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SOIL GENESIS AND CLASSIFICATION “Upon this handful of soil our survival depends. Husband it and it will grow our food, our fuel and our shelter and surround us with beauty. Abuse it and the soil will collapse and die, taking humanity with it.” – Vedas Sanskrit scripture-1500 BC

Authors:
  • ICAR-National Bureau of Soil Survey and Land Use Planning

Abstract and Figures

The domain of soil science study could be broadly divided in to two groups: pedology and edaphology. In edaphology, we intend to study the soil as a medium of plant growth, therefore, all physical, chemical and biological soil properties are of relevance. However, in pedology, main focus is to study about the process of soil formation, rocks and minerals, their weathering, soil forming factors and methods of soil survey, mapping and classification. Initially earth was a molten magma. Cooling and crystallization of this magma led to the formation of igneous rocks which is a source of sedimentary and metamorphic rocks. The composition of the rocks depends upon the type of minerals present in it. Weathering of the rocks results in to formation of parent material which constitutes about 45% volume of soil solid. Therefore, there is similarity between chemical composition of the soil to that of the earth's crust. The parent material is the state of soil system at the time zero of soil formation. Soil formation proceeds further by the combined interaction of soil forming factors and soil forming processes. Soil forming factors set the stage for the operation of processes of soil formation and have the influence on properties of the soils. The soil forming processes are the reorganization and rearrangement of mineral and organic soil constituents in to different horizons leading to development of soil. Initially soil was considered as weathered product of rocks by the geologists but V.V. Dokuchaev (Figure 3.1), known as ‘Father of Soil Science’, was the first to establish soil as a distinct natural body having a definite genesis and a distinct nature of its own. Like plants and animals, soil does not have any distinct physical body, therefore, pedon has been considered as a hypothetical soil body having minimum volume of soil to be observable. The real morphology of soil is present below the surface of the earth, therefore, society often fails to realize it. Soil is a highly heterogeneous body thus always difficult to be grouped in to different categories. Various classification systems have been prevalent during past but the ‘Soil Taxonomy’ is now an acceptable scientific system of soil classification. This chapter is intended to present a brief on the pedological aspects of soil science.
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Chapter 3
Soil Genesis and Classification
S.K. SINGH AND P. CHANDRAN
“Upon this handful of soil our survival depends. Husband it and it will grow
our food, our fuel and our shelter and surround us with beauty. Abuse it and
the soil will collapse and die, taking humanity with it.” – Vedas Sanskrit
scripture-1500 BC
3.1. Introduction
The domain of soil science study could be broadly divided in to two groups: pedology and
edaphology. In edaphology, we intend to study the soil as a medium of plant growth,
therefore, all physical, chemical and biological soil properties are of relevance. However,
in pedology, main focus is to study about the process of soil formation, rocks and minerals,
their weathering, soil forming factors and methods of soil survey, mapping and
classification. Initially earth was a molten magma. Cooling and crystallization of this
magma led to the formation of igneous rocks which is a source of sedimentary and
metamorphic rocks. The composition of the rocks depends upon the type of minerals
present in it. Weathering of the rocks results in to formation of parent material which
constitutes about 45% volume of soil solid. Therefore, there is similarity between chemical
composition of the soil to that of the earth's crust. The parent material is the state of soil
system at the time zero of soil formation. Soil formation proceeds further by the combined
interaction of soil forming factors and soil forming processes. Soil forming factors set the
stage for the operation of processes of soil formation and have the influence on properties
of the soils. The soil forming processes are the reorganization and rearrangement of
mineral and organic soil constituents in to different horizons leading to development of
soil. Initially soil was considered as weathered product of rocks by the geologists but V.V.
Dokuchaev (Figure 3.1), known as ‘Father of Soil Science’, was the first to establish soil as
a distinct natural body having a definite genesis and a distinct nature of its own. Like
plants and animals, soil does not have any distinct physical body, therefore, pedon has
been considered as a hypothetical soil body having minimum volume of soil to be
observable. The real morphology of soil is present below the surface of the earth, therefore,
society often fails to realize it. Soil is a highly heterogeneous body thus always difficult to
be grouped in to different categories. Various classification systems have been prevalent
during past but the ‘Soil Taxonomy’ is now an acceptable scientific system of soil
classification. This chapter is intended to present a brief on the pedological aspects of soil
science.
58 SOIL SCIENCE: AN INTRODUCTION
3.2. Historical Developments in Pedology
Pedology is a branch of soil science which has been evolved
through the creation of ideas in due course of time. It has also
been identified as the heart, soul and artistry of soil science.
Generally pedology has been subdivided into soil
morphology, soil forming factors, soil forming processes, soil
survey, soil characterization and analysis, soil classification,
soil geomorphology, soil mapping, and soil interpretation. A
brief history of temporal development in key concepts of
pedology is presented as under:
Figure 3.1. Vasily Vasilievich
Dokuchaev (1846-1903)
Period Key concepts
Pre-1880 Concept of soil as a medium for plant growth and as a weathered rock layer.
1880-1900 Initiation in developments of fundamental concepts in pedology; soil as a natural
body; soil horizons/profile; soil forming factors; early ideas of soil geography.
1900-1940 Global acceptance of concepts of soil as a natural body and soil forming factors;
development of first regional soil classification system; initiation of soil surveys;
identification of key soil-forming processes.
1940-1960 Factors of soil formation and genesis of soils clarified; efforts to develop globally
acceptable soil classification systems; intensified soil mapping.
1960-1990 Refinement of global soil taxonomic system; identification of pedon concept;
development of early soil models and soil cover pattern concept; recognition of co-
evolution of soils and landforms.
1990-present Increased understanding of soil processes; refinement of global soil models; further
refinement of global soil taxonomic systems; development of statistical and computer-
based soil formation systems; development of geo-informatics for land resource
management, environmental pedology, and hydropedology.
Source: Bockheim et al., (2005)
3.2.1. Evolution of Science of Pedology
A brilliant Russian team of soil scientists led by Dokuchaev (1870) reported unique
horizontal layers in soils which were associated with different combinations of climate,
vegetation and underlying soil material. The same sequences of layers were found in
widely separated geographical areas that had similar climate and vegetation. Thus, they
distinguished the soil from the undifferentiated weathered rocks or parent material below.
This was the beginning of science of pedology. Although, the Russian studies began as
early as 1870, they were known to only few scientists outside Russia until 1914, when
Great Soil Groups of the World” was published in German by K.D. Glinka, a member of
the Russian team. His work did not receive due attention of the West European and
American workers until the book of Russian scholar – Glinka (Great Soil Groups of the
World) was translated from German to English by C.F. Marbut in the year 1927.
3.2.2. Pedological Research in India
Pedological research in India had made a modest beginning in the later part of 20th
century which was mainly focussed on soil genesis, classification and mineralogy. Work
on soil survey started in 1950 in most states as a part of their agricultural development
programmes. With the establishment of the National Bureau of Soil Survey and Land Use
SOIL GENESIS AND CLASSIFICATION 59
Planning (NBSS&LUP) in the year 1976, a premier institute of pedology working under
the aegis of Indian Council of Agricultural Research (ICAR), pedological research received
an impetus in the country. During the last four decades, NBSS&LUP had made
outstanding research contributions through developing soil resource inventory at different
scales, undertaking studies on their genesis, revising existing theories and concepts, soil
correlations, soil mineralogy, paleo-pedology, carbon sequestration, modeling climate
change and related research in edaphology and pedology education in some State
Agricultural Universities.
3.3. Approaches of Soil Study
3.3.1. Pedological Approach
The origin of the soil, its classification and description are examined in Pedology (From
Greek word pedon, means soil or earth). Pedology is the study of soil as a natural body as
they occur in their natural environment. In this approach, we consider about origin of
soil which includes the study of rocks and rock-forming minerals, pedogenic factors and
processes involved in soil genesis, soil survey, soil description and soil classification. The
pedological information are is useful to engineers and town planners as to farmers.
3.3.2. Edaphological Approach
Edaphology (Greek word edaphos, means ground) is the study of soil from the stand point
of higher plants. This is an applied branch of soil science which is important for
agricultural production and involves study on properties of the soil which affect plant
growth, nutrition and yield of crops. Edaphological approach includes study of fertility,
chemistry, physics, microbiology and technology of soils. This helps to suggest
appropriate soil management for plant growth.
3.4. Origin and Composition of Earth’s Crust
3.4.1. Composition of Earth’s Crust
The earth’s crust is principally composed of mineral matter. This mineral matter is made
up of various elements either single or combined together to form compounds. Almost all
the elements known to us, except the inert gases, are present in the earth’s crust. Each
element is in combination with one or more elements to form a definite chemical
compound known as mineral. Many of these minerals in turn combine together to form
aggregates, which we know as rocks. Almost all the mineral matter is present in the form
of rocks in the earth’s crust. Thus, rocks are composed of elements, which in turn are
made up of atoms. Out of 115 elements known, 8 are sufficiently abundant as to constitute
about 99% by weight of the Earth’s Crust (up to 16 km).
Elemental composition of Earth’s crust (% by weight) is as under:
Non- metallic Oxygen (O2
- ) 46.60 74.32% ( ¾th)
Silicon (Si4+ ) 27.72
Metallic Aluminium (Al3+) 8.13 25.68% (¼th of the total)
Iron (Fe2+) 5.00
Calcium (Ca2+) 3.63
Sodium (Na+) 2.83
Potassium (K+ ) 2.59
Magnesium (Mg2+) 2.09
Others 1.41
60 SOIL SCIENCE: AN INTRODUCTION
Abundance of Rocks in the Earth’s Crust
The composition of the upper 5 km of the earth’s crust is as follows:
Sedimentary rocks
Shale 52%
Sandstone 15% 74%
Limestone and dolomite 7%
Igneous rocks
Granite 15% 18%
Basalt 3%
Other 8%
100%
The composition of earth’s crust as a whole differs significantly from the one described
above and comprised of the following:
Igneous Rocks 95%
*Sedimentary Rocks 05% (Shale - 4.0%, Sand stone - 0.75% and Lime stone - 0.25%)
*Although sedimentary rocks form only 5% of the total earth crust, yet they are important as they
occur to the extent of 74% (almost 3/4th) at or near the surface of the earth (upper 5 km). Igneous
rocks are dominant in deeper layers.
3.5. Characterization and Classification of Rocks
Definition of rock: A naturally occurring hard mass of mineral matter formed after the
solidification of molten magma comprising of two or more rock forming minerals e.g.
granite, basalt, and gneiss. But some may be monomineralic e.g. olivine, and dunite.
Petrology is the branch of science which study about the rocks. It consists of Petrography
which gives information about description of rocks and Petrogenesis which deals with
origin of the rocks.
3.5.1. Formation of Rocks
The processes involved in the formation of three major rocks (Figure 3.2) are as under:
1. Igneous rocks: Non-laminar, massive structure, formed by cooling and crystalization
of molten materials (magma) on or beneath the earth surface.
2. Sedimentary rocks: Formed from sediments showing different stages of formation
a) Weathering: Physical, chemical and biological weathering of primary rocks give
rise to quartz, secondary minerals and soluble substances (Ca, Mg, Fe, and salts .)
b) Transportation: Transportation of weathered material by water, wind, glaciers and
runoff .
c) Deposition or sedimentation: Transported sediments settled to form graded bedding.
