Class 7 Science Chapter 9 Important Questions

Class 7 Science Chapter 9 Important Questions Answers and Main Keywords of soil for the CBSE academic session 2020-2021 free to use. Important Extra questions Class 7 Science chapter 9 includes all the questions covering the entire textbook issued by NCERT.

Questions are taken from NCERT Textbook intext study material and exercises. Answers are prepared in simple format so that a student of class 7 can understand easily.

Class 7 Science Chapter 9 Important Questions for 2020-2021

Class: 7Science
Chapter: 9Soil
Contents:Extra Questions with Answers

Class 7 Science Chapter 9 Important Questions Set – 1

“Soil is an inseparable part of our life” – Comment.

Soil is one of the most important natural resources. It supports the growth of plants by holding the roots firmly and supplying water and nutrients. It is the home for many organisms. Soil is essential for agriculture. Agriculture provides food, clothing and shelter for all. Soil is thus, an inseparable part of our life.

A man digging a pit found that he could dig with ease initially but digging become difficult as he went deeper. He could not dig beyond a depth of 5 feet. Explain.

The man digging a pit could dig with ease initially because of the presence of top-soil and subsoil which is mainly comprising of humus and nutrients. But as he digs deeper, he finds it difficult to dig beyond a depth of 5 feet as lower layers are made up of small partially weathered rocks with cracks, crevices and with bedrock which make it hard to dig.

What is meant by soil erosion? What are its various causes and harms? How it can be checked? In which areas soil erosion is more severe?

The removal of land surface or the topsoil by water, wind or ice is known as soil erosion.
Plant roots firmly bind the soil. In the absence of plants, soil becomes loose. So, it can be easily moved by wind and flowing water.
For preventing soil erosion, deforestation and cutting of trees should be prevented and effort should be made to increase the green area. Erosion of soil is more severe in areas of little or no surface vegetation, such as desserts or barren lands.

Explain the soil profile.
The upper crust of the earth, which is capable of supporting vegetation is called soil. We usually see the surface of the soil. The sides of trench sides of a hill or steep river banks are clearly visible different zones or layers. The side-view of soil zones or layers is also as soil profile.
The uppermost layer of the soil is called the A-horizon or top-soil. It absorbs maximum water. The top soil consists of soil particles, minerals, water and humus. The decomposed organic matter or decayed plants and animals, remain forms a spongy and brownish or blackish material known as humus. The humus makes the soil very fertile. It also increases water holding capacity of the soil. Worms, insects, algae and fungi are found in the topsoil. You will also find the roots of plants in this layer. Plants get essential nutrients from the topsoil through roots.
The second layer, called B-horizon, is more compact and harder than the top-soil. It consists of sand, silt and clay. This layer contains very little organic matter. It is rich in soluble minerals and iron oxides.
The third zone, called C-horizon, consists of weathered rock. It is made up of small lumps of rocks with cracks and crevices. Below the C-horizon is found the bedrock.

Explain the water absorbing capacity of soil with the help of an activity.
Take a plastic funnel. Take a filter paper (or a piece of newspaper sheet), fold and place it. Weigh 50 g of dry, powdered soil and pour it into the funnel. Measure a certain amount of water in a measuring cylinder and pour it drop by drop on the soil. You can use a dropper for this purpose. Do not let all the water fall at one spot. Pour water all over the soil. Keep pouring water till it starts dripping. Subtract the amount of water left in the measuring cylinder from the amount you started with. This is the amount of water retained by the soil.
Record your results in your notebook in the following manner:
Weight of soil = 50g
Initial volume of water in the measuring cylinder = U mL
Final volume of water in the measuring cylinder = V mL
Volume of water absorbed by the soil = (U – V) mL
Weight of water absorbed by the soil = (U – V) g
(1 mL of water has weight equal to 1 g)
Percentage of water absorbed = (U – V)/50 x 100
Repeat this activity with different samples of soil. You observe that different soils have different absorbing capacity of water.

Class 7 Science Chapter 9 Important Questions Set – 2

What do you mean by “A-horizon” and “Bedrock” layer of the soil?

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. This provides shelter for many living organisms such as worms, rodents, moles and beetles.
Bedrock is the lowest layer. Below C-horizon layer is the bedrock, which is hard and difficult to dig with a spade. No flora and fauna found in this layer.

Write down the types of the soil on the basis of the proportion of particles of various sizes.

