Soil Taxonomy and Classification




Soil Taxonomy & Classification

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1.     1. 1 SOIL TAXONOMY & CLASSIFICATION By Prof. A. Balasubramanian Centre for Advanced Studies in Earth Science, University of Mysore, Mysore
2.     2. 2 Objectives: A soil is composed primarily of minerals which are produced from parent material that is weathered or broken into small pieces. Soils are the loose mineral or organic materials 25% air, 25% water, 45% mineral and 5% organic matter (humus, tiny living organisms and sometimes plant residue).
3.     3. 3 Like the classification systems for plants and animals, the soil classification system contains several levels of details, from the most general to the most specific types. The most general level of classification system is the soil order, of which there are 12 major types. The objective of this report is to provide the basics of soil taxonomy and the orders of soil that are most common throughout the world.
4.     4. 4 1.0 Introduction: Soil is a natural body comprised of solids (minerals and organic matter), liquid, and gases that occurs on the land surface. Soil occupies some space. Soil is characterized by both horizons, or layers.
5.     5. 5 These are distinguishable from the initial material as a result of additions, losses, transfers, and transformations of energy and matter or the ability to support rooted plants in a natural environment. Soil is not uniformly distributed. Properties of soil vary due to several soil forming factors. Properties vary very widely. Soil types are many.
6.     6. 6 To identify, understand, and manage soils, soil scientists have developed a set of soil classification or taxonomy systems. 1.1 The Soil Pedon: The term Pedon is used as a Unit of Sampling. A few soil properties can be determined from the surface.
7.     7. 7 To determine the nature of a soil, one must study its horizons, or layers. This study requires pits or some means of extracting samples of material from the surface to the base of the soil. A pedon will have the smallest volume for which one should describe and sample the soil to represent the nature and arrangement of its horizons and variability in the properties that are preserved in samples.
8.     8. 8 A pedon is comparable in some ways to the unit cell of a crystal. 1.2 The Epipedon The epipedon (Gr. epi, over, upon, and pedon, soil) is a horizon that forms at or near the surface and in which most of the rock structure has been destroyed.
9.     9. 9 It is darkened by organic matter or shows evidence of eluviation, or both. Rock structure includes fine stratification (less than 5 mm) in unconsolidated sediments (aeolian, alluvial, lacustrine, or marine) as well as saprolite derived from consolidated rocks in which the unweathered minerals and pseudomorphs of weathered minerals retain their relative positions to each other.
10.  10. 10 An epipedon is not the same as an A horizon. Soil Taxonomy recognizes eight epipedons. The most common pedons are the ochric, mollic, and umbric epipedons. In general, they are distinguished based on organic matter accumulation, color, thickness, and degree of base saturation.
11.  11. 11 2.0 Soil Taxonomy: Soil Taxonomy is a basic system of soil classification for making and interpreting soil surveys. This system helps to classify soils into various classes. The taxonomic classes defined in Soil Taxonomy group soils that have similar properties and that formed as a result of similar pedogenic processes.
12.  12. 12 Rather than define classes based directly on theories of soil genesis, however, the classes are based largely on the presence of diagnostic horizons and characteristics. These diagnostic horizons and characteristics reflect the important pedogenic processes (i.e., additions, removals, transfers, and transformations) that are either occurring now, or have occurred in the past, to produce the kinds of soil profiles we see today.
13.  13. 13 2.1 Soil Classification: Soil is not uniformly distributed. Properties of soil vary due to several soil forming factors. properties vary very widely. Soil types are many. Soil Classification concerns the grouping of soils with a similar range of properties (chemical, physical and biological) into units that can be geo-referenced and mapped.
14.  14. 14 Soils are a very complex natural resource, much more so than air and water. Early soil classification systems (Russian and USDA of the year 1938) focused on the environment and the soil forming factors to classify soils into zonal soils, azonal soils and intrazonal soils.
15.  15. 15 The difference between azonal and intrazonal soils was made on the basis of soil profile development. It is necessary to adopt a formal system of soil description and classification in order to describe the various materials found in ground investigation. Such a system must be meaningful and concise in an engineering context.
16.  16. 16 2.2 Classification of soil : Classification of soil is the separation of soil into classes or groups each having similar characteristics and potentially similar behaviour. A classification for engineering purposes should be based mainly on mechanical properties: permeability, stiffness, strength.
17.  17. 17 The class to which a soil belongs can be used in its description. A number of systems of classification have been evolved for categorizing various types of soil. The World Reference Base (WRB) is the international standard for soil classification system endorsed by the International Union of Soil Sciences.
