Soil Class 7 Notes Science

Soil Profile

Soil is formed when rocks are broken down by the action of wind, water and climate. This process is called weathering.

Soil is used in:

• Agriculture
• Gardens
• Pottery

Soil is formed when rocks are broken down by the action of wind, water and climate. This process is called weathering. The characteristic features of a soil depend upon the rocks from which it has been formed and the kind of plants that grow in it. Soil forms different layers of particles of different sizes. Each layer is different from the other in texture, colour and chemical composition. Even the thickness of each layer is not the same.

A vertical section that shows different layers of soil is called a soil profile. Each layer is called a horizon.

• Top soil or Horizon-A
• Middle layer or Horizon-B
• Horizon-C
• Bed rock

Horizon-A

• The topmost layer is dark in colour, and contains the remains of dead plants and animals. This rotting matter is called humus.
• This layer of soil is called topsoil or A-horizon.
• It is made up of humus and minerals, and makes the soil fertile.
• It is soft and porous, and can retain more water than the other layers.
• Many tiny organisms, such as beetles, worms and rodents, live in the topsoil.
• nts do not go down very deep and can be found in the topsoil.  The roots of small pla

When plastic bags are disposed off, they usually get embedded in the topsoil. Since plastic does not get decomposed naturally, the polythene bags pollute the soil and kill the organisms that live in it. Stop using polythene bags.

Horizon-B

• The layer below the topsoil is called B-horizon or the middle layer.
• The middle layer is less porous than the topsoil, and is, therefore, harder.
• It contains more minerals as compared to the topsoil, but less quantity of humus.

Horizon-C

• It is not as compact as the two layers above it. This layer is called C-horizon.
• It has cracks running through it.
• It is mostly made up of rocks.

Bedrock

• bedrock. in the soil profile is called The bottom-most layer
• This is far more solid in composition than the other layers and is very hard.
• lt to dig up this layer even with a spade.It is difficu

Soil Types and its Properties

Soil has some characteristics that form the basis for its classification into various types, and also the types of crops that are grown in it.
 Soil has some characteristics that form the basis for its classification into various types, and also the types of crops that are grown in it.

The rate of absorption is different for different types of soils. This phenomenon of absorption of water by soil is termed as percolation. The rate of absorption of a soil depends on its composition. As we know, soil is formed by the weathering of rocks. The amounts of sand and clay depend on the rock from which the soil particles have been formed. Soil is not similar everywhere. Soil is classified into various types based on the appearance and proportion of particles.

1. Sandy soil
2. Clay soil
3. Loamy soil

Properties of sandy soil:

• If the soil is made of a greater proportion of big particles, it is called sandy soil.
• The sand particles are relatively large. They cannot fit closely together.
• There are large spaces between them. These places are filled with air.
• Therefore, in this type of soil, water absorption is very high as the water passes quickly through these spaces.
• These soils are light, well aerated and dry.

Properties of clay soil:

1. It is made of a relatively high proportion of fine particles.
2. These soils have very less space between particles.
3. Because the particles are smaller in size, water can be trapped in the tiny gaps between them.
4. Clay soils are heavy and hold more water.

Properties of loamy soil:

• It is a mixture of sand, soil and silt.
• Silt particles are present between the particles of sand and soil.
• It also contains humus, and is, therefore, considered the best for the growth of plants.
• The percolation rate is between that of sandy soil and clay soil.

Climatic factors as well as the components of the soil determine the types of vegetation and crops that grow in a particular area.

• Cereals like wheat and gram are grown in clay and loamy soils, because these soils have better water retention capacity.
• Soils rich in clay and organic matter with good water retention capacity are ideal for paddy.
• Loamy soils, which drain water easily, are suitable for lentils and other pulses.
• For cotton, loamy or sandy soils are more suitable, because of their water draining and air logging capacity.
• Clay soils are rich in humus and fertile. These soils are very good for wheat.

Deforestation causes soil erosion. Plant roots firmly bind soil. In the absence of plants, soil becomes loose. Wind and flowing water move the loose soil away. Soil erosion is severe in areas of little or no surface vegetation, such as deserts and barren lands. Therefore, cutting of trees and deforestation should be prevented, and efforts should be made to increase greenery.