6. The United States Soil Classification System and Its Application in Arizona
Classification is fundamentally important to any science. Not only is it a means to impose order on diversity between and within objects and concepts, but classification also provides the avenue through which research can be addressed in a rigorously systematic manner. Classifications also have more practical applications. Classification of soils, for instance, is indispensible to the soil survey program of mapping the soils of Arizona. Soil surveys, in turn, can be used to apply the principle functions of soil science to agriculture, forestry and engineering to predict soil behavior under defined use and management or manipulation.
The soil classification system now used in the United States was developed by USDA Soil Conservation Service Soil Survey staff between 1951 and 1975. The system, published as Soil Taxonomy (Soil Survey Staff, 1975), was designed to classify all the world's soils because expanding soil survey programs demanded more precise definitions of soil properties than were possible with previous soil classification systems. An improved conceptual frame of reference also was needed so that research data could be more readily communicated, tested and applied between soils of one area to soils of another area where conditions of soil formation or genesis were similar (Simonson, 1962; Smith, 1963; Aandahl, 1965).
The soil classification system described in Soil Taxonomy (Soil Survey Staff, 1975) focuses on soil properties that for the most part can be measured quantitatively rather than on soil-forming processes or factors. Yet, the system certainly can not exclude soil genesis since many soil properties quantified have considerable significance in soil genesis. General objectives of the new system are to make the characteristics of various soils easier to remember, to make clearer the relationships among soils and between soils and other elements of the environment, and to provide a basis for developing principles of soil genesis and soil behavior that have prediction value.
The classification system as described in Soil Taxonomy (Soil Survey Staff, 1975) is too extensive to relate wholly in this publication. Nonetheless, the rest of the text in this chapter introduces the reader to a few of the basic elements of the system and describes its application in Arizona.
Soil classes in Soil Taxonomy (Soil Survey Staff, 1975) are defined by properties that can be measured quantitatively. Some properties used to classify soils are soil depth, moisture, temperature, texture, structure, cation exchange capacity, base saturation, clay mineralogy, organic matter content and salt content.
Certain soil horizons referred to as diagnostic horizons are the primary building blocks of the Soil Taxonomy (Soil Survey Staff, 1975) system. The diagnostic horizons that commonly are found in Arizona soils are listed in Table 6. Presence or absence of certain diagnostic horizons, which can be attributed to conditions of soil formation, is an important2 Although the processes by which diagnostic horizons formed are not always understood, the properties possessed by them are those that are significant to the behavior and management of soils.
|mollic epipedon||Surface horizon of accumulation of organic matter; dark colored|
|ochric epipedon||Surface horizon of limited accumulation of organic matter; light colored|
|argillic||Horizon of clay accumulation|
|natric||Horizona of clay and sodium accumulation|
|cambic||Horizon of pedogenic alteration usually expressed by soil structure or removal of calcium carbonate|
|calcic||Horizon of pronounced carbonate accumulation|
|gypsic||Horizon of pronounced gypsum accumulation|
|salic||Horizon of pronounced soluble salt accumulation|
|petrocalcic||Hard indurated horizon with calcium carbonate as the dominant cementing material|
|duripan||Hard indurated horizon with silica as the dominant cementing material|
|albic||Light colored horizon in which clay and organic matter have been significantly removed by leaching|
Epipedons are diagnostic horizons that have formed at the surface and occur nowhere else in the soil, unless the soil is buried under fluvial, aeolian or volcanic deposits. They are defined mostly in terms of soil color, content of organic matter and base saturation; i.e., relative amounts of bases such as calcium, magnesium, sodium and potassium as compared with hydrogen. Of the six epipedons currently recognized in the United States, only two are important in Arizona: the mollic and ochric. The mollic epipedon generally is darker and higher in organic matter than the ochric epipedon.
