Soil Classification Systems
While systems of classification are created to facilitate communication, they are artificial and have inherent limitations. They simplify the complexity and continuum of the world through necessary bias, whether rationally or irrationally applied. Soils are described by many types of classification systems, the first criteria being the definition of soil itself (See: How Soil is Defined). Agriculturists, engineers, geologists, and others each have different definitions based on the needs of their particular discipline or vocation. A soil to one may not be soil to another. Classification systems can be divided into two categories, vernacular and scientific. Vernacular systems have been used for millennia, while scientifically based systems are relatively recent developments (See: A Brief History of Soil Science). Aridic soils are soils that occur in an arid environment (See: Classifications of Arid Land Soils). If aridity has a high level of importance among those developing a particular soil classification system, then aridic soils will be clearly distinguished by that system, whether the system is vernacular or scientific.
Vernacular systems are developed by the land users. Their structure is either nominal, giving unique names to soils or landscapes, or descriptive, naming soils by their characteristics such as red, hot, fat, or sandy. Soils are distinguished by obvious characteristics, such as physical appearance (e.g., color, texture, landscape position), performance (e.g., production capability, flooding), and accompanying vegetation. These distinctions are often based on characteristics important to land management and largely they have been ignored by the scientific community until recently, with anthropologists and geographers being the first to document them. Vernacular systems can provide outsiders a language to communicate with local land users, especially regarding agricultural management and resource tenure. Vernacular systems can also provide technicians and scientists insight into natural resource management systems that can prove valuable in inventorying and developing local resources.
Ethnopedology, the term coined by Williams and Ortiz-Solorio (1981, p. 336), is the study of these vernacular systems. The anthropologist Harold C. Conklin first started documenting vernacular systems in the 1950s. Studies conducted by the Office of Arid Lands Studies, University of Arizona, show that vernacular systems in arid lands can be very detailed with comparable usefulness to scientific systems (e.g., the Peul's system in Mauritania) or conversely may have no pedologic usefulness (e.g., the Bedouin's system in Saudi Arabia which implies that differences in soils are not perceived to be important to their livelihood). In semi-arid California the vernacular system of Malibu encompasses only one soil, the infamous "Malibu blue clay" (called Diablo soils under the U.S. Department of Agriculture (USDA), system) that limits building development and reduces property values because septic-tank drainage-fields are not allowed on that soil.
Scientific systems are of two types, those based on processes of soil development or genesis and those based on quantifiable characteristics. Many systems have elements of both types. Criteria for distinguishing soils among these systems are by no means uniform. The structure of these systems can be hierarchical, descriptive, or nominal. Also, soil classification systems are not static. As knowledge is gained old systems and class names are changed for new ones, as with plant and animal classification systems. These differences in classification systems make it important to include descriptions of classified soils when reporting so that correlation to other systems is possible. Users of classification systems need to make sure that important characteristics have class limits narrow enough to be useful. Scientific systems, especially the hierarchical ones, are useful to exclude soils from consideration, but generally a detailed soil description is needed to make recommendations for soil use and management.
Process based classification systems were developed to explain how soil characteristics and appearance change with time. Soils form distinct layers as biological, physical, and chemical processes develop zones of material accumulation and zones of loss, also called horizons. This evolution is predicable as long as climate and other processes remain constant. This assumption of constancy poses problems for these types of classification systems, especially for those aridic soils that partially developed under humid conditions. It is possible to have more than one classification for the same soil, depending on which soil forming process is assumed to be expressing itself. Due to this and other problems, the USDA abandoned its system based on soil forming processes and developed the system layed out in Soil Taxonomy based on existing quantifiable characteristics. Engineers also use systems based on existing characteristics, for obvious reasons.
Criteria for distinguishing soils are not necessarily the same for different classification systems. Soil texture is a good example of the problems involved in correlating classifications between two or more systems. Texture describes the proportion of different size classes of the mineral part of the soil. For the same soil, texture-class names can differ depending on the classification system used. Even if the names are the same, the limits often differ, as with "sandy clay" of the USDA and French systems:
The problem continues with particle-size classes. For example, clay is defined as <0.002 mm in diameter by some systems and <0.005 mm by others. Other physical, chemical, and biological characteristics have similar discrepancies between systems which makes one to one correlation between systems nearly impossible unless a detailed soil description is available.
Hierarchical, descriptive, or nominal systems of classification each have their strengths and weaknesses. Rules of hierarchical systems allow one to determine the highest to lowest levels of classification without previously knowing any of the possible classifications. The resulting classification (e.g., clayey-skeletal, mixed, hyperthermic Typic Haplargid of USDA Soil Taxonomy), while providing much information, is far from what is usually needed to make management decisions. Also, these systems can make trivial distinctions when applied in areas of the world different from those where they were developed. For example, sandy West African soils have extremely low cation exchange capacities. Very small additions or losses of base cations can cause large differences in base saturation percentages that distinguish the orders Alfisols and Ultisols (highest level of USDA Soil Taxonomy).
