Class 7 Science Notes Chapter 9 (Soil) – Science Book

Okay, here are the detailed notes for NCERT Class 7 Science Chapter 9: Soil, tailored for government exam preparation.

NCERT Class 7 Science - Chapter 9: Soil - Detailed Notes for Government Exams

1. Introduction: What is Soil?

• Definition: Soil is the uppermost layer of the Earth's crust, primarily composed of a mixture of weathered rock particles, minerals, humus (decayed organic matter), water, air, and living organisms.
• Importance:
  • Agriculture: Anchors plants and provides essential nutrients and water for growth, forming the basis of food production.
  • Habitat: Home to numerous organisms like earthworms, bacteria, fungi, insects, and rodents.
  • Water Filtration: Acts as a natural filter for water.
  • Construction: Used as a building material (e.g., clay for bricks, pottery).
  • Supports Life: Directly or indirectly supports almost all life on land.

2. Soil Formation: Weathering

• Definition: Soil is formed by the slow process of weathering, which is the breaking down of large rocks into smaller particles.
• Process: It's a very gradual process, taking hundreds or thousands of years to form a thin layer of soil.
• Agents of Weathering:
  • Physical: Wind, water (rain, rivers, frost), temperature changes (expansion and contraction).
  • Chemical: Acid rain, reactions with minerals in rocks.
  • Biological: Plant roots growing into rock crevices, activities of lichens and microorganisms.
• Nature of Soil Depends On:
  • The parent rock from which it was formed.
  • The type of vegetation that grows in it.
  • Climatic factors.

3. Soil Profile

• Definition: A vertical section through different layers of the soil is called the soil profile. Each distinct layer is called a horizon.
• Horizons (Layers):
  • A-Horizon (Topsoil):
    • Uppermost layer.
    • Darkest in colour due to the presence of rich humus and minerals.
    • Soft, porous, and holds water well.
    • Most fertile layer, where most plant roots and soil organisms are found.
  • B-Horizon (Subsoil):
    • Lies below the topsoil.
    • Generally harder and more compact than topsoil.
    • Lighter in colour.
    • Contains less humus but is rich in minerals leached down from the A-horizon.
  • C-Horizon (Parent Material):
    • Lies below the subsoil.
    • Consists of small lumps of partially weathered rocks with cracks and crevices.
    • Represents the transition between soil and the underlying bedrock.
  • Bedrock:
    • The bottommost layer.
    • Hard, non-porous, solid rock.
    • Difficult to dig.
    • Source of the mineral particles for the soil above.

4. Composition of Soil

• Soil is a mixture of:
  • Rock Particles: Formed by weathering. Classified by size into gravel (largest), sand, silt, and clay (finest).
  • Humus: Decomposed organic matter from dead plants and animals. Makes soil fertile, dark, and improves water retention.
  • Minerals: Derived from the parent rock. Essential nutrients for plant growth.
  • Water: Held in the spaces between soil particles. Essential for plants and soil organisms.
  • Air: Occupies the spaces between soil particles not filled with water. Essential for root respiration and soil organisms.
  • Living Organisms: Bacteria, fungi, algae, protozoa, earthworms, insects, etc. Contribute to decomposition and soil aeration.

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

• The relative proportion of sand, silt, and clay particles determines the soil type and its properties.
  • Sandy Soil:
    • Composition: High proportion of larger sand particles (>70-80% sand).
    • Properties: Large air spaces, well-aerated, light, drains water very quickly (high percolation rate), low water-holding capacity, low nutrient content (less humus).
    • Feel: Gritty.
    • Suitability: Not ideal for most crops but suitable for some like groundnuts, millets, potatoes if irrigated and manured. Found in deserts and beaches.
  • Clayey Soil:
    • Composition: High proportion of very fine clay particles (>40-50% clay).
    • Properties: Particles packed tightly, very small air spaces, poorly aerated, heavy, high water-holding capacity, low percolation rate, sticky when wet, hard when dry, often rich in humus and nutrients.
    • Feel: Smooth when dry, sticky when wet.
    • Suitability: Good for crops needing standing water like paddy (rice). Also suitable for wheat and gram. Used for making pots, toys, and bricks.
  • Loamy Soil:
    • Composition: Balanced mixture of sand, silt, and clay particles. Also contains humus.
    • Properties: Considered the best soil for agriculture. Good water-holding capacity, adequate aeration, good drainage (moderate percolation rate), rich in humus, easy to plough.
    • Feel: Smooth, slightly gritty, slightly sticky.
    • Suitability: Ideal for growing a wide variety of crops like wheat, sugarcane, cotton, jute, pulses, and vegetables.

