Soil Class 7 Science Notes - Chapter 9

What Is Soil?

Soil

  • It forms the upper surface of the earth where life thrives and is the base of all agriculture.
  • Due to the physical and chemical weathering of rocks, it comprises of minerals and gravel, microbes, insects, decaying organic matter (humus), nutrients, water, and air.
  • There are different types of soil differing based on the geological composition, climate, rainfall the area receives and is regularly removed and formed by the activities (natural or human)

Are There Levels in Soil?

Humus

Humus is the organic component present in the soil formed by the decomposition of plant materials by microorganisms present in the soil. Humus is the rotting dead matter in the soil.

Weathering

Weathering is the breaking down or dissolving of rocks and minerals on Earths surface. Water, ice, acids, salt, plants, animals, and changes in temperature are all agents of weathering.

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

Each layer in a soil profile is defined as a horizon.

1. A- horizon or topsoil is the soft, porous layer which can absorb and hold more water.
2. B-horizon or middle layer is the more compact and harder layer.
3. C-horizon is the third layer made up of small rocks.
4. Bedrock is the fourth and hardest layer.

Soil erosion

The process of removal of land by water, wind or ice is known as erosion. The process of removal of soil by wind and water in the absence of plants is called as soil erosion.

Methods to prevent soil erosion are:

  • Deforestation should be avoided.
  • Plant more trees.

What are the Types of Soil?

Sandy Soil

The soil is classified on the basis of the proportion of particles of various sizes. Sand tends to have large particles and the particles are solid and have no pockets where water and nutrients can hold to it. Because of this, water and nutrients tend to run out, and because sandy soil lacks both water and nutrients, many plants have a difficult time surviving in this kind of soil.

Clayey Soil

The clayey soil consists of a larger proportion of finer particles. Clay soils contain more than 30 percent fine clay particles. Clays swell and shrink as they wet and dry, effectively cultivating themselves.

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

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.

Percolation rate of soil

  • Percolation rate of soil is used to determine the absorption rate of the soil.
  • The rate of percolation is the ratio of the amount of water in millimetres to the percolation time in minutes.

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

 

Moisture and Absorption

Moisture in Soil

  • Moisture is soil is the amount of water present in the soil.
  • Moisture in the soil can be removed by setting up the following experiment.

Take soil samples and put it in a boiling tube and heat it, we can observe water drops in the mouth of the test tube. This way we can remove moisture in the soil.

Absorption of water by soil

The amount of water absorbed by the soil when water is poured into the soil is called absorption capacity or absorption of water by soil.

Percentage of water absorbed

Percentage of water absorbed by soil = [ Initial weight or initial volume of water absorbed – Final weight or final volume of water absorbed/weight of soil ]×100
Weight of water absorbed by the soil = (u – v) g
Volume of water absorbed by the soil = (u – v) mL
where u is the initial volume of water/initial weight of water v is the final volume of water/final weight of water absorbed by the soil.

Crops

Crops and its relation to types of soil

Depending on the climatic factors and components of the soil different types of crops are grown in different regions.

  • Clayey soil and loamy soil – Good for growing cereals like wheat and gram.
  • Clayey soil, soils rich in organic matter and soils with good capacity to retain water – Good for growing paddy, wheat.
  • Loamy soil, soils with high draining capacity and can hold plenty of water – Good for growing Lentils (masoor) and other pulses, cotton.

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.
Hydroponic plants

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