CLASS 7 CHAPTER 9 SOIL

 SOIL NOTES 

Soil is one of the most important natural resources. It is the uppermost layer of earth’s crust which is blackish-brown in colour.

Soil Pollution
Sometimes people throw polythene bags and plastic articles in the soil. They pollute the soil and also hill the useful organisms living in the soil. Some other substances like chemicals and pesticides also pollute the soil.

Soil Profile
Soil is formed by breaking down of rocks by the action of wind, water or climate by the process called weathering. During this process, the rocks are worn away to form small particles by long exposure to the elements of weather.

Weathering is a very slow process as it takes thousands of years for a huge rock to turn into fine particles. The nature of the soil depends upon the rocks from which it has been formed and types of vegetation that grow on it.

The soil profile is a vertical section through different layers of the soil.

Soil profile consists of different layers of soil which are called horizon. Each horizon differs in texture, colour, depth and chemical composition. Different horizons of the soil are discussed further.

1. A-Horizon or Topsoil
It is the uppermost layer of soil and is usually dark in colour because of the presence of minerals and humus in it. It provides the nutrients in plants since the roots of most plants are confined to this layer. Humus is the dark brown coloured layer which consists of dead, rotting remains of plants and animals

Humus helps in making the topsoil porous. This layer is generally soft and retains more water in it. This layer provides shelter for many living organisms like earthworm, rodents, moles and beetles.

2. B-Horizon or Subsoil
This layer of soil lies just below the topsoil. It is made up of slightly bigger rock particles than that of the topsoil. It does not have much humus and is lighter in colour than topsoil. It is somewhat harder and more compact than the topsoil. This layer is less fertile than topsoil and contains few living organisms. The subsoil is rich in minerals and iron oxides.

3. C-Horizon or Substratum
The layer of soil which lies just below the subsoil is called C-horizon. It consists of small pieces of broken rocks with cracks and crevices, formed by the weathering of bedrock or parent rock.

4. Bedrock or Parent Rock
Below the C-horizon unweathered solid rock is found which is called as bedrock. It is non-porous and can produce the soil over a long period of time. Rainwater gets collected over it to form the water table.
This layer is hard enough and is difficult to dig with a spade.