The coarse particles settle first followed by finer particles down the stream.
d) Diagenesis: Transformation of unconsolidated sediments to rock with compaction
and cementation processes.
SOIL GENESIS AND CLASSIFICATION 61
3. Metamorphic rocks: Developed from the transformation of existing rocks (igneous and
sedimentary rocks) by the process of metamorphism, which means “change in form”.The
chemical or physical changes in their original form are brought by the heat and
pressure .The structure and mineral composition of metamorphic rocks depend upon
the composition of the original rock and the kind of metamorphism.
3.5.2. Classification of Rocks
The classification of different rocks alongwith examples are as under:
i) Igneous rocks
1. Basalt: Most abundant extrusive rock formed from molten magma. Fine grained, dark
coloured rocks, containing 50% feldspar and 50% ferromagnesian minerals (pyroxens and
olivine), basic in nature.
Figure 3.2. Rock cycle
62 SOIL SCIENCE: AN INTRODUCTION
2. Gabbro: Similar properties as basalt but coarse-grained.
3. Granite: Coarse textured, light colour, acidic in nature, contain 60-70% feldspar (40-
45% orthoclase, 20-25% plagioclase), 20-30% quartz and 3-10% ferromagnesian minerals.
4. Rhyolite: Similar to granite but fine-grained.
5. Pumice: Very light weight rock, specific gravity lower than water, thus it floats on
water.Texturally, it is like a sponge.
ii) Sedimentary rocks
1. Shale: Fine grained detrital rock made up of clay and silt sized particles.
2. Conglomerate: It is detrital rock made up of more or less rounded fragments. The rock
is termed as breccia, if the fragments are angular rather than rounded.
3. Lime stone: It contains mainly calcite mineral, which is formed by precipitation.
4. Sand stone: Mainly composed of quartz mineral. It is called arkose, if quartz and
feldspar are predominantly present. Its texture is intermediate between fine grained shale
and coarse grained conglomerate.
iii) Metamorphic rocks
1. Gneiss: Crystalline rock, bended appearance. Feldspar, quartz, mica, biotite and
muscovite are dominant minerals.
2. Schist: Finely foliated or laminated rock. Mica and chlorite are dominant minerals.
3. Marble: Non-foliated, crystalline rock. Calcite and dolomite are dominant minerals.
4. Slate: Very fine foliated rock, splits into thin smooth sheets. Mica, quartz and chlorite
are dominant minerals.
SOIL GENESIS AND CLASSIFICATION 63
Pre-existing rocks and their equivalent metamorphic rocks
Original /Pre-existing rocks Metamorphic rocks
Conglomerate, granite, syenite, gabbro, rhyolite, andesite Gneiss
Basalt Schist
Sandstone Quartzite
Limestone and dolomite Marble
Iron ores Haematite- schist
Coal Graphite
Shale Slate
3.6. Characterization and Classification of Rock-forming Minerals
The rocks which form the earth's crust upon weathering are made up of minerals. Thus,
study of rock-forming minerals is essential to understand the composition of rocks and
ultimately the soil.
Definition
A mineral is a naturally occurring homogeneous inorganic solid, composed of atoms
having orderly and regular arrangement with definite chemical composition and
characteristic geometric form.
The study of minerals is called mineralogy. There are over 4,900 known mineral species;
over 4,660 of these have been approved by the International Mineralogical
Association (IMA). The minerals influence the release of structural ions and the chemical
activity of exchangeable ions. Therefore, the mineralogical composition of soils has a
strong relevance to soil fertility.
Minerals can be described by various physical properties which relate to their chemical
structure and composition. Common distinguishing characteristics include crystal
structure, hardness, lustre, diaphaneity, colour, streak, tenacity, cleavage, fracture, and
specific gravity. More specific tests for minerals include reaction to acid, magnetism,
taste or smell, and radioactivity.
3.6.1. Classification of Minerals
A brief classification of minerals along with their characteristics and examples are as
under:
Minerals Characteristics Examples
Primary Formed by crystallization of molten magma Silicate minerals
minerals Inherited from igneous and metamorphic Quartz, feldspar, pyroxenes, amphiboles,
rocks hornblende, olivine, mica, muscovite and
Formed at elevated temperature biotite
Chemically remain unchanged.
Percentage in soil depends on its sand and
silt content.
· Oxygen and silicon together or with one or
more cations are combined to form silicate
minerals which are more than 90% in the
earth crust.
64 SOIL SCIENCE: AN INTRODUCTION
Minerals Characteristics Examples
Secondary Formed by weathering of primary Silicates Non-silicates
minerals minerals or inherited by soils from (clay Hematite (Fe2O3)
some sedimentary rocks Minerals) Goethite (FeO(OH)n H2O)
Most common is clay minerals Kaolins, Gibbsite {Al(OH)3}
Other secondary minerals found in Smectites. Calcite (CaCO3)
soils of arid and semi-arid (dry) Vermiculite, Dolomite {Ca.Mg(CO3)2}
regions are gypsum, calcite, apatite Mica Gypsum (CaSO4.2H2O)
etc. Chlorite Apatite {Ca3(PO4)2}
Accessory Occur only in small quantities in rocks but Non-silicate
minerals not essential Zircon Zr(SiO4)
Form a group of heavy minerals due to Zeolite Ca, Na,and K silicates
their high specific gravity Pyrite FeS2
Magnetite Fe3O4
Illemenite FeTiO3
Barytes BaSO4
Serpentine Mg6(Si4O10) (OH)8
Important rock forming minerals and their relative abundance in earth’scrust are as under:
A. Primary Minerals
Ferromagnesian minerals
(i) Olivine, pyroxene, and amphiboles 16.8%
(ii) Biotite and muscovite 3.6%
Non–ferromagnesian minerals
(iii) Feldspar (albite, anorthite, oligoclase) 61%
(iv) Quartz 11.6%
B. Secondary clay minerals and others 6.0%
3.7. Weathering of Rocks and Minerals
Weathering of rocks and minerals is caused by prevailing environment which mainly
occurs near the surface of the earth.Weathering of the bed rock, in the initial stage involves
physical and chemical breakdown of rocks and minerals resulting in to the formation of
loose, unconsolidated material called parent material. The soils are formed by the
combined action of soil forming factors and processes on a parent material.
Definition of weathering
Weathering refers to physical disintegration and chemical decomposition of rocks and
minerals resulting in the formation of parent material. It is basically a combination of
transformation and synthesis or the process of disintegration and decomposition of rocks
and minerals which are brought by physical, chemical and biological weathering, leading
to formation of parent material.
3.7.1. Classification of Weathering Processes
1. Physical Weathering: A mechanical process, causing disintegration of consolidated
massive rocks into smaller pieces under favourable climatic conditions through various
SOIL GENESIS AND CLASSIFICATION 65
agents viz. temperature, water, ice and wind. Agents of physical weathering and their role in
weathering is described as under:
Agents of physical General description
weathering
1. Temperature Due to mono-mineralic and poly-mineralic nature of rocks, a differential
expansion and contraction of minerals occurs owing to diurnal temperature
changes resulting in peeling off the surface layers from the rocks (breakdown
into small fragments). This phenomenon is called as exfoliation or onion-
type weathering. Examples include exfoliation in basalt and granite.
Cold temperature causes freezing of entrapped water inside the rock which
expands 9% in volume and exerts a pressure of about 1465 tonnes/m2 (t m-2)
resulting in the break-down of rocks.
2. Water Most pronounced weathering agent.
Action of flowing water leads to collision of rocks, resulting in formation of
smaller fragments which are transported and deposited at the far-off places.
Sediment-loaded water has tremendous capacity to cut the rocks and hard
surfaces; as a result, gorges, ravines and valleys are formed.
Water carries huge boulders which break into smaller fragments due to
continuous flowing in water with friction and/or abrasion.
3. Ice Moving ice or glaciers cause great deal of cutting and crushing of the
bedrocks. On moving, glaciers exert a tremendous pressure on rock over
which these pass. The loose material moves forward and gets deposited at
lower reaches after the melting of ice. This deposit is called Moraine.
The huge boulders seen in Kangra valley of Himachal Pradesh are examples
of the glacier movement and deposition.
4. Wind An important agent of transportation of suspended particles
Also exerts an abrasion effect which is more pronounced in the aridic
climates.
The rounded rock (Mushroom rock) remnants in the Aravalis are caused by
the wind erosion.
Poorly grained or single – grained deposits are more prone to the wind
erosion.
Action of wind as weathering agent could be prominently seen in Thar desert
of Rajasthan.
2. Chemical Weathering: Chemical weathering is a decomposition process which takes
place primarily at the junction of lithosphere and atmosphere called weathering front. It
leads to alteration or disappearance of some minerals and formation of new (secondary)
minerals. This process is generally dominant in tropical (high temperature and high
rainfall) than in arid climate and governed by various agents. The rate of chemical
weathering increases with increasing amounts of dissolved CO2 and other minerals in
water. The presence of organic and inorganic acids also accelerates the chemical
weathering. The chemical weathering has great role in conversion of primary minerals
(feldspars, mica, amphiboles etc.) into secondary minerals (kaolinite, montmorillonite, vermiculite
etc.). It is the most important process in respect to soil formation; however, its effectiveness
depends on types of the minerals present in the rock. For example, quartz (SiO2) responds
very slowly to chemical weathering as compared to olivine {(Fe Mg)2SiO4}.The details
pertaining to various chemical reactions involved in chemical weathering of rocks and
minerals are as under:
66 SOIL SCIENCE: AN INTRODUCTION
Chemical weathering General description Chemical reactions
1. Solution Water is a universal solvent and its solubility
action is enhanced with dissolved CO2, organic
and inorganic acids or salts in it.
Decomposition of rocks depends on composition
of rocks and solubility action of water.
For example, halite (NaCl) is readily soluble in
water whereas solubility of some silicate
minerals like quartz is very low.
In arid regions, soluble minerals remain in
rocks as such whereas in semi-arid and humid
regions, these are completely washed away.
2. Hydration Hydration means chemical combination of
water molecules with a mineral to form a new
mineral.
Hydration reactions occur primarily on the
surface and edges of mineral grains but may
pervade the entire structure in simple salts.
Due to hydration, there is swelling and
increase in the volume of minerals. The
minerals lose their lustre and become soft and
get readily weathered.
These absorb water, expand and tend to fall
apart.
The adsorbed water provides a bridge or entry
for the hydronium ions (H3O+) or hydrogen
ions (H+) to attack the structure.
This process is commonly evident in nature
with secondary minerals like aluminium oxide,
iron oxides and gypsum.
3. Hydrolysis Hydrolysis is one of the most important
processes of chemical weathering. This process
is due to the properties of dissociation of water
(H2O) into hydrogen (H+) and hydroxyl (OH-)
ions. It is a type of double decomposition
process, in which CO2, minerals and organic
acids-rich water get easily dissociated into H+
and OH- ions which chemically combine with a
mineral and form a new mineral.
Hydrolysis reactions may be considered as
the fore- runner in clay formation (secondary
clay minerals).
4. Oxidation Oxidation is combination of oxygen dissolved in
water or from atmospheric air with exposed
rocks or minerals.
It is an important chemical reaction occurring in
the well-aerated rock and soil materials where
oxygen supply is high and biological demand
is low.
