Soil CBSE Class 7 Science Notes Chapter 9

→ Soil is a naturally occurring substance that is valuable in sustaining life on earth.

→ Many microorganisms live in the soil.

Figure 1: Importance of Soil

1. Humus: It is a component of soil that is formed from the dead and decayed organic matter.
2. Weathering: It is the process of formation of soil by breaking down of rocks. It occurs by the action of wind, water, and climate.

Soil Profile

→ It is a vertical section through various layers of soil.

→ These various layers are known as horizons.

Figure 2: Soil Profile

There are four types of horizons:

1. A-horizon or topsoil: It is the top most soil, which is dark in colour and rich in humus. It is soft, porous, and has the ability to retain water.
2. B-horizon: It is the middle next layer of the soil profile. It has lesser humus and more minerals. The layer is harder and more compact.
3. C-horizon: It is the third layer made up of small lumps of rocks with cracks and crevices.
4. Bedrock: It is the lowermost hard layer and difficult to dig with a spade.

How is Soil Formed?

In the first stage of soil formation, the soil is generally non-porous in nature. Then it slowly turns into soil having air and water in the pores.

We can define soil as a mixture of rock particles and humus. Based on the size of the particles and the textures of the soil, it can be divided into various types.

Figure 3: Formation of Soil

Soil Types

→ Soil is a mixture of rock particles and humus.

→ Living organisms such as microbes, earthworm, and plant roots are found in soil.

Figure 4: Types of Soil

Classification of soil

1. Sandy soil
   1. Sand particles are large that cannot fit together.
   2. Larger spaces are present between particles.
   3. It is light, well aerated, and dry.
2. Clayey soil
   1. Clay particles are smaller and tightly packed together.
   2. It has little space for air and can hold water between particles.
3. Loamy soil
   1. It is the best top soil for the growth of plants.
   2. It is the mixture of sand, clay, and silt.
   3. It contains humus and has the right water holding capacity.
   4. Toys, pots, and statues are made up of clayey soil.
4. Silt soil
   1. The soil particles are smaller than that of sandy soils but larger than clayey soils.
   2. Silt Soil can hold water to some extent because of its fine quality.
   3. They are generally found near the water bodies like river banks and lakes.

Figure 5: Silt Soil

Properties of Soil

→ Percolation rate of water varies in different soil types.

Percolation rate (mL/min) = amount of water (mL) / percolation time (min)

Figure 6: Percolation rate

→ Percolation rate of water is highest in sandy soil and least in clayey soil.

→ The soil moisture and water absorption capacity of soil also varies among different soil types.

→ Loamy soil has the maximum accurate water holding capacity while sandy soil has the least.

→ Climatic factors such as wind, rainfall, temperature, light, and humidity affect the soil.

Soil and Crops

→ The soil capacity to hold water is important for the cultivation of various crops.

Crop

→ When same kinds of plants are cultivated on a large scale, it is called a crop.

It is of two types:

1. Kharif crops
   • Crops that are sown in rainy season (June to September)
   • Examples – soyabean, paddy, maize
2. Rabi crops
   • Crops that are grown during winter season (October to March)
   • Examples – wheat, gram, pea

Basic Crop Production Practices

1. Preparation of soil
2. Sowing
3. Adding manure and fertilizers
4. Irrigation
5. Protection from weeds (weeding)
6. Harvesting
7. Storage

Factors Leading to Soil Pollution

Various human activities and products pollute the soil. These pollutants destroy the fertility of the soil and harm the organisms living in the soil.

1. Polythene bags
2. Plastic products
3. Chemicals
4. Pesticides

→ The preventive measures that can be adopted to reduce soil pollution are as follows:

1. Treating the waste products before disposal
2. Reducing the use of polythene bags
3. Minimizing the use of products made of plastic
4. Minimizing the use of pesticides
5. Plastic particles and polythene bags are banned to avoid soil pollution.

Soil Erosion

→ When the top layer of soil gets removed it is called soil erosion.

→ The soil erosion mainly occurs when the soil is left loose without vegetation or when deforestation occurs.

→ In such a condition, the action of wind, water, and human activities can wash away the topsoil.

Conservation of Soil

→ Prevention of soil erosion is important for conserving the soil.

→ Methods of soil conservation include:

1. Terracing
2. Contour farming
3. Afforestation
4. Gully plugging
5. Building dams and check dams