Soil
Taxonomy & Classification
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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