- Email: [email protected]
CLASSIFICATION SYSTEMS | USA
CLASSIFICATION SYSTEMS/USA
portions of sediments), Synlithogenic (pedogenesis is synchronous to sediment deposition, as in alluvial and volcanic soils), and Organogenic (organic) soils are distinguished. Soil divisions – the second taxonomic level – comprise the soils characterized by the similar trend of pedogenesis and having the same major diagnostic horizon (as a rule, subsurface horizons are taken into account). Considerable changes are made at the level of soil types that are distinguished by the similarity in the system of the main diagnostic horizons, i.e., by the similar horizonation of the soil profile. Thus, the previously single type of Chernozem is differentiated into two types (with and without textural differentiation of the profile). Some new soil types (Gleyzems, Cryozems, Dark Vertic soils) have been added. The types of mountainous soils having the same genetic horizons as the corresponding soils of plains are united with the latter. Overall, 181 types of natural and agronatural soils are distinguished. Soil types are characterized by the similarity of the system of the main diagnostic horizons, except for the character of the parent material. Soil subtypes are distinguished as the soils having qualitative modifications of the main diagnostic horizons; as a rule, they represent intergrades between soil types. Quantitative criteria are used as soil differentiate at lower taxonomic levels (similar to the classification of 1997). Soil genera are separated by the peculiarities of their exchange complex and the chemistry of salinization. Soil species are distinguished on the basis of the degree of development of soil features taken into account at the type, subtype, or genera levels. Soil varieties take into account soil texture and stoniness. Soil phases are distinguished by the character of soilforming and underlying rocks and thickness of the fine-earth part of the soil profile. Further development of this system implies the creation of classification schemes for the mineralogical and textural peculiarities of soils and soil-forming rocks and for soil temperature and water regimes. However, the authors of the new classification argue that it would be difficult to develop appropriate systems for soil regimes because of the lack of adequate data for Russian soils. The development of a separate lithologic–mineralogical component reflecting soil features inherited from the parent rock is hampered by the lack of adequate criteria allowing us to distinguish between proper pedogenic alteration and the initial state of the parent material. At the same time, the need for ecological (environmental) characterization of soils is evident. Special ecological–landscape classifications should be developed, taking into account not only climatic and lithological indices but also the geomorphic position of soils, the character of natural and anthropogenic vegetation,
235
the presence of geochemical barriers in soils, and other indices important from the viewpoint of predicting soil behavior and elaborating the strategy of sustainable soil management. See also: Classification of Soils; Classification Systems: Australian; FAO; USA
Further Reading Afanasiev JN (1927) The Classification Problem in Russian Soil Science. Russian Pedological Investigations, commission V. Leningrad: Publishing Office of the Academy of Sciences of the USSR. Classification and Diagnostics of Soils of the USSR (1986). New Delhi: Oxonian Press. Gedroits KK (1966) Genetic Soil Classification Based on the Absorptive Soil Complex and Absorbed Soil Cations. Jerusalem: Israel Program for Scientific Translation. Glazovskaya MA (1983) Soils of the World, vol. 1. Soil Families and Soil Types. New Delhi: Amerind. Goryachkin SV, Tonkonogov VD, Gerasimova MI et al. (2002) Changing concepts of soil and soil classification in Russia. In: Eswaran H, Rice T, Ahrens R, and Stewart BA (eds) Soil Classification – A Global Desk Reference, pp. 187–200. Boca Raton, FL: CRC Press. Ivanova EN and Rozov NN (eds) (1970) Classification and Determination of Soil Types. Nos. 1–5. Jerusalem: Israel Program for Scientific Translation. Neustruev SS (1967) A Tentative Classification of SoilForming Processes as Related to Soil Genesis. Jerusalem: Israel Program for Scientific Translation. Shishov LL, Tonkonogov VD, Lebedeva II, and Gerasimova MI (2001) Russian Soil Classification System. Moscow: V. V. Dokuchaev Soil Science Institution. Strzemski M (1975) Ideas Underlying Soil Systematics. Warsaw, Poland: Foreign Scientific Publications Department of the National Center for Scientific, Technical and Economic Information.
USA D J Brown, Montana State University, Bozeman, MT, USA ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction ‘‘People who like sausage and respect the law shouldn’t watch either being made’’ (commonly attributed to Otto von Bismarck, German chancellor, 1871–1890).
The construction of the US Department of Agriculture’s Soil Taxonomy system has more in common with the legislative process and sausage-making than
236 CLASSIFICATION SYSTEMS/USA
with the development of a carefully reasoned philosophical doctrine. To be sure, there are philosophical justifications and rationalizations for Soil Taxonomy. But the key to understanding Soil Taxonomy, as with most systems of laws or rules, lies in understanding the history and evolution of the contemporary system. There are many thorough reviews of Soil Taxonomy, with summaries of the soil orders found in virtually every pedology text. US Soil Survey staff have published documents on the details of Soil Taxonomy and how it is supposed to work. This chapter is intended to complement and explain these sources, particularly the Keys to Soil Taxonomy. To the uninitiated, the Keys can appear overwhelmingly complex and difficult to interpret. This article provides a simple, clearly stated introduction for the uninitiated – though, to be fair, many experienced soil scientists struggle to understand Soil Taxonomy. The emphasis is on explanation, not description, with insight valued over completeness. Toward this end, I take a pragmatic stance – with both an appreciation for historical contingency and an irreverent focus on ‘where the rubber meets the road,’ how Soil Taxonomy is actually employed in practice. The fundamental sampling unit for Soil Taxonomy provides an illustration of the pragmatic approach. Officially, the pedon is the smallest soil unit, with a 1–3.5 m2 surface area and approximately 2 m depth. The polypedon, a contiguous set of pedons, is the fundamental unit for Soil Taxonomy. While there are philosophical arguments both for and against the pedon and polypedon concepts, these have little bearing on the way soils are actually classified. With few exceptions, in practice the exposed side of a soil pit (the profile) serves as the basic unit for description, sampling, and classification.
Soil Classification Criteria The criteria in Soil Taxonomy are designed to classify soils ‘‘on the basis of their characteristics and not on the basis of the supposed or partly proved causes which have produced the characteristics,’’ (a mandate from the early survey leader Curtis Fletcher Marbut). Yet while the classification system is superficially based on contemporary soil characteristics, historic soil-forming processes and regional atmospheric climate play a fundamental role in shaping the structure of Soil Taxonomy. The current role of climate and soil-forming processes can be traced to past classification systems. An appreciation of the history of soil classification in the USA greatly clarifies the seemingly obscure contemporary system. The most important taxonomic requirements for soil materials, and diagnostic horizons and diagnostic
characteristics are outlined below. For complete requirements, the reader is referred to the Keys to Soil Taxonomy. Soil Composition
The amount and type of chemically active surface in a soil are controlled by: (1) secondary clay minerals; (2) organic material; and (3) noncrystalline Fe and Al materials (imogolite, ferrihydrite, Al–humus complexes, and the semicrystalline allophane included by convention). We can subdivide the secondary clay minerals into: (a) high-activity clays (2:1 layer silicates, smectites, and vermiculites); and (b) lowactivity clays (kaolinite, Fe-oxyhydroxides, and Al-oxides). Likewise, organic materials can be classified according to the degree of decomposition, from least to most: (a) fibric; (b) hemic; (c) sapric. These differences in soil composition, which greatly affect soil management, are captured through various criteria in Soil Taxonomy and summarized in Figure 1. All soil materials can be classed into two mutually exclusive categories: (1) mineral soil materials; and (2) organic soil materials. Most accumulations of organic soil materials can be found in locations where the soil is saturated for at least 30 days year1. For these materials to be classed as organic, they must have more than 12–18% organic C, depending on clay content (0–60% respectively). Organic requirements depend on clay content because the contribution of organic matter to overall soil activity becomes more important as clay content declines. Organic soil materials can be further classified according to fiber content as fibric, hemic, or sapric (important for classifying organic soils). The term ‘andic soil properties’ refers to materials with significant amounts of noncrystalline Fe and Al materials, formed by the weathering of volcanic glass and usually associated with volcanic ash deposits. Key requirements for the andic designation include low bulk density, high phosphate retention, and significant amounts of noncrystalline Al and Fe (as measured by oxalate extraction). For coarser soils (30% coarse silt and sand), there is a tradeoff between volcanic glass
Figure 1 Schematic diagram of important soil constituents, with Soil Taxonomy designations where appropriate.
CLASSIFICATION SYSTEMS/USA
content and noncrystalline Al and Fe content required. To be termed andic, a soil must also have less than 25% organic C, with the result that both organic and mineral soils can qualify. Because the largely noncrystalline Fe and Al materials are very ‘active,’ they can be more important to the overall soil management than even large amounts of organic matter. While the laboratory measures required to identify andic properties are demanding, in practice they are rarely undertaken unless a volcanic ash parent material has been identified in the field. The types and amounts of clay minerals in a soil are vital for soil management. Low-activity clays (1:1 kaolinite, Fe-oxyhydroxides, and Al-oxides) predominate in highly weathered soils, often found in warmer and wetter climates. Within Soil Taxonomy these minerals are not identified with a distinctive class. However, low-activity clays (cation exchange capacity or CEC per kg clay 16 cmol) are key criteria for two diagnostic horizons, discussed in the next section. Smectitic clays can cause soils to shrink and swell, but they are identified in Soil Taxonomy largely through field observation of shrink–swell features in a profile – not mineralogy. Subsoil clay accumulation is a defining feature for two soil orders, and there are also texture modifiers in many diagnostics, as with the clay content for the determination of organic soil materials. Genesis and Soil Taxonomy ‘‘Soil surveys have created a new branch of soil science, that of soil anatomy’’ (C F Marbut (1921) The contribution of soil survey to soil science. Society for the Promotion of Agricultural Science Proceedings 41: 116–142).
