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
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
Q.3: Match the items in Column I with those in 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:
3. Parent rocks
Q.5: List the difference between clayey soil and 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.
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