Soils

Soil is the loose material of the earth's surface in which the

terrestrial plants grow. It is usually formed from weathered rock or
regolith change by chemical, physical and biological process.

Thus the soil may be considered as an entity, quite apart from the
rocks below it. It consist partly of mineral particles and partly to a
varying extent, of organic matter.

Composition of soil
Soil have four main constituents:

1. Mineral matter- it includes all mineral inherited from the parent

material as well as those formed by recombination from
substance in the soil solution.
2. Organic matter- it is derived mostly from decaying plant material
broken down and decomposed by the actions of animals and
microorganisms living in the soil. It is the organic portion that
differentiates soil from geological material occurring below the
earth's surface which otherwise may have many of the
properties of a soil.(Note: The end product of breakdown of
dead organic material is called humus.)
3. Air
4. Water
Normally, both air and water fill the voids in soil. Air and water in
the soil have a reciprocal relationship since both compete for the
same pore spaces.

For example, after a rain or if the soil is poorly drained, the pores
are filled with water and air is excluded. Conversely, as water
moves out of a moist soil, the pore space is filled with air.Thus the
relationship between air and water in soils is continually changing.

The ratio of the components by volume is

generally indicated as:

The exact ratio depends on various

factors like geographical location and the
historical treatment of soil – by humans,
by climate, by time.
How is soil formed?

Soil formation is a
process taking
many thousands
of years.
The Pedogenic Processes:
The above-explained conversion from rocks to soils happens via
four basic processes:
1. Additions: Most additions occur at the surface. The most
obvious ones include solar energy, water controlled by climate,
and organic material derived principally from the vegetation.
2. Losses: Losses occur both from the surface and from the deep
subsoil. For instance, water is lost by evapotranspiration and
carbon dioxide by diffusion at the surface and, on a more
catastrophic level, large masses of soil can be stripped by
erosion. Materials suspended or dissolved in water are the
main forms of losses
from the subsoil e.g.
leaching.
3. Translocation: It refers to
the physical movement of
material within the soil.
The material can be in the
solid, liquid or gaseous
form, the movement can
be in any direction from
and to any horizon. For
instance clay, organic
matter and iron and
aluminium hydrous
oxides are commonly
moved from the surface
horizon to a subsurface horizon. Conversely, in very dry
climates salts are moved upwards in solution by capillarity,
and in very cold climates solid mineral fragments are moved
upwards by frost action.
4. Transformation: Additions, losses and translocations all involve
movement . Transformations, on the other hand, involve the
 change of some soil constituent without any physical
displacement. Chemical and physical weathering and the
decomposition of organic matter are included here.

All these processes occur to a greater or lesser extent in all soils.

The properties that characterise one soil are the result of a
particular balance among all the processes. Other soils will be
different because they have been formed by groups of processes
having different balances.

• The two driving forces for these processes are:

climate (temperature and precipitation), and
organisms, (plants and animals).
• Passive factors:
Parent material is usually a rather passive factor in affecting
soil processes because parent materials are inherited from
the geologic world.
Topography (or relief) is also rather passive in affecting soil
processes, mainly modifying the climatic influences of
temperature and precipitation.

The major factors responsible for the formation of soil:

The major factors affecting the formation of soil are relief, parent
material, climate, vegetation and other life-forms and time.
Besides these, human activities also influence it to a large extent.

1. Parent Material

The parent material of soil may be deposited by streams or

derived from in-situ weathering. Soil inherits many properties from
the parent material from which it forms, for example, the mineral
composition, the colour, the particle size and the chemical
elements.
For Example,

• The peninsular soils reflect the parent rock very much.

• The ancient crystalline and metamorphic rocks which are
basically granite, gneiss and schist form red soils on weathering
because they contain iron oxide.
• Soils derived from lava rocks are black coloured.
• Sandy soils are derived from sandstone.
• At the same time, the soils of the northern plains are transported
and deposited from Himalayan and peninsular blocks, so they
have little relation to rock material in-situ.

2. Climate

The role of climate is to vary the inputs of heat and moisture. It

affects the rate of weathering of the parent rock. Hot and humid
environments, in general, witness the most rapid weathering of
parent materials.

