The document discusses the characteristics of soil profiles. It describes the various soil horizons from O (surface organic layer) to R (bedrock). The major horizons are O, A, E, B, C, and R. Each horizon has distinct properties based on its composition and depth. The document also discusses how soil profiles form over long periods from the interaction of climate, organisms, parent material, and relief in the area. The parent material could be sediments deposited by streams, wind, glaciers or developed from weathered bedrock. Together these factors determine the unique characteristics expressed in each soil profile.

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CHARACTERISTICS OF SOIL PROFILE
by
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science
University of Mysore
India

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1. Introduction:
Soils are structural and functional elements of
terrestrial (land-based) ecosystems.
Soils are formed by various geological process
through the interaction of geological, climatic
and biotic factors.
The soil forming process is very slow. It takes
normally thousands of years to form soils of a
thin layers. Soil is the fundamental source of
life for all living beings.

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Soils possess many physical and chemical
properties which exert great influence on the
distribution and development of vegetation and
life.
2. Soils are independent natural bodies
Soils are considered to be independent natural
bodies, each with a unique morphology and
resulting from a unique combination of climate,
living matter, parent rock materials, relief, and
time.

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The morphology of each soil, as expressed in its
profile, reflected the combined effects of the
particular set of genetic factors responsible for
its development.
3. Distribution of soil layers:
The vertical distribution of soils show some
variations due to duration and extent of
weathering, geomorphic conditions, and
strength of parent rocks.

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It also varies from place to place. There is also
minute vertical variation within the layers of
soil mass. Hence, they are called as soil
horizons.
A soil containing a set of distinct horizons is
called as a soil profile. Soil “horizons” are
typically distributed as layers parallel to the
ground surface. In some soils, they show some
evidences of their source rocks and the soil
forming processes.

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4. Soil Profile
Soils, are landscapes as well as profiles. If we
dig a massive trench (hole) of about 2 to 6 m
vertically downwards into the ground, we will
notice various layers of soil horizons. A look
at these layers from a distance, gives a cross-
sectional view of the ground (beneath the
surface) and the kind of soils and rocks that
make up the soil profile. This cross sectional
view is called as the Soil Profile.

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The profile is made up of layers, running
parallel to the surface, called Soil Horizons.
5. Soil Horizons
Each soil horizon(layer) may be slightly or very
much different from the other layer existing
above or below it. Each horizon also tells a
story about the makeup, age, texture and other
characteristics of that layer.

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The layers are divided as top soil layer, sub-soil
layer and the bed rock layers.
6. Soil Profile contains four major horizons:
There are more minor subdivisions in soil
horizons. Most of the soils have five major
horizons. These are designated as O, A,B,C,E
and R. The A,B,C horizons are further
subdivided into micro-layers as A1, A2, A3,
B1, B2, and B3. In some profiles, the letter E is
not used in the zonation process.

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7. The O-Horizon(humus + litter layer):
The O horizon is very common in many
surfaces with lots of vegetative cover.
It is the layer made up of organic materials such
as dead leaves and surface organisms, twigs and
fallen trees.
It has about 20% organic matter.

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It is possible to see various levels of
decomposition occurring here (minimal,
moderately, highly and completely decomposed
organic matter).
This horizon is often black or dark brown in
color, because of its organic content.
It is the layer in which the roots of small grass
are found.

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8. The A-Horizon( Top soil + Root Zone):
The A horizon may be seen in the absence of
the O horizon, usually known as the topsoil.
It is the top layer soils for many grasslands and
agricultural lands.
Typically, the A horizons are made of sand, silt
and clay with high amounts of organic matter.
This layer is most vulnerable to wind and water
erosion. It is also known as the root zone.

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9. The E-Horizon:
The E horizon is usually lighter in color, often
occurring below the O and A horizons. It is
often rich in nutrients that are leached from
the top A and O horizons. It has a lower clay
content. It is common in forested lands or areas
with high quality O and A horizons.

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10. The B-Horizon( Mineral dominated
zone):
The B-horizon has some similarities with the E-
horizon.
This horizon is formed below the O, A and E
horizons and may contain high concentrations
of silicate clay, iron, aluminum and carbonates.
It is also called the illuviation zone because of
the accumulation of minerals. It is the layer in
which the roots of big trees exist.

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11. The C-Horizon(saprolite layer):
C horizons are mineral layers which are not
bedrock and are little affected by pedogenic
processes and lack properties of O, A, E or B
horizons.
The C horizon lacks all the properties of the
layers above it.
It is mainly made up of broken bedrock and no
organic material.

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It has cemented sediment and geologic material.
There is little activity here although additions
and losses of soluble materials may occur.
The C horizon is also known as saprolite.
12. The R-Horizon:
The R horizon is bedrock horizon. It contains
materials that are compacted and cemented by
the weight of the overlying horizons.

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It is the hard layer of unweathered parent
material. All kinds are rock types exist as
basement.
13. Analysing Soil profiles:
First a vertical cut is to be made into the soil
like a deep trench.
After a vertical cut has been made with a spade,
the profile needs to be cleaned and dressed to
bring out the structure and other features
without interference of loose roots.

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Beginning at the top, fragments of the soil may
be broken off with a large knife or fork to
eliminate spade marks.
Dust and small fragments may be blown away
with a small tire pump. Brooms are also used
but may leave streaks. It may be helpful to
moisten the whole profile or parts of it with a
hand sprayer for uniformity of moisture content
and comparability of contrast.