The remaining nine diagnostic horizons form below the soil surface, although they may become exposed if surface horizons are removed. For the most part, subsurface diagnostic horizons develop from materials leached and accumulated from upper horizons, or they exhibit special features that are useful in differentiating soils. Argillic horizons, for example, are accumulations of clay derived from overlying horizons. Cambic horizons, on the other hand, are only slightly altered by soil-forming processes from the parent material.
Soil Taxonomy (Soil Survey Staff, 1975) has six categories as illustrated in Figure 39. These are, from top to bottom, order, suborder, great group, subgroup, family and series. The highest categories have the fewest classes and criteria separating classes, while the lowest categories have the most classes and criteria. The ‘‘soil type,’’ a soil series subdivision, used in previous classification systems is not a category in Soil Taxonomy (Soil Survey Staff, 1975), but is a phase within a mapping unit.
Ten classes are in the order level. Criteria used to differentiate orders are highly generalized and based more or less on the kinds and degrees of soil-forming processes. Mostly these criteria include properties that reflect major differences in the genesis of soils.
A suborder category is a subdivision of an order within which genetic homogeneity is emphasized. Soil characteristics used to distinguish suborders within an order vary from order to order. For example, soil moisture and temperature are the important factors that differentiate the suborders in the order Alfisols. The presence or absence of an argillic horizon, on the other hand, distinguishes the two suborders of the order Aridisols. Forty-seven suborders are recognized in the United States.
The great group category is a subdivision of a suborder. They are distinguished one from another by kind and sequence of soil horizons. All soils belonging to one of the suborders of Aridisols have argillic horizons. They also may have additional diagnostic horizons such as a petrocalcic as well as several others. Soils having these additional horizons are placed in separate great groups. About 185 great groups are recognized in the United States.
Great group categories are divided into three kinds of subgroups: typic, intergrade and extragrade. A typic subgroup represents the basic concept of the great group from which it derives. An intergrade subgroup contains soils of one great group, but have some properties characteristic of soils in another great group or class. These properties are not developed or expressed well enough to include the soils within the great group toward which they grade. Extragrade subgroup soils have aberrant properties that do not intergrade to any known soil. There are about 1,000 kinds of subgroups in the United States.
A soil family category is a group of soils within a subgroup that has similar physical and chemical properties that affect response to management and manipulation. The principal characteristics used to differentiate soil families are texture, mineralogy and temperature. Family textural classes, in general, distinguish between clayey, loamy and sandy soils. For some soils the criteria also specify the amount of silt, sand and coarse fragments such as gravel, cobbles and rocks.
The soil series is the narrowest category in Soil Taxonomy (Soil Survey Staff, 1975). Its general concept essentially is the same as the soil classification system that Soil Taxonomy (Soil Survey Staff, 1975) superceded (Simonson, 1964). Some series have been redefined, some subdivided and some newly established. These changes have produced more precise definitions of soil series and narrower ranges of properties used in those definitions. And more specific statements can be made about soils than were possible in the past. This, in turn, enhances the value of research and permits more effective application of existing knowledge to use of soil resources because information is more easily and accurately communicated. More than 14,000 soil series are recognized in the United States.
When a person first encounters Soil Taxonomy (Soil Survey Staff, 1975), one of the features most startling is the nomenclature. It is not unusual for such a person to throw up his or her hands in despair at the seemingly incoherent, barbaric and meaningless terms. Some critics of the system reserve their most vehement criticism of Soil Taxonomy (Soil Survey Staff, 1975) for its nomenclature. However, once a person becomes familiar with the construction of the nomenclature, he or she soon recognizes the utility and the advantages of the system.
The nomenclature was developed so that each class had a name that was mnemonic, that is, to help memory, and that would connote some properties of the soils of each class. The name also places a class in the system so that a person can recognize both the category of the class and the classes of the higher categories to which it belongs. Incidentally the system of nomenclature was developed primarily not by soil scientists but by classical linguists, Professors J.L. Heller, University of Illinois, and A.L. Leemans, State University of Ghent, Belgium.
