Class 7 Soil

Chapter 11

1. 11
Soil
KNOW THESE PoINTS BEFORE
YOU START
Learning Objectives
The students will be able to
Soil can be broadly classified as sandy soil, clayey soil and loamy soil.
Sandy soil mainly contains large-sized sand particles, is well-aerated, cannot hold
much water and nutrients, and can be ploughed easily.
Clayey soil mainly contains smaller-sized clay particles, traps very little air, can
hold water and nutrients well and is difficult to plough.
understand what is soil
and its importance.
+Study different layers of
soil.
know the varíous
components that make
up the soil.
Loamy soil contains sand, silt and clay in right proportions, has the right water
and nutrients holding capacity, is well-aerated and can be ploughed easily.
know about the different
Different types of soil
absorb water to different
extent
Percolation rate can be calculated using the formula:
types of soil.
+understand that different
soils have different water volume of water (mL)
holding capacities.
Percolation rate (ml/min)
percolation time (min)
know what is soil erosion
and how it is caused.
Soil erosion can lead to floods, famines and landslides.
We need to look after the soil as it takes millions of yearsto get formed.
Now, study the chapter in detail.
Consider the following questions and try to answer them:
How do the trees manage to stand in a forest year-after-year
What is the home of the organisms like earthworms?
What is the foundation of our buildings?
Agriculture provides food, shelter and clothing to us. What is
essential for agriculture?

2. Well! The answer to all this lies in the soil.
nutrients required by plants to grow. Soil layers contain particles of different sizes.
s also very important
as a habitat
-
a
home
Let us perform
an activity to
make .
a rough
LAYERS OF 90IL
Soil enable Soil enables plants to grow. Soil provides all Soil is made up of different layers. These
the nutri
or millions of soil organisms. It also provides estimate of the amount of different particles
anchorage to trees and vegetation.
present
in different layers of
soil.
Soil, like air and water, is one of the most
important natural resources. Soil is found all
around us -in gardens, school playgrounds, Take a glass container (you can use an empty jam jar).
at the parks, at construction sites and also in
the fields. It ies beneath our feet and is often
hidden by roads and buildings. Probably,
this is the reason we often fail to realise how
much soil affects our lives. We depend a lot
on soil. Soil is also essential for the forests of
the world. We get many useful things from
forests. These forests provide us timber and
are also the natural habitat of many plants
Activity
Put some soil in it. Fill 3/4th of the container with
clean water. Stir the mixture well and secure the lid
firmly. Leave the set-up
undisturbed (preferably for
a couple of days) in a safe place. Observe carefully
the different layers of soil. Do they look like this?
How many layers of soil can you observe? Do you
find any dead leaves or animal remains filoating on
water?
and animals.
Activity1
Collect different types of soil samples. You may find
them in your school, at construction sites, roadside or
a garden. Using a hand lens, observe these samples
carefully. You may notice pieces of grasses, pebbles
and small organisms in the soil. Make a list of all
the things, you find in the soil.
The dead and rotting remains of plants and
animals form humus. Let us now study about
the different layers of soil.
S. No. Things seen in the soil
Topsoil or A-horizon
The topmost layer of the soil is called
A-horizon.
It is rich in humus.
It is rich in minerals needed by plants for
their growth.
It is soft.
It is porous.
It is darker than other layers.
It can retain (hold) more water.