Most of the diagnostics for Soil Taxonomy have a basis in soil formation theory. Understanding and using Soil Taxonomy effectively requires at least a rudimentary understanding of soil-forming processes. Soil properties important for land management play a secondary role in this system. For example, surface texture is one of the most important soil properties from a management perspective, yet influences classification primarily at the lowest series level. The emphasis on soil formation in Soil Taxonomy can be traced to the introduction of the biological metaphor in early twentieth-century soil survey and classification. Curtis Fletcher Marbut, the inspirational leader of the US Soil Survey from 1913 until his death in 1935, modeled the science of soil survey on biology. Due largely to the tremendous success and influence of Darwin’s ideas, many natural sciences drew upon a biologic metaphor from the late nineteenth through the early twentieth century, including geomorphology, ecology, entomology, and sociology. The
237
study of profile formation was tied to the biological subfield of morphology – the study of embryonic development. And from this morphology, a hierarchical, ‘genetic’ soil taxonomy was developed analogous to biological taxonomy. Soils were grouped according to their mode of formation, not necessarily their contemporary properties. ‘‘No deviation from strict scientific [genetic] considerations for the sake of the so called practical use of the soil can safely be permitted.’’ For example, texture was not considered important for classification because it was considered ‘‘not primarily a product of soil development.’’ Like an old wine in new skins, the current Soil Taxonomy has evolved in many ways from early soil classification attempts, yet still retains the core of Marbut’s system. The genetic basis of Soil Taxonomy can be clearly observed in the requirements for diagnostic subsurface illuvial horizons (Table 1). (To present these horizons more clearly the requirements have been summarized: see Soil Taxonomy for precise requirements.) All but a few of these horizons require some evidence of illuviation in addition to compositional requirements. The argillic and natric require clay skins or an increasing fine-to-total clay ratio. The spodic requires an overlying albic or chemical evidence of illuviation. The calcic or gypsic requires observation of secondary (formed in soil) CaCO3 or gypsum, or an increase in CaCO3 relative to the parent material below. Among the nonilluvial diagnostic subsurface horizons (Table 2), the oxic horizon requires low CEC clays, and a paucity of weatherable minerals in the fine sand fraction (a genetic requirement). The cambic is a diagnostic horizon with indications of weak soil development. It is not necessary to demonstrate the genesis of these horizons, but the quantitative requirements for most diagnostic horizons are designed to capture genesis as best evidenced by contemporary soil properties. There is no diagnostic subsurface horizon in Soil Taxonomy to describe soil layers that are periodically saturated and chemically reduced. The term ‘aquic conditions’ refers to soils which are periodically saturated and reduced. In most cases, redoximorphic features (‘mottles’ or a ‘gleyed’ horizon) are used to indicate reducing conditions. An accumulation of organic soil materials can also indicate saturated conditions. Direct measurements of soil hydrology and/or a chemically reduced state can confirm aquic conditions, but due to the cost and time involved are rarely employed. Aquic conditions are deliberately vague, with more detailed requirements specified for each soil order. Confusing matters further, the term ‘aquic’ is also employed for the aquic soil moisture regime.
238 CLASSIFICATION SYSTEMS/USA
Table 1 Illuvial–eluvial diagnostic subsurface horizons Diagnostic horizon
Field
Genesis
Key requirements
Albic
E
Eluviation/leached
Argillic
Bt
Clay illuviation
Natric
Btn
Clay dispersion and illuviation
Spodic/ortstein
Chelation and illuviation
(Petro) calcic
Bs Bh Bhs Bhsm Bk, Bkm
Illuviation
(Petro) gypsic
By, Bym
Illuviation
Duripan
Bqm
Illuviation
Light-colored horizon below A High value, low chroma Clay-enriched subsoil Clay 3–8% (absolute) > A/E horizons Evidence of illuviation: – clay skins (macro/micro), or – increased fine : total clay ratio High sodium argillic Argillic requirements Columnar or prismatic structure Sodic soil (high proportion of Na) Illuvial Al-humus materials, often with Fe pH 5.9, organic C 0.6% Dark and/or red color with overlying albic If cemented, then ortstein Secondary CaCO3 accumulation 15% CaCO3 (or 5% if sandy) Evidence of illuviation: – 5% identifiable secondary CaCO3, or – 5% (absolute) > an underlying horizon If cemented/indurated ! petrocalcic Secondary gypsum accumulation 5% gypsum (CaSO4 2H2O) Thickness (cm) % gypsum 150 Evidence of illuviation: – 1% identifiable secondary gypsum If cemented/indurated ! petrogypsic Cemented/indurated silica accumulation Does not slake in weak acid Does slake in alkali solution
All diagnostic horizons have minimum thickness requirements, 2.5–15 cm. All pans or cemented horizons require lateral continuity, vertical cracks 10 cm apart.
The surface diagnostic horizons, called epipedons, also have a genetic basis (Table 3). The histic epipedon, for example, not only requires a surface accumulation of organic materials, but also has a genetic requirement that this organic material accumulated under saturated conditions. The mollic epipedon is designed to capture all surface soils formed under grasslands. Trees generally deposit organic detritus on the soil surface, resulting in surface ‘duff’ layers and thin accumulations of organic matter in the soil relative to grasslands where annual root turnover adds organic material to the rooting depth. The mollic epipedon criteria (e.g., 0.6% organic C) are designed to capture the lowest prairie organic matter accumulation, such as in Montana, so prairie soils in Iowa usually exceed the requirements many times over. One important point should be added to this discussion of diagnostic horizons: formal identification of these layers requires extensive, highly specified laboratory characterization. For a calcic horizon, CaCO3 must be greater than 15% as determined by the evolution of CO2 gas with acid treatment. For argillic horizons, clay films can be observed in the
field or through a microscopic analysis of thin sections. For the mollic epipedon, the organic C content must be measured in the laboratory, no matter how thick and dark the surface soil. In fact, there are only a handful of diagnostic horizons that can be determined based on field observations alone. In this way, the contemporary Soil Taxonomy differs greatly from the earlier US Soil Survey classification systems where ‘‘the criteria used to classify are those that can be observed or determined rapidly by simple tests in the field.’’ In practice, however, laboratory characterization is rarely performed. Given the time and expense of laboratory analyses, soil surveyors and others rely on experience, field observations, and a familiarity with similar profiles to make reasonable assumptions as to soil composition. Climate Zones in Soil Taxonomy ‘‘without soil climate as a criterion at some level in the taxonomic system, for example, Vertisols from Texas could be in the same class as Vertisols from North
CLASSIFICATION SYSTEMS/USA
239
Table 2 Other common diagnostic subsurface horizons or characteristics Diagnostic horizon
Field
Genesis
Key requirements
Aquic conditions
Bg
Redox
Kandic
Bt
? Clay, mineral weathering
Oxic
Bo
Mineral weathering
Salic
Bz
Multiple pathways
Fragipan
Bx
? Unclear
Placic
Bsm
Redox
Periodically saturated-reduced soil Redoximorphic features (gley, ‘mottles’) Saturated soil (directly measured) Chemical reduction (directly measured) Clay-enriched, weathered subsoil Clay increase relative to A/E Low-activity clays (kaolinite, oxides) – CEC 16 cmol kg1 clay Highly weathered soil Low-activity clays (kaolinite, oxides) – CEC 16 cmol kg1 clay Weatherable minerals 10% in fine sand No clay increase (not Kandic) Saline layer EC 30 dS m1 EC thickness (cm) 900 High-bulk-density brittle pan Dry material slakes in water Very coarse or weak structure, or massive At least firm rupture resistance Brittle, with no roots Thin Fe/Mn/organic pan Cemented with Fe, Mn, or organic matter Thin: 1 mm minimum, usually less than 25 mm Default subsurface horizon Some soil formation, but not enough to meet any other diagnostic requirements
Cambic
Multiple pathways
All diagnostic horizons have minimum thickness requirements, 2.5–15 cm. All pans or cemented horizons require lateral continuity, vertical cracks 10 cm apart. CEC, cation exchange capacity; EC, electrical conductivity.
Table 3 Most common diagnostic epipedons (surface horizons) Epipedon
Genesis
Key requirements
Mollic
Prairie grasses
Umbric
?
Histic
Wetland
Folistic
Litter layer
Melanic
Volcanic
Ochric
Forest/shrub
Thick, dark, organic-rich, fertile topsoil Dark: moist Munsell value and chrome 3 Organic C 0.6% Base saturation 50% Thickness 18 or 25 cm (see conditions), or meets requirements when mixed to 18 cm depth Low base saturation mollic Meets mollic requirements, except: Base saturation 50% Organic surface horizon Saturated 30 days year1 Organic soil material 20 cm thick Not thick enough to be an organic soil (40–60 cm) Nonwetland organic surface horizon Saturated <30 days year1 Organic soil material 15 cm thick, or 20 cm if sphagnum or bulk density <0.1 Thick, dark volcanic surface horizon Andic soil properties 30 cm thick Organic C 4% throughout, 6% average Dark: moist Munsell value and chroma 2 Default surface epipedon Does not meet the requirements of other epipedons
240 CLASSIFICATION SYSTEMS/USA Dakota’’ (Soil Survey Staff (1999) Soil Taxonomy. Washington, DC: US Department of Agriculture, Natural Resources Conservation Service, p. 94).