Class names were coined from Greek and Latin roots, for the most part, that are familiar because of their use in many common words. Order names end in ‘‘sols’’ (Table 7). A formative element from each order is the ending of names of the suborders, great groups and subgroups. The order formative elements are in Table 8 (See also Appendix D).
Names of suborders have two syllables. The last syllable is the formative element from the order. The first syllable is the suborder formative element that suggests certain characteristics about the soil. A few examples and meanings of suborder formative elements are in Table 9. One subgroup is the Argids. These are Aridisols, dry soils, with argillic horizons (arg + id = Argids).
FIGURE 39. The Hierarchical Soil Classification System
Continuing in the same manner, great group names are formed simply by putting a great group formative element in front of the suborder name. The prefix is connotative and suggests the unique features of the particular great group named. The great group Durargids, for example, include Aridisols with argillic horizons and duripans (dur + Argid = Durargids).
Names of subgroups are binomial. Each binomial has an adjective before the name of the great group to which the subgroup belongs. The term ‘‘Typic’’ is added to a great group name to form the subgroup name for soils that are typical or modal of the great group. Thus, Typic Durargids is the name of the modal subgroup of the Durargids great group. An intergrade subgroup also carries the name of its great group but it is modified by the adjectival form of the name of the class or classes towards which it grades. The adjective is formed by adding ‘‘ic’’ to the class name. For example, Durargids that have some properties of the Haploxerolls great group belong to the Haploxerollic Durargid subgroup. Extragrade subgroup names are formed by adding to the great group name a formative element that describes the extragradational feature. Abruptic Durargids, for example, include soils that have properties of Durargids but have, in addition, an abrupt transition between the A and B horizons characterized by a large difference in percentage of clay.
Family names are fairly common terms that denote in more detail the features of a particular subgroup. A fine, montmorillonitic, thermic soil family is one that is fine textured, contains montmorillonite as the dominant mineral in the clay and has a warm, or thermic mean annual soil temperature.
Soil series are abstract names sometimes taken from some local geographic feature near the site where the series was first established. Examples include the Gila, Graham, Mohave, Tubac and Moenkopie series. Other names are variants of these place names and still others simply are coined.
|Aridisols||Argids||Durargids||Typic Durargids||fine-loamy, mixed, hyperthermic||Suncity|
|Alfisols||Mineral soils relatively low in organic matter with relatively high base saturation. Contains horizon of illuvial clay. Moisture is available to mature a crop.|
|Aridisols||Mineral soils relatively low in organic matter. Contain developed soil horizons. Moisture is inadequate to mature a crop without irrigation in most years.|
|Entisols||Mineral soils lacking developed soil horizons. Moisture content varies.|
|Histosols||Soils composed mostly of organic matter. Moisture content varies.|
|Inceptisols||Mineral soils containing some developed horizons other than one of illuvial clay. Moisture is available to mature a crop.|
|Mollisols||Mineral soils with thick, dark surface horizons relatively high in organic matter and with high base saturation.|
|Oxisols||Mineral soils with no weatherable minerals. High in iron and aluminum oxides. Contain no illuvial horizons.|
|Spodosols||Soils that contain an illuvial horizon of amorphous aluminum and organic matter, with or without amorphous iron. Usually moist or well leached.|
|Ultisols||ineral soils with an illuvial clay horizon. Has low base saturation. Generally found in humid climates.|
|Vertisols||Clayey soils with deep wide cracks at some time in most years. Moisture content varies.|
The major difficulty in classifying soils is that soils are a continuum in which properties may change gradually with distance. Unlike the classification of plants and animals, where an individual ponderosa pine tree or Abert's squirrel is readily recognized, defining the basic soil entity or entities that are to be grouped into classes is a problem. In an attempt to solve this problem the basic diagnostic entity is defined in Soil Taxonomy (Soil Survey Staff, 1975) as being the smallest volume within the landscape needed to sample and describe the soil. This description must include the nature and arrangement of soil horizons and the variability of other properties. The term ‘‘pedon’’ is applied to this small volume of soil. A soil individual, called a polypedon, is composed of a group of contiguous pedons that belong to the same soil series. Through the soil series designation, the polypedon links soil bodies as they exist in nature with Soil Taxonomy (Soil Survey Staff, 1975) (Johnson, 1963) (See figure 40).