Rocks containing pyroxenes, hornblende,
biotite, glauconite and chlorite etc. are
susceptible to oxidation.
SOIL GENESIS AND CLASSIFICATION 67
Chemical weathering General description Chemical reactions
In olivine, ferrous form of iron gets oxidised to
ferric form. Due to this the rigidity of mineral
structure weakens and the mechanical
break-down becomes easier.
Due to oxidation, new minerals are also formed.
Minerals containing manganese as Mn2+ and
sulphur as S2- ions are also susceptible to
oxidation.
5. Reduction The process of removal or loss of oxygen is
called reduction.
Reduction is just opposite to oxidation and
occurs where a mineral is water-saturated,
oxygen supply is very low and biological
demand of oxygen (BOD) is very high, resulting
in conversion of the higher valent metals to the
low valent states (e.g. Fe3+ into Fe2+).
In the presence of Fe2+ ions in the system
sulphide (S2-) are also formed which impart a
green and blue colour to the soil.
Lepidocrocite (γ-FeOOH) is also formed in this
condition which imparts orange and yellow
mottles to soil matrix under reduced conditions.
The oxidation and reduction processes are
common in the minerals containing Fe, Mn and S.
6. Carbonation Carbonation is the combination of carbon
dioxide with any base.
When water reacts with CO2, it gives carbonic
acid (H2CO3), which is a very important agent
of chemical weathering of rocks and minerals.
The solubility of calcium bicarbonate (CaHCO3)
is considerably higher than that of calcite
(CaCO3).
3. Biological Weathering
Biological weathering is a change brought by living agents (humans and animals, higher
plants and their roots and micro-organisms) which is mainly controlled by prevailing
environment and are responsible for both physical and chemical changes.
When mosses or lichens grow on the exposed rocks, they facilitate the deposition of dust,
resulting in higher plants getting favourable environment to grow. As a result, roots of
higher plants penetrate inside the rocks and disintegrate them physically. The mosses
and lichens growing on rocks secrete chemical exudates which act chemically on the
minerals present in the rocks and tend to form new minerals. Burrowing animals,
movement of animals and human activities (cultivation, land-levelling, construction of
roads, buildings, railway lines etc.) also facilitate the physical weathering.
3.8. Weathering Sequence of Minerals
Minerals present in soil and parent materials under various environmental conditions
differ in their degree of weathering. A generalised sequence of the minerals in order of
their weathering resistance is as under:
68 SOIL SCIENCE: AN INTRODUCTION
Quartz (most resistant) >muscovite, K-feldspar >Na and Ca feldspar >biotite >hornblende
and augite >olivine >dolomite and calcite >gypsum (least resistant).
This sequence is comparable to weathering sequence as proposed by Goldich in the year
1938. The degree of weathering may vary depending upon the climate and other
environmental conditions.
3.9. Pedological Concepts
Land and Soil
Concept of land is a broader entity which includes not only the soil but also the living
organisms, air and water bodies within or on it and the rocks and parent materials below
it. Land by virtue of its vast resources of biosphere (flora and fauna), water, minerals and
soil constitute the foundation for the habitation and civilization. Soils are defined as a
heterogeneous mixture of mineral and organic materials on the land surface which serves
as a medium for the growth of plants.
Pedon (Gr. Pedon meaning ground)
The term ‘pedon’ has been proposed for small basic soil entity that is a part of continuum
mantling over land surface. It is smallest volume that can be classified as a ‘soil’. It is
very similar to soil profile but is three-dimensional. The shape of the pedon is roughly
hexagonal. Its area ranges from 1 to 10 m2 depending upon soil variability.
Soil Individual or Polypedon
Soil individual is collection of the similar types of pedons bounded on all sides by not
soil material or pedons of unlike characteristics. The minimum area of polypedon is more
than 1 km2 and an unspecified maximum area.
3.10. Soil Profile
A soil profile is a vertical section of soil where as individual layers (horizontal) in the soil
profile are called soil horizons. A soil profile is an historic record of all the soil forming
factors and processes which keeps the snap shot of soil development. It forms a unit of
study in pedological investigations and is an important tool for soil classification.
3.10.1. Designation for Horizons (affected by pedogenesis) and Layers (not
affected by pedogenesis)
Soil horizons are of two types: genetic horizons and diagnostic horizons. Discussion in
this section is restricted to the genetic horizons. The designation of genetic horizons (O,
A, E.. etc) only expresses the qualitative judgement about the changes that have taken
place in the soil material whereas diagnostic horizons (Mollic, Umbric, Ochric…etc.) are
quantitatively defined features that are used to differentiate between different categories
of Soil Taxonomy.
Genetic horizons can be broadly classified in to three categories:
1. Master horizons
2. Transitional and combination horizons
3. Subordinate distinction within master horizons
SOIL GENESIS AND CLASSIFICATION 69
3.10.1.1. Master Horizons
The capital letters are used to represent the master horizons. Earlier there were only six
master horizons (O,A,E,B,C,R); later on Soil Survey Staff added three more (L,M,W)
horizons. A brief description of master horizons are as under:
a) O-horizon or layer - A layer dominated by organic material. The O-horizon may be on
the top of either mineral soil or organic soils. An O-layer may be on the surface of
mineral soil or it may be at any depth below the surface, if it is buried. A horizon formed
by illuviation of organic material in to a mineral subsoil is not an O-horizon, although
some horizons that have formed in this manner contain considerable amount of organic
matter.
b) A-horizon - Mineral horizon that has formed at the surface or below an O-horizon. This
horizon shows accumulation of humified organic matter closely mixed with the mineral
fractions or properties resulting from cultivation, pasturing and similar kinds of
disturbances. Recent aeolian or alluvial deposits are not considered to be A-horizon unless
cultivated.
c) E-horizon - Mineral horizon in which the main feature is the loss of silicate clay, iron,
aluminium or some combination of these leaving behind a concentration of sand and silt
particles. An E-horizon is most commonly differentiated from an underlying B-horizon
by a colour of high value or lower chroma, or both, by coarser texture or a combination of
these properties. This horizon is lighter in colour and has less organic matter than the A-
horizon. This horizon is also called as a horizon of eluviation.
d) B-horizon - Horizon that has formed below an A, E or O-horizon. This shows one or
more of soil characteristics like: accumulation of clay, humus, iron, aluminium, carbonates,
gypsum, strong gleying, low chroma mottles and alterations associated with formation of
granular, blocky or prismatic structure. This horizon can be generally considered as a
layer of accumulation or illuviation .
e) C-horizon or layer - C horizon is the horizon which ideally represents the parent
material from which the soil above is formed. Since many soils are trans-located or
transported by water, ice or air, these soils may not have a parental legacy with the
horizon above.
f) R-layer - Strongly cemented to indurate bedrocks such as granite, basalt, quartzite, lime
stone, sandstone.
g) L-horizon or layer - It includes both organic and mineral liminic materials that are
deposited in water by precipitation or through the action of algae and diatoms. It includes
coprogenous earth or sedimentary peat (co), diatomaceous earth (di) and marl (m). This
layer is found only in the Histosols.
h) M-layer - Root limiting subsoil layers consisting of nearly continuous, horizontally-
oriented, human-manufactured materials. Examples of the materials designated by M are
geotextile liners, asphalt, concrete, rubber and plastic.
i) W-layer - This symbol indicates water layers within or beneath the soil. The water layer
is designated as Wf, if it is permanently frozen. The w (or Wf) designation is not used for
shallow water, ice or snow above the soil surface.
3.10.1.2. Transitional and Combination Horizons
Horizon dominated by properties of one master horizon but having subordinate properties
of another e.g. AB, EB, BE etc is called a transitional horizon. For example, an AB horizon
70 SOIL SCIENCE: AN INTRODUCTION
has characteristics of both A and B horizons, but it is more like the A horizon than the B
horizon. Combination horizon is comprised of two distinct parts that have recognizable
properties of the two kinds of master horizons e.g. A/B, B/E, B/C etc. The first symbol
indicates horizon with the greater volume than another.
3.10.1.3. Subordinate Distinction within Master Horizons and Layers
Lower case letters are used as suffixes to designate specific kind of master horizons and
layers. The term “accumulation” is used in many of the definitions of such horizons to
indicate that these horizons must contain more of the material in question than is
presumed to have been present in the parent material. The suffix symbols and their
meaning are as follows:
a : highly decomposed organic material
b : buried genetic horizons
c : concretions or nodules
d : physical root restriction
e : organic material of intermediate decomposition
f : frozen soil or water
ff : dry permafrost
g : strong gleying
h : illuvial accumulation of organic material and sesquioxide
i : slightly decomposed organic material
j : accumulation of jarosite
jj : evidence of cryoturbation
k : accumulation of carbonates
m : cementation or indurations
n : accumulation of sodium
p : tillage or other disturbances
q : accumulation of silica
r : weathered or soft bed rock
s : illuvial accumulation of sesquioxide and organic matter
ss : presence of slickensides
t : accumulation of silicate clay
v : plinthite
w : development of colour and structure
x : fragipan characteristics
y : accumulation of gypsum
z : accumulation of salts more soluble than gypsum
Source :Soil Survey Staff (2014) Keys to Soil Taxonomy 12th Ed. USDA, NRCS.
3.11. Soil Forming Factors (Pedogenic Factors)
Soil is a product of combined interaction of soil forming factors and soil forming processes
(Figure 3.3). Soil forming factors influence the properties of soil but direct relation between
them are very difficult to derive as all factors act simultaneously to form a soil. Dokuchaev
(1889) was the first to show that soil forms a definite pattern on the landscape and
establish that they develop distinct horizons under the influence of parent material, climate
and organism which he put as: S=f (p,cl,o). Later on Jenny (1941) added two more factors
i.e. relief and time. He stated that all five soil forming factors act simultaneously at one
point on the surface to produce soil are: climate, biosphere, relief or topography, parent material
and time. These are further classified into two groups i.e. active and passive soil forming
factors by J.S. Joffe (1949). It may be represented by following equation:
SOIL GENESIS AND CLASSIFICATION 71
Figure 3.3. Stages of soil formation
S = f (cl, b, r, p, t…..) Given by Jenny (1941)
where, S = any soil property
f = function of or dependent on
cl = climate
b = biosphere (flora and fauna)
r = relief or topography
p = parent material
t = time
…..= unspecified/unidentified factors
3.11.1. Classification of Pedogenic Factors
Active pedogenic factors provide the energy that act on the parent material to form soil.
Active factors are capable of changing the effect of passive factors, although during initial
stage of soil development, the effects of passive factors are quite evident. Examples are
climate and biosphere.
Passive pedogenic factors provide soil forming conditions and mass or real skeletan
which serve as a base on which active soil forming factors work for the development of
soil in due course of time. Examples are parent material, relief and time.
A brief enumeration of the effects of soil forming factors on soil characteristics is as
under:
Active soil forming factors
Classified by Joffe (1949)
Passive soil forming factors
72 SOIL SCIENCE: AN INTRODUCTION
Soil forming factors Description
1. Climate Perhaps the most influential factor in soil development
(Precipitation and Affects soil formation directly by supplying water and heat which help in
temperature) decomposition of minerals present in the parent material. Percolating water
acts as a conveyer belt to carry soluble material to lower depths where these
materials are deposited at various depths, thus forming the soil
horizons.
