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Topic: Soil Classification

Prof. Umesh Kumar Singh

Department of Environmental Science

School of Earth, Environmental and Biological Sciences

Central University of South Bihar, Gaya

Note: These materials are only for classroom teaching purpose at Central

University of South Bihar. All the data/figures/materials are taken from

several research articles/e-books/text books including Wikipedia and other

online resources.

Programme: M.Sc.(Environmental Science) Course: Soil Science

Semester: IV Code: MSESC4007E04

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• Pedology: The origin of the soil , its classification, and its

description are examined in pedology (pedon-soil or earth in

greek). Pedology is the study of the soil as a natural body and

does not focus primarily on the soil’s immediate practical use.

A pedologist studies, examines, and classifies soils as they

occur in their natural environment.

• Edaphology (concerned with the influence of soils on living things,

particularly plants ) is the study of soil from the stand point of

higher plants. Edaphologist considers the various properties of

soil in relation to plant production.

• Soil Profile: specific series of layers of soil called soil

horizons from soil surface down to the unaltered parent

material.

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• By area Soil – can be small or few hectares.

• Smallest representative unit – k.a. Pedon

• Polypedon

• Bordered by its side by the vertical section of soil …the soil profile.

• Soil profile – characterize the pedon. So it defines the soil.

• Horizon tell- soil properties- colour, texture, structure, permeability, drainage, bio-activity etc.

• 6 groups of horizons k.a. master horizons. O,A,E,B,C &R.

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Soil Sampling and Mapping Units

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Typical

soil

profile

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O

• OM deposits (decomposed, partially

decomposed)

• Lie above mineral horizon

• Histic epipedon (Histos Gr. – tissue)

• Oi (non to slightly decomposed)

• Oe (intermediately decomposed)

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A

• Top most mineral horizons

• Inorganic – OM

• Usually dark in colour

• If O horizon not there – k.a. – Ap (p= plow)

• A horizon- used to classify soil (a.k.a

diagonsistic epipedons) (Gr. Epi= over; pedon

= soil)

• Seven major epipedons are there.

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Diagnostic Surface Horizons

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E horizons

• Located under A

• Zone of eluviation (Clay, humus, Fe, Al)

• Deposit (eluvial deposit) is the remaining

material.

• Usually white and pale (k.a. albic horizon)

• AE

• EB

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B horizons

• Illuviation

• The B horizon is a zone of illuviation where

downward moving, especially fine material, is

accumulated. The accumulation of fine

material leads to the creation of a dense layer

in the soil.

• Accumulation from A, E (Clay, Fe, Al, Humus)

• Transition zone BC

• Diagnostic subsurface horizons.

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DIAGNOSTIC SUBSURFACE HORIZONS

• Cambic Horizon (Bw horizon)- recently young

• Argillic (Bt horizons) – enriched with clay

• Natric Horizons - sodium

• Albic Horizon- white in colour

• Oxic Horizon – highly weathered horizon

• Calcic Horizons,

• Gypsic Horizons, and

• Salic Horizons

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DIAGNOSTIC SUBSURFACE HORIZONS:

• Cambic Horizon (Bw horizon)

• A young B Horizon

• The cambic horizon can form quickly, relatively

speaking, because changes in the color and

structure, and some leaching will convert the

subsoil parent material into a cambic horizon.

• Cambic horizons are not illuvial horizons and,

generally, they are not extremely weathered. A

cambic horizon is a Bw horizon.

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DIAGNOSTIC SUBSURFACE HORIZONS

• Argillic (Bt horizons) and NatricHorizons (Btn horizons)

• The gradual illuvial accumulation of clay in a cambic horizon converts a cambic horizon (Bw horizon) into a Bt horizon. Typically, clay skins occur on the ped surfaces of Bt horizons

• A natric horizon is a special kind of argillic horizon. These horizons have typically been affected by soluble salts. The natric horizon is a Btn horizon; the n indicates the accumulation of exchangeable sodium.

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DIAGNOSTIC SUBSURFACE HORIZONS

• Albic Horizon

• The loss by eluviation of sesquioxides• (A sesquioxide is an oxide containing three atoms of oxygen with

two atoms (or radicals) of another element. For example, aluminium

oxide (Al2O3) is a sesquioxide),and clay, during the formation of spodic and argillic horizons, tends to leave behind a light-colored overlying eluvial horizon called the albichorizon.

• Albic is derived from the word white. The horizon is eluviated and is labeled an E horizon. The color of the albic horizon is due to the color of the primary sand and silt particles rather than to the particle coatings. Albic horizons are commonly underlain by spodic, argillic, or kandichorizons.

