NCERT Class VII: Science-Exercise Solutions

NCERT Book Exercise Page Number-105
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

Answer: (iii) minerals, organic matter, air and water. As these things are required for the soil to be more fertile, hence the soil contain them.
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
Answer: (ii) clayey soil. Water holding capacity is the amount of water that a given soil can hold for crop use.
Water holding capacity of a soil is a very important agronomic characteristic.
Soils that hold generous amounts of water are less subject to leaching losses of nutrients or soil applied pesticides.

Q.3. Match the items in Column I with those in Column II:

Q.4. Explain how soil is formed.
Answer: Soil is formed by weathering of rocks. Weathering is the breakdown of rocks by the action of air, wind and water. Soil formation is a slow process. It occurs all the time.
Soil formation is a two-step process:
(i) Weathering of rocks takes place. Rock is broken down into small particles.
(ii) These small particles mix with humus (organic matter) and form soil.
Q.5. How is clayey soil useful for crops?
Answer: Clayey soil is rich in humus and very fertile, so it is suitable for growing cereals like wheat and gram. Such soil is good at retaining water.

Q.6. List the differences between clayey soil and sandy soil.
Answer:

Q.7. Sketch the cross section of soil and label the various layers.

Answer: A vertical section through different layers of the soil is called the soil profile. Each layer differs in feel (texture), colour, depth and chemical composition.
These layers are referred to as horizons.
• The uppermost horizon is generally dark in colour as it is rich in humus and minerals. The humus makes the soil fertile and provides nutrients to growing plants. This layer is generally soft, porous and can retain more water. It is called the topsoil or the A-horizon.
• The next layer has a lesser amount of humus but more of minerals. This layer is generally harder and more compact and is called the B-horizon or the middle layer.
• The third layer is the C-horizon, which is made up of small lumps of rocks with cracks and crevices.
• Below this layer is the bedrock, which is hard and difficult to dig with a spade.

Q.8. Razia conducted an experiment in the field related to the rate of percolation.
She observed that it took 40 min for 200 mL of water to percolate through the soil sample. Calculate the rate of percolation.
Answer: $Rate of percolation = \frac{Amount of water(ml)}{ Percolation time(min)}
$= \frac{200}{40}
= 5ml/min
NCERT Book Exercise Page Number-106
Q.9. Explain how soil pollution and soil erosion could be prevented?
Answer: Prevention of Soil Pollution:
• Use manures instead of chemical fertilizers.
• Industrial waste should be treated before release.
• Avoid use of polythene and plastic or they should not get mixed in soil.
Prevention of Soil Erosion:
• Afforestation: large scale planting in place of cut down forests.
• Avoid over-grazing of grass lands.
• Use of step-farming in hill regions.