CBSE Class 7 Science Chapter 9 Notes Soil

Soil Class 7 Notes Understanding the Lesson

1. Soil is one of the most important natural resources for supporting life on the earth.

2. The rotting dead matter in the soil is called humus.

3. Soil is formed by breaking down of rocks by the action of wind, water and climate and this process is called weathering.

4. 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.

5. A-horizon is the uppermost layer. It is also known as the topsoil. This layer is rich in humus and minerals. It is dark, soft and porous. This provides shelter to many living organisms and roots of small plants are embedded entirely in the topsoil.

6. B-horizon is the layer which is just below the A-horizon and is known as the sub-soil. This layer is rich in minerals and is normally hard and compact.

7. C-horizon is the layer which is just below the B-horizon. This layer has partially weathered rock with a lot of cracks and a long narrow opening.

8. Bedrock is the bottom-most layer in soil profile and is known as the bedrock. This is the type of layer which is full of hard rock.

9. The mixture of rock particles and humus is called the soil. Soil can be classified into three main types based on the proportion of soil particles they are composed of. They are

• Sandy soil
• Clayey soil and
• Loamy soil.

10. Sandy soil contains a larger proportion of big particles. It is well aerated and water drains quickly in this type of soil.

11. Silt soil contain soil particles of size in between sand and clay. Silt feels smooth and powdery.

12. Clayey soil contains a larger proportion of fine particles. It has little air spaces between the particles. Water does not drain quickly in this type of soil.

13. Loamy soil contains almost equal proportion of large and fine particles. Loamy soil is a mixture of sand, clay and silt. It also has a lot of humus in it.

14. Following are the properties of soil:

• Soil colour
• Soil texture
• Water-holding capacity
• Percolation rate

15. Soil contains water in the form of moisture.
16. Different soils have different water holding capacity. It is highest in clayey soil and least in sandy soil.
17. Percolation rate of water is different in different types of soil. It is highest in sandy soil and least in the clayey soil.
18. The factors that affect soil are wind, rainfall, temperature, light and humidity. These factors also affect the soil profile and bring changes in the soil structure.
19. The removal of topsoil by the action of water and wind is called soil erosion. Following are some factors which lead to soil erosion:

• Large scale cutting of trees-deforestation.
• Overgrazing in forests.
• Deep ploughing and forests fires also lead to soil erosion.

20. Planting more and more trees or afforestation helps in preventing soil erosion as plant roots bind the soil.
21. Preventing overgrazing, doing terrace farming and planting windbreaks can also help in controlling soil erosion.

Class 7 Science Chapter 9 Notes Important Terms

Clayey: It is a type of soil which have large proportion of fine particles. It has little air space between the particles. Water does not drain quickly in these type of soil.

Humus: A dark-brown or black organic component of soil, formed by the decomposition of leaves and other plant material by soil microorganisms.

Loamy: This type of soil contains almost equal proportions of large and fine particles. It is a mixture of sand, clay and silt.

Moisture: Soil contains water in the form of moisture. Moisture content of a soil depends upon its type and source.

Percolation: Percolation means the movement of water through the soil. It is the rate at which water flows or trickles through porous soil.

Sandy: It is a type of soil which contains a larger proportion of big particles. It is gritty with lot of air spaces between its particles. It is well aerated. Water drains quickly in this type of soil.

Water retention: It is the property of soil to hold a given amount of water which can be used by the crop.