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DIAGNOSTIC SUBSURFACE HORIZONS

• Oxic Horizon

• The oxic horizon (Bo horizon) is a subsurface horizon at least 30 centimeters thick, that is in an advanced stage of weathering. It is not dependent on a difference in the clay content of subsoil versus the topsoil horizons.

• Oxic is derived from the word oxide.

• Oxic horizons consist of a mixture of iron and/or aluminum oxideswith variable amounts of kaolinite and highly insoluble accessory minerals such as quartz sand.

• Soils with oxic horizons have essentially reached the end point of weathering.

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DIAGNOSTIC SUBSURFACE HORIZONS

• Calcic, Gypsic, and Salic Horizons

• The calcic horizon is a horizon of calcium carbonate or calcium and magnesium carbonate accumulation.

• Calcic horizons develop in soils in which there is limited leaching and the carbonates are translocated downward. However, the carbonates are deposited within the soil profile because there is insufficient water to leach the carbonates to the water table. The symbol k indicates an accumulation of carbonates, as in Bwk or Ck.

• Gypsic horizons have an accumulation of secondary sulfates, and that is indicated with the symbol y. Cemented calcic and gypsic horizons are petrocalcic and petrogypsic horizons, respectively.

• Salic horizons contain a secondary enrichment of saltsmore soluble than gypsic and are indicated with the symbol z.

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Soil Profile = layers in soil observed with depth;

Individual layers are horizons

Bedrock

O horizon – contains litter on

surface, humus + OM beneath

A horizon – various stages of

breakdown of organic matter

B horizon – mineral soil in which organic

compounds have been converted into

inorganic

C horizon – unmodified

parent material

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Zonation in Soil Profile

Bedrock

A horizon – may show color gradient

as OM decreases – eluvial horizons =

leaching

B horizon – inorganic compounds leached

from A horizon accumulate here – illuvial

horizons = accumulation

Rain,

Leaching

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Eluviation: It is the mobilization and translocation of certain constituent’s viz. Clay, Fe2O3, Al2O3, SiO2, humus, CaCO3, other salts etc. from one point of soil body to another. Eluviation means washing out. It is the process of removal of constituents in suspension or solution by the percolating water from the upper to lower layers. The eluviation encompasses mobilization and translocation of mobile constituents resulting in textural differences. The horizon formed by the process of eluviation is termed as eluvial horizon (A2 or E horizon).

Illuviation: The process of deposition of soil materials (removed from the eluvial horizon) in the lower layer (or horizon of gains having the property of stabilizing translocated clay materials) is termed as Illuviation. The horizons formed by this process are termed as illuvialhorizons (B-horizons, especially Bt) The process leads to textural contrast between E and Bt horizons, and higher fine: total clay ratio in the Bt horizon.

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Soil Profile is determined by

• Vegetation

• Climate ---- rainfall and leaching

• Weathering ---- geological history, age

• Topography

• Parent Material

• Etc.

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Soil Classification

• Profile

• Texture

• Origin

• Vegetation

• Minerals

• Climate

• Age

• Etc.

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Order

Suborder

Great group

Sub group

Family

Series

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19,000

Soil Taxonomy Hierarchy

63

250

1400

8000

Kingdom

Phylum

Class

Order

Family

Genus

Species

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Soil Orders12 recognized, 10 used in agriculture

1. Histosols2. Vertisols3. Entisols4. Inceptisols5. Aridisols6. Mollisols7. Alfisols8. Spodosols9. Oxisols10. Ultisols11. Andisols12. Gelisols

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Soil OrdersUse in Agriculture

• Histosol

• Vertisol

• Entisol

• Inceptisol

• Aridisol

• Mollisol

• Alfisol

• Spodosol

• Oxisol

• Ultisol

Many types respond to

management (fertilizer,

irrigation)

Best

Poor

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Formative Syllables, Derivations, and Meanings of Soil Orders

Gelisols el Gk. Gelid very cold, pergelic soil

temp. regime

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Diagnostic Subsurface Horizons

Clays Organic Matter Oxides

Formation

Translocation

Transformation

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Mollic Horizon

• The word mollic is derived from mollify, which means to soften. The mollic horizon is a surface horizon that is soft rather than hard and massive when dry.

• Mollic epipedon formation is favored where numerous grass roots permeate the soil and a moderate to strong grade of structure is created.

• Except for special cases, mollic horizons are dark-colored, contain at least 1 percent organic matter, and are at least 18 centimeters thick.

• They are only minimally or moderately weathered and leached.

