YASRAJ STUDIES
DAV Class 7 Science Chapter 10 Question Answer – Soil
Introduction
Soil is an important natural resource. It provides plants with support, water, air, and nutrients. It is also a habitat for many living organisms. In this chapter, we learn about soil formation, soil profile, soil texture, different types of soil, soil properties, soil pollution, and soil erosion.
A. Fill in the Blanks
1. Various layers in the soil are called __________.
Answer: Soil horizons.
Explanation:
A vertical section of soil contains different layers. These layers differ in colour, texture, composition, and amount of organic matter. These layers are called soil horizons.
2. The intermediate particles, that are neither very rough, nor too smooth, are termed as __________.
Answer: Silt.
Explanation:
Sand particles are comparatively large and rough, while clay particles are very fine and smooth. Silt particles are intermediate in size between sand and clay.
3. __________ is the tendency of similar particles to stick to one another.
Answer: Cohesion.
Explanation:
The tendency of particles of the same substance to stick together is called cohesion. This property helps soil particles remain together.
4. Soil air is important for roots to carry out __________.
Answer: Breathing.
Explanation:
Roots need oxygen present in the air spaces of soil. They use oxygen for respiration, which releases energy required for growth and other activities.
5. __________ and __________ cause soil pollution.
Answer: Insecticides and fertilizers.
Explanation:
Excessive use of chemical fertilizers and insecticides can introduce harmful substances into the soil. This can reduce soil quality and harm soil organisms.
B. Match the Following
1. Potter's soil
Answer: Clay.
Explanation:
Clayey soil has good plasticity and cohesion, so it can be moulded into different shapes. Therefore, clay is commonly used by potters.
2. Organic material
Answer: Humus.
Explanation:
Dead plants and animals decompose and form a dark, nutrient-rich organic material called humus.
3. Vermicompost
Answer: Earthworm.
Explanation:
Earthworms help convert organic waste into nutrient-rich compost. This compost is called vermicompost.
4. Sandy soil
Answer: Well aerated.
Explanation:
Sandy soil has relatively large particles with large spaces between them. These spaces allow air to move easily through the soil.
5. Concrete floor
Answer: No percolation.
Explanation:
A concrete floor does not normally allow water to pass through it. Therefore, there is practically no percolation through a concrete floor.
Complete Matching Table
| Column A | Column B | Answer |
|---|---|---|
| 1. Potter's soil | (a) No percolation | (b) Clay |
| 2. Organic material | (b) Clay | (d) Humus |
| 3. Vermicompost | (c) Well aerated | (e) Earthworm |
| 4. Sandy soil | (d) Humus | (c) Well aerated |
| 5. Concrete floor | (e) Earthworm | (a) No percolation |
C. Tick (✓) the Correct Option
1. Which of the following is not an agent of physical weathering?
(a) Wind
(b) Temperature
(c) Water
(d) Organic acids
Answer: (d) Organic acids
Explanation:
Wind, water, temperature changes, and plant roots can cause physical weathering by breaking rocks into smaller pieces. Organic acids mainly take part in chemical weathering, in which minerals in rocks are chemically changed or dissolved.
2. Which of these is the lowest horizon of soil profile?
(a) Subsoil
(b) Bed rock
(c) Gravel
(d) Humus
Answer: (b) Bed rock
Explanation:
Bedrock is the hard layer of solid rock found below the other layers of a soil profile. It forms the lowest layer in the simplified soil profile studied in this chapter.
3. Which organism is known as nature's ploughman?
(a) Honeybee
(b) Rat
(c) Bird
(d) Earthworm
Answer: (d) Earthworm
Explanation:
Earthworms move through the soil and make small channels. These channels improve soil aeration and help water move through the soil. Hence, earthworms are called nature's ploughmen.
4. Which soil can retain more water?
(a) Sandy soil
(b) Clayey soil
(c) Loamy soil
(d) Polluted soil
Answer: (b) Clayey soil
Explanation:
Clayey soil contains very fine particles and has tiny spaces between them. It can therefore retain more water than sandy soil.
5. Coconut trees thrive in coastal region where the soil present is mainly
(a) Clayey soil
(b) Loamy soil
(c) Sandy soil
(d) Sandy loam
Answer: (d) Sandy loam
Explanation:
Coastal areas commonly contain sandy or sandy-loam soil. Coconut plants are well adapted to these soils, which generally provide good drainage.
D. Answer the Following Questions Briefly
1. State the meaning of the term 'soil'.
Answer:
Soil is the loose upper layer of the Earth's land surface. It contains mineral particles, organic matter, air, water, and living organisms and provides support for plant growth.
