zonal classification  These observations led N. M. Sivirtsev to propose in about 1900 that major kinds of soil could be classified as Zonal if their properties reflected the influence of climate and vegetation, as Azonal if they lacked well-defined horizons, and as Intrazonal if their properties resulted from some local factor such as a shallow ground water or unusual parent material. This concept was not accepted for long in Russia. It was adopted in the United States in 1938 as a basis for classifying soil but was dropped in 1965. This was because the Zonal soils as a class could not be defined in terms of their properties and because they had no common properties that were not shared by some Intrazonal and Azonal soils. It was also learned that many of the properties that had been thought to reflect climate were actually the result of differences in age of the soils and of past climates that differed greatly from those of the present. Zonality of soil distribution is important to students of geography in understanding differences in farming, grazing, and forestry practices in different parts of the world. To a very large extent, zonality is reflected but is not used directly in the soil classification used in the United States. The Entisols include most soils formerly called Azonal. Most of the soils formerly called Intrazonal are included in the orders of Vertisols, Inceptisols, and Histosols and in the aquic suborders such as Aquolls and Aqualfs. Zonal soils are mainly included in the other suborders in this classification. The soil orders and suborders have been defined largely in terms of the common properties that result from soil formation in similar climates with similar vegetation. Because these properties are important to the native vegetation, they have continuing importance to farming, ranching, and forestry. Also, because the properties are common to most of the soils of a given area, it is possible to make small-scale maps that show the distribution of soil orders and suborders with high accuracy.  Zonal properties  A few examples of zonal properties of soils and their relation to soil use follow. The Mollisols, formerly called Chernozemic soils, are rich in plant nutrients. Their natural ability to supply plant nutrients is the highest of any group of soils, but lack of moisture often limits plant growth. Among the Mollisols, the Udolls are associated with a humid climate and are used largely for corn (maize) and soybean production. Borolls have a cool climate and are used for spring wheat, flax, and other early maturing crops. Ustolls have a dry, warm climate and are used largely for winter wheat and sorghum without irrigation. Yields are erratic on these soils. They are moderately high in moist years, but crop failures are common in dry years. The drier Ustolls are used largely for grazing. Xerolls have a rainless summer, and crops must mature on moisture stored in the cool seasons. Xerolls are used largely for wheat and produce consistent yields. The Alfisols, formerly a part of the Podzolic soil group, are lower in plant nutrients than Mollisols, particularly nitrogen and calcium, but supported a permanent agriculture before the development of fertilizers. With the use of modern fertilizers, yields of crops are comparable to those obtained on Mollisols. The Udalfs are largely in intensive cultivation and produce high yields of a wide variety of crops. Boralfs, like Borolls, have short growing seasons but have humid climates. They are used largely for small grains or forestry. Ustalfs are warm and dry for long periods. In the United States they are used for grazing, small grains, and irrigated crops. On other continents they are mostly intensively cultivated during the rainy season. Population density on Ustalfs in Africa is very high except in the areas of the tsetse fly. Xeralfs are used largely for wheat production or grazing because of their dry summers. Ultisols, formerly called Latosolic soils, are warm, intensely leached, and very low in supplies of plant nutrients. Before the use of fertilizers, Ultisols could be farmed for only a few years after clearing and then had to revert to forest for a much longer period to permit the trees to concentrate plant nutrients at the surface in the leaf litter. With the use of fertilizers, Udults produce high yields of cotton, tobacco, maize, and forage. Ustults are dry for long periods but have good moisture supplies during a rainy season, typically during monsoon rains. Forests are deciduous, and cultivation is mostly shifting unless fertilizers are available. Aridisols, formerly called Desertic soils, are high in some plant nutrients, particularly calcium and potassium, but are too dry to cultivate without irrigation. They are used for grazing to some extent, but large areas are idle. Under irrigation some Aridisols are highly productive, but large areas are unsuited to irrigation or lack sources of water. Physical Properties  Physical properties of soil have critical importance to growth of plants and to the stability of cultural structures such as roads and buildings. Such properties commonly are considered to be (1) size and size distribution of primary particles and of secondary particles, or aggregates; (2) the consequent size, distribution, quantity, and continuity of pores; (3) the relative stability of the soil matrix against disruptive forces, both natural and cultural; (4) color and textural properties, which affect absorption and radiation of energy; and (5) the conductivity of the soil for water, gases, and heat. These usually would be considered as fixed properties of the soil matrix, but actually some are not fixed because of influence of water content. The additional property, water content—and its inverse, gas content—ordinarily is transient and is not thought of as a property in the same way as the others. However, water is an important constituent, despite its transient nature, and the degree to which it occupies the pore space generally dominates the dynamic properties of soil. Additionally, the properties listed above suggest a macroscopic homogeneity for soil which it may not necessarily have. In a broad sense, a soil may consist of layers or horizons, each consisting of roughly uniform soil materials of various types, which considered together may affect both mechanical properties and movement of water in a soil profile. From a physical point of view it is primarily the dynamic properties of soil which affect plant growth and the strength of soil beneath roads and buildings. While these depend upon the chemical and mineralogical properties of particles, particle coatings, and other factors discussed above, water content usually is the dominant factor. Water content depends upon flow and retention properties, so that the relationship between water content and retentive forces associated with the matrix becomes a key physical property of a soil.