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1) IntroductionFurther Reading:
Buol S.W., Hole F.D., McCracken R.J., and Southard R.J., 1997. Soil Genesis and Classification. Iowa State University Press.
Avery B.W., 1973. Soil Classification in the Soil Survey of England and Wales. J. of Soil Sci, 24: 324-338.
FAO-UNESCO, 1987. "Soils of the World" Food and Agriculture Organization and United Nations Educational, Scientific and Cultural Organization, Elsevier Science Publishing Co. Inc., New York, NY.
Glazovskaya M.A., 1983. Soils of the World. Vol.1. Soil Families and Soil Types. New Dehli, India: USDA and Nat. Sci. Found., Washington, DC. Transl. from the Russian.
Jenny H., 1941. Factors of Soil Formation. McGraw-Hill, New York.
Simonson R.W., 1959. Outline of a Generalized Theory of Soil Genesis. Soil Sci. Soc. Am. Proc. 23: 152-156.
Soil Survey Staff. 1997. Keys to Soil Taxonomy USDA - Soil Conservation Service. 7th ed., Washington D.C.
Wilding L.P., Smeck N.E., and Hall G.F., 1983. Pedogenesis and Soil Taxonomy I. Concepts and Interactions - II. The Soil Orders. Elsevier Sci. Publ., New York.
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1) Introduction
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Formation
One of the earliest land evaluation systems that incorporated a soil classification was established during the Vao dynasty (2357-2261 B.C.) in China. Soils were graded into nine classes, based on their productivity. It has been suggested that property taxes were based on the size of the individual land holding and soil productivity (Lee, 1921).
In former times (< 1600 A.C.), soil was solely considered as a medium for plant growth . Knowledge of soil behavior and crop growth was passed from generation to generation gained by observation . For example, in the Middle Ages it was well know that manure applied to soils improved crop growth. There was a close relationship between plant and animal production. For instance, the 'Plaggen cultivation' was practiced for a long time in Europe, which left 'Plaggen soils': The top of grassland was peeled off and used as litter in the stables . This material mixed with manure was applied to arable land to improve crop production. In 1840, the German chemist Justus von Liebig initiated a revolution in soil science and agriculture. He proved that plants assimilate mineral nutrients from the soil and proposed the use of mineral fertilizers to fortify deficient soils. Crop production was increased tremendously using mineral fertilizers. Another effect was the shift from extensive to intensive techniques in agriculture, which influenced soils. Thaer (1853) published a classification that combined texture as a primary subdivision with further subdivisions based on agricultural suitability and productivity. Several classifications based largely on geologic origin of soil material were also proposed in the 19th century (Fallou, 1862; Peters et al., 1885; Richtofen, 1886).
From the 1660s onwards, various members of The Royal Society of London proposed schemes of soil classification that incorporated elements of a natural or scientific approach in their criteria (Boyle, 1665; Lister, 1684). From this period on, the disciplines of agricultural chemistry (with a strong focus on soil fertility), geography, and geology provided a broad but somewhat fragmented background from which pedology emerged as a separate discipline in the late 19th century more or less independently in Russia (Dokuchaev and colleagues) and in the United States (Hilgard and colleagues).
In 1883, Dokuchaev carried out a comprehensive field study in Russia, where he described the occurance of different soils thoroughly using soil morphologic features. Due to his observations in the field he hypothesized that different environmental conditions result in the development of different soils. He defined soil as an independent natural evolutionary body formed under the influence of five factors , of which he considered vegetation and climate the most important. Dokuchaev is generally credited with formalizing the concept of the 'five soil forming factors', which provides a scheme for study of soils as natural phenomena. The soil classification developed by Dokuchaev and his colleagues (Glinka, Neustruyev) was based on the soil forming factors -> soil forming processes -> and diagnostic horizons / soil properties. The focus in his soil classification approach was in soil genesis, therefore the classification system is called 'genetic'.
In the United States, Hilgard (1892) emphasized the relationship between soils and climate, which is known as the climatic zone concept. Coffey (1912), produced the first soil classification system for the United States based on the soil genesis principles of Dokuchaev and Glinka. Marbut (1951) introduced the concepts of Coffey into soil survey programs in the U.S. carried out by the USDA (United States Department of Agriculture). Between 1912 and the 1960 the soil classification in the U.S. used a genetic approach. Jenny (1941) put together a detailed description of the five soil forming factors responsible for the development of different soils. In 1959, Simonson stressed that many genetic processes are simultaneously and / or sequentially active in any soil. Hence, a soil classification based on principles of soil genesis would not be favorable. Other soil scientists in the U.S. like Smith (director of Soil Survey Investigations for the USDA Soil Conservation Service) agreed, that soil genesis is very important for soil classification, but that genesis itself cannot be used as a basis for soil classification because the genetic processes can rarely be quantified or actually observed in the field.