The classification of a dogwood (cornus) would not change at any level if we dug up a species in Texas and replanted it in North Dakota. If we dug up a Vertisol in Texas, and ‘planted’ it in North Dakota, however, the classification of the soil would change by virtue of location, and likely at one of the highest levels of Soil Taxonomy. This geographic–climatic dimension of Soil Taxonomy reflects the origins of soil classification in nineteenth-century Russia. V.V. Dokuchaev and his student Sibertsev proposed that soils be studied from a ‘geologic–geographic’ perspective, laying the theoretical groundwork for contemporary pedology, the science behind soil survey. In the nineteenth century, soils were studied from a fertility perspective or assumed to be an extension of the geologic rock below. Dokuchaev argued for the study of soils from a natural history perspective, examining their formation and distribution over the surface of the Earth. From this vantage point, he further outlined five environmental factors that controlled the formation and distribution of soils: ‘‘climate, country age, vegetation, topography and parent rock.’’ Based on these ideas, Sibirtsev published a ‘zonal’ classification system in 1900 whereby typical profiles were described for the major ‘physiographic zones’ of Russia. ‘Intrazonal types’ deviated from the zonal norm due to the influence of one or two soil-forming factors, commonly topography (e.g., bog soils) and/or parent rock (e.g., solonetzes, saltaffected soils). ‘Azonal’ soils were dominated by parent materials (e.g., fresh alluvium). This classification system was brought into Europe and the USA through the writings of K.D. Glinka, who strongly emphasized the role of climate in soil formation and classification. Determined to free pedology from the discipline of geology, Glinka argued that, unlike geologic strata, soil formation and geography were controlled primarily by climate, which ‘‘provides justification for singling out the soils as a particular group of natural bodies, with which a special branch of science should be concerned.’’ The ‘zonal’ or ‘climatic’ concept had a strong influence on the early development of pedology in the USA and this influence can still be found in contemporary Soil Taxonomy. Climate is an important and diagnostic characteristic for both mineral and organic soils. One soil order (Aridisols) and most of the suborders are governed by soil moisture regimes, while soil temperature regimes become important at lower levels of Soil Taxonomy.
Another soil order, the Gelisols, is founded on a soil feature (permafrost) that is highly correlated with atmospheric climate. The requirements for determining soil moisture regimes (Table 4) are excruciatingly complex, with extensive soil monitoring required. In practice these measurements are rarely made because, according to soil taxonomy, ‘‘the intent in defining the soil moisture control section is to facilitate the estimation of soil moisture regimes from climatic data.’’ In other words, soil moisture regimes are typically determined from regional atmospheric climate data, without need for soil measurements. For the vast bulk of the Earth’s land surface, the soil moisture regime can be mapped in large swathes or ‘zones’ reminiscent of the Russian system. Local topography and hydrology can also, in some situations, influence the soil moisture regime. In particular, the aquic and peraquic moisture regimes (saturated soils) are usually controlled by local topography and groundwater hydrology and identified by the presence of redoximorphic features or an accumulation of organic soil materials. Soil temperature regimes (Table 4) are also estimated for the most part from atmospheric climate data, though if necessary, temperatures can be taken at a depth of 50 cm at monthly intervals. Adjustments are available to convert mean annual temperature (MAT) to mean annual soil temperature (MAST). The temperature and moisture regimes for a soil to be classified can also be obtained from other soils in the region. Nonalpine soils in Montana, for example, are generally assumed to have ustic moisture and frigid temperature regimes.
Structure and Nomenclature Most Soil Taxonomy users will never classify a soil themselves. However, understanding the processes by which soil is classified can greatly enhance the interpretation of soils already classified. Toward this end, the procedure for classifying and naming a soil is described below, followed by a worked example using observations and data from a profile in south Dakota, USA. How it Works
Modeled on biological taxonomy, Soil Taxonomy has a hierarchical organization (Figure 2). The order, suborder, great group, and subgroup levels are primarily governed by climate and soil genesis, with the family and series levels capturing important physical and chemical properties for crop growth, engineering, and land management. In some cases, soil genesis and management are related: for example, with
CLASSIFICATION SYSTEMS/USA
241
Table 4 Brief descriptions of temperature and moisture regimes Moisture regimes
Brief description
Temperature regimes
Brief description
Aridic/torric Xeric
Dry most of the year Pronounced dry season – summer
Cryic
Ustic
Pronounced dry season – winter
(Iso)frigid
Udic Perudic
Humid climate, no pronounced dry season Wet climate, year-round precipitation > evapotranspiration Saturated and reduced part of the year Saturated and reduced year-round
(Iso)mesic (Iso)thermic
MAT < 8 C Summer temperature < 6–15 C No permafrost MAT < 8 C Isofrigid; can also be cryic 8 C MAT < 15 C 15 C MAT < 22 C
(Iso)hyperthermic
22 C MAT
Aquic Peraquic
MAT, mean annual temperature. ‘Iso’ prefix indicates that the mean summer and mean winter temperatures differ by less than 6 C.
Figure 2 Hierarchical organization of Soil Taxonomy, with climatic–genetic versus management levels identified separately.
Vertisols, where shrink–swell processes are key for both profile genesis and land management. Unfortunately, this not always the case. The soil orders are summarized in Table 5. Most of the orders can be related closely to genetic factors of soil formation, though most are related to more than one factor. Soils are keyed out in order, starting with Gelisols and ending with Entisols. Implicit in the requirements for each order is the requirement that the soil not meet the requirements for preceding orders. For example, a soil with permafrost at 75 cm, a mollic epipedon, and base saturation greater than 75% for all horizons would be classified as a Gelisol as this order takes precedence over the Mollisol. This sequential approach to ‘keying out’ a soil profile is used for all levels of soil taxonomy except the lowest, the soil series.
In practice, where Soil Survey activity is largely complete, soils are often classified from the bottom up. At the local level, a soil profile can be described and compared to known soil series. The series that best ‘fits’ the examined profile can then be applied, and the established taxonomic designation for that series thus derived. The Soil Taxonomy nomenclature follows the hierarchical structure of the system, and allows users to glean basic soil information from the name alone. Soil names are constructed from right to left, with segments drawn primarily from the diagnostic criteria discussed previously. This nomenclature system can best be illustrated with an example: Order : Alfisol Suborder : Udalf Great group : Kandiudalf
Subgroup : aquic kandiudalf ! Aquic j kandi j ud j alf
242 CLASSIFICATION SYSTEMS/USA
Table 5 Summary of soil orders and key factors of soil formation Order
Important features and properties
Key factors of soil formation
Gelisols Histosols Spodosols
Permafrost within 1–2 m of surface Organic soils (usually in wetlands) Illuvial Fe-Al-humus subsurface horizon, usually sandy
Andisols Oxisols
Aridisols Ultisols Mollisols Alfisols
Amorphous ‘minerals’; volcanic glass Highly weathered soils; dominated by low-CEC clays (kaolinite and oxides) Shrink–swell activity, as evidenced by cracks and ‘slickenslides’ Arid climate Clay-enriched subsoil; low base saturation Thick, dark, organic-rich surface layer; high base saturation Clay-enriched subsoil, high base saturation
Climate: frozen soils Topography: usually in wetlands Vegetation and parent material: coniferous trees, sandy deposits Parent material: volcanic ash Climate and time: mineral weathering
Inceptisols Entisols
Minimal horizon development Unaltered parent material; usually geomorphic deposits
Vertisols
? Source of shrink–swell clays Climate Climate and time: base leaching, acidification Vegetation: prairie soils Vegetation, climate, and time: forest soils, moderately leached Time and climate: adolescent development Time: young
CEC, cation exchange capacity.
