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Revision Notes for Class 7 Science Chapter 9 Soil
Class
7 Science students should refer to the following concepts and notes for
Chapter 9 Soil in Class 7. These exam notes for Class 7 Science will be
very useful for upcoming class tests and examinations and help you to
score good marks
Chapter 9 Soil Notes Class 7 Science
Soil
Soil
is very important natural resource and a gift from mother earth. It is
capable of supporting life on earth. Soil forms a very thin layer on the
surface of the earth, ranging from few mm to 3 to 4 meters. It is in
this layer of the soil that the nutrients are produced and held along
with air and water. These nutrients nourish the plants and support their
growth. Plants, as we now provide food for all land animals directly or
indirectly through different food chains. Soil holds the roots of
plants firmly. Soil is also the home of many living organisms.
Thus,
there would be no life on earth without soil. But surprisingly, there
are places on the earth which have no soil at all. Large areas of
Antarctica and the Arctic are covered with snow and have no soil for
plants to grow on.
SOIL FORMATION
Soil
is formed when large rocks on the surface of the earth break into
smaller pieces by the action of natural agents such as rain, wind, the
roots of plants, etc.
Soil has not always existed
on earth. In very early times, millions of years ago, the ground was
mostly solid rock. It partially changed into soil by the process called
weathering. Thus, the process of breakdown of large rocks into smaller
pieces by action of natural force, such as sun, water, wind, glaciers,
roots of plants, etc., is called weathering. Weathering is a very slow
process and occurs continuously.
Some of the agents which bring about weathering of rocks are:
1. Physical components like rain, wind and temperature.
2. Biological components like plants, animals and microbes.
Physical Components
Rain:
Rain water is one of the causes of the weathering of rocks. It enters
the crevices of rocks. In winter, this water freezes and form ice. Since
water expands on freezing, the ice in the crevices of the rocks which
further break down to form even smaller pieces.
Wind:
The broken pieces roll down by the force of flowing water or strong
winds. These pieces further break down on hitting other rocks. The river
currents break these into still smaller pieces which mix with humus to
form soil.
Temperature: The
rocks get heated during the day and cooled at night at places which
show large variation in the day and night temperatures. Alternate
heating and cooling cracks the rocks and causes weathering. The rocks
expand during the day and contract at night. This continuous expansion
and contraction weakens the rocks, and they crack and crumble.
Minerals
present in the rocks react with the atmospheric oxygen during hot and
humid climates. Oxidation produces acid and gases which further break
the rocks to form soil.
Biological Components
Plants:
Roots of plants can penetrate to great depth for the absorption of
water and nutrients. They push their way across rocks in the earth’s
crust and exert lot of pressure on the rocks. Thus, cracks are formed in
the rocks and cause weathering. The acid produced by the roots of some
plants also play a role in breaking down rocks.
Animals
and Microbes: The organic acids formed by organisms bring about the
biological weathering of the rocks. The weathered mineral matter and
decomposed organic matter get mixed to from soil.
Atmospheric
weathering is brought about by the influence of temperature changes and
frost. Chemical weathering involves the action of atmospheric gases and
acids dissolved in water.
Weathering of rocks takes a
very-very, long time. It takes hundred of years to produce a few
centimeters of soil. Small plants such as mosses, lichens and ferns
grows in small cracks and fissures in rocks. When they die, the residue
mixes with the soil to become part of the humus. Humus is the nutrient
rich part of soil that makes its fertile.
AGENTS OF GRADATION
Any
natural force which is capable of transporting the weathered rock
material is called an agent of gradation. Running water, ground water,
glaciers, wind, sea waves are the chief agents of gradation. These
perform the work of erosion, transportation and deposition of rock
materials. They build different soil types and land forms.
Soil Profile
Soil
profile is the side view of the vertical section cut through the soil
to the underlying layer of solid rock (Fig. 1). The process of soil
formation directly leads to the development of the soil profile. The
colour of the soil changes as we dig deeper. Similarly, the size of the
rock pieces also changes. Soil profile shows up different layers called
horizons, which are called the A, B and C layers. These three horizons
correspond to the three main layers of soil, viz, top soil, subsoil and
parent rock followed by the impervious or nonporous layer of bed rock.
Top Soil or A-Horizon
This
is the upper most layer which is dark in colour containing organic
matter, the humus. It is soft and porous consisting of fine particles of
soil. Humus makes the soil fertile. Minerals like phosphorus,
potassium, magnesium and calcium are found in this layer. These minerals
are very useful nutrients for healthy growth of plants. Roots of small
plants are buried in this layer. This layer also provides shelter to
organism like worms, beetles and moles.
Subsoil or B-Horizon
Below
the top soil is sub-soil which contains weathered rock materials silt
and clay. It is generally coarse and is rich in minerals that have been
washed down from the top layer. Sub-soil is poor in humus, so it is
lighter in colour. It provides minerals to plants with deep roots.
Weathered Rock or C-Horizon
Below
the sub-soil is the zone of infertile, partly weathered rock. It does
not contain any humus and is not fertile. This layer is first stage in
the soil formation process eventually forms the upper two layers.
Bed Rock or Parent Rock or D-Horizon
The
fourth layer consists of unweathered, non-porous and impervious parent
rock. This is known as bed rock. This layer provides as a base to other
three layers. The soil forming materials are derived from weathering of
this layer.
Soil Composition
About
one-fifth of the area of the Earth’s surface is covered with soil. It
is the topmost layer of the Earth’s crust. But not all top soil is the
same. The composition varies for different types of soil and depends
upon the type of rock from which it is formed. The colour and size of
the soil particles also vary with the minerals present in the rocks.
