SOIL Class 7 Chapter 9
LEARNING OBJECTIVES-
-Knowledge
about soil and soil formation
- Soil
Profile
-Types of
soil
-Properties
of soil
-Moisture
content in the soil
-Absorption
of water by soil
-Soil and
Crops
Soil
erosion and soil conservation-
SOIL-
The natural covering of broken rock particles and humus of the surface of the earth that supports life is called soil. Soil is the outer layer of the earth crust.
Soil supports plants and animals. The sources of food are grown in soil.
Fuels, fodder, medicines and fibres are grown in soil. Water for drinking is
mainly obtained as underground water. Soil is a habitat for many living
organisms like, earthworm, ants, bacteria, fungi, wound worms etc. various
kinds of minerals are obtained from the soil like gold, copper, manganese etc.
FORMATION OF SOIL-Soil is a
mixture of sand, small rock particles and humus. The sand and small rock
particles are formed by physical, chemical and biological weathering of rocks. The humus is formed by humification of dead organic matters. The process of soil formation
is called pedogenesis. The study of
soil and its properties is called pedology.
Pedogenesis = Weathering of Rocks +
Humification
The wind, water, ice, temperature etc help in breaking of rocks. It is
physical weathering of rocks. The water, organic acid like carbonic acid formed
by decomposition of organic matter, chemical reactions in rocks and acid rain
causes the weathering of rock by chemical methods. The animals and roots of
plants are the biological agents for causing weathering of rocks.
SOIL PROFILE-
The vertical section of soil which shows differently coloured and textured
layers is called soil profile. The chemical composition and depth of
different layers of a soil profile also differ from each other. The layers of
soil profile are as following-
1. A-Horizon or top soil layer - It is the upper most layer of the soil.
It is dark in colour than other layers due to presence of humus (decomposed organic
matter). It is soft and porous. It allows water to move inside the soil (water
percolation) easily. It is rich in minerals, so plants grow easily. The water
holding capacity of this layer is also more due to presence of humus. In this
layer various kinds of insects, worms and roots of plants are found in this
layer.
2. B-Horizon or Subsoil layer – It is just below the A-horizon. It has
very less amount of humus. It is less fertile than A-horizon. It has very less
population of microorganisms. It is rich in minerals, harder and compact than
top layer. It is slightly light in colour because of more minerals and less
humus.
3. C- Horizon – It is just below the B-horizon. It has small rocks formed
due to weathering of bedrocks. The roots of trees do not rich to this layer.
Very little humus may be found somewhere in it.
4. R-Horizon or Bedrock or Parent rock- The bedrock undergoes weathering
to add rocks, sand and minerals to the soil layers. It is very hard and does
not allow the water to move down it.
Fig: Soil Profile
TYPES OF SOIL-The size
of rock particles are different in different soils. The colour of soil varies
according to the presence of minerals and their characteristics. The chemical
nature of soil classifies the soil also.
A. SOIL TYPES ON THE BASIS OF PARTICLE SIZE-
Following types of particles are found in the soil.
S.N. |
Soil Particle |
Particle size |
1 |
Clay |
Less than 0.2 mm |
2 |
Silt |
More than 0.2 mm |
3 |
Sand |
0.2 – 2.0 mm |
4 |
Gravel |
2.0 – 50 mm |
So, soil types are-
1. Sandy soil- The soil which is mainly made up of large sand particles
is called sandy soil. Clay and silt may be found very less in such a soil. It
is mainly the desert area soil like Thar desert, Rajsthan, Haryana, Gujrat and
Punjab. The soil is well aerated due to presence of quite large air-spaces in
between soil particles. It has fewer nutrients. Its water holding capacity is
less. It can be ploughed easily. It dries easily.
2. Clayey Soil- This soil has very small size clay particles. It smooth
and its water holding capacity is very good. The aeration of this soil is not
so good. The mineral nutrients are good in this soil. The ploughing of clayey
soil is difficult. Sometimes water logging is seen in this soil.
3. Loamy soil- It is one of the best quality of soil for agriculture. It is made up of sand(40%), silt (40%) and Clay (20%). Its water holding capacity is good. It' minerals content is also optimum. It is suitable for all types of crops but wheat, pulses are mainly grown in this soil.
B. SOIL TYPES ON THE BASIS OF PLACE OF FORMATION
1. Red soil
2. Black Soil
3. Alluvial Soil
4. Laterite soil
5. Mountain Soil
6. Desert Soil
C.SOIL TYPES ON THE BASIS OF CHEMICAL NATURE
1. Acidic soil- pH= 0-7
2. Basic (Alkaline Soil) - pH= 7-14
3. Neutral Soil – pH=0
Properties of Soil-
1. Colour of Soil- The colour
of soil is due to mineral contents and organic matters present in it. Red,
brown and yellow colours are due to different types of iron oxides in the soil.
