Science - Chapter 9 - Soil
Class - 7
NCERT - Important
Questions
Long Answer Type Questions
Question 1: How soil is formed?
Answer:
* The soil is formed by the breaking down of rocks by the
action of wind, water and climate. This process is called weathering. There are
two different types of weathering. Physical weathering and chemical weathering.
* Mechanical (physical) weathering is the breakdown of rock
into smaller particles due to such factors as freezing and thawing, release of
pressure, water absorption, salt crystal formation, land mass uplift, expansion
and contraction from the sun or fire, plant root growth, actions of animals,
abrasion, or other means that do not directly affect the rock's chemistry.
* Chemical weathering is the dissolution, carbonation, oxidation,
or hydrolysis of rock and mineral by chemical means only, mostly from reactions
with water or the acids contained in rainwater. Other materials are formed in
the process. Warm, tropical climates are ideal environments for chemical
weathering to take place as the chemical reactions are quickened by the
bountiful rain and warm temperatures.
Question 2: Soil is one of the most important natural resources.
Explain .
Answer: Soil is one of the most important natural resources
because -
* It supports the growth of plants by holding the roots firmly
and supplying water and nutrients.
* It is the home for many organisms.
* Soil is essential for agriculture. Agriculture provides
food, clothing and shelter for all.
Soil is thus an inseparable part of our life.
Question 3: Describe the different layers in soil profile.
Answer: Different layers of the soil are
referred to as horizons.
Upper most layer-
* The upper most layer, the top soil, also known as
A-horizon.
* It is generally dark
in colour and fertile as it is rich in humus and minerals.
*This layer is generally soft, porous and can retain more
water.
* This provides shelter for many
living organisms such as worms, rodents, moles and beetles.
* The roots of small plants are embedded entirely in the
topsoil.
Middle layer-
The next layer, middle layer is called B-horizon. It has a lesser amount of
humus but more of minerals. This layer is generally harder and more compact.
Lower layer-
* The third layer is called the C-horizon. It is made up of small lumps of rocks with cracks
and crevices.
* Below this layer is the bedrock, which is hard and difficult
to dig with a spade. Water can be held in the tiny gaps.
Question 4: Explain humus and its importance.
Answer:
* A non-living organic matter formed from remains of dead and
decay plants and animal by the activity of microorganism present in soil is
called Humus.
* Humus controls and regulates many soil functions, as well as
plant growth. The better the humus content in the soil the healthier and more
productive soil will be.
* Good humus count in soil will hold soil moisture longer,
thus lesser water is required for crop production, also good humus count
reduces wind erosion of soil.
Question 5: State the main agents of weathering.
Answer: The main agents of weathering are:
Temperature: Temperature changes that cause the
rock surface to break apart
Frost: Ice expands and produces pressure,
causing the crevices in rock to open
Air: Wind blow across rock surface and carry minute particle and
form layer
Water: - Continuous movement of rain water
causes breaking down of rock particles into fine particles
Living organism: - Lichen grow on the surface of
rocks and produce acid which corrode the rocky surface. Weathering by living
organism is called biological weathering.
Question 6: What is physical weathering?
Answer: Physical weathering is the breakdown of rock into
smaller particles due to factors such as freezing and thawing, release of
pressure, water absorption, salt crystal formation, land mass uplift, expansion
and contraction from the sun or fire, plant root growth, actions of animals,
abrasion, or other means that do not directly affect the rock's chemistry. It
is also called mechanical weathering.
Question 7: What is chemical weathering?
Answer:
* Chemical weathering is the
dissolution, carbonation, oxidation, or hydrolysis of rock and mineral by
chemical means only, mostly from reactions with water or the acids contained in
rainwater. Other materials are formed in the process.
* Warm, tropical climates are ideal
environments for chemical weathering to take place as the chemical reactions
are quickened by the bountiful rain and warm temperatures.
Question 8: Soil particles size is an important factor to determine
the property of soil. Explain.
Answer: Different particles of soil have
different sizes which affect its properties like:
1. Sand particles are quite large and
having large spaces between them. These spaces are filled with air. Water can
drain quickly through these spaces. So, sandy soils tend to be light, well
aerated and rather dry.
2. Clay particles, being much smaller,
pack tightly together, leaving little space for air. So, clay soils have little
air. But they are heavy as they hold more water than the sandy soils.