• Role of precipitation: In areas that experience a lot of rainfall,

water percolating down through soil tends to leach nutrients and
organic matter out of the upper layers, unless modified by other
soil components like plant roots.
E.g. the soils underlying tropical rain forests tend to be
nutrient-poor because of intensive leaching due to heavy
rains; most of the nutrients are stored in the lush vegetation
itself.
Conversely, in arid regions with little annual precipitation,
high rates of evaporation encourage the accumulation of
salts in the soil.

• Role of temperature: Solar energy, usually expressed as

temperature, controls the form of water falling onto the soil
 surface as well as in the soil. Also, it increases the rate of
reactions, such as chemical reactions, evapotranspiration and
biological processes. Wide fluctuations in temperature,
especially in the presence of water cause shrinking and swelling,
frost action and general weathering in soils.

E.g. Laterite soils are found in alternate wet and dry climate.
In Rajasthan, both granite and sandstone give birth to sandy
soil irrespective of parent rock because of high temperature
and wind erosion.
3. Biota (Flora, Fauna and Microorganisms): Biota, in conjunction
with climate, modifies parent material to produce soil.

• The kind and amount of plants and animals that exist bring
organic matter into the soil system as well as nutrient elements.
This has a great effect on the kind of soil that will form.

E.g. Soils formed under trees are greatly different from soils
formed under grass even though other soil-forming factors
are similar.

• The roots of plants also hold the soils and protect them from
wind and water erosion. They shelter the soils from the sun and
other environmental conditions, helping the soils to retain the
needed moisture for chemical and biological reactions.
5. Topography (Relief, Altitude and Slope): Topography is often
considered a passive factor modifying the effects of climate.

Topography redistributes the water reaching the soil surface.

Runoff from uplands creates wetter conditions on the lowlands, in
some cases saline sloughs or organic soils. Thus, as a
redistributor of the climate features, topography affects soil
processes, soil distribution and the type of vegetation at the site.

6. Time: Soils can take many years to form. Younger soils have
some characteristics from their parent material, but as they age,
the addition of organic matter, exposure to moisture and other
environmental factors may change its features. With time, they
settle and are buried deeper below the surface, taking time to
transform. Eventually, they may change from one soil type to
another.
The above factors are not mutually exclusive but interdependent.
For example, the kind of vegetation found at any one location on
the earth’s surface is dependent on climate, parent material,
topography, time and, in fact, soil. It is obvious that numerous
combinations of the factors are possible. This leads to many
different kinds of soils, each representing a certain combination of
the factors of soil formation.

Soil Profile
As we discussed earlier, soil development begins when plants and
animals colonize rocks or deposits of rock fragments. Once
organic processes start among mineral particles or rock
fragments, chemical and physical differences begin to develop
from the surface down through the parent material.

Initially, vertical differences result from surface accumulations of

organic litter and the removal of fine particles and dissolved
minerals by percolating water that deposits these materials at a
lower level.

Over time, as climate, vegetation, animal life, and the land surface
affect soil development, this vertical differentiation becomes
increasingly apparent.
If you could dig a massive trench, about 50-100ft vertically
downwards into the ground, you will notice that you would have
cut through various layers of soil types. A look at the layers from a
distance gives one a cross-section view of the ground (beneath
the surface) and the kind of soils and rocks it is made up of. This
cross section view of soil from the surface down to the parent
material is called a Soil Profile.

The Soil Profile is a product of the balance between the soil

system inputs (i.e. additions) and outputs (i.e. losses) and the
redistribution of (i.e. translocations), and chemical changes
(transformations) in the various soil constituents.

The soil profile is made up of layers, running parallel to the

surface, called Soil Horizons. These layers are distinguished by
their physical and chemical properties.

Most soils have three major horizons. These are A Horizon, B

Horizon and C Horizon. Aside these three, there are also the O, E
and R horizons.

1. O-Horizon: The O-horizon is very common to 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. In fact, the ‘O’ designation refers to this horizon’s high
content of organic debris and humus. It is often black or dark
brown in colour, because of its organic content. It is the layer in
which the roots of small grass are found.