3. What is weathering?
Topsoil
The breaking
down of
rocks into small
particles by
the
action of
water and
wind is called weathering.
Subsoil
COMPONENTS OF S0IL
Soilis madeup
of
different-sizedroc
particles.
These
ditferent-sized
particle
infuence the propertiesof soil. Depending on
their size, the rock particles are divided into
four groups.
These four groups
are:
Weathered
parent rock
Bedrock
Clay: Clay
consists of the smallest roc
particles. These small particles are packed
tightly together,
as a result of which, very
little air is present in clay. Water is held in
the tiny gaps between the particles.
Silt: Silt consists of a little bigger rock
particles
than clay.
It is not so smooth.
Fig. 11.1 Layers of soil
A-horizon is the home of many living
organisms like insects and worms. The roots
of plants grow in this layer of soil.
Subsoil or B-horizon
B-horizon lies just below the topsoil. It has
a very small population of living organisms
and is less fertile as compared
to A-horizon.
Sand: Sand consists of still larger rock
particles. Since sand particles are quite
large, they are not bound closely together
As a result of which, there is a lot of
empty
space
between them which is filled with air
The large spaces
between the sand particles
allows water to drain quickly.
It contains comparatively
less amount of
humus.
It is rich in minerals.
It is generally harder and more compact
than topsoil.
It is lighter in colour because of the Gravel: It consists of the largest rock
presence
of less amount of humus. particles.
C-horizon
Other components of soil are as follows:
C-horizon lies just below the B-horizon. It Humus: The organic matter like dead and
decaying
remains of plants and animals
present in the soil is called humus. Humus
is important because it makes the soil fertile
as it contains all the nutrients required
by the plants for their proper growth. It
also improves the water holding capac1ty
of the soil. Humus also provides
food to
earthworms, beetles, millipedes, etc.
is made up of small lumps of rocks. These
rocks have cracks in them, formed due to
weathering of bedrock.
Bedrock
Bedrock is also called the parent rock.
This
parent
rock produces
soil over a long
period of time. The parent rock is very
hard.
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4. Living organisms: Many bacteria, insects
Check PeINTJ
betwe en soil particles
Plants do not
grow
Air: The gaps between the soil
particles are
d worms ve in the soil
Water
is also
present in the
space
Check PeINT 1.
Match the following.
(a)Soil
dead and decaying organic
matter present
in soil
natural
habitat for many
plants and animals
lighter in colour
without water.
The
presence of
water can (b) Humus
be
shown by the
following activity.
() Earthworm
(d) B-horizon
filled uith air. Soil
organisms use air for
eniration. Let us pertorm a simple activity 2. Give one word for the following.
rich in humus
(e) A-horizon
friends of farmers
how that air 1s
present in soil.
to (a) Dead and rotting
remains of plants
and
animals
Activity
come soil in a beaker. Pour
water over it.
(b) Parent rock
(c) The darkest layer of soil
(d) An organism that lives in soil
(e) Breaking down of small particles by
the action
of water and wind
13h carefully.
Do you see some air
bubbles in
the
water coming
out of the soil?
TYPES OF SOIL
Based on the proportion of various particles
present in the soil, the soil can be broadly
classified into sandy soil, clayey soil and
loamy soil
Sandy soil
This soil mainly contains large-sized sand
particles. Clay
and silt
may
be present
in
traces. The characteristicsof sandy soil are:
We can now define soil on the basis of its The space between sand particles is quite
Components. large.
This is filled
up by
air. Sandy
soil is,
therefore, well-aerated.
Yes, soil is a naturally occurring
loose
covering
of
broken rock particles and humus on the earth's
surtace, which is capable
of supporting
life.
Sornething More
Earthworms
are called friends of farmers
because they help in improving the fertility of
Sol. When an earthworm moves, it turns the
SOIl
Over and mixes the soil layers.
This allows air
mto the soil. Earthworms also drag
the leaves down
Into
their burrows, which on rotting
add humus to the
SOIL. Their excreta (wastes) and eventually,
their dead
bodies also increase the soil fertility.
Fig. 11.2 Sandy soil

5. Due to large spaces between the sand
particles, the water drains out quickly. soil.
Because of this, sandy soil cannot hold
much water. Sandy soil, therefore, tends
to be light
and dry. This is a disadvantage
of sandy soil.
Sandy soil can be ploughed easily
Sandy soil cannot hold nutrients.
in loamy soil. Loamy
soil is the most fertil.
Clayey soil
This soil mainly contains smaller-sized clay
particles. Sand and silt are present
in traces.
The characteristics of clayey soil are:
Since the particle size is quite small, the
particles
are
compactly arranged.
Because
of this, very little air can be trapped in
clayey soil. This is a disadvantage of clayey
soil
Fig.
11.4 Loamy
soil
TABLE 11.1 DiFFERENCES BETWEEN SANDY,
CLAYEY AND LOAMY SOILS
Property
Sandy soil
Clayey soil Loamy soil
Smaller-sized Clay, sand and
silt present
in right
proportions
1. Main Large-sized
constituent sand particles clay particles
Clayey
soil can hold much water. Water
drains out very slowly through clayey soil. 2. Space Quite large
Quite less Sufficient
between
This can however lead to waterlogging
which can damage the crops. Clayey soil
tends to be heavy and holds more water
than sandy soil.
particles
Can hold
3.
Presence of Well-aerated Not
well-
aerated air suficient air
4. Water holding Cannot hold
much water
Can hold much Right water
water capacity holding capacity
Clayey soil is difficult to plough.
Clayey soil
can hold nutrients.
Cannot hold
nutrients
6. Ploughing Easy
to plough Difficult to
Can hold
nutrients
5. Nutrients Can hold
nutrients
Easy to plough
plough
The characteristics of loamy soil are as follows:
It has the right water holding capacity.
It can hold sufficient air for the growth of
plantsbecause of the presence of adequate
air
spaces between the particles.
It can be ploughed easily.
It can hold necessary nutrients required for
the growth of plants.
Fig. 11.3 Clayey soil
ABSORPTION OF WATER BY S0IL
Loamy soil
Loamy soil contains clay, sand and silt in Wehave learnt that different soils have different
right proportions. Humus is also present water holding capacities, i.e., different soils
108