Explanation:
Soil is a mixture of many components. Rock particles form the mineral part, while decayed plants and animals add organic matter. Air, water, and organisms present in soil also make it suitable for supporting life.
2. List the four main agents of physical weathering.
Answer:
The four main agents of physical weathering are:
- Temperature changes
- Water
- Wind
- Plants
Explanation:
These agents can break rocks into smaller pieces without necessarily changing their chemical composition. The small rock particles formed through weathering contribute to soil formation.
3. Write one difference between sand, clay and silt.
Answer:
| Soil particle | Size |
|---|---|
| Sand | Large |
| Silt | Medium |
| Clay | Very small |
Explanation:
The particles differ mainly in size. Their size affects properties such as water retention, drainage, aeration, and workability of soil.
4. Name the types of soil classified on the basis of soil texture.
Answer:
The three main types of soil based on texture are:
- Sandy soil
- Clayey soil
- Loamy soil
Explanation:
Soil texture depends on the relative amount and size of sand, silt, and clay particles present in the soil.
5. Give the meaning of the term 'Cohesion'.
Answer:
Cohesion is the tendency of particles of the same substance to stick to one another.
Explanation:
For example, clay particles have cohesion and can stick together. This property helps clay remain together when it is moulded.
6. List any three causes of soil pollution.
Answer:
Three causes of soil pollution are:
- Excessive use of chemical fertilizers
- Excessive use of insecticides and pesticides
- Disposal of harmful chemicals and toxic substances
Explanation:
These substances can contaminate the soil and affect plants, soil organisms, and the natural fertility of the soil.
E. Answer the Following Questions in Detail
1. Classify, and explain, the categories of soil on the basis of 'texture of the soil'.
Answer:
On the basis of texture, soil is mainly classified into sandy soil, clayey soil, and loamy soil.
Sandy Soil
Sandy soil contains a large amount of sand particles. The particles are relatively large, so there are large spaces between them. Water drains quickly through sandy soil and it is generally well aerated.
Clayey Soil
Clayey soil contains a large amount of very fine clay particles. It can retain a large amount of water. When wet, clay becomes sticky and can be moulded easily because it has good plasticity.
Loamy Soil
Loamy soil contains a suitable mixture of sand, silt, and clay. It also contains organic matter such as humus. It can hold enough water while allowing air and excess water to move through it.
Explanation:
The texture of soil determines many of its properties, such as water-holding capacity, drainage, aeration, and suitability for agriculture. Loamy soil is generally considered suitable for growing many crops because it provides a good balance of these properties.
2. Differentiate between infiltration rate and percolation rate.
Answer:
| Infiltration Rate | Percolation Rate |
|---|---|
| It is the rate at which water enters the soil from the surface. | It is the rate at which water moves through the soil. |
| It describes the entry of water into the soil. | It describes the downward movement of water through soil layers. |
Explanation:
When rainwater falls on the ground, some of it enters the soil. The rate at which it enters is called the infiltration rate. Once inside the soil, water moves through the spaces between soil particles. The rate of this movement is called the percolation rate.
3. Write a short note on 'composition of soil'.
Answer:
Soil consists of several components:
- Mineral particles: These are formed mainly from weathered rocks.
- Organic matter: Dead and decaying plants and animals form humus.
- Water: Water occupies some of the spaces between soil particles.
- Air: Air is present in the spaces between soil particles.
- Living organisms: Earthworms, insects, bacteria, fungi, and other organisms live in soil.
Explanation:
All these components are important. Plant roots need water and air, while humus provides nutrients and improves soil quality. Soil organisms also help in decomposition and maintaining soil fertility.
4. The type of crop that may be grown in a particular field depends on the type of soil. Give three examples in support of this statement.
Answer:
| Crop | Suitable Soil |
|---|---|
| Rice | Clayey soil |
| Wheat | Loamy soil |
| Cotton | Sandy-loam or loamy soil |
Explanation:
Different crops have different requirements for water, air, nutrients, and drainage. Since different soils have different properties, farmers choose crops according to the type of soil available.
5. What is soil erosion? How can it be avoided?
Answer:
Soil erosion is the removal of the fertile top layer of soil by agents such as wind and flowing water.
Soil erosion can be reduced by:
- Planting more trees.
- Maintaining grass and other vegetation.
- Avoiding unnecessary cutting of trees.
- Following proper farming methods.
- Taking measures to slow down the movement of water on farmland.
Explanation:
Topsoil contains important nutrients and organic matter needed for plant growth. When it is washed or blown away, soil fertility decreases. Plant roots help hold the soil particles together and therefore reduce erosion.
