Soil Forming Processes:

During soil formation from parent materials, the regolith undergoes many profound
changes. These changes are brought about by variations in four broad soil-forming
processes.
1. Transformation
2. Translocation
3. Addition
4. Losses
1. Transformation: Transformation occurs when soil constituents are chemically or
physically modified or destroyed, and others are synthesized from the precursor
materials.Transformation may involve in weathering of primary minerals, disintegrating,
and altering some to form various kinds of silicate clays. As other primary minerals
decompose the decomposition products recombine into new materials that include
additional types of silicate clays and hydroxides of iron and Aluminium. Other important
transformations involve the decomposition of organic residues and the synthesis of
organic acids, humus, and other products.
2. Translocation: Translocation involves the movement of inorganic and organic
material laterally within a horizon or vertically from one horizon up or down to another.
Water,either percolating down with gravity or rising by capillary action, is the most
common translocation agent. The materials moved within the profile include dispersed
day particles and dissolved organic substances. Translocations of minerals by soil
organisms also significantly influence soil genecies.Important examples include the
incorporation of surface organic litter into A and B horizons by certain earthworms, the
transport of B and C horizon materials to the surface by mound-building termites, and
the widespread burrowing actions of rodents.
3. Additions: Inputs to materials to develop soil profile from outside sources are
considered additions, some common examples of additions are given below:
●​ The input of organic matter from fallen plant leaves.
●​ Sloughed-off roots which contain caron, originated into the atmosphere.
●​ Dust particles which are falling on the surface of the soil. Wind may have blown
these particles from a source just a few meters away or across the ocean.
●​ In arid regions, salts or silica are dissolved in the groundwater and deposited
heart or at the soil surface when the rising water evaporates.
4. Losses: Materials are lost from the soil profile in many ways. They are given below:
●​ Leaching to the groundwater: Leaching and drainage cause the loss of water,
and dissolved substances such as salts or silica which are weathered from
parent minerals or organic, acids produced by microorganisms or plant roots.
●​ Erosion to surface materials: Erosion often removes the finer particles (such as
humus, clay, and silt) leaving the surface horizon relatively sandier and less rich
organic matter than before.
 ●​ Microbial decomposition: Organic matter is also lost by microbial decomposition.
●​ Grazing and harvesting: Grazing by animals or harvesting by people can remove
large amounts of both organic matter and nutrients from the topsoil.

Soil horizon

During its formation, the soil is arranged in different layers. Each of these layers is
called a soil horizon, and when these layers are arranged sequentially one above the
other, it forms the soil profile. In other words, the soil profile is the vertical section of the
soil exposed by a soil pit.

Soil Profile

■ “Soil profile is defined as the vertical section of the soil from the ground surface
downwards to where the soil meets the underlying rock.”
■ The soil is the topmost layer of the earth’s crust mainly composed of organic minerals
and rock particles that support life. A soil profile is a vertical cross-section of the soil,
made of layers running parallel to the surface. These layers are known as soil horizons.
■ The soil is arranged in layers or horizons during its formation. These layers or
horizons are known as the soil profile. The vertical section of the soil is exposed by a
soil pit. The soil colour and size of soil particles can easily identify the layers of soil.

Layers of Soil profile

The different layers of soil are:

●​ Topsoil
●​ Subsoil
●​ Parent rock
Each layer of soil has distinct characteristics. Soil profile helps in determining the role
of the soil as well. It helps one to differentiate the given sample of soil from other soil
samples based on factors like its color, texture, structure, and thickness, as well as its
chemical composition. The soil profile is composed of a series of horizons or layers of
soil stacked one on top of the other. These layers or horizons are represented by the
letters O, A, E, C, B, and R.

The O-Horizon
The O horizon is the upper layer of the topsoil which is mainly composed of organic
materials such as dried leaves, grasses, dead leaves, small rocks, twigs, surface
organisms, fallen trees, and other decomposed organic matter. This horizon of soil is
often black-brown or dark brown and this is mainly because of the presence of organic
content.
 The A-Horizon or Topsoil
This layer is rich in organic material and is known as the humus layer. This layer
consists of both organic matter and other decomposed materials. The topsoil is soft and
porous to hold enough air and water. In this layer, seed germination takes place and
new roots are produced which grow into a new plant. This layer consists of
microorganisms such as earthworms, fungi, bacteria, etc.

