Class 7 Notes

Class 7 Science Notes Chapter 9 (Chapter 9) – Examplar Problems Book

Philoid

Philoid

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Examplar Problems
Detailed Notes with MCQs of Chapter 9, 'Soil', from your Science Exemplar book. This is a crucial chapter, not just for your class exams but also for various government competitive exams where basic science concepts are tested. Pay close attention to the details.

Chapter 9: Soil - Detailed Notes for Competitive Exams

1. What is Soil?

  • Soil is the uppermost layer of the Earth's crust, primarily composed of weathered rock particles, minerals, humus (organic matter), water, air, and living organisms.
  • It is a vital natural resource essential for life, supporting plant growth which forms the base of most food chains.

2. Soil Profile:

  • A vertical section through different layers of the soil is called the soil profile. Each layer, distinct in feel (texture), colour, depth, and chemical composition, is called a horizon.
  • O Horizon (Organic Layer - often thin or absent): Surface layer rich in freshly fallen leaves, twigs, and decomposing organic matter (litter).
  • A Horizon (Topsoil):
    • Dark in colour due to the presence of humus.
    • Rich in nutrients, soft, porous, and holds water well.
    • Most plant roots grow here.
    • Supports germination and provides shelter for many living organisms (worms, rodents, beetles).
  • B Horizon (Subsoil):
    • Lies below the topsoil.
    • Generally harder and more compact than topsoil.
    • Lighter in colour.
    • Contains fewer nutrients and less humus compared to the A horizon, but richer in minerals leached down from the topsoil.
  • C Horizon:
    • Consists of small lumps of rocks with cracks and crevices, formed from weathered parent rock.
    • Lies below the subsoil.
  • Bedrock (Parent Rock):
    • The solid, unweathered rock layer beneath the C horizon.
    • Difficult to dig.
    • Water table is often found above or within this layer.

3. Soil Formation (Weathering):

  • Soil is formed by the slow process of weathering, which is the breaking down of large rocks into smaller particles.
  • Agents of Weathering: Wind, water (rain, rivers, ice), climate (temperature changes - freezing/thawing), and living organisms (plant roots, burrowing animals).
  • Factors Influencing Soil Formation:
    • Parent Rock: Determines the mineral composition and texture of the soil.
    • Climate: Temperature and rainfall affect the rate and type of weathering.
    • Topography: Slope of the land influences erosion and water drainage.
    • Organisms: Plants, animals, and microorganisms contribute organic matter and aid weathering.
    • Time: Soil formation is a very slow process, taking thousands of years.

4. Soil Composition:

  • Mineral Particles: Sand, silt, and clay derived from weathered rock.
  • Humus: Decomposed organic matter from dead plants and animals. Makes soil fertile and improves its texture and water-holding capacity.
  • Water: Held in pore spaces between soil particles; essential for plant life.
  • Air: Occupies pore spaces not filled with water; needed for root respiration and soil organisms.
  • Living Organisms: Bacteria, fungi, algae, protozoa, insects, earthworms, etc., contribute to decomposition and soil structure.

5. Types of Soil (Based on Particle Size Proportion):

  • Sandy Soil:
    • Particles: Large (greater proportion of big particles).
    • Texture: Gritty, coarse.
    • Aeration: Well-aerated (large spaces between particles).
    • Water Retention: Poor; water drains quickly (low water-holding capacity).
    • Nutrients: Low nutrient content.
    • Suitability: Not ideal for most crops, but suitable for some like groundnuts, millets. Found in deserts and beaches.
  • Clayey Soil:
    • Particles: Very fine (greater proportion of fine particles).
    • Texture: Smooth when wet, hard when dry.
    • Aeration: Poorly aerated (very little space between particles).
    • Water Retention: High water-holding capacity; can become waterlogged.
    • Nutrients: Rich in nutrients.
    • Suitability: Heavy and difficult to plough. Suitable for crops like paddy (rice), wheat, gram. Used for making pots, toys, and bricks.
  • Loamy Soil:
    • Particles: Mixture of sand, silt, and clay in relatively equal proportions, along with humus.
    • Texture: Ideal texture - not too sticky, not too loose.
    • Aeration: Well-aerated.
    • Water Retention: Good water-holding capacity (just right).
    • Nutrients: Rich in humus and nutrients.
    • Suitability: Considered the best soil for growing most crops (e.g., wheat, sugarcane, cotton, jute, pulses, vegetables).

6. Properties of Soil:

  • Percolation Rate: The rate at which water passes down through the soil.
    • Formula: Percolation Rate (mL/min) = Amount of water (mL) / Time taken for percolation (min)
    • Highest in sandy soil, lowest in clayey soil.
  • Water Absorption/Retention: The ability of soil to hold water.
    • Clayey soil has the highest water retention capacity.
    • Sandy soil has the lowest water retention capacity.
    • Loamy soil has moderate and optimal water retention.
  • Moisture Content: Soil naturally contains some water, known as moisture. This can be observed by heating soil in a test tube (water droplets form).
  • Texture: Determined by the relative proportion of sand, silt, and clay particles.
  • Colour: Varies depending on mineral content and humus (e.g., black soil is rich in humus/minerals, red soil indicates iron oxide).

