Learning Objectives
- Understand soil formation and soil profile
- Learn about types of soil
- Know about properties of soil
- Understand soil erosion and conservation
Key Concepts
Soil Formation
Soil is formed by weathering of rocks over thousands of years. Weathering is caused by wind, water, temperature changes, and biological activity (lichens, moss, plant roots).
Soil = Rock particles + Humus (decayed organic matter) + Water + Air + Living organisms
Soil Profile (Layers)
- A-horizon (Topsoil): Dark colour, rich in humus and minerals. Soft, porous. Contains living organisms. Best for plant growth.
- B-horizon (Subsoil): Harder, less humus. Contains minerals washed down from topsoil.
- C-horizon: Made of small pieces of rocks (partially weathered). No humus.
- Bedrock: Hard, unweathered rock at the bottom.
Types of Soil
- Sandy soil: Large particles, high percolation rate, low water retention. Not very fertile.
- Clayey soil: Very small particles, low percolation rate, high water retention. Heavy and sticky when wet.
- Loamy soil: Mixture of sand, silt, and clay with humus. Best for growing most plants. Good water retention and drainage.
Soil Properties
Percolation rate: Rate at which water passes through soil. Sandy > Loamy > Clayey.
Moisture: Amount of water held in soil. Clayey > Loamy > Sandy.
Absorption: Clayey soil absorbs more water than sandy soil.
Soil Erosion and Conservation
Soil erosion: Removal of topsoil by wind, water, or human activities (deforestation, overgrazing).
Conservation methods: Planting trees and grass, terrace farming on slopes, building check dams, avoiding overgrazing, mulching.
Summary
Soil forms from weathering of rocks. The soil profile has distinct layers (A, B, C horizons and bedrock). Sandy, clayey, and loamy are main soil types. Loamy soil is best for farming. Soil erosion must be prevented through tree planting and proper land management.
Important Terms
- Weathering: Breaking down of rocks into soil
- Humus: Decomposed organic matter in soil
- Soil profile: Vertical cross-section showing soil layers
- Percolation: Passage of water through soil
- Soil erosion: Removal of topsoil by wind/water
Quick Revision
- Soil = Rock particles + Humus + Water + Air + Organisms
- Layers: Topsoil โ Subsoil โ C-horizon โ Bedrock
- Sandy: large particles, drains fast. Clayey: tiny, holds water. Loamy: best mix
- Percolation: Sandy > Loamy > Clayey
- Prevent erosion: plant trees, terrace farming, check dams