Indirectly, it exerts the control over flora and fauna which supply energy in
the form of organic matter.
Rainfall is a more significant climatic element to determine the nature of soil.
For example
Arid climate : formation of saline soils
Cold and humid climate : formation of podzol soils
Warm and humid climate : formation of red laterite and lateritic soils
Temperature is second important factor of climate as it influences the chemical
reactions involved in soil formation.
According to van’t Hoff law “with every 10 oC rise in temperature, the speed
of chemical reaction increases by a factor of two or three. This process applies
to weathering of minerals which release soluble components and thus help in
soil development. For example,
Warm and humid climate of tropical regions – weathering of rocks
from few metres to 30-40 m
Cold temperate region –weathering of rocks from few cm to 1 m.
Cold and humid region – rate of organic matter decay is very slow.
Arid region – low content of organic matter, nitrogen and clay in soils.
2. Biosphere Vegetation is an active factor of soil formation which constitutes second
(Flora and fauna) component of soil solid i.e. organic matter. It is a source of food for soil
organisms and is one of the most important indicator of soil health.
Decomposition of organic matter produces soluble organic acids which help in
breakdown of the minerals present in parent rock, resulting in the release of
essential plant nutrients. It also influences soil properties by way of improving
water and nutrient holding capacity, colloidal properties of soil and reducing
tendency of crust formation and soil erosion by wind and water.
Macro-organisms such as rodents, moles, snails, earthworms, termites,
millipedes, centipedes etc., owing to their burrowing habit, cause mixing of
soil material. By doing so they retard the differentiation of soil horizons and as
a consequence retard the process of soil development.
Microorganisms such as bacteria, fungi, actinomycetes, protozoa and
nematodes inhabit in large numbers in soil and play an important role in the
decomposition of organic matter and indirectly it also helps in decomposition
of mineral matters through production of organic acids. Rapid decomposition
of soil materials (mineral and organic) helps in movement and deposition of
these constituents in different layers, leading to the development of soil.
3. Relief or topography Relief and topography denote configuration of land surface. Relief indicates
local slope variation while topography expresses elevation of land surface on a
broad scale.
Soil organic matter and clay move in the form of suspension with flowing
water from uplands and get deposited in the low lands, thus affecting the soil
formation. Generally, lowland soils are darker in colour, higher in pH, EC,
clay and organic matter content. Soil formation is very poor on steep slopes
due to erosion and the gently-sloping flat land is ideal for soil development.
Due to higher rate of water percolation and leaching of soil constituents on flat
lands, horizon differentiation is rapid and depth of soil is more as compared to
the upland and lowland positions. In lowland depressions, water is available
but percolation is very poor due to deposition of clay and organic matter
particles which clog the soil pores. Thus, soil development is very poor.
The term ‘catena’ was introduced by G. Milne (1935), for a sequence of
topographically-related soils which have comparable parent material, climate
and age but show different soil characteristics because of the variation in
relief.
SOIL GENESIS AND CLASSIFICATION 73
4. Parent material Jenny (1941) defined parent material as “the initial stage of soil system at the
time zero of soil formation” or it may be defined as a loose unconsolidated
mass of mineral matter formed upon the weathering of rocks which serves as
the raw material of soil formation.
Parent materials may be classified on the basis of agent of deposition as:
alluvium by water, lacustrine by lake, marine by ocean, dune/ loesses and aeolian
by wind, colluviums by gravity action and till/ moraine by ice.
The role of parent material in soil formation is passive as different kinds of
parent materials may produce similar soil when active factors (climate and
biosphere) in the area remain the same. However, in the initial stage of soil
formation, the soil properties are governed by the nature of parent material
but with the time the influence of parent material on soil properties gradually
diminishes.
The parent material determines the soil properties such as texture, structure,
water holding capacity and clay content. It may affect the downward
movement of water which is important for profile development.
The soils (calcimorphic and hydromorphic) where the composition of parent
material resists the effect of climate and vegetation are called as the
Endodynamorphic soils. The existence of these soils is temporary until the
chemical decomposition becomes active under the influence of climate and
vegetation. The Ectodynamorphic soils (podzol, laterite) develop a normal profile
under the influence of active soil forming factors (climate and biosphere).
5. Time Soil formation is a natural process which requires thousands of years to
develop a mature pedon. The period of time devoted by the nature from the
stage of parent material to the stage of formation of a mature soil (A, B, C
horizons) is considered as the pedologic time.
Rate of soil development (aging) varies among the soils. The warm humid
climate, flat to gently sloping topography, sandy parent materials are
favourable for soil formation, whereas, cold and arid climate, clayey parent
material, steep slope, flood plains and activities of burrowing animals retard
the pace of soil profile development.
3.12. Soil Forming Processes (Pedogenic Processes)
Most geological processes (upliftment of mountains/islands) operate very slowly in nature
but the pedogenic processes, although slow in terms of human life, but proceed at a
faster rate than the geological processes to convert the parent material in to a mature soil
showing distinct horizons (A,B,C). The pedogenic processes are extremely complex and
dynamic, involving many chemical and biological reactions and different processes or
combination of processes operate under the influence of soil forming factors. The basic
processes involved in operation of soil forming processes are:
1. Addition or gain of water, mineral and organic matter in the soil
2. Loss of the above materials from the soil
3. Transformation of mineral and organic materials within the soil
4. Transfer or translocation of energy and matter (mineral and organic)
All these promote horizon differentiation by way of transformation of soil materials
(mineral and organic matter) resulting in the release of soluble salts, carbonates,
sesquioxide, silica, clay minerals and organic acids and their subsequent translocation
with percolating water and its deposition at various depths.
3.12.1. Classification of Pedogenic Processes
Pedogenic processes are simply the reorganization and rearrangement of mineral and
organic soil constituents in to different layers under the influence of various pedogenic
factors. These can be broadly classified in to two groups :
74 SOIL SCIENCE: AN INTRODUCTION
1. Fundamental Pedogenic Processes
These processes are found in all soils and operate simultaneously
a. Humification It is the process of transformation of raw organic matter into
humus. When raw organic materials are added in soil, their
decomposition by various organisms produces simple organic acids.
Further bacterial polymerization of these organic acids produces a
dark brown colloidal substance resistant to microbial attack called
humus. The whole process may take about a decade to copmlete.
b. Eluviation/Emigration Process of removal (wash-out) of soil constitutents in suspension or
solution by the percolating water from the upper to lower layers,
resulting textural differences which leads to horizon differentiation
or development of soil.
Order of mobility of inorganic soil constituents expressed relative to
chloride, taken as 100 :SO4 (57.1) > Ca (3.0) > Na (2.40) > Mg (1.30) >
K (1.25) > SiO2 (0.20) > FeO3 (0.04) > Al2O3 (0.02)
c. Illuviation /Immigration Process of deposion of simpler soil constituents removed from upper
layers (eluvial horizon) in to the lower layer (zone of accumulation)
is termed as illuviation.
The horizons formed by this process is termed as illuvial horizon
(B horizon, especially Bt).
2. Specific Pedogenic Processes
The fundamental pedogenic processes collectively lead to the formation of a number of soils that are found
on the surface of the earth. These processes provide a skeleton for the operation of specific pedogenic
processes. In fundamental processes, if the pattern of deposition or removal of soil material is specific, then
these are kept under specific pedogenic process. Specific pedogenic processes can be categorised in to two
groups:
(a) Zonal Soil Forming Processes: The soil forming processes that are occurring under the prevailing
conditions of climate and biosphere (active factors). These attain an equilibrium with their surrounding
environment in due course of time.
1. Calcification This process occurs in arid and semi arid climates. Process of
precipitation and accumulation of calcium carbonate (CaCO3) after
mobilization from upper soil layers, in some part of the profile.
Calcification results in the formation of calcic horizon (k) (Figure
3.4). It can be represented by following equation:
Calcium compounds mobilized by acidification of water are kept in
solution as long as the supply of CO2 is maintained up to a critical
level. Below this level, the carbonates are precipited as white powdery
fillings in soil pores. Later on soil matrix is converted into an
indurated pan having hard spiny nodules of CaCO3.
2. Decalcification Reverse of calcification, that is the process of removal of CaCO3 or
Ca ions from the soil by leaching.
Figure 3.4. Process of calcification
SOIL GENESIS AND CLASSIFICATION 75
3. Podzolization The term podzol is derived from Russian word -Pod means under, zola
means ash like.
True podzols are not found in India except some reports of podzol
like soils (pseudopodzols) from high altitude areas of Himachal
Pradesh, Jammu & Kashmir, eastern and north eastern states.
Podzolization is a process of acid hydrolysis
Favourable factors
Climate: Cold–humid
Vegetation: Coniferous (acidic nature)
Parent material: Sandy (siliceous)
High rainfall coupled with sandy parent material favours intense
leaching. Basic cations like Na, Ca, K, Mg released during weathering
leach out of the profile. The pH of soil remains below 5.0 due to the
leaching of bases.
Under acidic environment and cold climate, polymerization of simple
organic acids is very slow due to sluggish bacterial activity. Thus
organic acids either alone or in combination with sesquioxides
(chelates) move to lower layers and deposited in B horizon (Bh) due
to increase in bacterial activities. Silica being insoluble at low pH
remains at upper layers (E horizon) and sesquioxides due to high
solubility under these conditions move down and deposited in B
horizon (Bs) where a slight increase in pH is encountered.
Thus, podzolization is a soil forming process in which humus and
sesquioxides become mobile, leach out from upper horizons and
get deposited in to lower horizons while silica remains accumulated
in upper horizons (Figure 3.5).
4. Laterization The term laterite is derived from latin word later means brick or tile.
Favourable factors
Climate: Warm–humid
Vegetation: Broad leaf tropical vegetation (basic nature)
Parent material: Basic parent materials that contain high iron e.g. Ferro-
magnesian minerals (pyroxene, amphibole, biotite, chlorite).
High rainfall favours intense leaching of basic cations (Na, K, Ca)
but due to basic nature of parent material and vegetation, soil pH
remains alkaline to neutral.
Silica being soluble in the alkaline conditions, leach out of the profile
whereas iron and aluminium oxides released during weathering
remain insoluble and coat the soil matrix and impart characteristic
red colour to the soil (Bs-horizon).
In this process most of the silica leach out of the soil whereas relative
proportion of iron and aluminium as oxides (sesquioxides) is high in
the solum (Figure 3.6).
The laterite (Oxisols) and lateritic (Ultisols) are mainly used for
shifting agriculture, low intensity grazing and growing plantation
crops with adequate nutrient supply.
(b) Intrazonal Soil Forming Processes: These pedogenic processes are more influenced by certain local
conditions such as relief or parent material than climate and vegetation e.g. Hydromorphic, halomorphic and
calcimorphic soils.
Figure 3.5. Process of
podzolization
Figure 3.6. Process of laterization
76 SOIL SCIENCE: AN INTRODUCTION
5. Gleization Glei (Russian word) refers to blue, grey, green clay
This process results in the development of a gley horizon (g) in some
part of profile (Figure 3.7) due to poor drainage condition (depression
land), impervious soil parent material, lack of aeration etc.