FIGURE 40. Diagram Showing Soil Pedon, Polypedon and Other Features Related to a Soilscape (Format after F. D. Hole, 1976)
|Order||Formative Element||Derivation 3||Mnemonicon|
|Spodosols||od||Gr.--spodos, wood ashes||Podzol; odd|
|Source: Soil Survey Staff, 1975.|
|aqu||A soil that is very wet or that has been artificially drained.|
|arg||A soil that has an illuvial horizon of clay.|
|fluv||A soil that is composed of recent alluvium.|
|orth||A soil that is the most representative.|
|psamm||A soil that has sandy texture, sand or loamy sand.|
|torr||A soil that is too dry to mature a crop without irrigation.|
|ud||A soil that is moist but not wet.|
|ust||A soil that is dry for long periods but moist in a growing season for 90 days or more.|
|xer||A soil that is moist in winter and dry in summer.|
Six of the 10 soil orders are officially recognized as being in Arizona. The presence of these six soil orders, Alfisols, Aridisols, Entisols, Inceptisols, Mollisols and Vertisols, reflects the wide range of soil-forming conditions in Arizona. In addition to these six orders about 65 ha (160 ac) of Histosols have been described in the floodplain of the Little Colorado River south of Joseph City (USDA Soil Conservation Service, 1976). Of the three remaining soil orders, Spodosols and Ultisols usually form in climates more humid than Arizona. Oxisols are not found outside of tropical regions with the exception of certain relict or exhumed Oxisols such as those in the Sacramento Valley-Sierra Nevada foothills in California (Singer and Nkedi-Kizza, 1980).
Alfisols. Alfisols are soils with light-colored surface layers and clayey subsurface (argillic) horizons. Alfisols, like the Mollisols, occur at higher elevations than Aridisols and are scattered throughout the semiarid and subhumid regions of Arizona. They generally are fairly old soils since probably more than 10,000 years are required to form their argillic horizons. They are mostly in the forested or wooded regions. Boralfs and Ustalfs are the two suborders of Alfisols recognized in Arizona.
Boralfs occur in the cool and cold mountain regions that primarily have coniferous vegetation. They have mean annual soil temperatures of less than 8 C (47 F) at a depth of 50 cm (20 in). Boralfs often are associated with Borolls, but they contain lower amounts of organic matter than Borolls.
Ustalfs occur in warmer and generally drier climates than Boralfs. They are scattered throughout the semiarid and subhumid regions of Arizona in association with Ustolls. Ustalfs usually are reddish and have some accumulations of carbonates in or below the subsoil. They generally lack diagnostic horizons other than ochric epipedons and argillic horizons. Ustalfs with high sodium content occur to a limited extent in the Willcox Playa vicinity.
Aridisols. Aridisols are the developed soils of dry regions. They have light-colored surface layers (ochric epipedons), generally have low amounts of organic matter and have at least one diagnostic subhorizon. Calcium carbonate usually is in some or all parts of the soil. Some soils are high in soluble salts. The two suborders of Aridisols, Orthids and Argids, are widely distributed in arid and semiarid Arizona.
Orthids show little or no textural change with depth. Most are calcareous throughout and many have a distinct accumulation layer of carbonates (calcic horizons). Some Orthids have hardpans cemented by carbonates (petrocalcic horizons) or by silica (duripans). Soils with layers of gypsum accumulation (gypsic horizons) occur to a limited extent in northern and eastern Coconino, Apache and Navajo counties and in the San Simon Valley and possibly elsewhere in Arizona.