The soil is classified on the basis of the proportion of particles of various sizes. If soil contains greater proportion of big particles it is called sandy soil. If the proportion of fine particles is relatively higher, then it is called clayey soil. If the amount of large and fine particles is about the same, then the soil is called loamy. Thus, the soil can be classified as sandy, clayey and loamy.
The best topsoil for growing plants is loam. Loamy soil is a mixture of sand, clay and another type of soil particle known as silt. Silt occurs as a deposit in river beds. The size of the silt particles is between those of sand and clay. The loamy soil also has humus in it. It has the right water holding capacity for the growth of plants.

Explain how does soil particles affect their properties.

The sizes of the particles in a soil have a very important influence on its properties. Sand particles are quite large. They cannot fit closely together, so there are large spaces between them.
These spaces are filled with air. We say that the sand is well aerated. Water can drain quickly through the spaces between the sand particles. So, sandy soils tend to be light, well aerated and rather dry. Clay particles, being much smaller, pack tightly together, leaving little space for air. Unlike sandy soil, water can be held in the tiny gaps between the particles of clay. So, clay soils have little air. But they are heavy as they hold more water than the sandy soils.

The contamination of soil with waste materials causes soil pollution. Does the use of pesticides in agriculture also contribute to the soil pollution? Explain.
The contamination of soil with waste materials especially used polythene bags and plastics, pesticides, fertilisers, acid rain industrial chemical wastes, etc., is called soil pollution.
Pesticides are the poisonous chemical substances which are sprayed on standing crops to save them from the harmful insects and diseases. Some of these pesticides also mix up with the soil in the fields and pollute it. The grains, fruits and vegetables grown in this polluted soil contain pesticides. When we eat such contaminated grains, fruits or vegetables, the pesticides present in them damage our health also.

How is soil formed?
Soil is formed by weathering or disintegration of parent rocks. The process by which huge rocks are broken down into small particles by different factors, called weathering. As it happens by:

    • i) Chemical weathering (especially hydrolysis and oxidation) is the first stage in the production of soils. Hydrolysis is the breakdown of rock by acidic water to produce clay and soluble salts. Oxidation is the breakdown of rock by oxygen and water, often giving iron-rich rocks a rusty-coloured weathered surface.
    • ii) Physical weathering which is degradation of rocks by physical agents like water, ice, wind, sun etc.
    • iii) Biological weathering is decomposition of parent rocks by bacteria and microorganisms.
    • iv) Weathering of rocks is a very slow process. It takes thousands of years to weather huge rocks into fine particles fit to make soil. And when dead plants and animals mixed (humus) with it becomes fertile soil.

Class 7 Science Chapter 9 Important Questions Set – 3

With the help of an activity, show that moisture presents in soil.

Take a boiling tube. Put two spoonful of a soil sample in it. Heat it on a flame and observe it. On heating, water in the soil evaporates, moves up and condenses on the cooler inner walls of the upper part of the boiling tube. In the other hand, on a hot summer day, the vapour coming out of the soil reflect the sunlight and the air above the soil seems to shimmer. After heating the soil, take it out of the tube. Compare it with the soil which has not been heated.

Continuously water-logged soils are disadvantageous for plant growth. Why?

Roots although underground possesses living cells that require oxygen for respiration and production of energy. They absorb oxygen that is present in the spaces between soil particles.
But in water-logged soils, water occupies spaces between soil particles and pushes the oxygen out into the atmosphere. Thus, roots are deprived of oxygen and this affects the plant growth.

Have you ever passed through a farmland during a hot summer day? Perhaps you noticed that the air above the land is shimmering. Why is it so?

If you pass through a farmland during a hot summer day, you will see that the air above the soil is shimmering this happens because the soil in the farmland contains some water. On a hot summer day, this water of soil evaporates to form water vapour. The water vapour coming out of the soil reflects the sunlight irregularly due to which the air above the soil seems to shimmer.

Write down the difference between sandy soil and clayey soil.
The main differences between sandy soil and clayey soil are given below:

    • a) Sandy soil contains mainly big rock particles whereas clayey soil contains mainly fine rock particles.
    • b) Sandy soil cannot hold much water but clayey soil has very good water holding capacity.
    • c) Sandy soil provides good aeration or air to plant roots but clayey soil is not able to trap enough air for the roots of plants.
    • d) Sandy soil is loose, light and non-sticky whereas clayey soil is compact, heavy and sticky.
    • e) Sandy soil is less fertile where is clayey soil is comparatively more fertile.