Climate, genesis, and key soil properties can be discerned from the four nomenclature segments above. The ‘alf’ term indicates that this profile has a clay increase in the subsoil, and does not meet the criteria for all of the preceding nine orders. The ‘ud’ term refers to the udic moisture regime. ‘Kandi’ is an abbreviation for the kandic diagnostic horizon, a clay-enriched, lowactivity (low CEC per kg clay) subsoil. ‘Aquic’ in the subgroup tag refers to aquic conditions lower in the profile. (By comparison, an ‘aqualf’ would have aquic conditions higher in the profile.) Given a familiarity with soil moisture regimes, order names, and diagnostic horizons, most Soil Taxonomy names can be similarly interpreted. Default terms are ‘orth,’ ‘hapl,’ and ‘typic’ for the suborder, great group, and subgroup respectively. For example, a ‘typic haplorthod’ is a spodosol with no additional features to describe in the second to fourth levels of Soil Taxonomy. Example Profile
The site description, field description, and essential lab characterization data for a Ferney profile in South Dakota are provided in Table 6. Using this information, the key diagnostic horizons are outlined with bold lines in Table 7. As can be seen from this example, diagnostic horizons do not necessarily correspond directly to field horizons. The mollic epipedon encompasses both the Ap and Bt horizons, while the argillic and natric diagnostic horizons are comprised of the Bt and Btkz horizons identified in the field. Furthermore, field designations do not directly correspond with the more precisely defined diagnostics. There are three B horizons with a ‘k’ designation for illuvial carbonate accumulation, for example, only one of which qualifies as a calcic horizon. There are
also three ‘z’ designations in the subsoil for salt accumulation, though the electrical conductivities of these horizons do not meet the salic horizon requirement. Both the Ap and Bt horizons meet the color value and chroma requirements as well as the organic carbon requirements for a mollic epipedon. The combined depth is 25 cm, which satisfies the thickness requirement for this epipedon, though the Ap alone would not. Similarly, the Bt and Btkz field horizons both meet the illuvial (observed clay skins) and clay increase requirements (1.2 eluvial horizon) for an argillic horizon, with a combined thickness of 51 cm. Horizons below could also meet the illuvial requirement (despite the lack of clay skins) through the examination of thin sections under a microscope, but these data are not available. The calcic horizon has both the overall quantity of CaCO3 required, and a 5% increase over an underlying horizon (evidence of illuviation). The bottom two horizons (C and Bkz2) might also meet the requirements for a gypic horizon, but confirmation of 1% secondary gypsum is lacking. Finally the Bt and Btkz horizons both meet the requirements for a natric horizon: (1) argillic requirements; (2) columnar or prismatic structure; and (3) sodium absorption ratio (SAR) 13. Using the diagnostic horizons obtained, together with climatic data, we can readily key out the soil profile (Table 8), indicating the degree to which the designated moisture regime borders on another regime (e.g., ustic bordering on aridic). Care must be taken to distinguish between ‘and’ and ‘or’ requirements, but with practice the Keys can be systematically applied to available data, yielding a Leptic Natrustoll in this case. Space constraints do not
Table 6 Ferney profile (based on Soil Survey Pedon no. 86P0004) Site description
Physiography: glaciated uplands Slope: 1% concave south-east facing Parent material: glacial till from mixed material
Drainage: moderately well drained Moisture regime: Ustic Moisture control section:
Temperature regime: frigid
dry 3/10 days with soil temperature >5 C
Clay (%)
Silt (%)
Sand (%)
Organic C (%)
CaCO3 (%)
Gypsum (%)
Base sat. (%)
pH (H2O)
CEC
CEC/clay
SAR
(mmolc kg1) (mmolc kg1) Ap – 0–13 cm; black (10YR 2/1) interior moist clay loam; moderate fine and medium subangular blocky structure; few fine accumulations of carbonate; strongly effervescent; neutral (pH ¼ 7.0); abrupt smooth boundary 23.7 47.7 28.6 3.24 – – 100 6.6 26.3 111 5 Bt – 13–25 cm; very dark brown (10YR 2/2) interior moist clay loam; strong medium columnar structure; continuous faint coats on tops of columns and continuous faint clay films on faces of peds; mildly alkaline (pH ¼ 7.6); clear wavy boundary 40.3 33.6 26.1 1.31 tr – 100 8.1 33.2 82 13 Btkz – 25–64 cm; dark grayish-brown (2.5Y 4/2) interior moist clay loam; moderate medium and coarse prismatic structure; continuous faint clay films on faces of peds; few fine carbonate concretions and many fine and medium salt masses; strongly alkaline (pH ¼ 8.8); clear wavy boundary 40.7 33.7 25.6 0.64 13 2 100 8.2 23.7 58 16 Bkz1 – 64–99 cm; dark grayish-brown (2.5Y 4/2) interior moist clay loam; few fine distinct mottles; weak medium and coarse prismatic structure; common fine carbonate concretions and common fine salt masses; strongly effervescent; strongly alkaline (pH ¼ 8.8); clear wavy boundary 38.2 35.0 26.8 0.27 17 1 100 8.4 21.2 56 21 Bkz2 – 99–127 cm; olive brown (2.5Y 4/4) interior moist clay loam; common fine prominent and many medium and coarse distinct mottles; weak coarse prismatic structure; common fine carbonate concretions, many coarse salt masses; strongly effervescent; moderately alkaline (pH ¼ 8.2); gradual wavy boundary 33.3 34.8 31.9 0.22 3 10 100 8.2 19.2 58 19 C – 127–152 cm; olive brown (2.5Y 4/4) interior moist clay loam; common fine prominent and common fine and medium distinct mottles; massive; common fine carbonate concretions, few salt masses; strongly effervescent; strongly alkaline (pH ¼ 8.8) 33.9 35.9 30.2 0.16 8 7 100 8.2 19.2 55 17 Note: Maximum electrical conductivity for profile is 14.2 dS m1. CEC, cation exchange capacity; SAR, sodium absorption ratio.
244 CLASSIFICATION SYSTEMS/USA
Table 7 Diagnostic horizons for Ferney profile
OC, Organic carbon; SAR, sodium absorption ratio.
Table 8 Keying out Ferney profile Order
Yes/no
Reason
A. Gelisol B. Histostol C. Spodosol D. Andisol E. Oxisol F. Vertisol G. Aridisol H. Ultisol I. Mollisol
No No No No No No No No Yes
No permafrost or gelic materials No organic soil materials No spodic horizon or ortstein No volcanic ash or andic materials No oxic or kandic horizon No surface cracks or slickenslides Ustic, not aridic moisture regime, no salic Base sat. >35% for all horizons 1a. Mollic epipedon 2. Base sat. >50% for all horizons
No No No No Yes
No albic horizon No aquic conditions at 40–50 cm depth 2. Argillic and calic horizons present Have Ustic, not Xeric moisture regime Ustic moisture regime
No Yes
No duripan Natric horizon present
Suborder
IA. Alboll IB. Aquoll IC. Rendoll ID. Xeroll IE. Ustoll Great group
IFA. Durustoll IFB. Natrustoll Subgroup (Natrustolls)
3a. Dry only 3 days out of 10 in moisture control section with temperature >5 C (not torric) 2a. Dry only 3 days out of 10 in moisture control section with temperature >5 C (not torric) 2a. No slickenslides or cracks 2b. Linear extensibility not 6.0 cm or greater IFBD. Glossic Vertic No No vertic properties (see above) IFBE. Vertic No No vertic properties (see above) IFBF. Aridic Leptic No 2a. Dry only 3 days out of 10 in moisture control section with temperature >5 C (not aridic) IFBG. Leptic Yes Visible salt crystals at 25 cm depth <40 cm Classification of profile to subgroup level: Leptic Natrustoll IFBA. Leptic Torretic IFBB. Torrertic IFBC. Leptic Vertic
No No No
allow classification to the family level, though assuming mineralogical data are available, this can also be accomplished by systematically following the Keys.
At the subgroup level, there are often statistically precise climatic requirements. These requirements indicate the degree to which the moisture regime
CLASSIFICATION SYSTEMS/USA
borders on another regime (e.g., Ustic bordering on Aridic). To simplify the classification for this example, the frequency of dry days during the growing season was provided in the site description, though this is rarely available in practice. As with climate regimes generally, these subgroup climatic modifiers are established regionally and can be obtained from the local soil survey staff.
Closing Thoughts In this chapter, I explained: (1) how US soil classification took its present form, and (2) how the contemporary system is actually used. Tracing the roots of Soil Taxonomy explains many peculiar features of the contemporary system. Past climatic and genetic classification ideas can be found beneath the utilitarian veneer of the current system. Understanding these historical ideas is vital to understanding contemporary US Soil Taxonomy, and how to use it.
List of Technical Nomenclature Base sat.
Percentage base saturation, exchangeable bases (CEC)1 100
CaCO3
Percentage of CaCO3 equivalent (measured by CO2 evoluation) measured relative to fine earth fraction (<2 mm)
CEC
Cation exchange capacity (mmolc kg1)
Clay (%)
Percentage of fine earth (<2 mm) with particle size less than 2 m
EC
Electrical conductivity (dS m1)
Gypsum
Percentage of CaSO42H2O equivalent measured relative to fine earth fraction (<2 mm)
Org. C
Percentage of organic C measured relative to fine earth fraction (<2 mm)
pH
Measure of solution acidity, log[Hþ]
SAR
Sodium absorption ratio ¼ Naþ {0.5 (Ca2þ þ Mg2þ)0.5}, based on saturated paste extract
Sand (%)
Percentage of fine earth (<2 mm) with particle size between 50 m and 2 mm
Silt (%)
245
Percentage of fine earth (<2 mm) with particle size less than 50 m
See also: Classification of Soils; Classification Systems: Australian; Russian, Evolution and Examples
Further Reading Ahrens RJ and Arnold RW (1999) Soil taxonomy. In: Summer ME (ed.) Handbook of Soil Science, pp. E117–136. Boca Raton, FL: CRC Press. Allen G (1978) Life Science in the Twentieth Century. Cambridge: Cambridge University Press. Baldwin M, Kellogg CE, and Thorp J (1938) Soil classification. In: Soils and Men, pp. 979–1001. Washington, DC: United States Government Printing Office. Bockheim JG and Gennadiyev AN (2000) The role of soilforming processes in the definition of taxa in Soil Taxonomy and the World Soil Reference Base. Geoderma 95: 53–72. Buol SW, Hole FD, McCraken RJ, and Southard RJ (1997) Soil Genesis and Classification. Ames, IA: Iowa State University Press. Cline MG (1949) Basic principles of soil classification. Soil Science 67: 81–91. Dokuchaev VV (1967) (originally published 1883)Russian Chernozem. Selected Works of V.V. Dokuchaev 1. Jerusalem Israel: Israel Program for Scientific Translations. Glinka KD (1963) (originally published 1931)Treatise on Soil Science (Pochvovedenie). Jerusalem, Israel: Israel Program for Scientific Translations. Haskett JD (1995) The philosophical basis of soil classification and its evolution. Soil Science Society of America Journal 59: 179–184. Marbut CF (1921) The contribution of soil survey to soil science. Society for the Promotion of Agricultural Science Proceedings 41: 116–142. Marbut CF (1927) A Scheme for Soil Classification, pp. 1–31. Washington, DC: First International Congress of Soil Science. American Organizing Committee. Sibirtsev NM (1966) Selected Works, 1. Jerusalem, Israel: Israel Program for Scientific Translations. Soil Survey Staff (1998) Keys to Soil Taxonomy. Washington, DC: US Department of Agriculture, Natural Resources Conservation Service. Soil Survey Staff (1999) Soil Taxonomy, A Basic System of Soil Classification for Making and Interpreting Soil Surveys. Washington, DC: US Department of Agriculture, Natural Resources Conservation Service.