Some kinds of rocks break into very small particles, whereas others
break into larger ones. Some basic components present in all types of
soils are:
1. Rock Particles: These are of different sizes and can be a part of clay, sand and gravel.
2. Minerals: Clay and sand contain minerals like silver, iron, magnesium, potassium, calcium necessary for healthy plant growth.
3. Humus:
Humus is made up of dead and decaying organic plant and animal remains.
Humus is formed by the action of microbes like bacteria and fungi on
these dead organisms. Humus adds to the fertility of the soil by
providing with organic nutrients. It improves the soil texture and water
holding capacity. It also prevents minerals from being washed away by
water.
4. Moisture:
Rain water seeps through the soil and collects above the bedrock. The
amount of water above bedrock determines the natural level of ground
water, which is called ‘The Water Table’. All three types of soil, viz.,
sandy, clayey and loamy contain some water in the spaces between the
soil particles.
This water is taken up to the plant leaves by
the root hairs through the xylem tissue of plants. The amount of water
contained in a particular type of soil is called its water holding
capacity and depends on the particle size of the soil.
5. Air:
Air is present in the spaces between the soil particles. Plants need
air to breathe or respire. Other living organisms present in the soil
also need air to survive and perform the vital function of converting
dead and decaying organic matter to humus.
Texture of Soil
Rock
break into smaller pieces of different sizes. This is why the size of
soil particles also vary. The texture of the soil thus depends on the
relative proportions of the various types of soil particles.
Depending on particle size, soil is of three types:
1. Sandy Soil 2. Clayey Soil 3. Loamy soil
Finally, depending upon the size of soil particles, soil can be classified into
Gravel 5.0 mm (diameter)
Fine gravel 2.0 mm (diameter)
Coarse sand 0.20 mm (diameter)
Fine sand 0.02 mm (diameter)
Silt 0.002 mm (diameter)
Clay Less than 0.002 mm (diameter)
Soil Types
Different
geographical regions have different types of soils with their own
characteristic colour, texture and contents. Six types of soils are
commonly found in India, as described below:
1. Red Soil or Red Latosol: This
soil is red in colour due to the presence of iron oxide It contains
quartz and clay particles but is deficient in nitrogen and humus. This
soil can be enriched with nutrients by the addition of manures and
fertilizers. This type of soil is found in the interior regions of
Kerala and Tamil Nadu, Southern Karnataka, Andhra Pradesh, Orissa and
Eastern Madhya Pradesh.
2. Black Soil or Regur: This
rich black loamy soil is rich in minerals, humus and is porous in
nature. It contains large proportions of iron and magnesium which are
derived from basaltic rocks - the rocks formed from the lava of volcanic
eruption This soil is ideally suited for growing sugarcane and cotton.
In India, regur soil is found in Maharashtra, Andhra Pradesh , Madhya
Pradesh and Gujarat.
3. Alluvial Soil or Khadar: It
is the most important and widespread of all the soil type It covers
approximately 40% of the land area . It is formed by the deposition of
silt which has been brought down by the great Himalayan rivers - Sutlej,
Ganga, Brahmaputra and their tributaries. In India it is found in the
plains of Haryana, Punjab, Uttar Pradesh, Bihar, West Bengal, Coastal
Orissa and Anthra Pradesh.
4. Desert Soil: This
soil contains large proportions of sand. It is greyish brown in colour,
porous in texture and is rich in soluble salt It does not have good
water holding capacity but it can become fertile when watered. Sandy
soil is mainly found in the Rann of Kutchh in Gujrat and in Rajasthan.
5. Mountain Soil: This is highly fertile soil and has the highest humus content among all the soils in India.
The
proportion of its contents vary from place to place. It is found in the
Himalayan region and North Eastern parts of India and some parts of the
Western Ghats and other hilly ranges such as the Malabar range in
Kerala..
6. Laterite Soil: It is red in colour
and is found in regions of heavy rainfall with highly humid tropical
climate It is clayey in texture and good for growing plantation crops
like tea, coffee and coconut. In India, it is mainly found in the
Westsern Ghats, parts of Tamil Nadu , Andhra Pradesh, Orissa and Assam.
Water Holding Capacity of Soil
The
amount of water absorbed and retained by the soil is called its water
holding capacity. It depends upon the amount of humus present in the
soil and the texture of the soil. Since the texture of soil varies from
place to place, its water holding capacity also varies.
Sandy Soil: Sandy soil has very poor water holding capacity because of the large spaces between the soil particles.
Clayey Soil: Clayey
soil has very good water holding capacity, so in areas that have a lot
of clayey soil, waterlogging is common. Waterlogging mean the collection
of stagnant water above the soil whenever there is rain. The water
logged soil cannot hold air for plant roots to breathe, hence plants
cannot grow well in it.
Loamy Soil: It has good capacity to hold and absorb water. This soil is ideal for agriculture.
Soil Quality and Suitability
Quality of soil depends upon:
1. Fertility of Soil: The
capacity of soil to sustain plant life is a fertility of soil. Soil
fertility can be enhanced artificially by adding to the soil.
a) Chemical fertilizers
b) Biomanures like cow dung compost and wormicast to the soil.
2. Soil Particles: On the basis of the particle size, soil can be classified as gravel, fine gravel, coarse sand, fine sand, silt and clay.
3. Soil Texture: Relative
proportion of different size of soil particles forms the texture of the
soil. On the basis of texture, soil can be classified as sandy, clayey
and loamy.
4. Water Content: Plants are
propagated mainly in soil and get their water supply from soil. Chief
source of water for soil is rainfall. As we have seen, the water holding
capacity of different types of soils varie