• The cation exchange capacity is 50 percent or more saturated with calcium, magnesium, potassium, and sodium when the cation exchange capacity is determined at pH 7.

• The major grassland soils of the world have mollic epipedons.

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Umbric and Ochric Horizons

• The umbric epipedon is similar to the mollic in overall appearance of thickness and color; however, it is more leached and has a saturation with basic cations calcium, magnesium, potassium, and sodium that is less than 50 percent when the cation exchange capacity is determined at pH 7.

• The epipedon is thinner than that of the umbric, is lower in organic matter, and is lighter in color. This epipedon is called ochric.

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Histic Horizon

• Where soil development occurs under conditions of extreme wetness, as in swamps or lakes, the epipedon is organic in nature and is, typically, a histic epipedon. Histic epipedons are o horizons.

• A histic epipedon must be water saturated for at least 30 consecutive days during the year, unless the soil has been drained.

• The degree of decomposition of the organic matter in histichorizons is indicated with the following symbols:

• i - for fibric,

• e - for hemic, and

• a - for sapric.

• Fibric (i) is the least decomposed and contains a large amount of recognizable plant fibers;

• sapric (a) is the most decomposed; and

• hemic (e) is of intermediate decomposition.

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Melanic Horizon

• Melanic epipedons are thick, black colored, and contain high

concentrations of organic matter, similar to some histic horizons.

• The organic carbon content is 6% or more but less than 25%.

• The organic matter is thought to result from the supply of large

amount of root residues from a graminaceous vegetation in

contrast to organic matter that results from a forest vegetation.

• Even though the organic matter content is high, the organic

matter in melanic horizons is mostly associated or complexed

with minerals, which results in properties dominated by the

mineral fraction rather than the organic fraction.

• The horizon is also characterized by low bulk density and high

anion exchange capacity.

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Anthropic and Plaggen Horizons

• The anthropic and plaggen horizons are formed by human activity.

The anthropic epipedon resembles the mollic horizon in color,

structure, and organic matter content; however, the phosphorus

content is 250 or more ppm of P2O5 equivalent soluble in 1 percent

citric acid solution. The anthropic horizon occurs where human

activity resulted in the disposal of bones and other refuse near

places of residence, or in agriculture at sites of long-continued use

of soil for irrigated.

• The plaggen epipedon is found in Europe where long-continued

manuring has produced a surface layer 50 centimeters or more

thick. During the Middle Ages, farmers in northwestern Europe

collected sods from forests where soils were very sandy. The

manure from the barn was applied to the land, which resulted in the

slow accumulation of a thick, sandy epipedon enriched with

organic matter.

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Entisols:

•Young and shallow soil

•A/C or A/R profiles

•Immature soils and no B Horizons

•Time of formation is too short

•Occurrence of steep slope

•Actively eroding slope

•Receiving frequent deposits from flood•Alluvial soils in India along the banks of river Ganges and Indus•Causes of delayed or absent development

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Gelisols:

• They are soils of very cold climates

which are defined as containing

permafrost within two meters of the

soil surface.

• The word "Gelisol" comes from the

Latin gelare meaning "to freeze", a

reference to the process of

cryoturbation that occurs from the

alternating thawing and freezing

characteristic of Gelisols.

• Structurally, Gelisols have no B

horizon and have an A horizon resting

on the permafrost.

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• Because soil organic matter accumulates in the upper layer, most Gelisols are black or dark brown in soil

color, followed by a shallow mineral layer. – underlaid

by Permafrost.

• Despite the influence of glaciation in most areas where Gelisols occur, chemically they are not highly fertilebecause nutrients, especially calcium and potassium, are very easily leached above the permafrost.

• The permafrost greatly restricts the engineering use of Gelisols, as large structures (e.g. buildings) subside as

the frozen earth thaws when they are put in place.

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Inceptisols:

• Inceptisols mark the beginning

of mature soils and have A/Bw/C

profile.

• B horizons are in the stage of

formation and are called as

Cambic Horizons (Bw).

Paddy soil

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Mollisols:

Mature soils A/Bk/C profiles

Mollisoils are formed by calcification

A horizon is a mollic epipedon

B- calcic horizon (Bk)

Mollisols are formed by calcification in semihumid climates with tall vegetations

Imp. Grassland soils

Very fertile soils in world

Great Plains of Canada, Steppes of Mongolia, Pampas of Argentina,

Ukraine k.a.- Chernozems (Black earth)

High OM and N content, More base saturation

Soil Physically and chemically excellent.