Under depleted oxygen condition, Fe-compounds are reduced to
soluble ferrous form. Most of the iron exists as Fe2+- organo complexes
in solution or as a mixed precipitate of ferric or ferrous
hydroxide, which imparts typical bluish or greyish colour to
horizons(g).
Distinct mottles of yellow to rusty brown colour on matrix of gley
horizons are often found where alternate oxidized and reduced
conditions prevail due to fluctuation of water table with the
season.The reddish mottles contain higher amount of iron than the
surrounding blue or grey matrix.
6. Salinization The process of accumulation of salts such as SO4
2-
and Cl- of Ca, and
Mg, in soil in the form of a salic horizon(z) leading to development
of saline soil (Figure 3.8).
A soil is considered as saline, if ECe (salinity of saturation extract) >
4.0 dS m-1, pH <8.5 and ESP <15. These soils are also called white
alkali soils and contain mostly Cl and SO4 of Ca and Mg with less
amounts of CO3 and HCO3 of Na.
These soils are formed under arid and semi-arid climate where
evaporation losses are more than precipitation. Salts released during
weathering accumulate in soil. Area having high and brackish ground
water, depression lands and poor drainage condition also leads to
the formation of saline soils. If ground water table is under capillary
fringe,upward movement of capillary water followed by evaporation
results in accumulation of salts at soil surface or in root zones.
Although, physical condition of saline soil is good due to presence
of Ca (cause flocculation) but high osmotic pressure due to excess
soluble salts in soil solution restrict availability of water and nutrients
to plants resulting in poor growth.
Saline soils could be reclaimed simply by leaching with good quality
irrigation water. If the leaching is not good then artificial drainage
network needs to be provided to flush out salts from affected areas.
7. Alkalization (Solonization) The process of accumulation of sodium ions on exchange complex of
the clay, resulting in the formation of alkali soils (sodic/solonetz).
These soils are also called black alkali soils and contain mostly CO3
and HCO3of Na and less amounts of Cl and SO4 of Ca and Mg.
All cations in soil solution are engaged in a reversible reaction with
the exchange sites on the clay and organic matter particles. As the
soil solution dries, most of the CO3 of Ca and Mg is precipitated in
soil solution much before the Na as sodium carbonate which is 100
times more soluble than Ca and Mg carbonates.Thus, the sodium
remains in soil solution and its adsorption on exchange sites (clay
and organic matter) is increased.
The dominance of Na on exchange sites causes dispersion of clay
leading to poor physical condition for plant growth and increase in
soil pH (>8.5). High pH adversely affects nutrients' availability to
plants and bring about the dissolution of soil organic matter which forms
thin friable layer on soil surface.This is followed by a dark coloured
clay illuviated impermeable horizons having a typical columner
structure which is a characteristic feature of sodic soil (Figure 3.9).
Figure 3.7. Process of gleization
Figure 3.8. Process of salinization
Figure 3.9. Process of alkalization
SOIL GENESIS AND CLASSIFICATION 77
The process of removal of Na from exchange sites is called
dealkalization or solodization.The amendment which is widely used
for amelioration of alkali soil is gypsum.The reaction could be
represented as under:
Other amendments like elemental sulphur and pyrites can also be
used.These amendments upon hydrolysis and oxidation form
sulphuric acid.This acid solubilises native calcium carbonates of soil
which replace the excess Na from the exchange sites with Ca resulting
in reclamation of soil.
8. Pedoturbation Process of mixing of soil materials not by the illuviation. Mixing to a
certain extent is evident in all soils.
Most common types of pedoturbation are:
Faunal pedoturbation: mixing of soil by animals
Floral pedoturbation: mixing of soil by plants
Agrillipedoturbation:mixing by the churing process caused by
swelling and shrinking of clays (evident in Vertislos).
Cryopedoturbation: Frost churning (only in Gelisol soil order)-
mixing due to freezing and thawing of soil material
3.13. Soil Classification Systems: Principles and Purpose
Principles
The principles on which soil classification is based (Buol et al. 1998) are:
Genetic Thread principle: The theories of soil genesis provide a framework for helping in
determining the significance and relevance of soil properties for use as differentiating
characteristics.
Principle of Accumulating Differentia: In a multiple category classification system,
differentiating characteristics accumulate from the higher levels of generalization to the
lower levels. Therefore, classes at the lower categories accumulate large number of
differentia and are, therefore, completely defined.
Principle of Wholeness of Taxonomic Categories: All individuals must be classified in
each category, as per the characteristics selected as differentiating at that level.
Ceiling of Independence Principle: A property used as a differentiating characteristic
(ceiling) in a category must not separate similar individuals in a lower category.
Purpose
The purpose of any classification system is to: a) Organize our knowledge, b) remember
properties of the objects, c) bring out and understand relationships among individuals
and classes of the population, d) learn new relationships among the group and e) establish
groups of the objects in a manner useful for practical and applied purposes.
3.14. Historical Developments in Soil Classification Systems
3.14.1. Early Systems of Soil Classification
Economic Classification: Grouping soils according to their productivity for taxation.
78 SOIL SCIENCE: AN INTRODUCTION
Physical Classification: Earliest system based on soil texture e.g. Sandy soils, loamy soils
etc.
Chemical Classification: Grouping of soils by their chemical composition e.g. Calcareous,
acid soils etc.
Geological Classification: Residual or sedentary soils which are developed in situ from the
underlying rocks and “transported soils” developed from sediments like alluvium,
colluvium etc.
Physiographic Classification: Here characteristics of landscapes and geomorphic terms
were considered such as levee soils, basin soils etc.
3.14.2. Modern System of Soil Classification
The first classification of soil, as proposed by Dokuchaev (1900), divides soils into three
categories: Normal, Transitional and Abnormal. These categories were later termed as Zonal,
Intrazonal and Azonal soils, respectively. The Zonal soils are those with fully developed soil
profiles, which are in equilibrium with the environmental conditions, such as climate and
vegetation: for example lateritic and red soils. The soils, where time has been a limiting
factor to develop horizons are termed as Azonal soils (Alluvial soils).Within the zonal
areas, some soil characteristics are modified by the influence of local conditions like
topography, specific parent material etc and such soils are termed as Intrazonal soils
(Halomorphic soils). Though Dokuchaev’s approach was based on sound principles of
soil genesis, but climate and vegetation were given more emphasis than soil properties in
grouping the soils.
3.14.2.1. Marbut’s Morphogenetic System: Marbut and associates, in USA accepted the
approach of the Russian schools, but introduced “Great Soil Groups” and advocated
classification of soils on the basis of their own intrinsic properties rather than on the basis
of soil-forming factors. He reduced emphasis on parent rock and evolved his scheme of
soil classification in successive steps. At the highest category level, zonal soils were
divided into 2 classes: Pedalfers (aluminium and iron accumulation) and Pedocals (calcium
carbonate). Pedalfers are highly leached soils of humid climates and the Pedocals are soils
of non-leaching environment of arid climate. The major limitation of Marbut’s system
was that it was based on assumed soil genesis and, therefore, many soil series recognized
in USA could not find an appropriate place in the system.
3.14.2.2. Baldwin and Associates' Genetic System: Baldwin and associates. in1938
emphasized the Marbuts morphogenetic system and returned to zonality concept but
gave more emphasis on soil as a three dimensional body. A new category, soil family was
introduced between Great Soil Group and Soil Series but they were not properly defined in
terms of soil properties. The system was revised by rearranging and developing new
“Great Soil Groups”. The soils were grouped in 3 Orders as
Zonal: Normal soils with characteristics reflecting the effect climate and vegetation on
well drained soils;
Intrazonal: Well developed soils showing influence of local factors such as age, parent
material and relief and
Azonal: Poorly developed soils.
These 3 orders were further subdivided into nine suborders on the basis of specific
climatic and vegetative regions. Each suborder was divided into Great groups based on
more specific conditions. The Great groups were further divided into Soil families, Soil
series and Soil types. The genetic system marked the beginning of the comprehensive
systems of soil classification.
SOIL GENESIS AND CLASSIFICATION 79
Major limitations of the genetic systems were:
The two highest categories were defined in genetic terms not on soil properties,
The Great soil group concepts were qualitative,
In definitions, more emphasis was given on properties of virgin soils which got
modified by use,
The nomenclature was evolved from many languages and it was difficult to name the
intergrades.
Many classification systems were developed in different parts of the world to serve the
needs of the people viz: FAO UNESCO legend of soil map of the world, French, Russian,
Chinese, Australian, German, Classification of Brazil, Canada, England, New Zealand,
South Africa etc. and the needs differed between the countries and changed with time.
This was also depended on the availability of data with them at that point of time.
International communication was affected as it required research in internationally
recognized classification or known terms. It is also important that soils to be identified by
names more easily recognized by indigenous population to attain recognition. Thus,
there was a need to identify the soils by more than one classification, though it is an
added responsibility of the pedologists. In India, such a task was very difficult at that
time because of the wide variations in soils due to tropical climate and other factors and
process of soil formations.
3.14.2.3. Soil Taxonomy: Comprehensive System of Soil Classification
The USDA soil classification system was developed through several approximations from
1951 onwards and the 7th approximation was published in 1960 and through a series of 4
supplements from 1965 to 68 and was finally published as Soil Taxonomy in 1975. The US
Soil Taxonomy is based on measurable soil properties. Although, the system departs
from the genetic classification, the properties that are the result of soil genesis are chosen
as differentiae because these carry the maximum number of accessory properties and
have geographic implications of susceptibility to mapping. The nomenclature of Soil
Taxonomy is coined, largely from Greek and Latin roots that fit in any modern European
language without translation. The name of each taxon clearly indicates the place of taxon
in the system and connotes some of its most important properties. The system
accommodates intergrades at subgroup level.
Soil Taxonomy was adopted in India in 1969 to classify the soils of India and since
then, it has been introduced in the course curriculum of Agricultural Universities for
teaching soil science. Since then the pedologists in India classified the soils as per US
Taxonomy and contributed immensely to develop rationale to the classification of tropical
soils.
Soil Taxonomy –Features
Soil Taxonomy has the following features
The comprehensive system is based on measureable soil properties. Soil classes were
defined in terms of present soil properties.
The system considers all such properties which affect soil genesis or are the outcome
of soil genesis.
The nomenclature were made, using coined words derived mainly from Greek and Latin.
A new category, i.e. subgroup, has been introduced to define the central concepts of
great groups and intergrades in order to express clearly that soils are a continuum
with gradual change in many properties.
80 SOIL SCIENCE: AN INTRODUCTION
One can remember individual groups of the objects without undue taxing of the
human memory.
3.14.2.3.1. Diagnostic Horizons and Features
(i) Diagnostic Horizons
A brief mention of important diagnostic surface and subsurface horizons, as used in the
system, is made here. The diagnostic surface horizons are called epipedons (Gr. epidermis,
skin; and pedon, soil). The epipedon includes the upper part of the soil darkened by
organic matter. Eight epipedons, viz. Mollic, Umbric, Anthropic, Ochric, Melanic, Folistic,
Histic and Plaggen, are recognized, but only three, i.e. Mollic, Ochric and Umbric, are of
importance in the soils of India.
(a) Diagnostic Surface Horizons (Epipedons)
1. Anthropic epipedon (Gr. Anthropikos, human being)
Colour same as Mollic/Umbric.
Developed by human altered or transported material with artefacts and human litters.