6. b
water to different extent. Let us do an
civity
to understand this. Soil has many pores or gaps
which allow
water to flow down. Rainwater also
passes
through
these
pores.
This water helps
the
plants to
grow, and also increases the level
of groundwater.
The pores present in soil are filled with
air. Air helps the animals living in soil to
Activity
samples
of ndy soil and
clayey soil in
equal
, say
50 g each. Place them in two
separate qua helled 'S' for sandy and 'C for
clayey. Dilute
jadr Daint (by mixing some water with red
some
paint)
This will act as coloured water. Now, prepare breathe.
two. o heakers containing each 100 mL coloured water.
Keep a stopwatch ready. Now, gently pour coloured Which type of soil, sandy or clayey,
will allow more of rainwater to seep
into the soil?
'S. Note the time taken for the coloured
water in jar
water to appear
fn the table given beloW. Repeat the activity with
at the base. Note
your observation
the clayey
soil in jar 'C'
Ooes the water take more or less time to
appear
Percolation rate of water in soil
at the base?
Activity
Look for a place which has a good amount of soil,
like a park or a garden. Dig at least 2 cm deep in
the ground. Now, place a glass pipe here, as shown
in the diagram. Measure 250 mL water and slowly
pour into the pipe. Start the stopwatch immediately
when you start pouring water into the pipe and note
down the time. Note the time again when all the
water percolates into the soil. You can now calculate
the percolation rate by using the formula:
JarS
Jar C
Time taken
volume of water (mL)
percolation time (min)
Percolation rate
Suppose the sample of soil taken by you takes
25 min for 250 mL water to percolate. The percolation
rate can be calculated using the above formula as:
What do you conclude from this activity?
The larger the soil particles, the less time it
takes for the water to pass because the space
between the particles
is larger.
250 ml
Percolation rate = = 10 mL/min
25 min
Can we try this activity with other
soil samples?
es, you may collect them from your
School lawn, playground, park, etc. You
Can
even try with sieved and unsieved
Soil and see the results!
Many characteristics of soil can be understood
Do you think the rate of percolation
of different soil samples will be the
Irom the above activity:
same?
09

7. of time, they
eat up
most of the
plant
(or grass)
of that land.
Ihis removes the
vegetation
cover from the topsoil.
T his bar
topsoil
can be easily
carried away by rair
or wind leading
to soil
erosion.
Deforestation: This
is another major caus
for soil erosion. Cutting
down of trees
on
large-scale is called deforestation. The t
of the trees bind the soil particles. Strong
winds and rainwater then carry away the
Check PeINT 2 -
Name the following.
1. Soil that can hold much water but is not
well-aerated. ain
2. Soil that has the right water holding capacity and
has sufficient air.
3. Soil that is well-aerated but cannot hold much
water.
4. Soil that is considered the most fertile.
5. Soil that is difficult to plough.
topsoil
and cause soil erosion.
90IL EROSION
As discussed earlier, the topsoil
is very
fertile.
Strong winds,
river water or heavy
rains
may
carry away
the topsoil.
The removal of topsoil
by
either strong winds, fHowing
river water or
rain is called soil erosion.
Soil erosion is more common in areas which
are not covered by
trees or grass
and are
bare. In the absence of any vegetation,
the
topsoil
becomes loose and gets easily
carried Fig. 11.6 Deforestation
Excessive ploughing of fields: If farmers
plough
or till their tields excessively
to
grow crops,
the soil in the field becomes
loose and can be easily eroded by ran and
strong wind.
away.
Fig. 11.5 Soil erosion
Soil does not get eroded if it is covered by
vegetation.
The roots of the plants bind
together
the topsoil particles
and prevent
soil
erosion. Fig. 11.7A farmer ploughing a field
Causes of soil erosion
Soil erosion can occur mainly due to:
Can frequent flooding of rivers lead
to soil erosion?
Overgrazing: When cattle are allowed to
graze on the same land tor a long period

8. ects of soil erosion
on can harm the
environment in
Effects
Sol
Wa Soil erosion can lead to
floods, imagine nas and can also
cause
landslides in hilly about 500
years
to get
formed! We just
cant
We need to look after the soil as it takes
vironment in millionsof years to get formed. Can you
imagine as little as 1 cm thick soil takes
areas.
we need to look after our soil
replace soil in our lifetime. Life on land
depends on soil. Soil at many places is getting Do we
andrdeners add
fertilisers to thedamaged
risk!
andpolluted.
Soilis,therefore, at
nts so s$O that they can
grow properly. But if
excess tertilisers Ps fertilisers
are used
over a
long time, Check PeINT 3.
risk!
damage the soil. Check PeINT 3.
they may
Fill in the blanks.
1.
Topsoilis
very
2. Removalof topsoilis called
3. Overgrazing removes
the topsoil
COver from
4. Cuttingdown trees on a large-scale is called
Fig. 11.8 A farmer adding fertiliser to crop
KNOW THESE TERMS
-
naturally occurring loose covering
of
broken rock particles and humus on
the earths surfacewhich is capable
of supporting life
Soil Humus organic
matter like dead and
decaying
remains of plants
and
animals present in the soil
Soil erosion
the removal of topsoil by
either
strong winds, fowing
river water
PRACTICE TIME
A. MCQs-Choose the correct answers.
3. Which of the following soils is the best for
1. Which of these soil particles contain the largest
rock particles?
(a) silt
growing crops?
(a) sandy
(c) loamy
b) clayey
d) none of these
b) clay
c)sand O(d) gravel
4. Which soil contains sand, silt and clay in right
2. The darkestlayer of soil is
a) subsoil
proportions?
(a) sandy
(c) clayey
b) loamy
6) bedrock
c) C-horizon Od) topsoil
d) none of these
11