Class 7 - Biology - Soil

Question 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



(iii) minerals, organic matter, air and water.

As these things are required for the soil to be more fertile, hence the soil contain them.



Question 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.



Question 3: 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



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



Question 4:

Explain how soil is formed.



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:


  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.



Question 5:

How is clayey soil useful for crops?



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.


Question 6: List the differences between clayey soil and sandy soil.


Answer 6:


Clayey soil

Sandy soil


Ratio of fine particles is higher.

Ratio of large particles is higher.


Soil retains water and becomes sticky and wet.

Loose particles, air permeate through the soil.


Suitable for crops like paddy, wheat and gram.

Suitable for vegetables, peanuts and cotton cultivation.


Difficult to plough when wet.

Dries up easily. Cannot retain water.



Question 7:

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



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.



Question 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.



 Rate of percolation =   Amount of water(ml)/  Percolation time(min) = 200/ 40 =  5ml/min



Question 9:

Explain how soil pollution and soil erosion could be prevented?



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.






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