7. Soil and Crops:

  • Different crops require different types of soil for optimal growth.
    • Clayey and Loamy soils: Suitable for cereals like wheat and gram (good water retention). Also good for paddy (rice) which requires standing water.
    • Loamy soils: Good for lentils (masoor) and other pulses (require good drainage).
    • Sandy-loam or Loam: Suitable for cotton (needs good drainage and aeration).

8. Soil Erosion:

  • The removal of the fertile topsoil by natural forces like strong wind, flowing water (rain, rivers), or human activities.
  • Causes:
    • Deforestation (cutting down trees).
    • Overgrazing by animals.
    • Improper farming techniques.
    • Floods and heavy rainfall.
    • Strong winds (especially in dry, bare areas).
  • Consequences: Loss of soil fertility, reduced agricultural productivity, desertification, floods, landslides.

9. Soil Conservation:

  • Protecting the soil from erosion and maintaining its fertility.
  • Methods:
    • Afforestation/Reforestation: Planting trees (roots bind the soil).
    • Terrace Farming: Cutting steps into hillsides to slow down water flow.
    • Contour Ploughing: Ploughing along the contours of a slope instead of up and down.
    • Shelterbelts: Planting rows of trees to block wind.
    • Cover Crops: Planting grasses or other crops to cover bare soil between main plantings.
    • Building Check Dams: To slow down water flow and prevent gully formation.
    • Preventing Overgrazing.

10. Soil Pollution:

  • Contamination of soil with harmful substances that reduce its quality and fertility.
  • Causes:
    • Excessive use of pesticides and insecticides.
    • Excessive use of chemical fertilizers.
    • Dumping of industrial waste and chemicals.
    • Disposal of non-biodegradable waste like plastics and polythene bags.
    • Acid rain.
  • Prevention: Reducing chemical use (promoting organic farming), proper waste disposal and management, treating industrial effluents before release.

Multiple Choice Questions (MCQs)

  1. Which horizon of the soil profile is richest in humus?
    (a) B-Horizon
    (b) C-Horizon
    (c) A-Horizon
    (d) Bedrock

  2. The process of breaking down rocks by the action of wind, water, and climate is called:
    (a) Percolation
    (b) Weathering
    (c) Erosion
    (d) Sedimentation

  3. Which type of soil has the highest water retention capacity?
    (a) Sandy soil
    (b) Loamy soil
    (c) Clayey soil
    (d) Gravel

  4. For growing cotton, the most suitable soil is:
    (a) Clayey soil
    (b) Sandy soil
    (c) Sandy-loam soil
    (d) Black soil rich in clay

  5. The rate at which water passes down through the soil is known as:
    (a) Absorption rate
    (b) Retention rate
    (c) Percolation rate
    (d) Evaporation rate

  6. Which of the following is NOT a primary agent of soil erosion?
    (a) Heavy rainfall
    (b) Strong wind
    (c) Deforestation
    (d) Afforestation

  7. Loamy soil is a mixture of:
    (a) Sand and gravel
    (b) Clay and gravel
    (c) Sand, silt, and clay
    (d) Silt and gravel

  8. Planting trees on a large scale to prevent soil erosion is called:
    (a) Deforestation
    (b) Terrace farming
    (c) Afforestation
    (d) Overgrazing

  9. If 200 mL of water takes 40 minutes to percolate completely through a soil sample, what is the percolation rate?
    (a) 5 mL/min
    (b) 0.2 mL/min
    (c) 8000 mL/min
    (d) 160 mL/min

  10. Which layer of the soil profile consists mainly of small lumps of rocks derived from the parent rock?
    (a) A-Horizon
    (b) B-Horizon
    (c) C-Horizon
    (d) O-Horizon

Answer Key for MCQs:

  1. (c) A-Horizon
  2. (b) Weathering
  3. (c) Clayey soil
  4. (c) Sandy-loam soil (Note: Black soil, often clayey or loamy, is also very good, but sandy-loam provides the best mix of drainage and retention mentioned in basic texts)
  5. (c) Percolation rate
  6. (d) Afforestation (This prevents erosion)
  7. (c) Sand, silt, and clay
  8. (c) Afforestation
  9. (a) 5 mL/min (Calculation: 200 mL / 40 min = 5 mL/min)
  10. (c) C-Horizon

Remember to thoroughly understand these concepts. Soil is fundamental to our environment and agriculture, making it a frequently tested topic. Good luck with your preparation!

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