The black soil is due to certain other minerals, mountain lava and humus
present in it.
2. Soil Structure- The soil
texture/ structure is different for different kind of soil. Many soils are
sticky while others do not stick with each other.
3. Percolation rate- The
movement of water down the soil layers is called as percolation or seepage. It
helps in recharging the underground water. Sandy soil has more water
percolation rate while clayey soil has least water percolation rate.
Percolation rate =Volume of water (in ml.)/Percolation time (in minute)
4. Absorption of water by soil- The water
holding / absorption capacity of the
soil is different for different soil. It is estimated by the formula as-
Percentage Absorption of water by soil = Volume of water absorbed by the
soil/ X 100
Weight of soil
5. Moisture content in the soil- The little
amount of water adhered to the soil particles is called soil moisture. In
summer season, the dry soil has negligible moisture. In rainy season, the soil
moisture will be high. When a little amount of soil is warmed in a test-tube,
little water vapour can be seen on inner wall of beaker.
Activity
1- To find out the
Percolation Rate of soil
Requirements- Clan ground of soil, hollow glass pipe, measuring
cylinder, 250 ml water and a Stopwatch.
Procedure-Clean the ground and dig a small hole. Place a glass
pipe in that hole and fill it up with soil. Now pour 250 ml of water and start
noting down the time take in percolation of water with the help of a stop
watch.
Observation- Approximately in 25 minutes 250 ml of water
percolated through the soil.
So, we can calculate the percolation rate by the following formula-
Percolation rate =Volume of water
(in ml.)/Percolation time (in minute)
= 250
ml/25 minutes
=10 ml/min.
Repeat this activity with sandy and clayey soil also. The sandy soil will
have more percolation rate and clayey soil will show comparatively less
percolation rate.
Activity
2- To calculate water
absorption/water holding capacity of the soil-
Requirements- 100 gm dry powered Soil, a funnel, measuring cylinder,
dropper, filter paper, 100 ml water and calibrated conical flask.
Procedure- The filter paper was folded as like a cup and place
in the funnel. This was then placed in the mouth of conical flask. The 100 ml
water was very slowly poured with the help of dropper.
Observation- The water that came down in the conical flask was 45
ml. It means 55 ml water was absorbed by the dry soil. So, the water absorption
capacity of soil was calculated by the formula-
Percentage Absorption of water by soil = Volume of water absorbed by the soil/Weight of soil X 100
=55 ml/100 gm X 100
=55 %
Repeat this activity with sandy and clayey soil also. The sandy soil will
have less water holding capacity and clayey soil will show comparatively more
water absorption capacity.
Soil and Crops- The crop to be grown in a particular soil depends on the following factors-
a. The type of soil.
b. The climate of that region.
c. The mineral and humus contents of the soil.
1. Sandy Soil- It has low
water holding capacity. The humus content is also low. The crops that grow well
are wheat and legumes like gram, pea, vegetables, chili etc. Coconut grows in
sandy soil of coastal areas.
2. Clayey Soil- Its water
holding capacity is maximum. It is well for growing paddy, sugarcane, jute etc.
3. Loamy Soil- Its moderate
water holding capacity, optimum humus and minerals content make loamy soil good
for almost all type of crops. Paddy, Wheat, Legumes, Vegetables, Cotton, etc.
grow well in loamy soil.
Soil Erosion- The top
soil is very fertile. The carrying away of fertile topsoil by river water,
heavy rain, strong wind, animals etc. from one place to another is called soil
erosion. The grasses and roots of trees bind the soil and prevent soil
erosion.
Impacts of Soil erosion-
A. Soil erosion causes flood.
B. Soil erosion results into famine.
C. Soil erosion causes landslide in hilly areas.
Causes of soil erosion-
1. Overgrazing
2. Deforestation
3. Excess ploughing of cop fields
Soil Conservation- The judicious
and economic use of soil is called soil conservation. Soil conservation is done
to make soil usable for coming generations. Following are the common methods of
soil conservation-
1. Afforestation-Plantation of trees on large scale.
2. Contour/step farming in hilly areas.
3. Avoiding overgrazing and excess ploughing of fields.
4. Construction of dams and river embankments.
Soil Pollution- The
adding of harmful toxic substances which make the soil unfit for use is called
soil pollution. Chemicals like paints, chemical fertilizers, polythene,
plastic, salts, radioactive wastes, oil, grease, dye, acids, industrial wastes
etc are some harmful substances which harm the soil.
The soil quality, percolation rate, fertility and water holding capacity
are affected due to it.
EXERCISE QUESTIONS-
Q.1
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