<25 cm thick.
n value is <0.7.
2. Folistic epipedon (Gr. folia - leaf)
Layer with high organic matter saturated with water for <30 days and
BD <0.1 Mg m-3 or organic carbon 8 to 16% depending on clay content.
3. Histic epipedon (Gr. histos - tissue)
>20-30% organic matter depending on clay and >30 days water saturated.
<30 cm thick if drained and 45 cm if not drained.
4. Melanic epipedon (Gr. melas, melan – black)
A thick black horizon with high organic carbon ( >6%).
Associated with andic properties.
Melanic index of < 1.7
5. Mollic epipedon (L. mollis - soft)
Surface 18 cm contain >1% organic matter.
Dark coloured (value < 5 and chroma < 3).
Structure neither massive nor hard.
Base saturation >50%.
Moist >3 months
n value <0.7
6. Ochric epipedon ( Gr. ochros, pale)
Light in colour (value >5 moist >3 dry).
<1% organic matter.
Hard or very hard when dry.
Do not qualify for any other horizon.
7. Plaggen epipedon (Ger. plaggen, sod)
Man-made horizon > 50 cm thick.
>0.6% organic carbon.
Moist >3 months.
SOIL GENESIS AND CLASSIFICATION 81
8. Umbric epipedon (L. umbra, shade)
Similar to mollic, but <50% base saturated.
Moist >9 months per year.
(b) Diagnostic Subsurface Horizons
The subsurface diagnostic horizons are formed below the soil surface.
Agric horizon (Layer, field)
Formed under plough layer as dark lamellae.
Andydritic horizon
A horizon >15 cm thick with 5% or more CaSO4 anhydrite.
Colour hue 5Y, chroma 1 or 2 and value 7 or 8.
Calcic (L calcis, lime)
An illuvial horizon of CaCO3 accumulation.
>15 cm thick and not cemented.
15% or more CaCO3 in soil
>5% CaCO3than underlying layer.
Duripan ( L durus, hard)
A subsurface horizon cemented by SiO2.
Air dry peds do not slake in water but in hot potassium hydroxide (KOH).
Glossic horizon (Gr. glossa, tongue)
Remnant of an argillic or kandic horizon from which clay and free iron oxides were
removed.
>5 cm thick.
Kandic horizon (modified from kandite)
Argillic horizon with or without clay skins.
Dominated low activity clay (LAC) soils.
CEC <16 and ECEC >12 cmol(p+)kg-1 of clay.
Oxic horizon (F. oxide)
> 30 cm thick, high content of low charge minerals.
CEC and ECEC <16 and 12 cmol(p+)kg-1 clay.
No argillic horizon.
<10% weatherable minerals in fine sand.
Petrogypsic horizon (Gr. petra, rock and gypsic)
Strongly cemented, >5 mm thick gypsum horizon.
Sombric horizon
An illuvial humus horizon with colour and base saturation of Umbric epipedon.
Albic horizon (L-albus, white)
Eluvial horizon, >1 cm thick formed by light coloured sand and silt.
Argillic horizon (L argilla, white clay)
Clay enriched B horizon with clay skins or clay films.
Clay content varies with the clay of the layer above.
>15 cm thick or at least 1/10th of upper horizon depending on texture.
82 SOIL SCIENCE: AN INTRODUCTION
Cambic (L cambiae, to exchange)
A colour or structural B horizon formed due to physical movement or chemical
weathering.
> 15 cm thick.
Fragipan (L fragilis, brittle and pan )
A layer >15 cm thick, brittle when moist and hard when dry.
Air dry fragment slake in water.
Gypsic horizon (L gypsum)
>15 cm thick horizon of gypsum enrichment.
>5% gypsum than other horizon.
Natric horizon ( L natrium, sodium)
An argillic horizon with prismatic or columnar structure.
Exchangeable sodium percent (ESP) >15.
More exchangeable Na and Mg than Ca plus exchange acidity at pH 8.2.
Petrocalcic horizon ( Gr. Petra, Rock and clacic)
A hard, >10 cm thick calcic horizon.
Roots cannot penetrate.
Salic horizon
A salt-enriched horizon >15 cm thick.
Electrical conductivity of saturated extract (ECe) >30 dS m-1.
Product of ECe and thickness in cm >900.
Spodic horizon
Illuvial horizon of free sesquioxides and organic matter.
>85% spodic material and >2.5 cm thick.
(c) Other Diagnostic Characteristics and horizons for soils
Andic soil properties
Presence of significant amount of allphone, imogolite, ferrihydrite or aluminium-
humus complexes.
Secondary carbonates
Calcium carbonates that have been formed in soil not from parent material.
Lithological discontinuities
Significant changes in particle size or mineralogy within a soil.
Plinthite
A humus-poor, sesquioxide-rich, red horizon which hardens irreversibly to iron stone.
Weatherable minerals
All 2:1 type minerals (except Al-interlayered chlorite) and sand and silt sized minerals.
Aquic conditions
Soils experiencing continuous or periodic saturation and reduction indicated by
redoximorphic features.
Permafrost
Soil material remains below 0 oC for > 2 years.
SOIL GENESIS AND CLASSIFICATION 83
Coefficient of linear extensibility (COLE)
Ratio of difference between moist length (Lm) at 33 kPa (moisture at field capacity)
and dry length (Ld) to its dry length i.e. (Lm-Ld)/Ld
Linear extensibility (LE)
It is a product of thickness and COLE of a particular horizon and its sum upto 100 cm
or shallower.
n value
It is used to predict degree of subsidence.
It is (A-0.2R) / (L+3H); A = % of water in the field condition; R = (silt + sand) %, L =
clay %, H = organic matter %
Slickensides
Polished and grooved surfaces developed by one soil mass sliding over another.
Common in swelling clay soils.
Cryoturbation
Mixing of the soil by frost churning.
Lithic contact
A boundary between soil and consolidated material (rock) below.
Sulfuric horizon
Mineral or organic soil with pH <3.5 due to presence of yellow mottles of Jarosite
[KFe3+
3(OH)6(SO4)2].
(ii) Soil Moisture Regimes (SMR)
The SMR refers to the presence or absence of plant available water in the soil during the
year. The soil moisture control section (SMCS) is defined by “an upper boundary equal to
the depth to which a dry soil will be moistened by 2.5 cm of water within 24 hours and a
lower boundary equal to the depth to which a dry soil will be moistened by 7.5 cm of
water within 48 hours (Figure 3.10).
Figure 3.10. Soil moisture control section (SMCS) and depth models based on particle size class (modified
from Sehgal 1996).
84 SOIL SCIENCE: AN INTRODUCTION
Figure 3.11. Soil moisture regimes
Soil moisture regime is used to determine placement of a soil at the suborder level. A
brief of SMR is given in figure 3.11.
(iii) Soil Temperature Regimes (STR)
The STRs are ranges of soil temperatures within which biological activities of different
degree prevail. At freezing limits and at higher temperatures the biotic activities are
severely restricted. The classes of soil temperature regimes are calculated at a depth of 50
cm and are shown in figure 3.12.
Figure 3.12. Soil temperature regime
SOIL GENESIS AND CLASSIFICATION 85
3.14.2.3.2. Structures/Categories in Soil Taxonomy
Soil Taxonomy is a multi-category system and in such a system each category contains all
members of the population (Figure 3.13) Higher level categories are more generalised
and abstracted than the lower level categories. Once the classes are separated at higher
category level these remain separated throughout the lower categories of the system i.e.
the classes are mutually exclusive without overlap. The hierarchical system has six
categories which can be grouped as higher categories (order, suborder and great group)
and lower categories (subgroup, family and series).The various categories of soil taxonomy
along with their differentiating characteristics are presented in table 3.1.
3.14.2.3.3. Nomenclature
The nomenclature used in Soil Taxonomy is based on coined words from Greek or Latin
languages. The basic principles followed in coining the names, are that the name should:
i) be easily remembered, ii) suggest some properties of the object, iii) suggest the place of
a taxon in the system, iv) be as short as possible, v) be as euphonic as possible, and vi) fit
readily in as many languages as possible.
The names of the classification units are combinations of syllables. Each part of the
name conveys a concept of soil character or genesis. For example Aridisol (L. aridus
means dry; solum, soil) - the soils of dry places; Vertisol (L. verto means turn; solum, soil)-
the soils which churn or invert. Detail information about formative elements and their
meaning at each category level is available elsewhere).
Order : A formative element is abstracted from each order – it starts with first vowel and
ends with last consonant preceding the connecting vowel, with common ending ‘sol’
Figure 3.13. Soil Taxonomy - Categories in an hierarchical model
86 SOIL SCIENCE: AN INTRODUCTION
Table 3.1. Categories and their differentiating characteristics
Category Differentiating Characteristics and Description
Order 12 orders (AVAGAMI HOUSE*-A compound word to remember the names).
based on morphology
presence or absence of major diagnostic horizons.
Suborder 68 suborders within 12 orders.
Mostly based on soil properties associated with moisture and temperature regimes, parent
material and vegetation.
Great-Group The great group are >300 in number.
Divided based on presence or absence of diagnostic horizons and its arrangement and on
base status, soil moisture regimes (if not taken at suborder level).
Sub-Group More than 2,000.
The “Typic” is used as the central concept of great group;
Others are used to indicate intergrades.
Family Soil properties most important for plant growth are used as differentia. (like texture,
mineralogy and STR) averaged over the control section or solum.
Series It is the lowest category in the system.
A collection of soils uniform in characteristics and arrangement of horizons (like colour,
texture, structure, consistence, pH and EC).
Most useful for making land-use plans of a small area.
Named as the place where it was first recognized
*A - Alfisols, V -Vertisols, A - Andisols, G - Gelisols, A - Aridisols, M - Mollisols, I - Inceptisols, H -
Histosols, O - Oxisols, U - Ultisols, S - Spodosols, E - Entisols
with connecting “o” for Greek or “i” for Latin. This formative element is used as an
ending for all Suborder, Great Group, Subgroup and Family within an order.
Gelisol – el Andisol – and Aridisol – id Alfisol – alf
Histosol – ist Oxisol – ox Ultisol – ult Inceptisol – ept
Spodosol – od Vertisol – ert Mollisol – oll Entisol – ent
Sub Orders: Sub order consists of 2 syllables
i. First is suggestive of the class (sub Order)
ii. Second is name of the Order
Ust- alf, Hum- ult , Xer- ert, Fluv- ent
iii. Great Group: The name of great group are made by prefixing an additional prefix
Hapl ustalf , Kandi humult, Usti fluvent
iv. Sub Group: These names are coined prefixing one more adjective with the Great
Group
Alfisol – Order
Ustalfs – Suborder
Haplustalfs Great Group
Typic Haplustalfs Subgroup
v. Family: Texture, mineralogy class and temperature regime in order are included to
the subgroup to coin the family name
Ex: Fine, mixed, isohyperthermic family of Typic Haplustalfs
vi. Series: Series are named after a town, village, river, (geographic name), near where
they were first recognized. Jalandhar series, Nabibagh series.
In detail maps surface features like slope, texture, stoniness and gravel, calcareousness
which are important for land use are given along with series name to denote a surface
phase.
SOIL GENESIS AND CLASSIFICATION 87
3.14.2.3.4. Key to Soil Orders
One must use the key (Figure 3.14) systematically to key out the soils through elimination.