What are the various components of soil?

There are two types of components of soil:

    1. Organic components: Organic wastes, dead animals, plants and decompositions products.
    2. Inorganic components: Minerals which are derived from fragmentation and weathering of rocks. The pore spaces from formed between the mineral particles of soil are filled with water and gases.

Class 7 Science Chapter 9 Important Questions Set – 4

Why is horse dung mixed in soil which is used for making “matkas and “surahis”?

When surahis and matkas are baked, the burnt horse dung opens up the pores in the soil. So, the water percolates out of the matkas or surahis, evaporates and cools the water inside the matka.

What makes soil a shelter for many living organisms?

Soil contains organic material such as starch, cellulose, fats and proteins and remains of dead organisms. These give energy to the living organisms. Air, water and minerals present in soil also give enough energy to living organisms to sustain their lives.

Show that dry soil also contains some amount of water.

Take a boiling tube. Take some amount of soil in it. Now heat tube on a flame and observe. On heating, water in the soil evaporates, moves up and condenses on the cooler inner walls of the upper part of the boiling tube. This activity shows that dry soil also contains water.

Describe in brief the steps followed in making earthen pots.
For making earthen pots, following steps are followed:

    • i) Soil is collected from barren lands and placed in a tank. It is cleared of all pebbles etc., and soaked for about 8 hours.
    • ii) The soil is then kneaded after mixing horse dung in it.
    • iii) The kneaded soil is given appropriate shape by placing on the wheel. Then the final shape is given by hand.
    • iv) The items are coloured after 3 days of drying. Then all the items are baked in high temperature.

How the soil pollution and soil erosion can be prevented?
    • Prevention of soil pollution:
      i) Use manners institute of chemical fertilizers.
      ii) Industrial waste should be treated before release.
      iii) Avoid use of polythene and plastic or they should not get mixed in soil.
    • Prevention of soil erosion:
      i) Afforestation, largest scale planting in place of cut down forest.
      ii) Avoiding over-grazing of grass lands.
      iii) Use of step-farming in hill regions.

Class 7 Science Chapter 9 Important Questions Set – 5

What are “chemical fertilizers”? Write their harmful effects.

The chemical substances which are rich in nutrients and used to maintain fertility are called fertilizers. They also produce harmful effects. They remain in soil and spoil the porous structure of the soil which is essential for plant growth. They also kill the soil organisms like earthworms. Earthworms burrows and loosens the soil and make it porous.

Why is soil erosion relatively less in dense forests as compared to barren, open fields?

In dense forests, the tree cover or canopy prevents rainwater from directly falling on the ground or soil. Also, the roots of vegetation bind the soil particles and hold them together. As a result, soil erosion is minimised.
But in barren open fields, the soil is exposed to the falling rain. The soil particles become loose due to the impact of raindrops and the flow of water carries them away. The flowing water further erodes the soil surface aggravating erosion.

Is it a good practice to remove grass and small plants that are growing in an open, unused field?

No, it is not good practice to remove grass and small plants growing in an open, and unused field because the plants cover the soil surface. Their roots bind the soil particles, holding and adhering them in place.
It helps in preventing the top-soil from being washed off during heavy rain, floods and winds. In this way, soil erosion is prevented at top-soil layer is preserved for growing more plants.

Which types of crops are grown in clayey soil and loamy soil?
Clayey and loamy soils are both suitable for growing cereals like wheat, and gram. Such soils are good at retaining water. For paddy, soils rich in clay and organic matter and having a good capacity to retain water are ideal. For lentils (masoor) and other pulses, loamy soils, which drain water easily, are required. For cotton, sandy- loam or loam, which drain water easily and can hold plenty of air, are more suitable. Crops such as wheat are grown in the fine clayey soils, because they are rich in humus and are very fertile.

Why does in towns and cities generally the borewells have to be dug very deep to get water as compared to borewells dug in villages?

The borewells have to be dug very deep in towns and cities to get water as compared to those in villages because:

    • i) Excessive use of water in towns, depletes the groundwater.
    • ii) Towns and cities have asphalted roads and vast areas of soil are concreted.

As a result, rainwater cannot percolate to recharge groundwater and the groundwater level further decreases.
Villages have larger areas to open soil surface and fewer asphalted roads and concrete surfaces. Thus, larger soil surface area is available for rainwater to percolate into the soil easily and recharge the groundwater. As a result, even shallow borewells yield water.