portions of sediments), Synlithogenic (pedogenesis is synchronous to sediment deposition, as in alluvial and volcanic soils), and Organogenic (organic) soils are distinguished. Soil divisions – the second taxonomic level – comprise the soils characterized by the similar trend of pedogenesis and having the same major diagnostic horizon (as a rule, subsurface horizons are taken into account). Considerable changes are made at the level of soil types that are distinguished by the similarity in the system of the main diagnostic horizons, i.e., by the similar horizonation of the soil profile. Thus, the previously single type of Chernozem is differentiated into two types (with and without textural differentiation of the profile). Some new soil types (Gleyzems, Cryozems, Dark Vertic soils) have been added. The types of mountainous soils having the same genetic horizons as the corresponding soils of plains are united with the latter. Overall, 181 types of natural and agronatural soils are distinguished. Soil types are characterized by the similarity of the system of the main diagnostic horizons, except for the character of the parent material. Soil subtypes are distinguished as the soils having qualitative modifications of the main diagnostic horizons; as a rule, they represent intergrades between soil types. Quantitative criteria are used as soil differentiate at lower taxonomic levels (similar to the classification of 1997). Soil genera are separated by the peculiarities of their exchange complex and the chemistry of salinization. Soil species are distinguished on the basis of the degree of development of soil features taken into account at the type, subtype, or genera levels. Soil varieties take into account soil texture and stoniness. Soil phases are distinguished by the character of soilforming and underlying rocks and thickness of the fine-earth part of the soil profile. Further development of this system implies the creation of classification schemes for the mineralogical and textural peculiarities of soils and soil-forming rocks and for soil temperature and water regimes. However, the authors of the new classification argue that it would be difficult to develop appropriate systems for soil regimes because of the lack of adequate data for Russian soils. The development of a separate lithologic–mineralogical component reflecting soil features inherited from the parent rock is hampered by the lack of adequate criteria allowing us to distinguish between proper pedogenic alteration and the initial state of the parent material. At the same time, the need for ecological (environmental) characterization of soils is evident. Special ecological–landscape classifications should be developed, taking into account not only climatic and lithological indices but also the geomorphic position of soils, the character of natural and anthropogenic vegetation,
235
the presence of geochemical barriers in soils, and other indices important from the viewpoint of predicting soil behavior and elaborating the strategy of sustainable soil management. See also: Classification of Soils; Classification Systems: Australian; FAO; USA
Further Reading Afanasiev JN (1927) The Classification Problem in Russian Soil Science. Russian Pedological Investigations, commission V. Leningrad: Publishing Office of the Academy of Sciences of the USSR. Classification and Diagnostics of Soils of the USSR (1986). New Delhi: Oxonian Press. Gedroits KK (1966) Genetic Soil Classification Based on the Absorptive Soil Complex and Absorbed Soil Cations. Jerusalem: Israel Program for Scientific Translation. Glazovskaya MA (1983) Soils of the World, vol. 1. Soil Families and Soil Types. New Delhi: Amerind. Goryachkin SV, Tonkonogov VD, Gerasimova MI et al. (2002) Changing concepts of soil and soil classification in Russia. In: Eswaran H, Rice T, Ahrens R, and Stewart BA (eds) Soil Classification – A Global Desk Reference, pp. 187–200. Boca Raton, FL: CRC Press. Ivanova EN and Rozov NN (eds) (1970) Classification and Determination of Soil Types. Nos. 1–5. Jerusalem: Israel Program for Scientific Translation. Neustruev SS (1967) A Tentative Classification of SoilForming Processes as Related to Soil Genesis. Jerusalem: Israel Program for Scientific Translation. Shishov LL, Tonkonogov VD, Lebedeva II, and Gerasimova MI (2001) Russian Soil Classification System. Moscow: V. V. Dokuchaev Soil Science Institution. Strzemski M (1975) Ideas Underlying Soil Systematics. Warsaw, Poland: Foreign Scientific Publications Department of the National Center for Scientific, Technical and Economic Information.
USA D J Brown, Montana State University, Bozeman, MT, USA ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction ‘‘People who like sausage and respect the law shouldn’t watch either being made’’ (commonly attributed to Otto von Bismarck, German chancellor, 1871–1890).
The construction of the US Department of Agriculture’s Soil Taxonomy system has more in common with the legislative process and sausage-making than
236 CLASSIFICATION SYSTEMS/USA
with the development of a carefully reasoned philosophical doctrine. To be sure, there are philosophical justifications and rationalizations for Soil Taxonomy. But the key to understanding Soil Taxonomy, as with most systems of laws or rules, lies in understanding the history and evolution of the contemporary system. There are many thorough reviews of Soil Taxonomy, with summaries of the soil orders found in virtually every pedology text. US Soil Survey staff have published documents on the details of Soil Taxonomy and how it is supposed to work. This chapter is intended to complement and explain these sources, particularly the Keys to Soil Taxonomy. To the uninitiated, the Keys can appear overwhelmingly complex and difficult to interpret. This article provides a simple, clearly stated introduction for the uninitiated – though, to be fair, many experienced soil scientists struggle to understand Soil Taxonomy. The emphasis is on explanation, not description, with insight valued over completeness. Toward this end, I take a pragmatic stance – with both an appreciation for historical contingency and an irreverent focus on ‘where the rubber meets the road,’ how Soil Taxonomy is actually employed in practice. The fundamental sampling unit for Soil Taxonomy provides an illustration of the pragmatic approach. Officially, the pedon is the smallest soil unit, with a 1–3.5 m2 surface area and approximately 2 m depth. The polypedon, a contiguous set of pedons, is the fundamental unit for Soil Taxonomy. While there are philosophical arguments both for and against the pedon and polypedon concepts, these have little bearing on the way soils are actually classified. With few exceptions, in practice the exposed side of a soil pit (the profile) serves as the basic unit for description, sampling, and classification.
Soil Classification Criteria The criteria in Soil Taxonomy are designed to classify soils ‘‘on the basis of their characteristics and not on the basis of the supposed or partly proved causes which have produced the characteristics,’’ (a mandate from the early survey leader Curtis Fletcher Marbut). Yet while the classification system is superficially based on contemporary soil characteristics, historic soil-forming processes and regional atmospheric climate play a fundamental role in shaping the structure of Soil Taxonomy. The current role of climate and soil-forming processes can be traced to past classification systems. An appreciation of the history of soil classification in the USA greatly clarifies the seemingly obscure contemporary system. The most important taxonomic requirements for soil materials, and diagnostic horizons and diagnostic
characteristics are outlined below. For complete requirements, the reader is referred to the Keys to Soil Taxonomy. Soil Composition
The amount and type of chemically active surface in a soil are controlled by: (1) secondary clay minerals; (2) organic material; and (3) noncrystalline Fe and Al materials (imogolite, ferrihydrite, Al–humus complexes, and the semicrystalline allophane included by convention). We can subdivide the secondary clay minerals into: (a) high-activity clays (2:1 layer silicates, smectites, and vermiculites); and (b) lowactivity clays (kaolinite, Fe-oxyhydroxides, and Al-oxides). Likewise, organic materials can be classified according to the degree of decomposition, from least to most: (a) fibric; (b) hemic; (c) sapric. These differences in soil composition, which greatly affect soil management, are captured through various criteria in Soil Taxonomy and summarized in Figure 1. All soil materials can be classed into two mutually exclusive categories: (1) mineral soil materials; and (2) organic soil materials. Most accumulations of organic soil materials can be found in locations where the soil is saturated for at least 30 days year1. For these materials to be classed as organic, they must have more than 12–18% organic C, depending on clay content (0–60% respectively). Organic requirements depend on clay content because the contribution of organic matter to overall soil activity becomes more important as clay content declines. Organic soil materials can be further classified according to fiber content as fibric, hemic, or sapric (important for classifying organic soils). The term ‘andic soil properties’ refers to materials with significant amounts of noncrystalline Fe and Al materials, formed by the weathering of volcanic glass and usually associated with volcanic ash deposits. Key requirements for the andic designation include low bulk density, high phosphate retention, and significant amounts of noncrystalline Al and Fe (as measured by oxalate extraction). For coarser soils (30% coarse silt and sand), there is a tradeoff between volcanic glass
Figure 1 Schematic diagram of important soil constituents, with Soil Taxonomy designations where appropriate.
CLASSIFICATION SYSTEMS/USA
content and noncrystalline Al and Fe content required. To be termed andic, a soil must also have less than 25% organic C, with the result that both organic and mineral soils can qualify. Because the largely noncrystalline Fe and Al materials are very ‘active,’ they can be more important to the overall soil management than even large amounts of organic matter. While the laboratory measures required to identify andic properties are demanding, in practice they are rarely undertaken unless a volcanic ash parent material has been identified in the field. The types and amounts of clay minerals in a soil are vital for soil management. Low-activity clays (1:1 kaolinite, Fe-oxyhydroxides, and Al-oxides) predominate in highly weathered soils, often found in warmer and wetter climates. Within Soil Taxonomy these minerals are not identified with a distinctive class. However, low-activity clays (cation exchange capacity or CEC per kg clay 16 cmol) are key criteria for two diagnostic horizons, discussed in the next section. Smectitic clays can cause soils to shrink and swell, but they are identified in Soil Taxonomy largely through field observation of shrink–swell features in a profile – not mineralogy. Subsoil clay accumulation is a defining feature for two soil orders, and there are also texture modifiers in many diagnostics, as with the clay content for the determination of organic soil materials. Genesis and Soil Taxonomy ‘‘Soil surveys have created a new branch of soil science, that of soil anatomy’’ (C F Marbut (1921) The contribution of soil survey to soil science. Society for the Promotion of Agricultural Science Proceedings 41: 116–142).
