Class VII, CBSE Science
Solutions of NCERT Textbook Exercise Questions
Chapter Exercise
Important Questions Only
Q.1: In
addition to the rock particles, the soil contains:
(i)
air and water
(ii)
water and plants
(iii)
minerals, organic matter, air and water
(iv)
water, air and plants
Solution: (iii)
Q.2: The water holding capacity is the highest in:
(i)
sandy soil
(ii)
clayey soil
(iii)
loamy soil
(iv)
mixture of sand and loam
Solution:
(ii)
Q.3: Match the items in Column I with those in Column II:
Column I
|
Column II
|
(i) A home for living
organisms
(ii) Upper layer of the
soil
(iii) sandy soil
(iv)
Middle layer of the soil
(v) Clayey soil
|
(a) Large particles
(b) All kinds of soil
(c) Dark in colour
(d) Small particles and
packed tight
(e) Lesser amount of humus
|
Solution:
(i) b (ii) c (iii) a (iv) e (v) d
Q.4: Explain
how soil is formed.
Solution:
Soil formation
Soil
is formed from the rocks due to the action of either or some of these processes
-
(a)
Earthquakes
or Volcanic Eruptions: There is a hard layer of rock just below the
earth surface. Due to natural processes like earthquakes and volcanic eruptions
these big chunk of rocks are broken into smaller pieces and come on the surface
of the earth.
(b)
Weathering
of rocks: due to fast
blowing winds, falling of snow, actions of glaciers and running water of rains
etc. the fine particles from the surface of rocks are carried away to different
places. During this process, which is very slow and takes thousands of years,
the sizes of these particles are further reduced. This whole process is called
as ‘physical weathering’.
(c)
Corrosion
or Decomposition of rocks:
The smaller particles of rocks obtained due to weathering get further
decomposed on long exposure to air and moisture. These processes range from
oxidation, reduction, hydration, hydrolysis and carbonation. Lichens and other
plants help in the process, e.g. rocks of minerals like feldspar and mica
combine with water through the process of hydration and become soft and easily
disintegrable. Silicate rocks on hydrolysis produce silicate clays. These are
very slow process known as ‘chemical weathering’.
(d)
Addition
of Humus: Finally the
decomposed parts of dead plants and organisms get mixed up with the soil formed
above. This gives the soil enough energy in the form of organic matter.
There
are other factors also influencing soil formation such as:
1.
Climate
2.
Vegetation
3.
Parent
rocks
4.
Time
Q.5: List the difference between clayey soil and sandy soil.
Solution:
Clayey Soil
|
Sandy Soil
|
1. Proportion
of fine particles is large.
2. It has the
highest porosity among all type of soils. Pores present are mostly ‘closed
pores’.
3. Least
permeable and so, least rate of percolation of water.
4. Has the
maximum water holding capacity.
|
1. Proportion of big particles is
large.
2. Pores present are of continuous type
i.e. ‘open pores’.
3. More permeable than clayey soil and
also has highest water percolation rate.
4. Because of open pores has poor water
holding capacity.
|
Q.9:
Explain how soil pollution and soil erosion could be prevented.
Solution:
Prevention of Soil
Pollution: Following
steps are necessary to prevent the soil pollution -
1. There should be a complete ban on the
use of polythene and plastic bags.
2. Waste products and chemicals should be
treated before they are released into the soil.
3. The use of fertilizers and pesticides
should be minimized.
Prevention of Soil
Erosion: Soil erosion
can be prevented by taking the following steps -
- Planting trees (afforestation)
- Protecting the existing forests and trees
- Holding suitable minerals and organic matter in proper amounts
- Maintaining porous structure of the soil
- Control and reclamation of ravines and shifting of cultivation.
Further study on Chapter 9,
NCERT Science
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