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Moist soils are somewhat darker than dry ones
and often the colors are more intense.
If the whole profile is dry, it may be useful to
moisten one-half, vertically, to show the
contrast between the dry and moist soil.
14. Parent Materials of Soils
The term Solum includes both A and B
horizons that have developed from the parent
material by the processes of soil formation.

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Parent material refers to the unconsolidated
mass from which the solum develops.
Generally, the parent materials of soils can be
grouped into four classes:
(1) Those formed in place through the
disintegration and decomposition of hard
country rocks,
(2) those formed in place from soft or
unconsolidated country rocks,

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(3) those that have been transported from the
place of their origin and re-deposited either
before they became subject to important
modification by soil-building forces and
4) organic deposits.
Materials produced by weathering of hard rocks
in place are distinguished according to the
nature of the original rocks and the character of
the weathered material itself.

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15. Transported Materials
The most important group of parent materials is
the very broad group, made up of materials that
have been moved from the place of their origin
and redeposited during the weathering
processes.

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16. Materials Moved and Redeposited by
Water
Alluvium: The most important of the materials
moved and redeposited by water is alluvium. It
consists of sediments deposited by streams. It
may occur in terraces well above present
streams or in the normally flooded bottoms of
existing streams.

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Generally, the alluvium may be divided into
two main groups according to origin:
(1) Local alluvium, like that at the base of
slopes and along small streams flowing out of
tiny drainage basins of nearly homogeneous
rock and soil material, and
(2) general alluvium of mixed origin, as that
along major stream courses.

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Colluvium:
Colluvium is strictly the material moved
primarily under the influence of gravity and
they include poorly sorted material near the
base of strong slopes that has been moved by
gravity, frost action, soil creep, and local wash.

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Lacustrine deposits:
These deposits consist of materials that have
settled out of the quiet water of lakes.
Those laid down in fresh-water lakes associated
with glacial action are commonly included as a
subgroup under glacial drift.

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Marine sediments:
These sediments have been reworked by the sea
and later exposed either naturally or through the
construction of dikes and drainage canals.
Beach deposits:
These deposits, low ridges of sorted material,
often gravelly, cobbly, or stony, mark the shore
lines at old levels of the sea or lakes.

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17. Materials Removed and Redeposited by
Wind
The wind-blown materials are generally divided
into two classes, mainly in accordance with
texture.
Those that are mainly silty are called loess,
and those that are primarily sand are called
aeolian sands. Typically, deposits of loess are
very silty but contain significant amounts of
clay and fine sand.

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Most loess deposits are pale brown to brown,
although gray and red colors are common.
Characteristically, sand dunes, especially in
humid regions, consist of sand, especially fine
or medium sand, that is very rich in quartz and
low in clay-forming minerals.

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18. Materials Moved and Redeposited by
Glacial Processes
Several classes of materials moved and
redeposited by glacial processes are as follows:
Glacial drift:
Glacial drift consists of all the material picked
up, mixed, disintegrated, transported, and
deposited through the action of glacial ice or of
water. This is resulting primarily from the
melting of glaciers.

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In many places the glacial drift is covered with
loess.
Till or glacial till:
This includes that part of the glacial drift
deposited directly by the ice with little or no
transportation by water.
It is generally an unstratified, unconsolidated,
heterogeneous mixture of clay, silt, sand,
gravel, and sometimes boulders.

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Till may be found in ground moraines, terminal
moraines, medial moraines, and lateral
moraines.
Till varies widely in texture, chemical
composition, and the degree of weathering
subsequent to its deposition.
Glaciofluvial deposits:
These deposits are made up of materials
produced by glaciers and carried, sorted, and

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deposited by water that originated mainly from
the melting of glacial ice.
The most important of these is glacial out wash.
Especially near the moraines, poorly sorted
outwash materials may exist in kames, eskers,
and crevasse-fills.
Glacial beach deposits:
These consist of gravel and sand and mark the
beach lines of former glacial lakes.

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Depending upon the character of the original
drift, they may be sandy, gravelly,
cobbly, or stony.
19. Materials Moved and Redeposited by
Gravity:
Colluvmm is the unsorted or slightly sorted
material at the base of slopes, accumulated
largely as rock fragments that have fallen down
the slope under the influence of gravity. In its
extreme form this material is called talus.

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Rock fragments are angular in contrast to the
rounded, water-worn cobbles and stones in
alluvial terraces and glacial outwash.
20. Organic Materials : In moist situations
where organic matter forms more rapidly than it
decomposes, peat deposits are formed.
These peats become, in turn, parent material for
soils.

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If the organic remains are sufficiently fresh and
intact to permit identification of plant forms,
the material is regarded as Peat.
Besides the accumulation of minerals through
the decomposition of vegetation, large amounts
of mineral matter may be introduced into peat
formations by wash from surrounding uplands,
by wind, and as volcanic ash.

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Nearly raw peat may contain 50 percent mineral
matter as volcanic ash with only a small
influence on the character of the peat.
The color, texture, compactness, and other
characteristics of peat materials in soils need to
be analysed carefully.
The principal general classes of peat, mainly
according- to origin, are (1) woody, (2) fibrous,
(3) moss, (4) sedimentary, and (5) colloidal.

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Characteristics and Origin of the Parent
Material:
The hardness, lithological composition, and
permeability of the material directly beneath the
solum are especially important.
Some soils might have been developed from
stratified parent material; others might have
developed from uniform material.