Most of the diagnostics for Soil Taxonomy have a basis in soil formation theory. Understanding and using Soil Taxonomy effectively requires at least a rudimentary understanding of soil-forming processes. Soil properties important for land management play a secondary role in this system. For example, surface texture is one of the most important soil properties from a management perspective, yet influences classification primarily at the lowest series level. The emphasis on soil formation in Soil Taxonomy can be traced to the introduction of the biological metaphor in early twentieth-century soil survey and classification. Curtis Fletcher Marbut, the inspirational leader of the US Soil Survey from 1913 until his death in 1935, modeled the science of soil survey on biology. Due largely to the tremendous success and influence of Darwin’s ideas, many natural sciences drew upon a biologic metaphor from the late nineteenth through the early twentieth century, including geomorphology, ecology, entomology, and sociology. The
237
study of profile formation was tied to the biological subfield of morphology – the study of embryonic development. And from this morphology, a hierarchical, ‘genetic’ soil taxonomy was developed analogous to biological taxonomy. Soils were grouped according to their mode of formation, not necessarily their contemporary properties. ‘‘No deviation from strict scientific [genetic] considerations for the sake of the so called practical use of the soil can safely be permitted.’’ For example, texture was not considered important for classification because it was considered ‘‘not primarily a product of soil development.’’ Like an old wine in new skins, the current Soil Taxonomy has evolved in many ways from early soil classification attempts, yet still retains the core of Marbut’s system. The genetic basis of Soil Taxonomy can be clearly observed in the requirements for diagnostic subsurface illuvial horizons (Table 1). (To present these horizons more clearly the requirements have been summarized: see Soil Taxonomy for precise requirements.) All but a few of these horizons require some evidence of illuviation in addition to compositional requirements. The argillic and natric require clay skins or an increasing fine-to-total clay ratio. The spodic requires an overlying albic or chemical evidence of illuviation. The calcic or gypsic requires observation of secondary (formed in soil) CaCO3 or gypsum, or an increase in CaCO3 relative to the parent material below. Among the nonilluvial diagnostic subsurface horizons (Table 2), the oxic horizon requires low CEC clays, and a paucity of weatherable minerals in the fine sand fraction (a genetic requirement). The cambic is a diagnostic horizon with indications of weak soil development. It is not necessary to demonstrate the genesis of these horizons, but the quantitative requirements for most diagnostic horizons are designed to capture genesis as best evidenced by contemporary soil properties. There is no diagnostic subsurface horizon in Soil Taxonomy to describe soil layers that are periodically saturated and chemically reduced. The term ‘aquic conditions’ refers to soils which are periodically saturated and reduced. In most cases, redoximorphic features (‘mottles’ or a ‘gleyed’ horizon) are used to indicate reducing conditions. An accumulation of organic soil materials can also indicate saturated conditions. Direct measurements of soil hydrology and/or a chemically reduced state can confirm aquic conditions, but due to the cost and time involved are rarely employed. Aquic conditions are deliberately vague, with more detailed requirements specified for each soil order. Confusing matters further, the term ‘aquic’ is also employed for the aquic soil moisture regime.
238 CLASSIFICATION SYSTEMS/USA
Table 1 Illuvial–eluvial diagnostic subsurface horizons Diagnostic horizon
Field
Genesis
Key requirements
Albic
E
Eluviation/leached
Argillic
Bt
Clay illuviation
Natric
Btn
Clay dispersion and illuviation
Spodic/ortstein
Chelation and illuviation
(Petro) calcic
Bs Bh Bhs Bhsm Bk, Bkm
Illuviation
(Petro) gypsic
By, Bym
Illuviation
Duripan
Bqm
Illuviation
Light-colored horizon below A High value, low chroma Clay-enriched subsoil Clay 3–8% (absolute) > A/E horizons Evidence of illuviation: – clay skins (macro/micro), or – increased fine : total clay ratio High sodium argillic Argillic requirements Columnar or prismatic structure Sodic soil (high proportion of Na) Illuvial Al-humus materials, often with Fe pH 5.9, organic C 0.6% Dark and/or red color with overlying albic If cemented, then ortstein Secondary CaCO3 accumulation 15% CaCO3 (or 5% if sandy) Evidence of illuviation: – 5% identifiable secondary CaCO3, or – 5% (absolute) > an underlying horizon If cemented/indurated ! petrocalcic Secondary gypsum accumulation 5% gypsum (CaSO4 2H2O) Thickness (cm) % gypsum 150 Evidence of illuviation: – 1% identifiable secondary gypsum If cemented/indurated ! petrogypsic Cemented/indurated silica accumulation Does not slake in weak acid Does slake in alkali solution
All diagnostic horizons have minimum thickness requirements, 2.5–15 cm. All pans or cemented horizons require lateral continuity, vertical cracks 10 cm apart.
The surface diagnostic horizons, called epipedons, also have a genetic basis (Table 3). The histic epipedon, for example, not only requires a surface accumulation of organic materials, but also has a genetic requirement that this organic material accumulated under saturated conditions. The mollic epipedon is designed to capture all surface soils formed under grasslands. Trees generally deposit organic detritus on the soil surface, resulting in surface ‘duff’ layers and thin accumulations of organic matter in the soil relative to grasslands where annual root turnover adds organic material to the rooting depth. The mollic epipedon criteria (e.g., 0.6% organic C) are designed to capture the lowest prairie organic matter accumulation, such as in Montana, so prairie soils in Iowa usually exceed the requirements many times over. One important point should be added to this discussion of diagnostic horizons: formal identification of these layers requires extensive, highly specified laboratory characterization. For a calcic horizon, CaCO3 must be greater than 15% as determined by the evolution of CO2 gas with acid treatment. For argillic horizons, clay films can be observed in the
field or through a microscopic analysis of thin sections. For the mollic epipedon, the organic C content must be measured in the laboratory, no matter how thick and dark the surface soil. In fact, there are only a handful of diagnostic horizons that can be determined based on field observations alone. In this way, the contemporary Soil Taxonomy differs greatly from the earlier US Soil Survey classification systems where ‘‘the criteria used to classify are those that can be observed or determined rapidly by simple tests in the field.’’ In practice, however, laboratory characterization is rarely performed. Given the time and expense of laboratory analyses, soil surveyors and others rely on experience, field observations, and a familiarity with similar profiles to make reasonable assumptions as to soil composition. Climate Zones in Soil Taxonomy ‘‘without soil climate as a criterion at some level in the taxonomic system, for example, Vertisols from Texas could be in the same class as Vertisols from North
CLASSIFICATION SYSTEMS/USA
239
Table 2 Other common diagnostic subsurface horizons or characteristics Diagnostic horizon
Field
Genesis
Key requirements
Aquic conditions
Bg
Redox
Kandic
Bt
? Clay, mineral weathering
Oxic
Bo
Mineral weathering
Salic
Bz
Multiple pathways
Fragipan
Bx
? Unclear
Placic
Bsm
Redox
Periodically saturated-reduced soil Redoximorphic features (gley, ‘mottles’) Saturated soil (directly measured) Chemical reduction (directly measured) Clay-enriched, weathered subsoil Clay increase relative to A/E Low-activity clays (kaolinite, oxides) – CEC 16 cmol kg1 clay Highly weathered soil Low-activity clays (kaolinite, oxides) – CEC 16 cmol kg1 clay Weatherable minerals 10% in fine sand No clay increase (not Kandic) Saline layer EC 30 dS m1 EC thickness (cm) 900 High-bulk-density brittle pan Dry material slakes in water Very coarse or weak structure, or massive At least firm rupture resistance Brittle, with no roots Thin Fe/Mn/organic pan Cemented with Fe, Mn, or organic matter Thin: 1 mm minimum, usually less than 25 mm Default subsurface horizon Some soil formation, but not enough to meet any other diagnostic requirements
Cambic
Multiple pathways
All diagnostic horizons have minimum thickness requirements, 2.5–15 cm. All pans or cemented horizons require lateral continuity, vertical cracks 10 cm apart. CEC, cation exchange capacity; EC, electrical conductivity.
Table 3 Most common diagnostic epipedons (surface horizons) Epipedon
Genesis
Key requirements
Mollic
Prairie grasses
Umbric
?
Histic
Wetland
Folistic
Litter layer
Melanic
Volcanic
Ochric
Forest/shrub
Thick, dark, organic-rich, fertile topsoil Dark: moist Munsell value and chrome 3 Organic C 0.6% Base saturation 50% Thickness 18 or 25 cm (see conditions), or meets requirements when mixed to 18 cm depth Low base saturation mollic Meets mollic requirements, except: Base saturation 50% Organic surface horizon Saturated 30 days year1 Organic soil material 20 cm thick Not thick enough to be an organic soil (40–60 cm) Nonwetland organic surface horizon Saturated <30 days year1 Organic soil material 15 cm thick, or 20 cm if sphagnum or bulk density <0.1 Thick, dark volcanic surface horizon Andic soil properties 30 cm thick Organic C 4% throughout, 6% average Dark: moist Munsell value and chroma 2 Default surface epipedon Does not meet the requirements of other epipedons
240 CLASSIFICATION SYSTEMS/USA Dakota’’ (Soil Survey Staff (1999) Soil Taxonomy. Washington, DC: US Department of Agriculture, Natural Resources Conservation Service, p. 94).
