Soil - Revision Notes

 CBSE Class 7 Science

Revision Notes 
Chapter – 9
Soil

• Soil is the uppermost crust that covers the earth. It is a mixture of rock particles and humus.
• Soil is important for life on the earth.
   
• Formation of Soil:
  (i) Soil is formed by weathering or disintegration of parent rocks.
  (ii) Physical weathering is degradation of rocks by physical agents like water, ice, wind, sun, etc.
  (iii) Chemical weathering is chemical decomposition of rocks.
  (iv) Biological weathering is decomposition of parent rocks by bacteria and microorganisms.
   
• Soil Profile: It is the vertical section of soil from the ground surface to the parent rock. Soil profile is a section through different layers of the soil, Various layers are called horizons.
   
• Constituents of Soil: Soil consists of both living and non-living matter. These constituents make the soil fertile.
   
• Types of Soil: Soil is of different types: clayey, loamy and sandy.
  (i) Sandy soil: It contains soil particles with a diameter of 0.2 to 2.0 mm. It comprises of around 60% sand along with some clay. It has very low water retention capacity. It is not rich in humus.
  
  (ii) Clayey soil: It contains soil particles with a diameter of less than 0.2 mm. It has an excellent water retention capacity and air circulation is sufficient.
  
  (iii) Loamy soil: It contains a good mixture of sand, clay and humus. It has a good water retention capacity and air circulation is sufficient.
   
• Percolation rate of water is different in different types of soil.  It is highest in the sandy soil and least in the clayey soil.
   
• Different types of soils are used to cultivate different types of crops. Clay and loam are suitable for growing wheat, gram and paddy. Cotton is grown in sandy loam soil.
   
• Soil holds water in it, which is called soil moisture.  The capacity of a soil to hold water is important for various crops.
• Clayey soil is used to make pots, toys and statues.