Class:-7, Science Chapter:- 9 Soil Notes, NCERT Solutions & Frequently Asked Questions

 Science

Class:-7 Chapter:- 9

Soil

Notes, NCERT Solutions & Frequently Asked Questions

-- Notes --

⭐Soil:- It forms the upper surface of the earth where life exists 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).

⭐Major functions of the soil are:- 

• It supports plant growth by holding the roots firmly and supplying water and nutrients to the plants.

 • It acts as a natural habitat for many organisms like an earthworm, fungi, bacteria, ants, etc.

 • It is also essential for agriculture which provides us with food, clothing and shelter for all.

 • It supplies water and nutrients to plants.

⭐Properties of Soil:- Soil possesses various properties like:-

• It contains air.

• It can hold water or moisture.

• It can absorb or soak water

• It allows water to pass down   through it.

⭐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.

a) A-Horizon or Topsoil:- It is the soft, porous layer which can absorb and hold more water.

b) B-Horizon or Middle Layer or Subsoil:- It is the more compact and harder layer.

c) C-Horizon or Substratum:- It is the third layer made up of small rocks.

d) Bedrock or Parent Rock:- Itis the fourth and hardest layer.

⭐Types of Soil:- The soil is classified on the basis of the proportion of particles of various sizes.

a) Sandy Soil:- 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.

b) 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.

c) 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.

⭐Soil Pollution:- Sometimes people throw polythene bags and plastic articles in the soil. They pollute the soil and also hill the useful organisms living in the soil. 

• Some other substances like chemicals and pesticides also pollute the soil.

• To prevent the soil pollution, the use of polythene bags must be banned, and the waste products and chemicals must be treated before they are released into the soil. 

• The use of pesticides in the agricultural field must also be minimised in order to prevent soil pollution.

⭐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.

⭐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)

• Percolation rate is highest in sandy soil because it is very loose. 

•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 in Soil:- Moisture is soil is the amount of water present in the soil.

• Moisture in the soil can be removed by heating a soil sample in a container. 

⭐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.

⭐ Water Retention:- The ability of soil to hold water is called water retention. The space between soil particles is called pores provide the passage for gases and moisture within the soil.

• The ability of soil to retain water is strongly related to the particle size. Water molecules hold more lightly to the fine particles of the clayey soil than the coarser particle of sandy soil.

• Sandy soil will absorb less water and allows more water to percolate.

• A clayey soil will absorb more water but allow less water to percolate.

• Sandy soil will absorb less water than clayey soil because of the large spaces between the soil particles. The area where, there is a lot of clay in the soil, stagnant water collects above the soil whenever it rains.

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

• 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.

• Clayey soil and loamy soil are good for growing cereals like wheat and gram.

a) Clayey soil:- Soils rich in organic matter and soils with good capacity to retain water are called clayey soil.

  -- Good for growing paddy, wheat.

b) Loamy soil:- Soils with high draining capacity and can hold plenty of water are called Loamy soil.

  -- 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.

-- NCERT Solutions --

Tick the most suitable answer in questions 1 and 2.

Question 1.- In addition to the rock particles, the soil contains

i) air and water

ii) water and plants

iii) minerals, organic matter, air and water

iv) water, air and plants

Solution:- iii) minerals, organic matter, air and water.

Question 2.- The water holding capacity is the highest in

i) sandy soil

ii) clayey soil

iii) loamy soil

iv) mixture of sand and loam

Solution:- ii) clayey soil

Question 3.- Match the items in Column I with those in Column II:

Solution:- 

Question 4.- Explain how soil is formed.

Solution:- Soil is formed due to the weathering of rocks. Weathering is a process in which physical breakdown and chemical decomposition of minerals takes place primarily by wind, water and climatic changes. In the weathering process, rocks are converted to small pieces, which eventually turn to soil particles to form a layer of soil.

Question 5.- How is clayey soil useful for crops?

Solution:- Clayey soil is useful for crops for the following reasons:-

•It has excellent water holding capacity. 

•Clayey soil is rich in organic matter. 

•Clayey and loamy soils are suitable for growing cereals like wheat. 

•Water holding-capacity of clayey soil supports the growth of crops like paddy which require more water to grow.

Question 6.- List the differences between clayey soil and sandy soil.

Solution:-

Question 7.- Sketch the cross-section of soil and label the various layers.

Solution:-

Question 8.- Razia conducted an experiment in the field related to the rate of percolation. She observed that it took 40 min for 200 mL of water to percolate through the soil sample. Calculate the rate of percolation.

Solution:- 

Question 9.- Explain how soil pollution and soil erosion could be prevented

Solution:- Soil pollution can be controlled by the following measures:-

• By reducing the use of plastics, we can reduce soil pollution.

• By controlling the use of chemical fertilisers and pesticides, we can control soil pollution.

• Soil erosion can be controlled by taking the following steps

• Planting more and more trees will result in the reduction of soil erosion.

By stopping deforestation and avoiding overgrazing of animals.

Question 10.- Solve the following crossword puzzle with the clues given:

Across

2. Plantation prevents it.

5. Use should be banned to avoid soil pollution.

6. Type of soil used for making pottery.

7. Living organism in the soil.

Down

1. In desert soil erosion occurs through.

3. Clay and loam are suitable for cereals like.

4. This type of soil can hold very little water.

5. Collective name for layers of soil.

Solution:-

Across

2. Erosion

5. Polythene

6. Clay

7. Earthworm

Down

1. Wind

3. Wheat

4. Sandy

5. Profile 

-- Frequently asked Questions --

Question 1.- What is ‘Percolation Rate’?

Answer:- Percolation is the movement of water through soil, and the percolation rate is the speed at which that movement occurs.

Question 2.- What are the some of the properties of ‘Humus’?

Answer:-

a) Improves fertility of soil

b) Contains 60% of carbon

c) Contains additional essential nutrients

Question 3.- How can we control ‘Soil erosion’?

Answer:- 

a) Plant more trees 

b) Mulching 

c) Maintain a perennial plant cover.  

- By Durgesh Pandey