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SpodosolsA/E/Bh or Bhs/C profiles

• Formation process is Podolization

• Occurs in cool humid regions under the influence

of coniferous forest or mixed conifer-hardwood

vegetation

• Under the influence of acid leaching- Al, Fe

compounds or humus are translocated to the B

Horizons creating spodic B Horizon.

• When B horizon enriched with Humus called as

Bh Horizon = Humod (Hum= Humus and od =

formative element from spodozols).

• When mixture of Fe and Al compounds and

humus is accumulated Bhs horizon is formed.

• Under certain condition if only Fe is accumulated

it is Bs horizon and called as Ferrod .

• FAO called this order as Podozols (Russian Pod =

under and Zola = white layer of ash) [Taiga forest]

• Soils are very acidic, need adequate fertilizers and

lime for crop production

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Podzolization

• Cool climates

• Forested areas generally

• Strongly leached

• Acidic soils

• Low CEC and low fertility

• Generally an E or spodic horizon

of leached/bleached gray/white

• Large surface humus layer

• Iron and Aluminum in B horizon

• Poor farming areas

• NE US

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AlfisolsA/E/Bt/C - mature soils

• Formed by the combination of Podolozation and laterization in cool to warm temperature humid regimes usually under hardwood forests.

•Soils are affected by more drastic leaching than mollisols and are therefore in a more advanced stage of profile development.

•Surface soil colour- gray-brown to reddish brown-a.k.a. Gray-brown podozolic soil.

•Because of eluviations process B horizon is enriched with illuvial clay called as agrillic horizon (Bt horizon) [a.k.a. luvisols L. Lou= to wash, illuvial clay]

•Alfisols are highly productive soils as base saturation is >35% , illuvial clay – unfavorable conditions.

•Liming need to neutralize moderate acidic conditions.

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Ultisols

•Mature soils – A/E/Bt/C profiles

•Formed by a combination of laterizationand podolization with more emphasis on laterization in warm humid temperature regions to humid tropics where leaching process is very much pronounced.

•Soils are highly weathered and A horizon accumulate varying amounts of Fe oxides which gives yellow colour to red colours.

•Enrichment of B horizons with illuvialclay cause agrillic horizons (Bt horizons).

•Due to drastic leaching process soil has low base status <35% .

•Adequate liming, OM, Fertilizers, proper management- soils can be quite productive.

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OXISOLS (A/B/C profiles)

Oxisols are the most intensively

weathered soils, consisting largely of

mixtures of quartz, kaolinite, oxides of

iron and aluminum, and some organic

matter. [hydrous oxides of clay and

sesquioxides]

Generally, Oxisols represent the most

naturally infertile soils for agriculture

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Laterites

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Laterization

Formed by laterization

process in warm humid and

tropical regions.

More weathered soils more

than ultisols.

Soils are highly leached and

are acidic in reaction and

low in bases.

Depending on the iron

content some oxisols may

convert it into laterites .

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Laterite = highly developed oxisol.

Forms in a hot, humid

climate. Soil is deep red,

hard and infertile.

Plants recycle nutrients in a thin A and O horizon.

Soil types: oxisols

Laterite formation on gneiss,

Kerala, India

45Oxisols being used for building materials in India

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Aridisols

•PET >> Precipitation

•Desert soils

•Pale, light color near surface

•Long periods of soil moisture deficit

•Little if any organic matter

•High CEC

•Very fertile if add water

•Salinization is common

•Here the white color is from salts that have

precipitated in the soil

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Aridisols

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VertisolsConditions that give rise to the

formation of Vertisols are: (1) parent

materials high in, or that weather to

form, a large amount of expanding clay,

usually smectites; and (2) a climate with

alternating wet and dry seasons.

Expansion or swelling of soil in the

vicinity of cracks results in great lateral

and upward pressures, which cause a

slow gradual movement of soil upward

between areas that crack. This soil

movement produces a microrelief called

gilgai.

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Genesis of Vertisols

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Summary:

• 3 orders with unique parent materials

(Andisols, Histisols, Vertisols).

• 3 orders with unique environments

(Aridisols, Gelisols, Oxisols).

• 3 orders by age of development (Entisols,

Inceptisols, Ultisols).

• 3 orders by unique vegetative influence

(Alfisols, Mollisols, Spodosols).

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References:1. N.C Brady -The Nature and Properties of Soil

2. McBride - Environmental Chemistry of Soils

3.Dutta -Principles and Practice of Soil Science

Note: These materials are only for classroom teaching purpose at Central

University of South Bihar. All the data/figures/materials are taken from

several research articles/e-books/text books including Wikipedia and other

online resources.