The classification of a dogwood (cornus) would not change at any level if we dug up a species in Texas and replanted it in North Dakota. If we dug up a Vertisol in Texas, and ‘planted’ it in North Dakota, however, the classification of the soil would change by virtue of location, and likely at one of the highest levels of Soil Taxonomy. This geographic–climatic dimension of Soil Taxonomy reflects the origins of soil classification in nineteenth-century Russia. V.V. Dokuchaev and his student Sibertsev proposed that soils be studied from a ‘geologic–geographic’ perspective, laying the theoretical groundwork for contemporary pedology, the science behind soil survey. In the nineteenth century, soils were studied from a fertility perspective or assumed to be an extension of the geologic rock below. Dokuchaev argued for the study of soils from a natural history perspective, examining their formation and distribution over the surface of the Earth. From this vantage point, he further outlined five environmental factors that controlled the formation and distribution of soils: ‘‘climate, country age, vegetation, topography and parent rock.’’ Based on these ideas, Sibirtsev published a ‘zonal’ classification system in 1900 whereby typical profiles were described for the major ‘physiographic zones’ of Russia. ‘Intrazonal types’ deviated from the zonal norm due to the influence of one or two soil-forming factors, commonly topography (e.g., bog soils) and/or parent rock (e.g., solonetzes, saltaffected soils). ‘Azonal’ soils were dominated by parent materials (e.g., fresh alluvium). This classification system was brought into Europe and the USA through the writings of K.D. Glinka, who strongly emphasized the role of climate in soil formation and classification. Determined to free pedology from the discipline of geology, Glinka argued that, unlike geologic strata, soil formation and geography were controlled primarily by climate, which ‘‘provides justification for singling out the soils as a particular group of natural bodies, with which a special branch of science should be concerned.’’ The ‘zonal’ or ‘climatic’ concept had a strong influence on the early development of pedology in the USA and this influence can still be found in contemporary Soil Taxonomy. Climate is an important and diagnostic characteristic for both mineral and organic soils. One soil order (Aridisols) and most of the suborders are governed by soil moisture regimes, while soil temperature regimes become important at lower levels of Soil Taxonomy.
Another soil order, the Gelisols, is founded on a soil feature (permafrost) that is highly correlated with atmospheric climate. The requirements for determining soil moisture regimes (Table 4) are excruciatingly complex, with extensive soil monitoring required. In practice these measurements are rarely made because, according to soil taxonomy, ‘‘the intent in defining the soil moisture control section is to facilitate the estimation of soil moisture regimes from climatic data.’’ In other words, soil moisture regimes are typically determined from regional atmospheric climate data, without need for soil measurements. For the vast bulk of the Earth’s land surface, the soil moisture regime can be mapped in large swathes or ‘zones’ reminiscent of the Russian system. Local topography and hydrology can also, in some situations, influence the soil moisture regime. In particular, the aquic and peraquic moisture regimes (saturated soils) are usually controlled by local topography and groundwater hydrology and identified by the presence of redoximorphic features or an accumulation of organic soil materials. Soil temperature regimes (Table 4) are also estimated for the most part from atmospheric climate data, though if necessary, temperatures can be taken at a depth of 50 cm at monthly intervals. Adjustments are available to convert mean annual temperature (MAT) to mean annual soil temperature (MAST). The temperature and moisture regimes for a soil to be classified can also be obtained from other soils in the region. Nonalpine soils in Montana, for example, are generally assumed to have ustic moisture and frigid temperature regimes.
Structure and Nomenclature Most Soil Taxonomy users will never classify a soil themselves. However, understanding the processes by which soil is classified can greatly enhance the interpretation of soils already classified. Toward this end, the procedure for classifying and naming a soil is described below, followed by a worked example using observations and data from a profile in south Dakota, USA. How it Works
Modeled on biological taxonomy, Soil Taxonomy has a hierarchical organization (Figure 2). The order, suborder, great group, and subgroup levels are primarily governed by climate and soil genesis, with the family and series levels capturing important physical and chemical properties for crop growth, engineering, and land management. In some cases, soil genesis and management are related: for example, with
CLASSIFICATION SYSTEMS/USA
241
Table 4 Brief descriptions of temperature and moisture regimes Moisture regimes
Brief description
Temperature regimes
Brief description
Aridic/torric Xeric
Dry most of the year Pronounced dry season – summer
Cryic
Ustic
Pronounced dry season – winter
(Iso)frigid
Udic Perudic
Humid climate, no pronounced dry season Wet climate, year-round precipitation > evapotranspiration Saturated and reduced part of the year Saturated and reduced year-round
(Iso)mesic (Iso)thermic
MAT < 8 C Summer temperature < 6–15 C No permafrost MAT < 8 C Isofrigid; can also be cryic 8 C MAT < 15 C 15 C MAT < 22 C
(Iso)hyperthermic
22 C MAT
Aquic Peraquic
MAT, mean annual temperature. ‘Iso’ prefix indicates that the mean summer and mean winter temperatures differ by less than 6 C.
Figure 2 Hierarchical organization of Soil Taxonomy, with climatic–genetic versus management levels identified separately.
Vertisols, where shrink–swell processes are key for both profile genesis and land management. Unfortunately, this not always the case. The soil orders are summarized in Table 5. Most of the orders can be related closely to genetic factors of soil formation, though most are related to more than one factor. Soils are keyed out in order, starting with Gelisols and ending with Entisols. Implicit in the requirements for each order is the requirement that the soil not meet the requirements for preceding orders. For example, a soil with permafrost at 75 cm, a mollic epipedon, and base saturation greater than 75% for all horizons would be classified as a Gelisol as this order takes precedence over the Mollisol. This sequential approach to ‘keying out’ a soil profile is used for all levels of soil taxonomy except the lowest, the soil series.
In practice, where Soil Survey activity is largely complete, soils are often classified from the bottom up. At the local level, a soil profile can be described and compared to known soil series. The series that best ‘fits’ the examined profile can then be applied, and the established taxonomic designation for that series thus derived. The Soil Taxonomy nomenclature follows the hierarchical structure of the system, and allows users to glean basic soil information from the name alone. Soil names are constructed from right to left, with segments drawn primarily from the diagnostic criteria discussed previously. This nomenclature system can best be illustrated with an example: Order : Alfisol Suborder : Udalf Great group : Kandiudalf
Subgroup : aquic kandiudalf ! Aquic j kandi j ud j alf
242 CLASSIFICATION SYSTEMS/USA
Table 5 Summary of soil orders and key factors of soil formation Order
Important features and properties
Key factors of soil formation
Gelisols Histosols Spodosols
Permafrost within 1–2 m of surface Organic soils (usually in wetlands) Illuvial Fe-Al-humus subsurface horizon, usually sandy
Andisols Oxisols
Aridisols Ultisols Mollisols Alfisols
Amorphous ‘minerals’; volcanic glass Highly weathered soils; dominated by low-CEC clays (kaolinite and oxides) Shrink–swell activity, as evidenced by cracks and ‘slickenslides’ Arid climate Clay-enriched subsoil; low base saturation Thick, dark, organic-rich surface layer; high base saturation Clay-enriched subsoil, high base saturation
Climate: frozen soils Topography: usually in wetlands Vegetation and parent material: coniferous trees, sandy deposits Parent material: volcanic ash Climate and time: mineral weathering
Inceptisols Entisols
Minimal horizon development Unaltered parent material; usually geomorphic deposits
Vertisols
? Source of shrink–swell clays Climate Climate and time: base leaching, acidification Vegetation: prairie soils Vegetation, climate, and time: forest soils, moderately leached Time and climate: adolescent development Time: young
CEC, cation exchange capacity.