6. Properties of Soil

• Percolation Rate:
  • Definition: The speed at which water passes down through the soil.
  • Measurement: Percolation rate (mL/min) = Amount of water (mL) / Time taken for percolation (min).
  • Dependence: Highest in sandy soil, lowest in clayey soil. Loamy soil has a moderate rate.
• Moisture Content:
  • Definition: The amount of water present in the soil.
  • Demonstration: Heating soil in a test tube shows water evaporating and condensing on cooler parts.
  • Importance: Essential for plant life. Varies with soil type and weather.
• Water Absorption/Retention:
  • Definition: The ability of soil to hold water.
  • Dependence: Clayey soil holds the most water, sandy soil holds the least. Loamy soil holds a moderate amount.
  • Importance: Crucial for supplying water to plants between rainfalls or irrigations.

7. Soil and Crops

• Different crops require different soil types for optimal growth, depending on their water and nutrient needs.
  • Clayey and Loamy Soils: Suitable for cereals like wheat and gram (require good water retention). Also good for paddy (rice) which needs soil capable of holding standing water (clayey soil is particularly good).
  • Sandy-Loam or Loam: Suitable for cotton (requires good drainage and aeration).
  • Loamy Soils: Best for lentils (masoor) and other pulses (require good drainage).
• Soil fertility can be improved by adding manure or fertilizers. pH of the soil also affects crop suitability.

8. Soil Erosion

• Definition: The removal of the fertile topsoil layer by natural agents like wind, water (heavy rain, floods, rivers), or due to human activities.
• Causes:
  • Deforestation: Cutting down trees exposes the soil. Tree roots bind the soil.
  • Overgrazing: Removal of grass cover by animals exposes the soil.
  • Strong Winds: Blow away loose topsoil, especially in dry areas.
  • Heavy Rains & Floods: Wash away topsoil, especially on slopes.
  • Improper Farming Practices: Ploughing on slopes can accelerate erosion.
• Effects:
  • Loss of fertile topsoil, reducing agricultural productivity.
  • Desertification (in severe cases).
  • Water pollution (eroded soil silts up rivers and lakes).
  • Reduced groundwater recharge.

9. Soil Conservation

• Definition: Protecting the soil from erosion and maintaining its fertility.
• Methods:
  • Afforestation & Reforestation: Planting trees and restoring forests. Tree roots bind soil.
  • Shelterbelts: Planting rows of trees to reduce wind speed and prevent wind erosion, especially in coastal and dry regions.
  • Terrace Farming: Cutting steps into hillsides to slow down water flow and reduce erosion on slopes.
  • Contour Ploughing: Ploughing parallel to the contours of a slope instead of up and down.
  • Building Dams and Embankments: To control floods and check riverbank erosion.
  • Preventing Overgrazing: Managing livestock grazing to maintain grass cover.
  • Cover Cropping: Planting crops like grass or legumes to cover the soil between main crop seasons.

Key Takeaways for Exams:

• Know the definitions: Weathering, Humus, Soil Profile, Horizon, Loam, Percolation, Soil Erosion, Soil Conservation.
• Understand the formation of soil (Weathering process).
• Be able to describe the Soil Profile and the characteristics of each horizon (A, B, C, Bedrock).
• Compare the properties (particle size, aeration, water retention, percolation rate) of Sandy, Clayey, and Loamy soils.
• Know which soil type is best suited for which major crops mentioned in the chapter (Wheat, Gram, Paddy, Cotton, Pulses).
• Understand the formula and concept of Percolation Rate.
• Identify the causes of Soil Erosion and methods of Soil Conservation.
• Recognize the importance of soil for life and agriculture.