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Soil: Origin, Types, and Other Characteristics – UPSC Biogeography Notes
Overview: Soil is one of the most important natural resources on Earth. It supports plant growth, regulates water, stores nutrients, and sustains life. Understanding the formation, characteristics, and types of soil is essential for UPSC Geography, Environment, and Agriculture sections.
Table of Contents
- 1. What is Soil?
- 2. Origin and Formation of Soil
- 3. Factors of Soil Formation
- 4. Processes of Soil Formation
- 5. Soil Profile and Horizons
- 6. Major Types of Soil
- 7. Characteristics of Soil
- 8. UPSC Quick Revision Notes
- 9. Previous Year Question
1. What is Soil?
Soil is the uppermost layer of the Earth’s crust, composed of minerals, organic matter, water, air, and living organisms. It is a product of long-term weathering and biological activity.
2. Origin and Formation of Soil
Soil originates from the weathering of rocks. Over time, physical, chemical, and biological processes break down rocks into fine particles, which mix with organic material to form soil.
This entire process is known as pedogenesis (soil formation).
3. Factors of Soil Formation
Five major factors influence soil formation:
- (1) Parent Material: The rock from which soil develops. Determines mineral content.
- (2) Climate: Temperature and rainfall control weathering and organic decomposition.
- (3) Organisms: Plants, animals, microbes add organic matter and help soil mixing.
- (4) Relief (Topography): Slope affects erosion and drainage.
- (5) Time: Longer the period, more mature the soil.
4. Processes of Soil Formation
Soil is formed through several physical, chemical, and biological processes:
(A) Physical Processes
- Temperature changes → expansion and contraction of rocks.
- Frost action → cracks rocks.
- Water and wind → erosion and deposition.
(B) Chemical Processes
- Hydration: Minerals absorb water and expand.
- Oxidation: Formation of iron oxides (red colour).
- Carbonation: CO₂ reacts with minerals to form carbonates.
- Solution: Minerals dissolve in water.
(C) Biological Processes
- Plants add organic matter (humus).
- Microorganisms decompose organic matter.
- Burrowing animals mix soil layers.
5. Soil Profile and Horizons
A soil profile is a vertical section showing different layers called horizons.
The main horizons are:
- O Horizon: Organic matter (leaves, plant remains).
- A Horizon (Topsoil): Mineral-rich with humus; most fertile layer.
- B Horizon (Subsoil): Accumulated minerals like iron, clay.
- C Horizon: Weathered parent material.
- R Horizon: Bedrock.
6. Major Types of Soil
Soils differ based on climate, parent rock, vegetation, and geography. Below are the major types covered in NCERT and UPSC syllabus (global + India-focused):
(A) Zonal Soils (Mature Soils)
- Laterite Soil: Found in humid tropics; rich in iron/aluminum; low fertility.
- Podzols: Found in cold regions; acidic; poor in nutrients.
- Chernozem (Black Earth): Highly fertile; mid-latitude grasslands.
- Tundra Soil: Frozen, with little organic matter.
(B) Intrazonal Soils
- Calcium-rich (Pedocals): Found in dry regions; lime accumulation.
- Saline and Alkaline Soils: Poor drainage; salt accumulation.
- Peaty and Marshy Soils: High organic matter; wet areas.
(C) Azonal Soils
- Young soils formed recently.
- Found in river floodplains, deltas, and glacial deposits.
(D) Major Soil Types in India (Important for UPSC)
- Alluvial Soil: Fertile; river plains; agriculture-rich.
- Black Soil (Regur): Clayey; cotton-growing; found in Deccan Plateau.
- Red Soil: Rich in iron; poor in nitrogen.
- Laterite Soil: Tropical regions; low fertility.
- Desert Soil: Sandy; low organic matter; Rajasthan.
- Mountain Soil: Thin, shallow; found in Himalayas.
7. Characteristics of Soil
(A) Soil Texture
- Proportion of sand, silt, and clay.
- Determines water-holding capacity.
- Clayey → holds water; Sandy → drains quickly.
(B) Soil Structure
- Arrangement of soil particles.
- Types → blocky, platy, granular.
(C) Soil Colour
- Red → iron oxide.
- Black → organic matter or volcanic origin.
- Yellow → hydrated iron oxide.
(D) Soil pH
- Acidic → high rainfall regions.
- Alkaline → arid regions.
(E) Soil Fertility
- Determined by nutrient content → N, P, K.
- Humus improves fertility.
8. UPSC Quick Revision Notes
Soil formation factors → climate, parent rock, organisms, relief, time.
Processes → weathering (physical, chemical, biological).
Profile → O, A, B, C, R horizons.
Zonal soils → laterite, podzol, chernozem.
Intrazonal soils → pedocals, saline soils, peaty soils.
Azonal soils → immature; floodplains & deltas.
Indian soils → alluvial, black, red, laterite, desert, mountain.
Texture → sand/silt/clay ratio; Structure → particle arrangement.
Fertility depends on humus + nutrients (NPK).
9. Previous Year Question
Q. Which one of the following factors influences soil formation the most?
(a) Relief
(b) Parent Material
(c) Climate
(d) Time
Answer: (c) Climate
Related Keywords
Soil Formation UPSC Notes, Types of Soil, Biogeography Notes, Indian Soils, Soil Profile, Pedogenesis, Geography for UPSC.
Sources: NCERT Class XI – India Physical Environment & Fundamentals of Physical Geography, FAO Soil Classification (simplified), Goh Cheng Leong.
Next Topic → Soil Erosion and Conservation – UPSC Geography Notes