Climate, genesis, and key soil properties can be discerned from the four nomenclature segments above. The ‘alf’ term indicates that this profile has a clay increase in the subsoil, and does not meet the criteria for all of the preceding nine orders. The ‘ud’ term refers to the udic moisture regime. ‘Kandi’ is an abbreviation for the kandic diagnostic horizon, a clay-enriched, lowactivity (low CEC per kg clay) subsoil. ‘Aquic’ in the subgroup tag refers to aquic conditions lower in the profile. (By comparison, an ‘aqualf’ would have aquic conditions higher in the profile.) Given a familiarity with soil moisture regimes, order names, and diagnostic horizons, most Soil Taxonomy names can be similarly interpreted. Default terms are ‘orth,’ ‘hapl,’ and ‘typic’ for the suborder, great group, and subgroup respectively. For example, a ‘typic haplorthod’ is a spodosol with no additional features to describe in the second to fourth levels of Soil Taxonomy. Example Profile
The site description, field description, and essential lab characterization data for a Ferney profile in South Dakota are provided in Table 6. Using this information, the key diagnostic horizons are outlined with bold lines in Table 7. As can be seen from this example, diagnostic horizons do not necessarily correspond directly to field horizons. The mollic epipedon encompasses both the Ap and Bt horizons, while the argillic and natric diagnostic horizons are comprised of the Bt and Btkz horizons identified in the field. Furthermore, field designations do not directly correspond with the more precisely defined diagnostics. There are three B horizons with a ‘k’ designation for illuvial carbonate accumulation, for example, only one of which qualifies as a calcic horizon. There are
also three ‘z’ designations in the subsoil for salt accumulation, though the electrical conductivities of these horizons do not meet the salic horizon requirement. Both the Ap and Bt horizons meet the color value and chroma requirements as well as the organic carbon requirements for a mollic epipedon. The combined depth is 25 cm, which satisfies the thickness requirement for this epipedon, though the Ap alone would not. Similarly, the Bt and Btkz field horizons both meet the illuvial (observed clay skins) and clay increase requirements (1.2 eluvial horizon) for an argillic horizon, with a combined thickness of 51 cm. Horizons below could also meet the illuvial requirement (despite the lack of clay skins) through the examination of thin sections under a microscope, but these data are not available. The calcic horizon has both the overall quantity of CaCO3 required, and a 5% increase over an underlying horizon (evidence of illuviation). The bottom two horizons (C and Bkz2) might also meet the requirements for a gypic horizon, but confirmation of 1% secondary gypsum is lacking. Finally the Bt and Btkz horizons both meet the requirements for a natric horizon: (1) argillic requirements; (2) columnar or prismatic structure; and (3) sodium absorption ratio (SAR) 13. Using the diagnostic horizons obtained, together with climatic data, we can readily key out the soil profile (Table 8), indicating the degree to which the designated moisture regime borders on another regime (e.g., ustic bordering on aridic). Care must be taken to distinguish between ‘and’ and ‘or’ requirements, but with practice the Keys can be systematically applied to available data, yielding a Leptic Natrustoll in this case. Space constraints do not
Table 6 Ferney profile (based on Soil Survey Pedon no. 86P0004) Site description
Physiography: glaciated uplands Slope: 1% concave south-east facing Parent material: glacial till from mixed material
Drainage: moderately well drained Moisture regime: Ustic Moisture control section:
Temperature regime: frigid
dry 3/10 days with soil temperature >5 C
Clay (%)
Silt (%)
Sand (%)
Organic C (%)
CaCO3 (%)
Gypsum (%)
Base sat. (%)
pH (H2O)
CEC
CEC/clay
SAR
(mmolc kg1) (mmolc kg1) Ap – 0–13 cm; black (10YR 2/1) interior moist clay loam; moderate fine and medium subangular blocky structure; few fine accumulations of carbonate; strongly effervescent; neutral (pH ¼ 7.0); abrupt smooth boundary 23.7 47.7 28.6 3.24 – – 100 6.6 26.3 111 5 Bt – 13–25 cm; very dark brown (10YR 2/2) interior moist clay loam; strong medium columnar structure; continuous faint coats on tops of columns and continuous faint clay films on faces of peds; mildly alkaline (pH ¼ 7.6); clear wavy boundary 40.3 33.6 26.1 1.31 tr – 100 8.1 33.2 82 13 Btkz – 25–64 cm; dark grayish-brown (2.5Y 4/2) interior moist clay loam; moderate medium and coarse prismatic structure; continuous faint clay films on faces of peds; few fine carbonate concretions and many fine and medium salt masses; strongly alkaline (pH ¼ 8.8); clear wavy boundary 40.7 33.7 25.6 0.64 13 2 100 8.2 23.7 58 16 Bkz1 – 64–99 cm; dark grayish-brown (2.5Y 4/2) interior moist clay loam; few fine distinct mottles; weak medium and coarse prismatic structure; common fine carbonate concretions and common fine salt masses; strongly effervescent; strongly alkaline (pH ¼ 8.8); clear wavy boundary 38.2 35.0 26.8 0.27 17 1 100 8.4 21.2 56 21 Bkz2 – 99–127 cm; olive brown (2.5Y 4/4) interior moist clay loam; common fine prominent and many medium and coarse distinct mottles; weak coarse prismatic structure; common fine carbonate concretions, many coarse salt masses; strongly effervescent; moderately alkaline (pH ¼ 8.2); gradual wavy boundary 33.3 34.8 31.9 0.22 3 10 100 8.2 19.2 58 19 C – 127–152 cm; olive brown (2.5Y 4/4) interior moist clay loam; common fine prominent and common fine and medium distinct mottles; massive; common fine carbonate concretions, few salt masses; strongly effervescent; strongly alkaline (pH ¼ 8.8) 33.9 35.9 30.2 0.16 8 7 100 8.2 19.2 55 17 Note: Maximum electrical conductivity for profile is 14.2 dS m1. CEC, cation exchange capacity; SAR, sodium absorption ratio.
244 CLASSIFICATION SYSTEMS/USA
Table 7 Diagnostic horizons for Ferney profile
OC, Organic carbon; SAR, sodium absorption ratio.
Table 8 Keying out Ferney profile Order
Yes/no
Reason
A. Gelisol B. Histostol C. Spodosol D. Andisol E. Oxisol F. Vertisol G. Aridisol H. Ultisol I. Mollisol
No No No No No No No No Yes
No permafrost or gelic materials No organic soil materials No spodic horizon or ortstein No volcanic ash or andic materials No oxic or kandic horizon No surface cracks or slickenslides Ustic, not aridic moisture regime, no salic Base sat. >35% for all horizons 1a. Mollic epipedon 2. Base sat. >50% for all horizons
No No No No Yes
No albic horizon No aquic conditions at 40–50 cm depth 2. Argillic and calic horizons present Have Ustic, not Xeric moisture regime Ustic moisture regime
No Yes
No duripan Natric horizon present
Suborder
IA. Alboll IB. Aquoll IC. Rendoll ID. Xeroll IE. Ustoll Great group
IFA. Durustoll IFB. Natrustoll Subgroup (Natrustolls)
3a. Dry only 3 days out of 10 in moisture control section with temperature >5 C (not torric) 2a. Dry only 3 days out of 10 in moisture control section with temperature >5 C (not torric) 2a. No slickenslides or cracks 2b. Linear extensibility not 6.0 cm or greater IFBD. Glossic Vertic No No vertic properties (see above) IFBE. Vertic No No vertic properties (see above) IFBF. Aridic Leptic No 2a. Dry only 3 days out of 10 in moisture control section with temperature >5 C (not aridic) IFBG. Leptic Yes Visible salt crystals at 25 cm depth <40 cm Classification of profile to subgroup level: Leptic Natrustoll IFBA. Leptic Torretic IFBB. Torrertic IFBC. Leptic Vertic
No No No
allow classification to the family level, though assuming mineralogical data are available, this can also be accomplished by systematically following the Keys.
At the subgroup level, there are often statistically precise climatic requirements. These requirements indicate the degree to which the moisture regime
CLASSIFICATION SYSTEMS/USA
borders on another regime (e.g., Ustic bordering on Aridic). To simplify the classification for this example, the frequency of dry days during the growing season was provided in the site description, though this is rarely available in practice. As with climate regimes generally, these subgroup climatic modifiers are established regionally and can be obtained from the local soil survey staff.
Closing Thoughts In this chapter, I explained: (1) how US soil classification took its present form, and (2) how the contemporary system is actually used. Tracing the roots of Soil Taxonomy explains many peculiar features of the contemporary system. Past climatic and genetic classification ideas can be found beneath the utilitarian veneer of the current system. Understanding these historical ideas is vital to understanding contemporary US Soil Taxonomy, and how to use it.
List of Technical Nomenclature Base sat.
Percentage base saturation, exchangeable bases (CEC)1 100
CaCO3
Percentage of CaCO3 equivalent (measured by CO2 evoluation) measured relative to fine earth fraction (<2 mm)
CEC
Cation exchange capacity (mmolc kg1)
Clay (%)
Percentage of fine earth (<2 mm) with particle size less than 2 m
EC
Electrical conductivity (dS m1)
Gypsum
Percentage of CaSO42H2O equivalent measured relative to fine earth fraction (<2 mm)
Org. C
Percentage of organic C measured relative to fine earth fraction (<2 mm)
pH
Measure of solution acidity, log[Hþ]
SAR
Sodium absorption ratio ¼ Naþ {0.5 (Ca2þ þ Mg2þ)0.5}, based on saturated paste extract
Sand (%)
Percentage of fine earth (<2 mm) with particle size between 50 m and 2 mm
Silt (%)
245
Percentage of fine earth (<2 mm) with particle size less than 50 m
See also: Classification of Soils; Classification Systems: Australian; Russian, Evolution and Examples
Further Reading Ahrens RJ and Arnold RW (1999) Soil taxonomy. In: Summer ME (ed.) Handbook of Soil Science, pp. E117–136. Boca Raton, FL: CRC Press. Allen G (1978) Life Science in the Twentieth Century. Cambridge: Cambridge University Press. Baldwin M, Kellogg CE, and Thorp J (1938) Soil classification. In: Soils and Men, pp. 979–1001. Washington, DC: United States Government Printing Office. Bockheim JG and Gennadiyev AN (2000) The role of soilforming processes in the definition of taxa in Soil Taxonomy and the World Soil Reference Base. Geoderma 95: 53–72. Buol SW, Hole FD, McCraken RJ, and Southard RJ (1997) Soil Genesis and Classification. Ames, IA: Iowa State University Press. Cline MG (1949) Basic principles of soil classification. Soil Science 67: 81–91. Dokuchaev VV (1967) (originally published 1883)Russian Chernozem. Selected Works of V.V. Dokuchaev 1. Jerusalem Israel: Israel Program for Scientific Translations. Glinka KD (1963) (originally published 1931)Treatise on Soil Science (Pochvovedenie). Jerusalem, Israel: Israel Program for Scientific Translations. Haskett JD (1995) The philosophical basis of soil classification and its evolution. Soil Science Society of America Journal 59: 179–184. Marbut CF (1921) The contribution of soil survey to soil science. Society for the Promotion of Agricultural Science Proceedings 41: 116–142. Marbut CF (1927) A Scheme for Soil Classification, pp. 1–31. Washington, DC: First International Congress of Soil Science. American Organizing Committee. Sibirtsev NM (1966) Selected Works, 1. Jerusalem, Israel: Israel Program for Scientific Translations. Soil Survey Staff (1998) Keys to Soil Taxonomy. Washington, DC: US Department of Agriculture, Natural Resources Conservation Service. Soil Survey Staff (1999) Soil Taxonomy, A Basic System of Soil Classification for Making and Interpreting Soil Surveys. Washington, DC: US Department of Agriculture, Natural Resources Conservation Service.