18.  18. 18 2.3 The more common classification systems are enlisted below: a) Geological Classification b) Classification by Structure c) Classification based on Grain-size d) Unified Soil Classification System e) Preliminary Classification by soil types.
19.  19. 19 2.4 Standard Class. systems: Australian Soil Classification. / Canadian system of soil classification./ French soil classification./ FAO soil classification (1974-1998)/ International Committee on Anthropogenic Soils (ICOMANTH) / Unified Soil Classification System. USDA soil taxonomy/ Indian Standard Classification (IS: 1498-1970).
20.  20. 20 3.0 Geological Classification: Soil types may be classified on the basis of their geological origin. The origin of a soil may refer either to its constituents or to the agencies responsible for its present status. Based on constituents, soil may be classified as: Inorganic soil/ Organic soil. Based on the agencies responsible for their present state, soils may be classified under following types:
21.  21. 21 Residual Soils. Transported Soils: Alluvial or sedimentary soils/Aeolian soils /Glacial soils / Deposited Soils = Lacustrine soils/Marine soils. 4.0 Classification by Structure: Depending upon the average grain-size and the conditions under which soils are formed and deposited in their natural state, they may be categorized into following types on the basis of their structure:
22.  22. 22 Soils of single-grained structure /Soils of honey-comb structure/ Soils of flocculent structure. 5.0 Classification based on grain-size In the grain-size classification, soils are designated according to the grain-size or particle-size.
23.  23. 23 Terms such as gravel, sand, silt and clay are used to indicate certain ranges of grain-sizes. Since natural soils are mixtures of all particle- sizes, it is preferable call these fractions as sand size, silt size, etc. The range of particle sizes encountered in soils is very large: from boulders with dimension of over 300 mm down to clay particles that are less than 0.002 mm.
24.  24. 24 Some clays contain particles less than 0.001 mm in size which behave as colloids, i.e. do not settle in water. In the Indian Standard Soil Classification System (ISSCS), soils are classified into groups according to size, and the groups are further divided into coarse, medium and fine sub- groups.
25.  25. 25 The grain-size range is used as the basis for grouping soil particles into boulder, cobble, gravel, sand, silt or clay. Very coarse soils Boulder size > 300 mm Cobble size 80 - 300 mm Coarse soils Gravel size (G) Coarse 20 - 80 mm Fine 4.75 - 20 mm Sand size (S) Coarse 2 - 4.75 mm Medium 0.425 - 2 mm Fine 0.075 - 0.425 mm Fine soils Silt size (M) 0.002 - 0.075 mm Clay size (C) < 0.002 mm
26.  26. 26 Gravel, sand, silt, and clay are represented by group symbols G, S, M, and C respectively. 5.0 Unified soil classification system: Unified soil classification system was originally developed by Casagrande (1948) and was known as airfield classification system.
27.  27. 27 It was adopted with some modification by the U.S. Bureau of Reclamation and the U.S. Corps of Engineers. This system is based on both grain size and plasticity characteristics of soil. Coarse grained soils are those with more than 50% of the material larger than 0.075mm size. Coarse grained soils are further classified into gravels (G) and sands (S).
28.  28. 28 The gravels and sands are further divided into four categories according to gradation, silt or clay content. Fine grained soils are those for which more than 50% of soil finer than 0.075 mm sieve size. They are divided into three sub-divisions as silt (M), clay ( C ) and organic salts and clays (O). based on their plasticity nature they are added with L, M and H symbol to indicate low plastic, medium plastic and high plastic respectively.
29.  29. 29 These are the Examples with their notations: GW – well graded gravel GP – poorly graded gravel GM – silty gravel SP – poorly graded sand SW – well graded sand SC – clayey sand SM – silty sand OH – organic silt and clays of high plastic. CL – clay of low CI – clay of medium
30.  30. 30 plastic plastic CH – clay of higher plastic ML – silt of medium plastic MI – silt of medium plastic MH – silt of higher plastic OL – organic silt and clays of low plastic OI – organic silt and clays of medium plastic
31.  31. 31 Fine grained soils have been sub-divided into three subdivisions of low, medium and high compressibility instead of two sub-divisions of the original Unified Soil Classification System. 6.0 Preliminary Classification by soil types: Familiarity with common soil types is necessary for an understanding of the fundamentals of soil behaviour.
32.  32. 32 In this approach, soils are described by designation such as Boulders, Gravel, Sand, Silt, Clay, Rock flour, Peat, China Clay, Fill, Bentonite, Black Cotton soil, Boulder Clay, Caliche, Hardpan, Laterite, Loam, Loess, Marl, Moorum, Topsoil and Varved Clay.