Argids have an accumulation of translocated clay in the subsurface layers (argillic horizons). Argids usually are on older landscapes. Soil genesis research findings indicate that at least 10,000 years are required for Argids to form (Soil Survey Staff, 1975). In fact, many soil scientists believe that argillic horizons in Aridisols developed in the past under a more humid climate. Argids may have other diagnostic horizons as well. Calcic horizons are the most common, but a few Argids have petrocalcic horizons or duripans. Sodium has accumulated in some Argids near the Willcox Playa, in the Gila River Valley near Casa Grande and in a few other areas.
Entisols. Entisols are soils that show little or no evidence of horizon development. They may have thin surface horizons with some accumulation of organic matter, but they lack enough alteration of parent materials to form other horizons. Entisols generally are in young landscapes where time has not been sufficient for soils to develop. Some Entisols may occur on older landscapes if they are composed of materials resistant to weathering, or if the climate has been too dry for appreciable soil formation.
Fluvents formed in recently deposited alluvium in floodplains and near stream channels on alluvial fans or piedmont slopes. Fluvents lack horizon development because of flooding at fairly frequent intervals, leaving too little time for significant soil formation between alluvial depositions. Fluvents are a significant proportion of the irrigated agricultural land in Arizona.
Most Orthents in Arizona are shallow over rock, usually less than 50 cm (20 in). They typically occur on steep slopes where soil material is removed so fast that time is insufficient for significant horizon development. Other Orthents include deeper and older soils that lack horizon development because of dry environment.
Psamments are sandy Entisols. Soil textures are uniformly coarse, either sand or loamy sand. They include soils of stabilized sand dunes such as in Yuma, Navajo and Apache counties. Psamments also formed in sandy alluvium.
Inceptisols. Inceptisols are relatively young soils that lack horizons of illuvial clay accumulation (argillic horizons). They differ from Entisols because of weak to moderate profile horizonation. The horizons of Inceptisols result mostly from slight to moderate alteration of the parent material. These alterations may be expressed by soil structure development, carbonate removal and hydrolytic weathering to produce clay, form iron oxide minerals and accumulate organic matter. Inceptisols are limited in Arizona primarily to subhumid regions. Only the Ochrepts suborder of Inceptisols is officially recognized in Arizona, but soils of the Andepts suborder have been identified (Hendricks and Davis, 1979).
Ochrepts recently were recognized officially in the soil survey of central Coconino County (Taylor, 1982). Ochrepts also are recognized in several of the national forests (personal communication, Owen Carlton, 1983). Most Ochrepts identified in Arizona are shallow and characterized by weakly developed B horizons (cambic horizons). Because they occur mostly on relatively young geomorphic surfaces, time has limited horizon development.
Mollisols. Mollisols have thick, dark-colored surface horizons (mollic epipedons). Mollisols in Arizona occur at higher elevations under semiarid and subhumid climates in landscapes covered with grass or grass-tree mixtures, although in some areas they are mostly tree covered. Aquolls, Borolls and Ustolls are the three suborders of Mollisols in Arizona.
Borolls are cool and cold Mollisols in Arizona's higher elevations that have a mean annual soil temperature of less than 8 C (47 F) at a depth of 50 cm (20 in). Borolls include dark-colored forest and mountain meadow soils and are most common along the Mogollon Rim and in the higher mountain ranges throughout Arizona. Some Borolls have argillic horizons. Others have formed from calcareous parent materials and have calcic horizons.
Ustolls occur under warmer and drier conditions at elevations below Borolls. They are fairly widely distributed throughout Arizona under conditions a little cooler and more moist than those of Aridisols. Some Ustolls have argillic horizons. Others have calcic horizons and a few have petrocalcic horizons.
Vertisols. Vertisols are clayey soils that have deep wide cracks at some time during the year and that are associated primarily with volcanic (basalt) rocks in Arizona. They occur mostly in semiarid to subhumid regions in Yavapai, Coconino, Apache and Navajo counties and in the semiarid San Bernardino Valley of southeastern Cochise County.