Descriptive soil classification systems are commonly developed for single purpose application. For example, the Fertility Capability Soil Classification System uses a string of upper and lower case letters to represent characteristics important in soil fertility management for crops. These systems are relatively simple, easy to interpret, but of limited value.
Nominal systems are much like vernacular systems but with the rigor of scientific descriptions and engineering capabilities. These systems can be detailed enough to use as a basis for management decisions, but only at a regional level. Comparing numerous soils under this system would be an onerous task. The USDA's system of Soil Series is an example of a nominal system that predates Soil Taxonomy. Soil Series have been modified so that they fit within one Soil Taxonomy classification and linked to this hierarchical system. Together, Soil Taxonomy and Soil Series systems provide powerful tools for identifying, understanding and managing soils. This merging of hierarchical and nominal systems can be copied in other countries, especially in developing countries by using existing vernacular systems.
To demonstrate how this all fits together, the aridic soil "Malibu blue clay" (vernacular name) was established by the USDA in 1910 as Diablo soil series (old nominal scientific system) and classified as a Grumusols (old hierarchical scientific system). Since then the Diablo soil series description has been modified (new nominal scientific system) and classified as fine, smectitic, thermic family of Aridic Haploxererts (current hierarchical scientific system).
Partial List of Soil Classification Systems
The French systems have been widely used in former colonies and current territories.
Originally developed as a legend to its soils map of the world it has been applied throughout the world by United Nations sponsored projects and soil classifiers trained to use this system. Many countries have modified this system to fit their particular needs.
American Association of State Highways and Transportation (AASHTO) (1978). Standard specifications for transportation materials and methods of sampling and testing. AASHTO Designation: M 145-73, 12th ed. Washington, D.C.: AASHTO.
Avery, B.W. (1973). Soil classification in the soil survey of England and Wales. Journal of Soil Science 24:324-38.
Bakker, H. de and J. Schelling (1966). Systeem van bodemclassificatie voor Nederland: De hogere niveaus. Wageningen, The Netherlands: Center for Agricultural Publications and Documentation. 217 p.
Baldwin, M., C.E. Kellogg, and J. Thorp (1938). Soil classification. In Soils and men: Yearbook of agriculture. Washington D.C.: U.S. Department of Agriculture. pp. 979-1001.
Basinski, J.J. (1959). The Russian approach to soil classification and its recent development. Journal of Soil Science 10:14-26.
Butler, B.E. (1980). Soil classification for soil survey. Oxford: Clarendon Press. 129 p.
Costa de Lemos, R. (1968). The main tropical soils of Brazil: Approaches to soil classification. FAO World Soil Resources Report 32. Rome: Food and Agriculture Organization of the United Nations. pp. 95-106.
Commission de Pédologie et de Cartographie des Sols (CPCS) (1967). Classification des sols. l'Institut national de la recherche agronomique (INRA). Paris: Laboratoire de Géologie-Pédologie de l'ENSA.
Canada. Department of Supply and Services. (DSS) (1978). History of soil classification in Canada. Canada Department Agriculture Publication; no. 1646. Canada: DSS. 164 p.
Duchaufour, P. (1988). Pédologie. Paris: Masson. 224 pp.
Federal Aviation Administration (FAA) (1967). Airport Paving. Washington D.C.: FAA.
Finkl, C.W. (1982). Soil classification. Stroudsburg, Penn.: Hutchinson Ross Publishing Company. 391 p.
Jamangne, M. (1967). Bases et techniques d'une cartographie des sols. Vol. 18. Versailles: l'Institut national de la recherche agronomique.
Marbut, C.F. (1935). Soils of the United States. In Atlas of American agriculture. Part III. Washington D.C.: U.S. Department of Agriculture. pp. 1-98.
Mückenhausen, E. (1965). The soil classification system of the Federal Republic of Germany. Pedologie. Spec. Ser. 3:57-74.
Northcote, K.H. (1962). The factual classification of soils and its use in soil research. 7th International Congress of Soil Science, Transactions. New Zealand. pp. 291-297.
Thorp, J. and G.D. Smith (1949). Higher categories of soil classification. Soil Science 67:117-126.
United States Army (1967). The Unified Soil Classification System. Technical Memorandum; no. 3-357, U.S. Army Waterways Experiment Station, Office of the Chief of Engineers.
Williams, B. J. and C. A. Ortiz-Solorio (1981). Middle American folk soil taxonomy. Annals of the Association of American Geographers. 71(3)335-357.
World Bank (1994). Desertification: Implementing the Convention. Washington D.C.: The World Bank, Environmental Department. pp. 47.Text by Joe Tabor