The keys are required to avoid confusion in placing the soils in different orders. For
example, presence of mollic epipedon is not a single criterion for Mollisols as mollic
epipedon with other subsurface horizon may qualify for other soil orders. Presence of
argillic horizon may qualify for either Alfisol or Ultisol or any other. Therefore, keys are
important and to be strictly followed in the Soil Taxonomy.
Figure 3.14. Simplified key for classifying soils in different Orders of Soil Taxonomy
88 SOIL SCIENCE: AN INTRODUCTION
3.14.2.3.5. Differentiating Properties of Soil Orders
In Soil Taxonomy, there are twelve Soil Orders and they replace the Zonal, Intrazonal
and Azonal orders of old system. These soils represent broadly the geographic areas that
roughly coincide with the bioclimatic boundaries. The differentiating properties of 12 soil
orders are given below.
Soil Order Major Properties
Gelisols (L. Occur in areas of cold region: Artic, antarctic or high mountains.
Gelare: to freeze) Cryoturbation is an important process.
Soil solution movement is restricted due to freezing temperature.
Not reported in India but may occur in snow-covered Himalayas.
Histosols Organic matter rich (>20%) soils with peaty horizon under permanent water saturated
(Gr. Histos: environment – Histic epipedon.
tissue) Organic matter accumulated is more than decomposition.
Spodosols Cool, humid climate with siliceous parent material.
(Gr. Spodos: An illuvial horizon of sesquioxides and humus formed under a wood ash coloured
wood-ash) eluvial E horizon mainly of silica (spodic horizon).
Not reported in India.
Andisols Soils developed on volcanic ash.
(from Japanese: Dark coloured soils with low bulk density.
black soil) High content of allophane minerals and thus low bulk density (1Mg m-3) and fluffiness
(Andic properties).
Easy to cultivate.
Not reported in India.
Oxisols Deeply weathered soils of humid tropics with brick red colour
(Fr. Oxide: Uniform profile with low amount of weatherable minerals.
oxide) Indurated, mottled horizon (Plinthite) may be present.
Dominated by kaolinite and sesquioxides (Oxic).
Not reported in India.
Vertisols Concept is derived from soil morphology.
(L. Verto: invert) >50 cm thick, black and other dark coloured soils.
Swell when moist and shrink on drying which induces deep wide cracks with gilgai
macro-relief or intersecting slickensides.
High clay content (>30%) with smectite type minerals.
Occur in Peninsular India.
Aridisols Occur in arid climates (arid and semi-arid).
(L.Aridus: dry) Soils dry for most part of the year.
Salt accumulation at surface or subsurface (salic/gypsic or calcic) horizon.
Found in western and north-western part of India.
Ultisols Base poor soils of humid tropical climate (higher rainfall and temperature).
(L. Ultimum: Argillic or kandic horizon with low base saturation (<35%).
last) Advanced stage of weathering.
Occur in southern India and north-eastern regions.
Mollisols Dark coloured base rich soils (>50% base saturated).
(L. Mollis: soft) Soft, well structured, organic matter rich epipedon (mollic).
Developed in subhumid to humid climate under grassland or forest vegetation.
In India, occur in Terai region of Uttarakhand and some other states.
Alfisols Soils developed on stable landscape and formed under soils of humid and subhumid
(from pedalfer climate.
of Marbut) Base rich soils (>35% base saturated) with clay-enriched (argillic) horizon.
Evidence of clay illuviation.
More strongly weathered than Inceptisols but less than Ultisols.
Widely distributed in India.
Inceptisols Soils in an early stage of development.
(L. Inceptum: Alteration of parent material to develop structure, textural variation or colour.
beginning) May have one or more diagnostic horizons (cambic, umbric or mollic).
Do not have an argillic horizon.
Found throughout India and are important soils.
Entisols Recently developed mineral soil with no diagnostic horizon.
(recent) Low degree of soil development due to less time.
Occur in all states of India.
SOIL GENESIS AND CLASSIFICATION 89
3.14.2.4. The World Reference Base for Soil Resources (WRB)
There are many systems in vogue to group the soils but there is no universally acceptable
system for soil classification for hundreds of years. Though it is very difficult for any one
system to adequately meet all the global, regional and local objectives, yet it is necessary
to have one system acceptable and understandable to all pedologists working at different
regions.
The World Reference Base (WRB) is the International Standard Taxonomic Soil
Classification System adopted by the International Union of Soil Sciences (IUSS). It was
developed by the International Soil Reference and Information Centre (ISRIC) in
collaboration with FAO and IUSS and replaces the FAO Soil Classification.
The WRB has taken concepts from many Classification System including Soil
Taxonomy FAO Soil Map Legend of 1988, and Russian concepts. The classification is
based mainly on soil morphology which gives imprints of pedogenesis. A major difference
with Soil Taxonomy of USDA is that soil climate is not taken as a part of the system
though climate influences soil characteristics and are important for interpretation. In
general, the diagnostic criteria match with existing systems so that correlation is possible
with national and previous international systems. The third edition of WRB has been
published recently (IUSS Working Group, WRB, 2014).
3.15. Soils of India
India has a total geographical area of 328.79 Mha. The country exhibits great diversity in
physiography and it influence the climate and vegetation. The climate and vegetation
influenced the geological formations of igneous, sedimentary and metamorphic rocks
occurring in the country and this resulted in development of different types of parent
materials and finally the soils. Thus, the difference in dominating soil forming factors
and processes resulted in development of a wide variety of soils in the country.
3.15.1. Soils
The first soil map of India was developed in 1932 by Scholaskaya, a Russian scientist, on
the basis of the then Russian pedological principles. Later soils of India were divided into
24 major soil groups by Govindarajan (1965) and Raychaudhuri and Govindarajan (1971)
revised this soil map with equivalents of US system of soil classification. The National
Bureau of soil survey and Land Use Planning (1982) prepared a soil map of India (1:7
million scale) using units of Soil Taxonomy with the information available at that time. In
2002, the Bureau further developed the soil map of India on 1:1 scale by categorical and
cartographic generalization from state soil maps on 1:250K scale published periodically
from 1989 onwards (NBSS&LUP Staff 2002). India has a variety of landforms, geology
and climatic conditions and therefore, exhibit diverse types of soils. The soils of India can
be grouped in two systems viz. genetic system and Soil Taxonomy. The former is based
on genetic factors and processes whereas the latter is based on scientific and measurable
soil properties which reflect the soil genesis. Recent publication indicated that 7 soil
orders are occurring in the country. The orders Spodosols, Oxisols, Gelisols and Andisols
(Table 3.2) were not reported.
The major soils of India can be grouped as alluvial, black, red, laterite and lateritic
and desert soils (Figure 3.15). The salt-affected soils are interspersed in some of the above
major soil groups. In terms of Soil Taxonomy, they key out to 7 orders, 23 suborders, 75
great groups and 215 subgroups and 1247 families (Table 3.2). Since this chapter is
90 SOIL SCIENCE: AN INTRODUCTION
Table 3.2. Major soils of India and the number of categories in different orders
Sl. Order Common names Sub Great Sub- Family Area Percent
no. (Tentative) Order Group Group (%)
(Mha)
1 Inceptisols Alluvial soils* 2 2 8 55 129.34 39.4
2 Entisols Alluvial soils (Recent)** 3 9 26 93 78.75 24.0
3 Alfisols Red soils, alluvial soils (old), 3 13 21 61 42.20 12.9
salt-affected soils
4 Vertisols Black soils 3 6 14 50 26.62 8.1
5 Aridisols Desert soils 3 14 44 174 13.35 4.1
6 Ultisols Red, laterite and lateritic soils 4 12 59 447 8.41 2.5
7 Mollisols Forest soils, Terai soils 5 19 43 367 1.64 0.5
8 Others 27.97 8.5
Total 7 23 75 215 1247 328.28 100
* All other soil groups except desert soils; ** Includes all other soil groups except laterite and lateritic soils.
Figure 3.15. Major soil groups of India
SOIL GENESIS AND CLASSIFICATION 91
intended for graduate students and soil taxonomic groups even at suborder level are
large in number, we stick here to the old major soil grouping.
3.15.2 Major Soils of India
3.15.2.1. Alluvial Soils
Derived from alluvial deposits.
Occupy about 113 Mha in the Indo-Gangetic plains (IGP), Brahmaputra valley and
coastal area.
Distributed in Punjab, Haryana, Uttar Pradesh, Uttarakhand, Bihar, West Bengal,
Assam and East and West Coast (Figure 3.15).
Alluvial soils are rich in plant nutrients but deficient in organic carbon and nitrogen.
Classified as Entisols, Inceptisols, Alfisols and Aridisols as per Taxonomy.
3.15.2.2. Black Soils (Block Cotton Soils)
Very dark, cracking clay-dominated soils.
Locally called regur (central India), karail (U.P.) and bhal (Gujarat).
Developed from weathered alluvium of Deccan Basalt or basic parent material under
arid, semi-arid and subhumid climates.
Occupy in about 55 Mha and occur in Maharashtra, Madhya Pradesh, Gujarat,
Rajasthan, Chhattisgarh and some parts of Karnataka and Tamil Nadu (Figure 3.15).
Swell when wet and shrink on drying developing deep wide cracks and slickensides
(in subsurface layers).
High clay varying from 30-60%, high CEC (35-55 cmol(p+)kg-1 and base rich, calcareous
and pH varies from 7.5 to 9.5.
Black colour is due to clay-humus complexes and presence of titaniferrous magnetite
mineral.
Inherently fertile but problem in workability and development of subsoil sodicity
(ESP >5) in some pockets
Classified as Vertisols and Vertic intergrades (subgroup) of Inceptisols and Alfisols.
3.15.2.3. Red Soils
Red soils occupy about 85.7 Mha in the states of Tamil Nadu, Karnataka, Goa,
Maharashtra, Madhya Pradesh, Eastern and north-eastern regions of Bihar, Jharkhand,
West Bengal, Assam and Uttar Pradesh (Figure 3.15).
Shallow to very deep, well drained soils found in semi-arid to warm humid tropical
climates.
Developed from crystalline granite and metamorphic rocks like gneisses and schists
of Archean period.
The colour is due to coating of ferric oxides on soil particles rather than high iron
content – red when oxide is anhydrous (FeO - hematite) and yellow when hydrated
(FeO-OH - limonite).
Slightly acidic to slightly alkaline, low CEC and base saturation lower than black and
alluvial soil but more than laterite and lateritic soils.
Poor in organic carbon, N and P and respond well to K-management
Classified as Alfisols, Ultisols, Entisols and Inceptisols.
92 SOIL SCIENCE: AN INTRODUCTION
3.15.2.4. Laterite and Lateritic Soils
Formed by laterization process (alkali hydrolysis).
Buchanan, 1807, a British Geographer first coined the word laterite (Brick red) for
highly ferruginous, vesicular deposits found in Malabar hills of Kerala.
Formed in tropical and subtropical climate with alternate wet and dry season.
Occur in about 18 Mha in the southern states, Western Ghats of Maharashtra, Orissa,
some parts of West Bengal and north-east regions (Figure 3.15).
The soils are red in colour (5YR to 2.5YR), very deep, high clay content.
