Soil CBSE Class 7 Science Notes Chapter 9
Topics in the
chapter
· Introduction
· Soil Profile
· Soil Types
· Properties of
Soil
· Soil and Crop
· Basic Crop
production Practices
· Factors leading
to Soil Pollution
Introduction
→ 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 |
• 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:
(i) 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.
(ii) 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.
(iii) C-horizon: It is the third layer made up of small
lumps of rocks with cracks and crevices.
(iv) 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.
• Soil → Rock particle + Humus
→ Living organisms such as microbes,
earthworm, and plant roots are found in soil.
 |
| Figure 4: Types of Soil |
Classification of soil
(i) Sandy soil
→ Sand particles are large that cannot
fit together.
→ Larger spaces are present between particles.
→ It is light, well aerated, and dry.
(ii) Clayey soil
→ Clay particles are smaller and
tightly packed together.
→ It has little space for air and can hold water between particles.
(iii) Loamy soil
→ It is the best top soil for the
growth of plants.
→ It is the mixture of sand, clay, and silt.
→ It contains humus and has the right water holding capacity.
→ Toys, pots, and statues are made up of clayey soil.
(iv) Silt
soil→ The soil
particles are smaller than that of sandy soils but larger than clayey soils.
→ Silt Soil can hold water to some extent because of its fine quality.
→They are generally found near the water bodies like river banks and lakes.
Figure 5: Silt Soil
Properties
of Soil
→ The soil
particles are smaller than that of sandy soils but larger than clayey soils.
→ Silt Soil can hold water to some extent because of its fine quality.
→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.
(i) Kharif crops
·
Crops that are
sown in rainy season (June to September)
Examples – soyabean, paddy, maize
(ii) Rabi crops
·
Crops that are
grown during winter season (October to March)
Examples – wheat, gram, pea
Basic
crop production practices
- Preparation of soil
- Sowing
- Adding manure and fertilizers
- Irrigation
- Protection from weeds (weeding)
- Harvesting
- 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.
• Some of the important soil pollutants are given below.
→ Polythene bags
→ Plastic products
→ Chemicals
→ Pesticides
• The preventive measures that can be
adopted to reduce soil pollution are as follows.
→ Treating the waste products before
disposal
→ Reducing the use of polythene bags
→ Minimizing the use of products made of plastic
→ Minimizing the use of pesticides
→ 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 situation, strong winds and flowing water or
rainwater takes away the topsoil and therefore decrease its quality.
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