33.  33. 33 6. 1 Soil Taxonomy has six categories: Soil Taxonomy has six categories. These are, from top to bottom, order, suborder, great group, subgroup, family and series. Ten classes are in the order level. Criteria used to differentiate orders are highly generalized and based more or less on the kinds and degrees of soil-forming processes.
34.  34. 34 Mostly these criteria include properties that reflect major differences in the genesis of soils. A suborder category is a subdivision of an order within which genetic homogeneity is emphasized. Soil characteristics used to distinguish suborders within an order vary from order to order. The great group category is a subdivision of a suborder.
35.  35. 35 They are distinguished one from another by kind and sequence of soil horizons. All soils belonging to one of the suborders will have more horizons. Soils having these additional horizons are placed in separate great groups. Great group categories are divided into three kinds of subgroups: typic, intergrade and extragrade.
36.  36. 36 A typic subgroup represents the basic concept of the great group from which it derives. An intergrade subgroup contains soils of one great group, but have some properties characteristic of soils in another great group or class. These properties are not developed or expressed well enough to include the soils within the great group toward which they grade.
37.  37. 37 Extragrade subgroup soils have aberrant properties that do not intergrade to any known soil. A soil family category is a group of soils within a subgroup that has similar physical and chemical properties that affect response to management and manipulation. The principal characteristics used to differentiate soil families are texture, mineralogy and temperature.
38.  38. 38 Family textural classes, in general, distinguish between clayey, loamy and sandy soils. For some soils the criteria also specify the amount of silt, sand and coarse fragments such as gravel, cobbles and rocks. The soil series is the narrowest category in soil taxonomy .
39.  39. 39 7.0 Soil Orders: To identify, understand, and manage soils, soil scientists have developed a soil classification or taxonomy system. Like the classification systems for plants and animals, the soil classification system contains several levels of detail, from the most general to the most specific.
40.  40. 40 The most general level of classification in the United States system is the soil order. There are 12 orders of soils categorized by the U.S. Department of Agriculture. Each order is based on one or two dominant physical, chemical, or biological properties that differentiate it clearly from the other orders.
41.  41. 41 Each order is based on one or two dominant physical, chemical, or biological properties that differentiate it clearly from the other orders. The 12 soil orders all end in "sol" which is derived from the Latin word "solum" meaning soil or ground. Most of the orders also have roots that tell you something about that particular soil. For example, "molisol" is from the Latin "mollis" meaning soft.
42.  42. 42 7.1 Gelisols: Gelisols (from the Latin gelare – to freeze) are soils that are permanently frozen (contain “permafrost”) or contain evidence of permafrost near the soil surface. Gelisols are found in the Arctic and Antarctic, as well as at extremely high elevations.
43.  43. 43 Permafrost influences land use through its effect on the downward movement of water and freeze-thaw activity (cryoturbation) such as frost heaves. Permafrost can also restrict the rooting depth of plants. Gelisols make up about 9% of the world’s glacier-free land surface.
44.  44. 44 7.2 Histosols: Histosols (from the Greek histos – tissue) are dominantly composed of organic material in their upper portion. The Histosol order mainly contains soils commonly called bogs, moors, peat lands, muskegs, fens, or peats and mucks.
45.  45. 45 These soils form when organic matter, such as leaves, mosses, or grasses, decomposes more slowly than it accumulates due to a decrease in microbial decay rates. This most often occurs in extremely wet areas or underwater; thus, most of these soils are saturated year-round. Histosols can be highly productive farmland when drained; however, drained Histosols can decompose rapidly and subside dramatically.
46.  46. 46 They are also not stable for foundations or roadways, and may be highly acidic. Histosols make up about 1% of the world’s glacier-free land surface. 7.3 Spodosols: Spodosols (from the Greek spodos – wood ash) are among the most attractive soils.
47.  47. 47 They often have a dark surface underlain by an ashy gray layer, which is subsequently underlain by a reddish, rusty, coffee-colored, or black subsoil horizon. These soils form as rainfall interacts with acidic vegetative litter, such as the needles of conifers, to form organic acids. These acids dissolve iron, aluminum, and organic matter in the topsoil and ashy gray (eluvial) horizons.
48.  48. 48 The dissolved materials then move (illuviate) to the colorful subsoil horizons. Spodosols most often develop in coarsely textured soils (sands and loamy sands) under coniferous vegetation in humid regions of the world. They tend to be acidic, and have low fertility and low clay content. Spodosols occupy about 4% of the world’s glacier-free land surface.
49.  49. 49 7.4 Andisols: Andisols (from the Japanese ando – black soil) typically form from the weathering of volcanic materials such as ash, resulting in minerals in the soil with poor crystal structure. These minerals have an unusually high capacity to hold both nutrients and water, making these soils very productive and fertile.