The E-Horizon
This layer is composed of nutrients leached from the O and A horizons. This layer is
more common in forested areas and has lower clay content.

The B-Horizon or Subsoil

■ It is the subsurface horizon, present just below the topsoil and above the bedrock.
It is comparatively harder and more compact than topsoil. It contains less humus,
soluble minerals, and organic matter. It is a site of deposition of certain minerals
and metal salts such as iron oxide.
■ This layer holds enough water than the topsoil and is lighter brown due to the
presence of clay soil. The soil of horizon A and horizon-B is often mixed while
ploughing the fields.
■ The C-Horizon or Saprolite
■ This layer is devoid of any organic matter and is made up of broken bedrock. This
layer is also known as saprolite. The geological material present in this zone is
cemented.
■ The R-Horizon
■ It is a compacted and cemented layer. Different types of rocks such as granite,
basalt and limestone are found here.

SOIL PHYSICAL PROPERTIES

Physically, soils are composed of mineral and organic particles of varying sizes. The
particles are arranged in a matrix that results in about 50 percent pore space occupied
by water and air. This produces a three-phase system of solids, liquids, and gases.
Essentially, all uses of soils are greatly affected/influenced by certain physicalproperties.
Texture,structure, consistency,porosity, density, color, and temperature.

Soil texture
The physical and chemical weathering of rocks and minerals results in a wide range in
size of particles from stones, to gravel, to sand, to silt, and to very small clay particles.
The particle-size distribution determines the soil's coarseness or fineness,or the soil's
texture. Specifically, texture is the relative proportions of sand, silt, and clay in a soil.
Soil texture refers to the composition of the soil in terms of the amounts of small (clays),
medium (silts), and large (sands) size particles. The primary particles of sand, silt,and
clay make up the inorganic solid phase of the soil.These particles often become
aggregated together with each other and other parts of the soil, most importantly
soil organic matter.

The Soil Separates

Soil separates are the size groups of mineral particles less than 2 millimeters (mm) in
diameter or the size groups that are smaller than gravel.The diameter and the number
and surface area per gram of the separates are given in Table 3.1.
Sand is the 2.0 to 0.05 millimeter fraction and, according to the United States
Department of Agriculture (USDA) system, the sand fraction is subdivided into very fine,
fine, medium, coarse, and very coarse sand separates.
Silt is the 0.05 to 0.002 millimeter (2 microns) fraction. At the 0.05 millimeter particle
size separation, between sand and silt, it is difficult to distinguish by feel the individual
particles.In general, if particles feel coarse or abrasive when rubbed between the
fingers, the particles are larger than silt size.

Soil Textural Classes

Once the percentages of sand, silt, and clay have been determined, the soil can be
placed in one of 12 major textural classes.Different texture types like “loam” or “sandy
loam” are described by their proportions of sand, silt, and clay. For example, a clay loam
has proportions of about 35% silt, 30% sand, and 35% clay (the point in the diagram
just above the ‘O’ of clay loam) with a range of values that define the light green “clay
loam” zone. To avoid confusion in using the diagram, pay attention to the direction of the
percentage labels around the outside, which align with the lines you should use to
assess a point on the diagram, and also their relation to the 100% types of clay,silt, and
sand.The texture of a soil is expressed with the use of class names.The sum of the
percentages of sand, silt, and clay at any point in the triangle is 100.A soil containing
equal amounts of sand, silt, and clay is a clay loam. The area outlined by the bold lines
in the triangle defines a given class.For example, a loam soil contains 7 to 27 percent
clay, 28 to 50 percent silt, and between 22 and 52 percent sand.Soils in the loam class
are influenced almost equally by all three separates-sand,silt, and clay.For sandy soils
(sand and loamy sand), the properties and use of the soil are influenced mainly by the
sand content of the soil.For clays (sandy clay, clay, silty clay), the properties and use of
the soil are influenced mainly by the high clay content
Significance of Soil Textural Class
Soil texture has agricultural applications such as determining crop suitability and to
predict the response of the soil to environmental and management conditions such as
drought or calcium (lime) requirements.