2. The A-Horizon: The A horizon, immediately below the O horizon,

is usually known as the topsoil. It is the top layer soil for many
grasslands and agricultural lands. In general, A horizons are dark
because they contain decomposed organic matter.

3. The E-Horizon: The E horizon is usually lighter in colour, often

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
and is common in forested lands or areas with high-quality O and
A horizons.

4. The B-Horizon: Below the E-horizon is the B-horizon, a zone of

accumulation, where much of the nutrients removed from the A
and E horizons are deposited. It is the layer in which the roots of
big trees end. There is a close relationship between the A and B
horizons. Translocations, as well as, many biological and chemical
reactions take place between them. The B horizon, however, tends
to be more stable than the A for short term differences.

5. The C-Horizon: The C horizon is the weathered parent material

from which the soil has developed. This layer is the first stage in
the soil formation process and eventually forms the above two
layers. The C horizon is also known as saprolite.

6. The R-Horizon: It is the unweathered parent material.

Soil Characteristics
Knowing a soil’s water, mineral, and organic components and their
proportions can help us determine its productivity and what the
best use for that soil may be. Several soil properties that can be
readily tested or examined are used to describe and differentiate
soil types. The most important properties are discussed below:

1. Colour: A soil’s colour is generally related to its physical and

chemical characteristics. E.g.

Soils rich in humus tend to be dark because decomposed organic

matter is black or brown. Soils with high humus content are
usually very fertile, so dark brown or black soils are often referred
to as ‘rich’. [Note – Some dark soils may be dark because of other
soil forming factors and may have little or no humus]
Red or yellow soils typically indicate the presence of iron.
2. Texture: The soil texture refers to the coarseness/fineness of the
mineral matter in the soil. It is determined by the proportion of the
sand, silt and clay particles:

Clay: Particle Size – diameters

less than 0.002 millimetre
Silt: Particle Size – diameters
between 0.002 millimetres to
0.05 millimetres.
Sand: Particle Size – diameters
between 0.05 and 2 millimetres.
[Rocks larger than 2 millimetres are regarded as pebbles, gravel,
or rock fragments and technically are not soil particles.]

Note: Clay being the finest of all plays the most important role in
soil chemistry (offers more surface area).
The proportions of each of these soil
fractions determine soil texture and
its properties.

The soil texture directly affects:

• The soil water content

• Water flow
• Retention of nutrients
• Extent of aeration
Loamy Soil: Loamy soil is
the one in which none of the
three (sand/silt/clay)
dominates the other two. In
particular, loamy soil has
about 40% sand, 40%silt,
and 20% clay.

Good Soils = Clay + Humus.

The clay-humus complex is
essential for a fertile soil as it provides it with a high water and
nutrient holding capacity. Humus acts as a cement binding the soil
particles together and thus reducing the risk of erosion.

3. Structure :

While the soil texture describes the size of soil particles, soil
structure refers to the arrangement of the soil particles. The way in
which sand, silt, clay and humus bond together is called soil
structure. Structure can partially modify the effects of soil texture.

Some structural characteristics of soil:

• Permeability – The ease with which liquids/gases can pass

through rocks or a layer of soil is called permeability. It depends
on the size, shape and packing of particles. It is usually greatest
in sandy soils and poor in clayey soils.
• Porosity – The volume of water which can be held within a soil is
called its porosity. It is expressed as a ratio of volume of voids
(pores) to the total volume of the material.

• Most porous rocks are

permeable with the
exception of clay in
which pore spaces are so
small that they are often
sealed with groundwater
held by surface tension.
Another exception –
granite is non-porous but
permeable. It is a crystalline rock and hence non-porous. Its
individual crystals absorb little or no water but the rock may
have numerous joints/ cracks through which the water can pass
rendering it permeable.
• A soil with high organic content also tends to have high porosity.

4. Soil Chemistry – Acidity or Alkalinity:

An important aspect of soil chemistry is acidity, alkalinity
(baseness), or neutrality.
Low pH values indicate an acidic soil, and a high pH indicates
alkaline conditions. Most complex plants grow only in the soils
with levels between pH 4 and pH 10 but optimum pH varies with
the plant species.

