Download CHAPTER - 9 SOIL

CHAPTER - 9
SOIL
1) Importance of soil :i) Soil is an important natural resource.
ii) Soil helps in the growth of plants by holding the
roots and supplying water and nutrients.
iii) Soil is essential for agriculture which provides us
food, clothing and shelter.
iv) Soil is a habitat for several organisms like ants, earth
worms, snails, centipeds, millipeds, snakes, rats, beetles
etc.
Living organisms found in the soil
2) Soil pollution :Soil pollution is caused by plastics , chemicals, pesticides
and other waste products.
Soil pollution can be reduced by :i) Avoiding use of plastics.
ii) Treating of wastes and chemicals before releasing into
the soil.
iii) Reducing use of pesticides.
v) Afforestation – planting more trees.
3) Layers of soil :Activity :Take a glass tumbler three quarters filled with water. Add
a handful of soil in a glass tumbler. Stir it well with a glass
rod and leave it undisturbed for some time. We can see
different layers of particles of different sizes.
The rotting dead plant and animal remains at the top is
humus. Below it are layers of clay, sand and gravel.
humus
water
clay
sand
gravel
4) Soil profile :A vertical section through different layers of soil is called
the soil profile.
The layers are called horizons. Each layer is different in
texture, colour and chemical composition.
A – horizon (Top soil)
B – horizon
C – horizon
Bedrock
A – horizon is the uppermost layer of the soil. It is also
called top soil. It is dark in colour and rich in humus. It
provides nutrients to plants. It is soft, porous and retains
more water.
B – horizon is the second layer. It has less humus but has
more minerals. It is harder and more compact.
C – horizon is the third layer. It is made up of small lumps
of rocks.
The lowermost layer is the bedrock made up of very hard
rocks.
A – horizon (Top soil)
B – horizon
C – horizon
Bedrock
5) Formation of soil and types of soils :a) Formation of soil :Soil is formed formed by the breaking down of rocks into
smaller particles by the action of wind, water and climate.
This process is called weathering.
Soil is a mixture of rock particles and humus.
b) Types of soil :There are three main types of soil. They are sandy soil, clayey soil
and loamy soil.
i) Sandy soil :- In sandy soil the particles are large in size. They have
more space between the particles filled with air. They do not retain
water and are light and dry.
ii) Clayey soil :- In clayey soil the particles are smaller. They have less
space between the particles and has less air. They retain more water
and are heavy and wet.
iii) Loamy soil :- In loamy soil the particle size is between those of sand
and clay. It contains humus and has the right water holding capacity.
So it is the best soil for the growth of plants.
Sandy soil
Loamy soil
Clayey soil
6) Properties of water :i) Percolation rate of water :- (How fast water
passes through the soil)
Water percolates faster through sandy soil
and slower through clayey soil.
Activity :Take a PVC pipe about 20 cm long and 5 cm
diameter. Place it about 2 cm in the ground.
Pour 200 ml water slowly into it and note the
time taken for the water to percolate. The
rate of percolation of water can be calculated
by using the formula
Amount of water (mL)
Rate of percolation =
Percolation time (min)
ground
ground
ii) Moisture in the soil :Soil contains moisture. The amount of
moisture is different in different types of
soils. Clayey soil contains most moisture
And sandy soil contains least moisture.
Activity :Take some soil in a boiling tube and fix
it to a stand. Heat the soil with a burner.
Drops of water are formed in the boiling
tube. The water from the soil evaporates
and condenses in the inner walls of the
boiling tube. This shows that soil contains
moisture.
water drops
soil
iii) Absorption of water by the soil :Soil absorbs water. Clayey soil absorbs most water and sandy soil
absorbs least water.
Activity :Take a funnel. Fold a filter paper and keep it in the
funnel. Weigh 50g dry soil and pour it in the
dropper
funnel. Measure a certain amount of water in a
soil
measuring cylinder. Pour it drop by drop all over
filter paper
the soil till it starts dripping. Subtract the amount
of water left in the measuring cylinder from the
......................... ...............
original amount. This is the amount of water
..........
funnel
absorbed by the soil. Calculate the percentage of
water absorbed by the soil.
Weight of soil = 50 g
Initial volume of water = U mL
Final volume of water = V mL
Volume of water absorbed by the soil = (U – V) mL
(U – V)
Percentage of water absorbed =
X 100
50
beaker
7) Soil and crops :Different types of crops are grown in different types of
soils.
Clayey and loamy soils are suitable for growing wheat and
pulses.
Clayey soil containing organic matter is suitable for
growing paddy.
Sandy and loamy soils are suitable for growing cotton.
Wheat
Peas
Paddy
Cotton