50.  50. 50 Andisols include weakly weathered soils with much volcanic glass, as well as more strongly weathered soils. They typically occur in areas with moderate to high rainfall and cool temperatures. They also tend to be highly erodible when on slopes. These soils make up about 1% of the glacier-free land surface.
51.  51. 51 7.5 Oxisols: Oxisols (from the French oxide – oxide) are soils of tropical and subtropical regions, which are dominated by iron oxides, quartz, and highly weathered clay minerals such as kaolinite. These soils are typically found on gently sloping land surfaces of great age that have been stable for a long time.
52.  52. 52 For the most part, they are nearly featureless soils without clearly marked layers, or horizons. Because they are highly weathered, they have low natural fertility, but can be made productive through wise use of fertilizers and lime. Oxisols are found over about 8% of the glacier- free land surface.
53.  53. 53 7.6 Vertisols : Vertisols (from the Latin verto – turn) are clay- rich soils that contain a type of “expansive” clay that shrinks and swells dramatically. These soils therefore shrink as they dry and swell when they become wet. When dry, vertisols form large cracks that may be more than one meter (three feet) deep and several centimeters, or inches, wide.
54.  54. 54 The movement of these soils can crack building foundations and buckle roads. Vertisols are highly fertile due to their high clay content; however, water tends to pool on their surfaces when they become wet. Vertisols are located in areas where the underlying parent materials allow for the formation of expansive clay minerals. They occupy about 2% of the glacier-free land surface.
55.  55. 55 7.7 Aridisols: Aridisols (from the Latin aridus – dry) are soils that occur in climates that are too dry for “mesophytic” plants (plants adapted to neither too wet nor too dry environments)to survive. The climate in which Aridisols occur also restricts soil weathering processes.
56.  56. 56 Aridisols often contain accumulations of salt, gypsum, or carbonates, and are found in hot and cold deserts worldwide. They occupy about 12% of the Earth’s glacier-free land area, including some of the dry valleys of Antarctica.
57.  57. 57 7.8 Ultisols: Ultisols (from the Latin ultimus – last) are soils that have formed in humid areas and are intensely weathered. They typically contain a subsoil horizon that has an appreciable amount of translocated clay, and are relatively acidic.
58.  58. 58 Most nutrients are held in the upper centimeters of Ultisol soils, and these soils are generally of low fertility although they can become productive with additions of fertilizer and lime. Ultisols make up about 8% of the glacier-free land surface.
59.  59. 59 7.9 Mollisols: Mollisols (from the Latin mollis – soft) are prairie or grassland soils that have a dark colored surface horizon, are highly fertile, and are rich in chemical “bases” such as calcium and magnesium.
60.  60. 60 The dark surface horizon comes from the yearly addition of organic matter to the soil from the roots of prairie plants. Mollisols are often found in climates with pronounced dry seasons. They make up approximately 7% of the glacier-free land surface.
61.  61. 61 7.10 Alfisols: Alfisols (from the soil science term Pedalfer – aluminum and iron) are similar to Ultisols but are less intensively weathered and less acidic. They tend to be more inherently fertile than Ultisols and are located in similar climatic regions, typically under forest vegetation.
62.  62. 62 They are also more common than Ultisols, occupying about 10% of the glacier-free land surface. 7.11 Inceptisols: Inceptisols (from the Latin inceptum – beginning) exhibit a moderate degree of soil development, lacking significant clay accumulation in the subsoil.
63.  63. 63 They occur over a wide range of parent materials and climatic conditions, and thus have a wide range of characteristics. They are extensive, occupying approximately 17% of the earth’s glacier-free surface.
64.  64. 64 7.12 Entisols: Entisols (from recent – new) are the last order in soil taxonomy and exhibit little to no soil development other than the presence of an identifiable topsoil horizon. These soils occur in areas of recently deposited sediments, often in places where deposition is faster than the rate of soil development.
65.  65. 65 Some typical landforms where Entisols are located include: active flood plains, dunes, landslide areas, and behind retreating glaciers. They are common in all environments. Entisols make up the second largest group of soils after Inceptisols, occupying about 16% of the Earth’s surface.
66.  66. 66 8.0 Conclusion: Classification is basically important to any science. Classification provides the avenue through which research can be addressed in a rigorously systematic manner. Classifications also have more practical applications. Classification of soils is necessary for all soil survey program and mapping the soils of any region.
67.  67. 67 Soil surveys employ the principles, functions of soil science to agriculture, forestry and engineering to predict soil behavior for different use , management or manipulation.

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