Solved examples for class 7 science : Chapter 9

    Soil

    I. Multiple Choice Questions (MCQs).
    Q.1. Earthworm is generally called the
    a) earthy material
    b) worm cast
    c) farmer’s friend
    d) all of these

    Answer: (c) farmer’s friend
    Q.2. The science dealing with study of soil is called
    a) pedology
    b) virology
    c) phycology
    d) mycology
    Answer: (a) pedology
    Q.3. The arrangement of horizons of soil in a vertical section of earth crust is called
    a) soil layers
    b) soil division
    c) soil profile
    d) none of these
    Answer: c) soil profile
    Q.4. Sandy soil contains
    a) large proportion of sand particles and small proportion of clay
    b) large proportion of clay particles and small proportion of sand
    c) large proportion of sand particles and small proportion of silt
    d) none of these
    Answer: (a) a large proportion of sand particles and small proportion of clay

    Q.5. The largest particles of the soil are
    a) silt
    b) clay
    c) sand
    d) gravel

    Answer (c) sand
    II. Fill in the blanks by selecting appropriate words from the given list :
    List : horizon, cast, soil, weathering, soliem
    Q.1. The word soil is derived from a Latin word ______________ meaning earthy material in which plants grow.
    Q.2. Earthworms make burrows in the soil and excrete it in the form of worm ______________.
    Q.3. The process of soil formation from the rocks is called ______________.
    Q.4. Each layer of soil profile is called a ______________.
    Q.5. Mixing of different percentage of sand, silt and clay particles result in the formation of different types of ______________.

    Answer:
    1- soliem,
    2- cast
    3- weathering
    4- horizon
    5- soil
    III. Match the items of Column A with those of Column B.
    .

    Answer: 1- (b), 2- (e) 3- (d) 4-(a) 5-(c )
    IV. Write ‘T’ for the true and ‘F’ for the false statements.
    Q.1. Soil is a source of water and nutrients for the plants.
    Q.2. Soil is the home to many organisms like bacteria, fungi, protozoans and burrowing animals.
    Q.3. The process of soil formation from the rocks is called soil profile.
    Q.4. The nature of soil depends upon the type of parent rock and type of plant that grows on it.
    Q.5. Generally soil in the moist climate is basic whereas in dry climate it is acidic.

    Answer: -1-T, 2-T 3-F, 4-T, 5-F
    V. Very Short Answer Questions.
    Q.1. What are the factors responsible for formation of soil?
    Answer: Factors responsible for formation of soil are air, water, temperature and other climatic factors.
    Q.2. Define biological weathering.
    Answer: Some microorganisms such as bacteria, fungi, algae, plants, etc. Also causes weathering of rocks and formation of soil called biological weathering.
    Q.3. What are the factors on which nature of soil depends?
    Answer: The nature of soil depends upon the type of parent rock and the type of plant that grows in it.

    Q.4. Write the name of different horizons of soil profile.
    Answer: Different horizons of soil profile are
    i Horizon
    ii Horizon
    iii Horizon

    Q.5. Write different types of soil.
    Answer: Different types of soil are
    (a) Sandy soil
    (b) Clayey soil
    (c) Loamy soil

    VI. Short Answer Questions.
    Q.1. How is soil formed?
    Answer: Soil is formed by the breaking down of rocks into smaller pieces by various breaking agents like air, water, temperature and other climatic factors

    Q.2. What do you mean by soil profile?
    Answer: If you see a vertical section of soil upto two metres or upto the parent rock, it shows different layers. Each layer is called a horizon. The arrangement of these horizon is called as soil profile.
    Q.3. How does change in pH affect the soil?
    Answer: The pH scale measures acidic or basic property of a substance. The pH scale ranges from 0 to 14. A pH 7 on the pH scale is called neutral pH. A pH less than 7 is acidic pH and more than 7 is basic pH. Different crops require different soil pH for absorption of nutrients from soil. pH of the soil can be tested by pH paper. Generally soil in moist climate is acidic whereas in dry climate soil is basic. Generally soil in moist climate is acidic whereas in dry climate soil is basic.