Acidic (<6.0), with low CEC [12-16 cmol(p+)kg-1], ECEC [10-16 cmol(p+)kg-1clay] and
base saturation (20-40%).
Deficient in almost all nutrients but can be managed well.
Liming is essential for better crops.
Taxonomically classified as Ultisols and Alfisols.
True laterite is equivalent to Oxisols in Soil Taxonomy, but Oxisols are not found in
India in spite of all the favourable conditions for its formation.
Acidic nature of soils does not allow desilication and therefore, formation of Oxisols
from Ultisols as proposed in soil genesis model is not true.
3.15.2.5. Salt-affected Soils
Salt-affected soils occur in arid and semi-arid climate (<850 mm rainfall) and occupy
about 6.74 Mha.
Distributed in almost all states except NE region (Figure 3.16).
Major part of the IGP and part of coastal areas are saline.
Major characteristics to identify them are
Soil characteristics Saline Alkali Saline-Alkali (Sodic)
pH <8.5 >8.5 <8.5
ECe >4 <4 >4
ESP <15 >15, (>5*) >15, (>5*)
SAR <13 >13 >13
* >5 in case of black soils (Vertisols). ECe= Electrical conductance of saturation paste
Salinity is due to chlorides and sulphates of Na+, Ca2+ and Mg2+ and salts are developed
from weathering, groundwater, irrigation water and sea.
In dry season, soil solution concentrates and causes precipitation of carbonates
(CaCO3).
This CaCO3 precipitation (pedogenic carbonate) increases SAR, ESP and pH in soil.
Cultivated for salt-tolerant crops like, rice, wheat, sugarcane, barley, oats, cotton, etc.
and a variety of grasses like Karnal grass, Bermuda grass.
Leaching of salts with good quality water along with adequate drainage network of
removal of leached water is the practice for reclamation.
Sodic soils are reclaimed by gypsum application.
In sodic soils, drainage is the major problem due to dispersion of clay and clogging of
soil pores.
Classified as Alfisols, Inceptisols, Entisols and Vertisols
SOIL GENESIS AND CLASSIFICATION 93
Figure 3.16. Salt-Affected Soils (Source : NAAS-ICAR 2010)
3.15.2.6. Desert Soils
These are soils of arid climate (both hot and cold), occupy about 26.3 Mha in western
part of Rajasthan, Gujarat, Haryana and Punjab (Figure 3.15).
Cold arid soil occurs in Leh, Ladakh region of Jammu and Kashmir.
In hot dry deserts aeolian action (drawn by wind) carry sand and deposits in the
direction of wind to form thick sandy surface.
Light soils with <10% clay, and very less profile development.
Moderately to strongly alkaline (pH 7.9-9.0), low in organic matter, high in salts, and
calcareous
Low in nutrients and water retention capacity.
Inter-dunal valleys are cultivated for millets and pulses.
Through canal irrigation, soils in Rajasthan are cultivated for many crops.
Classified as Aridisols and Entisols.
94 SOIL SCIENCE: AN INTRODUCTION
3.15.2.7. Forest and Hill Soils
These soils are developed under forest canopy.
Forest soils are found in Himachal Pradesh, Jammu and Kashmir, Uttarakhand,
Sikkim, Madhya Pradesh, Kerala, NE regions and Andaman & Nicobar Islands (Figure
3.15).
Soils of cool climates, under coniferous forests of Himalaya are strongly acidic, high
in organic carbon, moderately high in clay, <50% base saturated and classified as
Alfisols, Ultisols and Inceptisols.
Brown forest soils under sub-humid to humid climate are neutral to slightly acidic,
rich in organic carbon, moderate CEC (15-25 cmol(p+)kg-1) and >60% base saturated
and classified as Mollisols, Inceptisols and Entisols.
3.15.2.8. Peat and Marshy Soils
Developed under humid climate and occur in localized pockets of Kerala and eastern
region of tidal swamps (Figure 3.15).
High amount of organic matter with presence of pyrites (FeS2).
Generally submerged (reduced) but sulphuric acid is formed when oxidized and soil
becomes extremely acidic (pH <4).
These soils are called acid sulphate soils or cat clays, saline peat soils of Kerala are
called “kari soils”.
Keeping water table above pyrite layers rice is grown successfully.
Marshy soils are found in tidal swamps in West Bengal (Sunderbans), coastal areas of
Orissa, Tamil Nadu, and Goa with mangrove vegetation.
Questions
1. What do you understand by land and soil? Draw a neat sketch of soil profile
indicating master horizons. Explain A and E horizons.
2. Define rocks and minerals. Give a neat diagram of rock cycle operating in nature.
3. What do you understand by the term weathering? How is weathering helpful in soil
formation?
4. Explain the direct and indirect effect of climate on soil formation.
5. What are the active and passive factors of soil formation? Describe the process of
calcification operating in soil.
6. What do you understand by factors and processes of soil formation? Describe the soil
characteristics developed under upland, midland and low land situations.
7. Discuss the influence of local relief variation on soil properties.
8. Explain the role of water percolation in development of morphological features in soil
profile.
9. What are the salient features of Soil Taxonomy? What do you understand by diagnostic
and genetic soil horizons? Describe in brief about L, M and W horizons.
10. Differentiate between fundamental and specific soil forming processes. Describe the
soil forming processes that operate under following conditions-
i) Cold humid climate, sandy parent material and coniferous vegetation.
ii) Hydromorphic condition.
SOIL GENESIS AND CLASSIFICATION 95
11. Describe the concept of pedon and polypedon. How do the pedons differ from the
soil profile?
12. What are salient feature of soil taxonomy? Describe its merits and demerits.
13. What do you understand by endodynamorphic and ectodynamorphic soils? Explain
the process of laterization operating in soil.
14. Enlist the conditions which retard the process of soil development.
15. Explain the role of parent material on soil properties and soil profile differentiation.
16. What are various categories in soil taxonomy? Enlist different orders of soil taxonomy.
17. Classify major soil groups of India. Describe in brief the laterite and lateritic soils.
18. Explain in brief i) Gilgai micro relief, ii) Pedoturbation, iii) Slickensides, iv)
Hapladization, v) Redoximorphic features, vi) Role of CO2 in calcification, vii)
Formation of bluish matrix and rusty brown mottles in Hydromorphic soils, viii)
Formation of sodic soils, ix) Cryoturbation, and x) Formation of sodic soil
19. Differentiate between the followings
(a) Podzolization and Laterization
(b) Active and Passive soil forming factors
(c) B – horizons and E – horizons
(d) Pedalfer and Pedocal
(e) Alfisols and Ultisols
(f) Zonal and Intrazonal soils
(g) Active and passive factors of soil formation
(h) Genetic and diagnostic soil horizons
References
Baldwin, M., Kellogg, C. E. and Thorp, J. (1938) Soil classification. In Soils and Men. Year Book of
Agriculture, USDA, Washington D.C., USA, pp 979-1001.
Bockheim, J.G., Gennadiyev, A.N., Hammer, R.D. and Tandarich, J.P. (2005) Historical develop-
ments of key concepts in pedology. Geoderm, 124, 23-36.
Buchanan, F. (1807) A Journey from Madras through the Countries of Mysore, Canara and Malabar. T.
Cadell & W. Davies / Black, Parry & Kingsbury London.
Dokuchaiev, V.V. (1900) Cited by Glinka, K.D. (1927) Dokuchaev’s ideas in the development of pedol-
ogy and cognate sciences. Academy of Sciences, USSR, Pedology 1, Leningrad.
Goldich, S. S. (1938) A Study in Rock Weathering. Journal of Geology 46, 17–58.
Govinda Rajan, S.V. (1971) Soil Map of India, 1:7 m. In Review of Soil Research in India, (J.S. Kanwar
and S.P. Raychaudhuri, Eds), ICAR, New Delhi.
Jenny, H. (1941) Factors of Soil Formation. McGraw-Hill, New York.
Marbut, C.F. (1927) A new scheme for soil classification. Proceedings and Paper of the First Interna-
tional Congress of Soil Science.
Marbut, C.F. (1935) Soils of the United States. In Atlas of American Agriculture, United States
Department of Agriculture, Washington, D.C., USA.
Milne, G. (1935) Some suggested units for classification and mapping, particularly for east African
soils. Soil Research 4.
96 SOIL SCIENCE: AN INTRODUCTION
Soil Survey Staff (1975) Soil Taxonomy : A basic system of soil classification for making and
interpreting soil surveys. Agric. Handbook No. 436, United States Department of Agriculture,
Washington, D.C., U.S.A.
Soil Survey Staff (1999) Soil Taxonomy – A Basic System of Soil Classification for Making and Interpret-
ing Soil Surveys. 2nd Edition. Agriculture Handbook No. 436, United States Department of
Agriculture, Washington, D.C., U.S.A.
Suggested Further Readings
Brady, N.C. and Weil, R.R. (2002). The Nature and Properties of Soils 13th edn. Pearson Education
Pvt. Ltd., New Delhi.
Buol, S.W., Southard, R.J., Graham, R.C. and McDaniel, P.A. (2011). Soil Genesis and Classification
6th edn, Wiley-Blackwell, A John Wiley & Sons Inc., U.K.
... Soil gleyization is a process of soil formation due to lack of oxygen (under an anaerobic environment) that results in the development of a gley horizon in the soil profile. This process is highly dependent on poor drainage conditions that result from land depressions with low topographic elevation, continuous water recharge with water-logged conditions, impervious soil parent materials, and lack of aeration (Brammer and Brinkman, 1977;Lin et al., 2007;Singh and Chandran, 2015). The gleyed soil is oxygen-poor and contains much reductive material, which weakens the biological activity of soil, inhibits the mineralization of organic matter, restricts plant root growth, and results in late maturity and low yield grain crops (Rowell, 1988;Verheye, 2007;Liu et al., 2015). ...
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Thoroughly updated and now in full color, the 15th edition of this market leading text brings the exciting field of soils to life. Explore this new edition to find: A comprehensive approach to soils with a focus on six major ecological roles of soil including growth of plants, climate change, recycling function, biodiversity, water, and soil properties and behavior. New full-color illustrations and the use of color throughout the text highlights the new and refined figures and illustrations to help make the study of soils more efficient, engaging, and relevant. Updated with the latest advances, concepts, and applications including hundreds of key references. New coverage of cutting edge soil science. Examples include coverage of the pedosphere concept, new insights into humus and soil carbon accumulation, subaqueous soils, soil effects on human health, principles and practice of organic farming, urban and human engineered soils, new understandings of the nitrogen cycle, water-saving irrigation techniques, hydraulic redistribution, soil food-web ecology, disease suppressive soils, soil microbial genomics, soil interactions with global climate change, digital soil maps, and many others Applications boxes and case study vignettes bring important soils topics to life. Examples include “Subaqueous Soils—Underwater Pedogenesis,” “Practical Applications of Unsaturated Water Flow in Contrasting Layers,” “Soil Microbiology in the Molecular Age,” and "Where have All the Humics Gone?” Calculations and practical numerical problems boxes help students explore and understand detailed calculations and practical numerical problems. Examples include “Calculating Lime Needs Based on pH Buffering,” “Leaching Requirement for Saline Soils,” "Toward a Global Soil Information System,” “Calculation of Nitrogen Mineralization,” and “Calculation of Percent Pore Space in Soils.”
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