References
Boyle, 1665. ?????
Coffey G.N., 1912. A Study of the Soils of the United States. U.S. Dept. of Agriculture Bur., Soils Bull. 85. U.S. Govt. Printing Office, Washington D.C.
Dokuchaev V.V., 1883. Russian Chernozems (Russkii Chernozems). Israel Prog. Sci. Trans., Jerusalem, 1967. Transl. from Russian by N. Kaner. Available form U.S. Dept. of Commerce, Springfield, VA.
Fallou F.A., 1862. Pedologie oder allgemeine und besondere Bodenkunde, Dresden, Germany.
Hilgard E.W., 1892. A Report on the Relations of Soil to Climate. U.S. Dept. of Agriculture. Weather Bull. 3: 1-59.
Jenny H., 1941. Factors of Soil Formation. McGraw-Hill, New York.
Lee, 1921 ??????
Liebig J. von, 1840. Chemistry in its Application to Agriculture and Physiologie. Playfaeir, Taylor and Waton, London.
Lister 1684 ????????
Marbut C.F., 1951. Soils: Their Genesis and Classification. A Memorial Volume of Lectures given in the Graduate School of the United States Dept. of Agriculture in 1928. Publ. 1951 by Soil Sci. Soc. Am., Madison, WI.
Peters and Peters, 1885 ????????
Richtofen F.F. von, 1886. Fuehrer fuer Forschungsreisende. Berlin.
Simonson R.W., 1959. Outline of a Generalized Theory of Soil Genesis. Soil Sci. Soc. Am. Proc. 23: 152-156.
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3) Soil Formation
The soil classification based on external environmental factors and an assumed genesis as differentiating characteristics led to dissatisfaction among U.S. soil scientists in the 1950's and 1960's. It was stressed that there is a lot of uncertainty involved dealing with soil forming factors and changing environmental conditions in a landscape. Many processes go on in any soil, often offsetting one another. Hence, it is often difficult to identify the processes in soils because most soils are polygenetic. Furthermore, it is difficult to assess the relative importance of each soil forming factor contributing to the development of a distinct soil class. There was much concern of not being able to classify certain soils with adequate agreement because of uncertainties or disagreement concerning their genesis. The knowledge about processes was and still is limited, because we do not completely understand soil processes. It became obvious that a more 'objective' soil classification scheme should be developed to replace the 'subjective' one.
As a consequence a completely new system of soil classification was developed: The U.S. Soil Taxonomy (Soil Survey Staff, 1975). It is termed a 'natural' soil classification system, which attempts to organize the division of soils from a more holistic appraisal of soil attributes. The pedon is used as the sampling unit for soil classification and mapping in schemes employing the U.S. Soil Taxonomy. Diagnostic horizons are used to define different soil taxa. Since that time, the Keys to Soil Taxonomy have undergone six published revisions. In this course the 6th edition of Keys to Soil Taxonomy (Soil Survey Staff, 1994) is used.
The principles of Soil Taxonomy are:
Classify soils on basis of
properties
Soil properties should be readily
observable and / or measurable
Soil properties should either
affect soil genesis or result from soil genesis
Soil processes are not refused in the Soil Taxonomy but the focus is on soil properties rather than on processes. The rationale behind this is that processes are important to produce soil properties and that those are used for classification, i.e. genesis 'lies behind' the soil taxa. Soil processes provide a framework for understanding the Soil Taxonomy, for example, the soil forming process of decomposition and humification forms an A horizons. The soil properties recognizable in the field are a dark color of the A horizon with a granular soil structure . The soil properties selected should be observable or measurable, though instruments may be required for observation or measurement. Properties that can be measured quantitatively are to be preferred to those that can be determined only qualitatively.
References
Soil Survey Staff. 1975. Soil Taxonomy. U.S. Dept. Agric. Handb. 436. U.S. Govt. Printing Office, Washington, DC.
Soil Survey Staff. 1994. Keys to Soil Taxonomy U.S.D.A. - Soil Conservation Service. 6th ed., Washington DC.
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3) Soil Formation
There are many diverse approaches to soil classification in different countries. Most schemes have in common a hierarchical structure, i.e. a structure in which individuals are collected in small groups, the small groups belonging to larger groups, a.s.o. The criteria to be used in establishing the different categories are:
The inferred genesis of the
soil, on the assumption that the soil is in equilibrium with its
environment and reflects the influence of prevailing soil forming
factors, or:
The soil properties , being
the outward expression of the processes which shaped the soil.
No matter how divergent current soil classifications are in principle, language and format, two main levels of generalization stand out:
A lower level encompassing
the surveyor's series, which are narrow classes in harmony with the
landscape.
A higher level of broad
classes (conceptual classes) with somewhat overlapping boundaries,
each of which embodies a central, usually pedogenetic concept.
Examples for modern soil classifications used in different countries:
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