• In arid and semi-arid regions, soils tend to be alkaline and soils

in humid regions tend to be acidic.
• To correct soil alkalinity and to make the soil more productive,
the soil can be flushed with irrigation water.
• Strongly acidic soils are also detrimental to plant growth, but
soil acidity can generally be corrected by adding lime to the soil.

Soils of India
India has varied relief features, landforms, climatic realms and
vegetation types. These have contributed to the development of
various types of soils in India.

Various classifications adopted to study the Indian Soils:

1. In ancient times, soils used to be classified into two main

groups:

• Urvara (i.e. fertile), and

• Usara (i.e. sterile)

2. In the 16th century A.D., soils were classified on the basis of

their inherent characteristics and external features such as texture,
colour, the slope of land and moisture content in the soil.

• Based on texture, main soil types were identified as sandy,

clayey, silty and loam, etc.
• On the basis of colour, they were red, yellow, black, etc.

3. The National Bureau of Soil Survey and the Land Use Planning
an Institute under the control of the Indian Council of Agricultural
Research (ICAR) did a lot of studies on Indian soils. In their effort
to study soil and to make it comparable at the international level,
the ICAR has classified the Indian soils on the basis of their nature
and character as per the United States Department of Agriculture
(USDA) Soil Taxonomy.
Chief characteristics of these are:

• Entisols – Immature soils that lack the vertical development of

horizons. These soils are often associated with recently
deposited sediments from wind, water, or ice erosion. Given
more time, these soils will develop into another soil type.
• Inceptisols – young soils that are more developed than
entisols.
• Vertisols – heavy clay soils that show significant expansion and
contraction due to the presence or absence of moisture. These
are common in areas that have shale parent material and heavy
precipitation.
• Aridisols – soils that develop in very dry environments.
• Ultisols – associated with humid temperate to tropical climates.
Warm temperatures and the abundant variability of moisture
enhance the weathering process and increase the rate of
leaching in these soils.
• Mollisols – soils common to grassland environments

4. On the basis of genesis, colour, composition and location, the

soils of India have been classified into:

(i) Alluvial soils

• Alluvial soil are widespread in the northern plains and the river
valleys. These soil cover about 40% of the total area of the
country. They are transported and deposited by rivers &
streams.

• They are generally rich in potash but poor in phosphorous

• In the upper middle Ganga plain two diffident types of alluvial

Soil have developed viz. Khadar & Bhangar.
• Khadar is the new alluviam and is deposited by floods annually,
which enriches the soil by depositing fine silts

• Bhangar represents a system of older alluviam deposited away

from the flood plains.

• Both the Khadar and Bhangar soils contains calcareous

concretions ( kankars)

• The colour of alluvial soil varies from the light grey to ash grey.
Alluvial soils are intensively cultivated.

• Alluvial soils of the northern plains → transported soils →

therefore lack humus → lack nitrogen [That is why we need to
use nitrogenous fertilisers in the northern plains!]. Exception: the
Ganga-Brahmaputra delta region is rich in humus.

• In certain areas, these soils are covered with unproductive wind-

borne soil called Loess.
• Lack nitrogen (But these soils are capable of fixing nitrogen very
rapidly through leguminous crops (peas, beans, cloves etc.)
• Suitable Crops: Wheat, rice, maize, sugarcane, pulses, oilseeds,
fruits and vegetables, leguminous crops.
(ii) Black soils
• It covers most of the Deccan plateau which includes parts of
Maharashtra, Madhya Pradesh, Gujrat, Andhra Pradesh and
some parts of Tamil Nadu.

• These soils are also known as the Regur soil or the Black
cotton soil. Internationally, these are known as ‘tropical
 chernozems’. These soils are famous for the cultivation of
cotton.

• They are generally clayey, deep and impermeable.

• The Black soil retains the moisture for very long time because of
slow absorption and loss of moisture.

• The Black soil are rich in lime, iron, magnesia and alumina.they
also contain potash, but they lack in phosphorous, nitrogen &
organic matter.

• The colour of the soil ranges from deep black to grey.

• In the southern and eastern parts of the country where rainfall is

heavy, black soils often occur in close proximity to red soils.
Black soils occupy valleys and low-level areas whereas the red
soils occur on higher slopes and hill tops. Mixed black and red
soils occur in Coimbatore, Madurai, Tirunelveli (Tamil Nadu) and
Bundelkhand region.

