Soil Class 7 Science Notes - Chapter 9
Soil Class 7 Notes Science - Chapter 9
What Is Soil?
Soil
It is the foundation of all agriculture and makes up the upper surface of the earth, where life is most abundant.
- It consists of minerals and gravel, microbes, insects, decomposing organic matter (humus), nutrients, water, and air due to the physical and chemical weathering of rocks.
- There are various types of soil, which vary depending on the geological makeup, climate, and amount of rainfall that an area receives. These soils are also regularly removed and created by activities, whether they be natural or human.
Levels in Soil
Humus
The organic component of soil known as humus is created when soil-dwelling microorganisms break down plant matter. The decaying dead matter in the soil is known as humus.
Weathering
The breakdown or dissolution of rocks and minerals on the surface of the Earth is known as weathering. Agents of weathering include water, ice, acids, salt, plants, animals, and variations in temperature.
Soil profile
The soil profile is a composition of different layers of soil in a vertical section.
Each layer is different from others because of its texture, colour, depth and chemical composition.
Horizon
A horizon is a term used to describe each layer in a soil profile.
- The horizon, also known as topsoil, is a porous, supple layer that can hold more water.
- The layer that is denser and harder is called the B-horizon or middle layer.
- The third layer of small rocks is known as the C-horizon.
- The fourth and hardest layer is bedrock.
Soil erosion
The removal of land surface by water wind or ice is known as erosion.
- As the plant roots bind to the soil.
- Soil erosion is mainly caused by the large scale cutting of forest trees and plants.
- Erosion of soil is more severe in the areas of little or no surface vegetation like the desert or barren land.
Therefore, cutting of trees and deforestation must be prevented.
The effects of soil erosion are, famines, flood desertification and damage or spoilage of environment.
What are the Types of Soil?
Sandy Soil
These are made up of larger particles enough than that of silt and can be easily seen. These are coarse to touch due to their larger size.
Clayey Soil
These are formed by the smallest rock particles. These are so small that we cannot see a single clay particle and it feels smooth.
Loamy Soil
When the proportion of larger and finer particles are equal, then that type of soil is called as loamy soil.
Loamy soil is a mixture of silt, clay and sand which is the best topsoil for growing plants.
Percolation Rate
Percolation is the process of filtering liquid while passing through a filter.
- Rainfall seeps underground through percolation, where water travels downwards through the tiny spaces between rocks and soil particles.
- The water eventually saturates the underlying rock much like water fills the tiny holes of a sponge.
- This helps to replenish aquifers under the ground.
The rate of percolation can be calculated by using the following formula:
Percolation rate (mL/min) =
Percolation rate is highest in sandy soil because it is very loose. On the other hand, clayey soil is very compact and therefore has the lowest rate of percolation. The rainwater moves or reaches to well faster and in a greater amount from sandy soil.
Moisture and Absorption
Moisture in Soil
The amount of water in the soil is referred to as moisture.
By conducting the subsequent experiment, moisture in the soil can be eliminated.
We can see water drops in the test tube's mouth when we take soil samples, place them in a boiling tube, and heat them. By doing so, we can dry out the soil.
Absorption of water by soil
Even though soil contains moisture, it can still soak up or absorb a lot of water. However, soil has a limit to how much water it can hold. Absorption percentage refers to the soil's ability or capacity to absorb a specific amount of water. The calculation is as follows:
Percentage of water absorbed = `\frac{Amount of water ( inmL)}{Amount of Soil(g)}` `\times 100`
Crops
Crops and its relation to types of soil
- Different types of soil are found in different parts of India.
- Soil is mainly affected by wind, rainfall, temperature, light and humidity.
- Some climatic factors also affect the soil profile and bring changes in the soil structure.
- The plants that grow on the surface of the earth are called vegetation.
- It includes green grass, herbs, shrubs, bushes, crop plants and trees.
- Cereals like wheat and gram can be grown in both loamy and clayey soils. These soils are effective at retaining water.
- Clay and organic matter-rich soils with a high capacity to hold water are ideal for paddy.
- Loamy soils with good water drainage are necessary for growing lentils (masoor) and other pulses.
Sandi-loam or loam, which can hold a lot of air and easily drain water, are better suited for cotton.
Wheat and other crops are grown in the fine clayey soils because they are fertile and rich in humus.
Hydroponic plants
Plants, which do not need soil to grow instead these plants need water to grow. These types of plants are known as hydroponic plants.
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