3. Loamy soil is a mixture of sand, clay and another type of
soil particle known as silt. The size of the silt particles is between those of
sand and clay. The loamy soil also has humus in it. It has the good water
holding capacity for the growth of plants.
Question 9: Define (a) loamy soil (b) sandy soil .
Answer:
(a) Loamy soil is a mixture of sand, clay and another type of
soil particle known as silt. The size of the silt particles is between those of
sand and clay. The loamy soil also has humus in it. It has the good water
holding capacity for the growth of plants
(b) If soil contains greater proportion of big particles it is
called sandy soil.
In sandy soil Sand particles are quite large and
having large spaces between them. These spaces are filled with air. Water can
drain quickly through these spaces. So, sandy soils tend to be light, well
aerated and rather dry.
Question 10: What is percolation of water? How it is different for
different types of soil?
Answer:
* The gravity flow of groundwater
through the pore spaces in rock or soil is called percolation of water. The
rate of absorption is different for different types of soils. This phenomenon
of absorption of water by soil is termed as percolation. Percolation rate of
water is different in different types of soil. It is highest in the sandy soil
and least in the clayey soil. The rate of absorption of a soil depends on its
composition.
Percolation rate= amount of water
(mL)/ Percolation time (min)
Question 11: State some ways to prevent soil erosion.
Answer:
There are many ways to prevent soil erosion
* By planting more trees and increasing the vegetation cover.
* By prevented Cutting of trees and deforestation and taking
efforts to increase the green areas.
* By maintaining soil in its natural condition by using less
manure or by crop rotation
* By arranging proper irrigation
Question 12: Differentiate between alluvial soil and mountain soil
Answer:
* Alluvial Soils are mainly found in the plains of northern
India. These soils have low phosphorous and nitrogen content and found in the
north western regions of the country.
* Mountain soils are considered as a significant variety of
soil in the Himalayan region of the country. They are mainly found in dry and
cold district in the northern region of India.
Question 13: Explain different types of soil found in India.
Answer: Following are the different types
of soil found in India
a. Laterite Soils:
* This typical soil is found in those regions which receive
heavy rainfall. This soil is poor in lime content and hence it is more acidic.
This soil contains least moisture content.
* It is basically red in colour because of the presence of
iron oxides. It is more popular in the coastal regions of Ratnagiri District
and Malabar.
b. Black Soil:
It is black in colour due to presence of salt and
large amount of humus. This soil becomes sticky when is wet. It does not
contain adequate nitrogen but it contains sufficient phosphorous required for
the growth of the plants. It is generally found in hilly areas.
c. Red and Yellow Soils:
Red and yellow soils are found in areas, which receive
low rainfall. They contain huge concentration of iron oxides that are
responsible for giving the reddish or yellow colour.
d. Alluvial Soils:
Alluvial Soils are mainly found in the plains of
northern India. These soils have low phosphorous and nitrogen content and found
in the north western regions of the country
e. Mountain Soils:
Mountain soils are considered as a significant variety
of soil in the Himalayan region of the country. They are mainly found in dry
and cold district in the northern region of India.
Question 14. Explain how soil pollution and soil erosion could be
prevented.
Answer: Steps to prevent soil pollution are:
* The polythene bags and plastics should be banned.
* Waste products and chemicals should be treated before they
are released into the soil.
* The use of pesticides should be minimised.
* Use of organic manure, fertilizers, and milder pesticides
for agricultural activities should be encouraged.
Steps to prevent soil erosion are
* Planting more and more trees.
* Terrace farming.
* Retaining walls can be built around the area of erosion to
prevent water run-off.
* Making people aware of pros and cons of deforestation.
Question 15. Describe the different layers of soil profile.
Answer: Different layers of soil profile are:
* The uppermost horizon is generally dark in colour as it is
rich in humus and minerals. The humus makes the soil fertile and provides
nutrients to growing plants. This layer is generally soft, porous and can
retain more water. It is called the topsoil or the A-horizon. This provides
shelter for many living organisms such as worms, rodents, moles and beetles.
The roots of small plants are embedded entirely in the topsoil.
* The next layer has a lesser amount of humus but more of
minerals. This layer is generally harder and more compact and is called the
B-horizon or the middle layer.
* The third layer is the C-horizon, which is made up of small
lumps of rocks with cracks and crevices.
* Below this layer is the bedrock, which is hard and difficult
to dig with a spade.
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