    Q.4. Discuss three horizons of the soil profile briefly.

    Answer: A soil profile consists of three layers or horizons:
    (a) Top soil or A-horizon – This is the uppermost horizon. It is dark in colour because of the presence of rotten dead matter called humus. This horizon is the most fertile one. (b) B-Horizon – This horizon is present just below the A-horizon. This horizon contains less amount of humus and more amount of inorganic salts. Roots of the tall trees can reach to this horizon.

    (c) C-Horizon – It is the lowest horizon. It consists of small lumps of rocks. Beneath the C-horizon, bed rock is present which cannot be easily dug up.

    Q.5. What is the importance of soil? Discuss in brief.
    Answer: Followings are the importance of soil
    • Soil provides nutrients and water to the plants.
    • Many minerals that are used in industry are present in soil. Examples of such minerals are calcite, bauxite, gypsum, etc.
    • Soil is used for making bricks and huts. It is also used in pottery.
    • Many organisms live in soil. It is the home for insects, worms, bacteria, fungi and burrowing animals.

    Q.6. What are the causes of soil erosion and their preventions? Discuss briefly.

    Answer: Soil erosion is brought about by wind and water. In the absence of vegetation soil becomes loose. So, wind and water can erode this soil easily.
    The other factors that increase the soil erosion are overgrazing, poor-farming methods, flood, forest fire, etc.
    Prevention of forest fire:
    (a) Afforestation – Roots of plants bind to the soil.
    (b) Reducing the destruction of vegetation.
    (c) Crop rotation.
    (d) By adopting terrace farming methods for hilly areas.
    (e) Controlling the forest fire, etc.
    VII. Long Answer Questions.
    Q.1. How are different types of soil formed? Describe each one in brief.
    Answer: Sandy Soil: This type of soil is usually formed from rocks such as limestone, granite and shale. It contains large proportion of sand particles and small proportion of clay. Particles are easily visible and have a gritty structure.

    Clayey Soil: contains greater proportion of clay particles and small proportion of sand. Small sized clay particles are tightly packed together. So, clayey soil has poor space for air. Unlike sandy soil it can hold water in its tiny gaps.

    (c) Loamy soil It is a mixture of sand, clay and one more type of soil particles called silt. This soil contains right proportion of air and water for the growth of the plants. This soil is the best for growing plants.

    Q.2. What are the properties of soil? Discuss with examples.
    Answer: Following are the properties of soil
    (a) Texture – Texture of soil depends on relative proportion of sand, silt and clay particles present in it. Sandy soil has a fritty texture. Loamy soil crumbles when rubbed between the fingers. Clayey soil makes a shiny coating on the fingers. Clayey soil can be moulded into different shapes. Hence it can be used for making pots.

    (b) Percolation rate of water – Many spaces are present among the soil particles that allow water to percolate through it. Percolation rate can be calculated by following formula :

    For example if 200 ml of water is poured over a soil sample, 150 ml of water percolates through this soil sample within 15 minutes. Then

    (c) Water retention capacity- Water retaining capacity of soil depends upon two factors (i) the size of particles present in it and (ii) humus content. One can find out the amount of water absorbed by the soil very easily.

    Q.3. How will you find out the percentage of water absorbed by the soil?

    Answer: To find out the percentage of water absorbed by soil we need soil, water, beaker, funnel, filter paper, measuring cylinder.

    Procedure: Take a funnel and a filter paper. Fold the filter paper and set it in the funnel as shown in the figure. Now take 50 g of soil and pour it into the funnel. Now take water (Let u ml) in a measuring cylinder and pour it drop by drop on the soil. Keep pouring water till it starts dripping. Note down the amount of water left in measuring cylinder. Suppose it is v ml. Then, the amount of water absorbed by 50 g of soil = (u –v) ml.
    Weight of water absorbed by the soil = (u – v) g (1 ml of water has weight equal to 1 g)

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