(iii) Red and Yellow soils

• Red soil develops on crystalline igneous rocks in areas of low
rainfall in the eastern and Southern part of the Deccan plateau.
Along the piedmont zone of the western ghat, long stretch of
area is occupied by red loamy soil.

• Yellow and Red soils are found in the parts of Odisha and
Chhattisgarh and in the Southern part of middle Ganga plains.

• Locally called ‘Chalka’ in Andhra Pradesh

• Soil develops a reddish colour due to wide diffusion of is on in

crystalline and metamorphic rocks. It looks yellow when it
 occurs in a hydrated form.

• They awe generally poor in nitrogen, phosphorous and humus

(iv) Laterite soils

• These soils develop with high temperature and high rainfall.
These are the result of intense leaching due to tropical rains.

• These soils are poor in organic matter, nitrogen, phosphate and

calcium while iron oxide and potash are in excess.

• Red laterite soil in Tamil Nadu, Andhra Pradesh and Kerala are
more suitable for tree crops like cashew nut. These soils are
also suitable for tea plantations.These soils are widely get as
bricks for use in house constructions.

• These soils are commonly found in Karnataka, Kerala, Tamil

Nadu, Madhya Pradesh and the hilly area of Odisha and Assam.

(v) Arid soils

• Arid soil ranges from red to brown in colour. They are generally
sandy in structure and saline in nature.

• Due to the dry climate, high temperature and accelerated

evaporation they lack in moisture and humus. Nitrogen is
insufficient and phosphate content is normal.

• Lower horizons of the soils are occupied by 'kankar' layers

because of the increasing calcium content downwards. Arid soil
are characteristically developed in Western Rajasthan.

(vi) Saline soils

• They are also known as Usara soils.Various local names for
 saline soils are Reh, Kallar, and Chopan, Rakar, Thur, Karl etc.

• They occur in arid and semi arid regions and waterlogged and
swampy areas

• They are more widespread in Western Gujrat, deltas of the

eastern coast and in Sunderbans areas of West Bengal.

• They contain a large portion of sodium, potassium and

magnesium and The's they are infertile. They lack in nitrogen
and calcium.

• In the areas of intensive cultivation with extensive use of

irrigation, especially in areas of green revolution, the fertile
alluvial soils are becoming saline.

• In Punjab s Haryana farmers are advised to add gypsum to

solve the problem of salinity.

(vii) Peaty soils

• They are found in areas of heavy rainfall and high humidity
where there is a good growth of vegetation.

• It occurs widely in the northern part of Bihar, Southern part of

Uttaranchal and the coastal areas of West Bengal, Odisha and
Tamil Nadu.

• Large quantity of dead organic matter accumulates in these

areas and this gives in rich humus and organic content to the
soil.

• Soils are normally heavy and black in colour. at many places

they are alkaline also.
(viii) Forest soils
• They are formed in the forest areas where sufficient rainfall is
available.

• They are loamy, silty on Valley side and coarse & grained in the
upper slopes.

• In the Snow bound areas of Himalayas, they experience

denudation and are acidic with low humus content. The soil
found in lower valleys are fertile.

5. Another way of classifying rocks is on the basis of dominant soil

forming factors:

Zonal Soil – These soils occur in broad geographical areas or

zones.
• They are influenced more by the climate and vegetation of the
area rather than the rock-type.
• They are mature, as a result of stable conditions over a long
period of time.
• For example – red soils, black soils, laterite soils, desert soils
etc.
•
Azonal Soil – It is that soil which has been developed by the
process of deposition by the agents of erosion.
• It means that it has been made by the fine rocky particles
transported from the far-off regions.
• These are immature soils and lack well-developed soil profiles.
This may be due to the non-availability of sufficient time for
them to develop fully or due to the location on very steep slopes
which prohibits profile development.
• For Example – alluvial and loess soils.

Intrazonal Soil – These soils occur within other zonal soils.

• It is a well-developed soil reflecting the influence of some local
factor of relief, parent material, or age rather than of climate and
vegetation.
• For example, calcerous soil (soils which develop from
limestone), peat soil.