Vertisols generally are clayey in all horizons. The clay is composed dominantly of montmorillonite. This mineral causes the soil to undergo considerable shrinking and swelling during drying and wetting cycles. As a result, the soils crack and move both horizontally and vertically with changes in moisture content to produce a mixing action. This action yields uniform textures with depth and causes stones and other rock fragments to move to the soil surface. The soil movement makes Vertisols troublesome for engineering uses and can seriously affect the growth of trees. Building foundations on Vertisols may crack and fences, power lines, highways and trees often become misaligned or variously tilted.
Soil families were established based on criteria significant to soil use, management and behavior. Although nine families are defined in Soil Taxonomy (Soil Survey Staff, 1975), only three, soil temperature, particle size and mineralogy, are discussed here. These three are the most widely used in soil family groupings. The other six are applied only to a limited number of soils.
Soil Temperature Families. Four soil temperature families are recognized in Arizona: hyperthermic, thermic, mesic and frigid. They correspond to the four soil temperature regimes defined and discussed in the on climate. Soil Taxonomy (Soil Survey Staff, 1975) also contains four other temperature families: isohyperthermic, isothermic, isomesic and isofrigid. These families experience less difference between mean winter and summer soil temperatures.
Soil temperature has a strong influence on plant growth. For example, commercial citrus production is limited primarily to hyperthermic soils and cotton production is restricted to hyperthermic and thermic soils. The boundary between the pinyon-juniper woodland and the montane coniferous forest zones in the Southwest generally follows the mesic-frigid boundary, although exceptions are known.
Particle Size Families. Particle size refers to the grain size distribution of the whole soil and includes stones and gravel as well as sand, silt and clay particles. All 11 particle size family classes defined in Soil Taxonomy (Soil Survey Staff, 1975) occur in Arizona soils because of the wide range of soil-forming conditions. Their presence reflects the great diversity of soil parent materials and, to a lesser extent, that of climate in Arizona.
Each particle size family is defined by properties that tend to group soils in relation to use and management. For example, soils high in silt are susceptible to erosion. Many soils in southern Arizona floodplains are high in silt. The erodibility of these soils may have been a factor in downcutting some streams during the past 80 to 100 years. Soils high in silt thus are placed in families different from more stable soils low in silt. Plastic and nonplastic soils are distinguished by percent clay content. Particle size families that are plastic have more than 18 percent clay and those that are nonplastic have less than 18 percent clay content. Clayey soil families have more than 35 percent clay and tend to create difficulties in tillage, seedling survival and, in some instances, engineering behavior. Skeletal families, those containing significant amounts of coarse fragments such as gravels, cobbles and stones, usually create problems in soil-plant relations and tillage operations. But they may be potential sources of gravel for construction.
Mineralogy Families. Mineralogy families are defined mostly by the mineral composition of selected size fractions of the soil. With a few exceptions the clayey soils in Arizona are classified either as montmorillonitic or mixed. Montmorillonitic soils, the Vertisols for example, present problems in use, management and behavior not encountered in other soils.
Most medium and coarse-textured soils are in mixed mineralogy families. This indicates that the content of quartz and other resistant minerals is less than 90 percent of the sand fraction and that no other mineral dominates. Consequently, Arizona soils generally have a high inherent fertility, but are deficient in nitrogen, which is not derived from
Soils exceptionally high in carbonates or gypsum are classified as carbonatic or gypsic. Soils high in carbonates tend to cause chlorosis in plants and, from an engineering standpoint, ameliorate some properties of noncarbonate clays, such as amount of shrink-swell. High quantities of gypsum may cause rapid corrosion of uncoated steel pipes and concrete structures. Gypsiferous soils materials also are undesirable for foundations and for use in hydraulic structures such as canal and irrigation embankments.
2. *The smallest natural body that can be defined as a thing complete in itself is an individual. A class is a group of individuals that have been grouped together on the basis of certain selected characteristics. A class is distinguished from all other classes by differences in these characteristics. In a hierarchical classification system classes of a lower category are grouped together according to common properties to form a class of a higher category.
3. *L.=Latin G.=Greek Fr.=French
4. *Formative elements are used in more than one suborder.