6. Why is it said that 'plasticity' and 'cohesion' play an important role in the workability of soil?
Answer:
Plasticity is the ability of soil to be shaped or moulded without breaking apart. Cohesion is the tendency of similar soil particles to stick together.
Clayey soil has good plasticity and cohesion. Therefore, it can be moulded into different shapes and retain its shape. This makes clay useful for making pots and other earthen objects.
Explanation:
Sandy soil does not hold together strongly when wet, while clay particles stick together more firmly. Therefore, clay is much easier to mould and work with.
Value-Based Questions
Read the situation and answer the following questions.
The school principal compares the role of parents and teachers in the growth of children with the role of physical and chemical agents in the weathering of soil.
1. State any two of the values that parents/teachers generally display during the growth of their children.
Answer:
Two values generally displayed by parents and teachers are:
- Care and concern
- Guidance and support
Explanation:
Parents and teachers help children develop by caring for them, guiding them, teaching them, and encouraging them to become responsible individuals.
2. Write, in brief, the role of (i) Plants and animals (ii) Solvent action of water in the 'weathering' of the soil.
(i) Plants and Animals
Answer:
Plant roots can enter cracks in rocks and gradually widen them. Plants and organisms can therefore contribute to the breaking down of rocks. Animal activity can also disturb and loosen soil and rock material.
Explanation:
Growing roots exert pressure on cracks in rocks. Over time, this can help break rocks into smaller pieces and contribute to soil formation.
(ii) Solvent Action of Water
Answer:
Water can dissolve certain minerals and soluble substances present in rocks. This weakens and breaks down the rocks.
Explanation:
Rainwater and groundwater may dissolve soluble materials present in rocks. This is an important part of chemical weathering.
Something To Do
1. Visit a potter; observe the texture of soil used; can pots be made using sand? How does plasticity differ in sand and clay?
Answer:
A potter generally uses clayey soil for making pots.
Pots cannot be easily made from ordinary sand because sand has very little plasticity and its particles do not hold together strongly.
Clay has much greater plasticity. It can be moulded into different shapes and retain those shapes.
Explanation:
The high plasticity and cohesion of clay make it suitable for pottery, whereas sandy soil is loose and does not mould easily.
2. Take three pots with sand, clay, loamy soil; sow wheat or green gram seeds; water equally; record plant growth; infer.
Answer:
The seeds may show different growth in the three types of soil because each soil has different water-holding capacity, aeration, drainage, and nutrient content.
A suitable conclusion is:
- Sandy soil drains water quickly.
- Clayey soil retains more water but may have less aeration.
- Loamy soil provides a good balance of water, air, and nutrients and is generally suitable for many crops.
Explanation:
Plant growth depends on several factors, including water, air, nutrients, and soil conditions. Equal watering does not necessarily produce equal growth because the soils hold and drain water differently.
3. Group of six: Find the main crops in any three Indian states and soil types; make PowerPoint presentation.
Answer:
A sample table for the activity is given below:
| State | Main Crop | Soil Type |
|---|---|---|
| Bihar | Rice | Clayey soil |
| Punjab | Wheat | Loamy soil |
| Tamil Nadu | Rice | Loamy soil |
Explanation:
Different states have different soil and climatic conditions. Farmers select crops that can grow well under the conditions of their region.
This information can be arranged into slides for a PowerPoint presentation with the state name, crop, soil type, and pictures.
Important Points to Remember
- Soil is an important natural resource.
- Soil is formed mainly through the weathering of rocks and the addition of organic matter.
- The different layers of soil are called soil horizons.
- Sand particles are relatively large.
- Silt particles are intermediate in size.
- Clay particles are very fine.
- The three main soil types based on texture are sandy, clayey, and loamy soil.
- Sandy soil is generally well aerated and allows water to drain quickly.
- Clayey soil can retain a large amount of water.
- Loamy soil is generally suitable for growing many crops.
- Cohesion is the tendency of similar particles to stick together.
- Plasticity is the ability of soil to be moulded without breaking apart.
- Earthworms are called nature's ploughmen.
- Excessive use of chemical fertilizers and insecticides can cause soil pollution.
- Soil erosion removes fertile topsoil.
- Planting trees and maintaining vegetation can help control soil erosion.
- Soil conservation is important for agriculture and the environment.
Conclusion
Soil is essential for plant growth and supports many forms of life. Its composition and texture determine its properties and suitability for different crops and purposes. By understanding soil, we can use it properly, prevent pollution and erosion, and conserve this valuable natural resource for the future.
