Friday, September 21, 2012

CBSE Class 7 - Science - CH9 - Soil

SOIL
NCERT Solutions, MCQs, Q&A

Q1(MCQ): In addition to the rock particles, the soil contains

(a) air and water
(b) water and plants
(c) minerals, organic matter, air and water
(d) water, air and plants

Answer: (c) minerals, organic matter, air and water

Q2(MCQ): The water holding capacity is the highest in

(a) sandy soil
(b) clayey soil
(c) loamy soil
(d) mixture of sand and loam



Answer: (b) clayey soil

Q3(MCQ): Soil profile refers to an arrangement within a soil of 

(a) its horizontal layout
(b) vertical layout
(c) Size of soil particles
(d) None of these

Answer: (a) its horizontal layout

Q4(MCQ): Which topsoil is best suited for growing plants

(a) Sandy soil
(b) Loamy Soil
(c) Clayey Soil
(d) None of these

Answer: (b) Loamy Soil


Q5(MCQ): Which soil horizon contains humus?

(a) A-Horizon
(b) B-Horizon
(c) C-horizon
(d) Bedrock

Answer: (a) A-Horizon

Q6: Match the items in Column I with those in Column II:

Column I                                       Column II
(i). A home for living organisms(a). Large particles
(ii). Upper layer of the soil(b). All kinds of soil
(iii). Sandy soil(c). Dark in colour
(iv). Middle layer of the soil(d). Small particles and packed tight
(v). Clayey soil(e). Lesser amount of humus

Answer:

Column I                                       Column II
(i). A home for living organisms(b). All kinds of soil
(ii). Upper layer of the soil(c). Dark in colour
(iii). Sandy soil(a). Large particles 
(iv). Middle layer of the soil(e). Lesser amount of humus
(v). Clayey soil(d). Small particles and packed tight

Q7: Explain how soil is formed.

Answer: Soil is formed by weathering of rocks. Weathering is the break down 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:
  1. Weathering of rocks takes place. Rock is broken down into small particles.
  2. These small particles mix with humus (organic matter) and form soil.
Q8: Define Soil.

Answer: Soil is the uppermost layer of the earth's crust. It is capable of supporting life. It is composed of various types of minerals (nitrates, phosphates, potassium etc.) and particles (gravels, sand, humus etc.). It is essential for the existence of plants and animals.

Q9: How is clayey soil useful for crops?

Answer: Clayey soil has an important characteristic that it can retain water and moisture. Therefore it is suitable for growing cereals like wheat, and gram. Clayey soils rich in humus and water gives ideal conditions for paddy to grow.

Q10: What is Humus? What is its significance?

Answer: Humus is the organic matter present in the soil formed by decomposition of dead plant and animal remains. It is an important element of the topsoil that determines the soil fertility.

Q11: List the differences between clayey soil and sandy soil

Answer:
Clayey Soil                      Sandy Soil
1. Ratio of fine particles is higher 1. Ratio of large particles is higher
2. Soil retains water and becomes sticky and wet.2. Loose particles, air permeate through the soil.
3. Suitable for crops like paddy, whet and gram3. Suitable for vegetables, peanuts and cotton cultivation.
4. Difficult to plough when wet.4. dries up easily. Cannot retain water.
. .


Q13: Sketch the cross section of soil and label the various layers

Answer: A soil profile is a arrangement of the successive layers of the soil that would appear when we have vertical section view:

A-Horizon:  Also called topsoil. Upper most layer of soil. The colour of this layer is the darkest, since it is rich in humus and minerals. It is soft, porous and hold more water than the other layers. Plants get most of the nutrients and water from this layer. It is also a home of many living organisms like rodents, snakes, moles, worms etc.

B-Horizon: Second most layer from the top. Lighter in colour. May contain little humus. Rich in minerals and serves as water reservoir.

C-Horizon: Consists of small pieces of original rock broken due to weathering. Hard and compact than the above two soils.

Bedrock: The bottom layer is the unweathered solid rock.

Q14: 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 = Amount of water (mL) ÷ Percolation Time (min)
 = 200 ÷ 40 = 5mL/min.

Q15: What is Water absorption rate?

Answer: Water absorption rate of the soil is defined as the capacity of the soil to take in water from the surface or surrounding areas.

Q16: Why is the uppermost layer of soil dark in colour?

Answer: The colour of this layer is dark, since it is rich in humus and minerals.

Q17: What is soil erosion? What are the causes of soil erosion?

Answer: The removal of topsoil from a region by water and wind is called soil erosion.  It is a natural process and may spoil the environment. It may cause flash floods and it can turn lush green forest into a desert. Following are the causes of soil erosion:
  • Deforestation
  • Over-grazing
  • Frequent flooding of rivers
  • Forest Fires
  • Poor farming methods


Q18: What is soil pollution? What causes soil pollution?

Answer: The contamination of soil due to excess use of chemicals and harmful waste products like fertilizers, insecticides, industry waste is called soil pollution.
Following are the causes of soil pollution:
  • Excessive use of chemical fertilizers and pesticides.
  • Dumping of waste material.
  • Acid Rain
  • Leakage of Radiohazard material.
  • Polythene and plastic pollute soil and kill organisms that live in soil.
 
Q19: Explain how soil pollution and soil erosion could be prevented.

Answer:
Use of gabion to control erosion
Prevention of Soil Erosion
  1. Afforestation: large scale planting in place of cut down forests.
  2. Avoiding over-grazing of grass lands.
  3. Use of step-farming in hill regions.
  4. Constructing embankment or gabions at river banks.
Prevention of Soil Pollution:
  1. Avoid excess use of chemical fertilizers. Use manures instead.
  2. Industrial waste should be treated before release.
  3. Do not throw polythene and plastic bags.
  4. Avoid use of polythene and plastic or they should not get mixed in soil.


(In progress ...)

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