Soils: Types and Distribution for UPSC Prelims
1. Introduction to Soils
Soil is the uppermost layer of Earth's crust, composed of mineral particles, organic matter, water, and air. It forms through weathering of rocks and decomposition of organic material over time. Soil characteristics vary based on parent material, climate, topography, organisms, and time—collectively known as soil-forming factors or pedogenic factors.
Key Facts
- Composition: 45% minerals, 25% water, 25% air, 5% organic matter
- Functions: Medium for plant growth, water storage, habitat for organisms
- Formation Rate: 1cm takes 200-400 years to form naturally
- Importance: Basis for agriculture, carbon sequestration, biodiversity
Important Facts for UPSC
- Soil is a non-renewable resource on human timescales.
- India has 8 major soil types with distinct characteristics.
- Soil health is crucial for food security and climate resilience.
Soil Forming Factors (Pedogenic Factors)
- Parent Material: Rock or sediment from which soil forms
- Climate: Temperature and precipitation affect weathering
- Organisms: Plants, animals, microbes contribute organic matter
- Topography: Slope and elevation influence erosion/deposition
- Time: Duration of soil-forming processes
Soil Forming Processes
| Process |
Description |
Result |
| Leaching |
Downward movement of minerals by water |
Nutrient-poor topsoil |
| Illuviation |
Deposition of leached materials in lower layers |
Hardpan formation |
| Humification |
Decomposition of organic matter |
Dark, nutrient-rich topsoil |
| Laterization |
Leaching in tropical climates |
Iron/aluminum-rich soils |
Important Facts for UPSC
- Black soils form from basaltic parent material (Deccan Traps).
- Laterite soils are common in high-rainfall tropical regions.
- Podzolization occurs in cool, moist coniferous forests.
3. Soil Classification Systems
USDA Soil Taxonomy
The most widely used system with 12 soil orders:
| Order |
Characteristics |
Distribution |
| Alfisols |
Clay-rich subsoil, fertile |
Temperate forests |
| Andisols |
Volcanic ash parent material |
Volcanic regions |
| Aridisols |
Dry, saline, low organic matter |
Deserts |
| Entisols |
Recently formed, little development |
Floodplains, dunes |
| Mollisols |
Thick, dark topsoil, very fertile |
Grasslands (prairies) |
Indian Council of Agricultural Research (ICAR) Classification
Classifies Indian soils into 8 major types:
- Alluvial
- Black (Regur)
- Red and Yellow
- Laterite
- Arid/Desert
- Saline/Alkaline
- Peaty/Marshy
- Forest/Mountain
Important Facts for UPSC
- India follows both USDA and ICAR classification systems.
- Mollisols are considered the world's most fertile soils.
- Oxisols (tropical soils) are highly weathered and nutrient-poor.
4. Major Soil Types of the World
| Soil Type |
Characteristics |
Major Regions |
Crops Grown |
| Chernozem |
Black, high humus, very fertile |
Russian Steppe, Ukraine |
Wheat, barley |
| Laterite |
Red, iron/aluminum rich, acidic |
Tropics (India, Brazil) |
Tea, coffee, cashew |
| Podzol |
Ash-gray, acidic, leached |
Taiga regions |
Coniferous forests |
| Loess |
Wind-deposited silt, fertile |
China, Mississippi Valley |
Corn, wheat |
Zonal Soil Distribution
- Tropical: Laterite, Oxisols (high temp, rainfall)
- Temperate: Alfisols, Mollisols (moderate climate)
- Arid: Aridisols (low rainfall, high evaporation)
- Cold: Gelisols (permafrost)
Important Facts for UPSC
- Chernozem soils contain up to 16% organic matter.
- Laterite soils harden irreversibly when exposed to air (used as bricks).
- Loess deposits can be 100m thick in China's Loess Plateau.
5. Soil Types in India
| Soil Type |
% Area |
Regions |
Characteristics |
| Alluvial |
43% |
Indo-Gangetic plains, deltas |
New (Khadar) and old (Bangar), fertile |
| Black (Regur) |
16% |
Deccan Plateau |
Clay-rich, swell when wet, rich in Ca/Mg |
| Red & Yellow |
18% |
Eastern/Southern plateau |
Iron oxide, porous, acidic |
| Laterite |
7% |
High-rainfall areas |
Leached, brick-like when dry |
Special Indian Soils
- Karewa: Lacustrine deposits in Kashmir Valley (saffron cultivation)
- Terai: Fertile at foothills of Himalayas (high organic matter)
- Bhabar: Porous at Himalayan foothills (rivers disappear)
Important Facts for UPSC
- Black soils are ideal for cotton (called 'Black Cotton Soil').
- Alluvial soils are most extensive and agriculturally productive.
- Laterite soils cover parts of Kerala, Karnataka, and Odisha.
6. Soil Profile and Horizons
Soil Horizons
| Horizon |
Depth |
Characteristics |
| O (Organic) |
Top |
Fresh/decomposing organic matter |
| A (Topsoil) |
10-30cm |
Mineral + organic matter, most biological activity |
| E (Eluviation) |
Variable |
Leached zone (light-colored) |
| B (Subsoil) |
30-100cm |
Accumulation of clay, iron, aluminum |
| C (Parent) |
100-200cm |
Partially weathered bedrock |
| R (Bedrock) |
Below C |
Unweathered parent material |
Important Facts for UPSC
- Not all horizons are present in every soil profile.
- A-horizon is most important for agriculture.
- Laterite soils often lack distinct horizons due to intense leaching.
7. Soil Erosion and Degradation
Types of Soil Erosion
- Sheet Erosion: Uniform removal of topsoil
- Rill Erosion: Small channels (<30cm deep)
- Gully Erosion: Large channels (>30cm deep)
- Wind Erosion: Common in arid regions
Causes of Soil Degradation
| Cause |
Impact |
Examples |
| Deforestation |
Loss of protective cover |
Himalayan slopes |
| Overgrazing |
Removal of vegetation |
Rajasthan deserts |
| Waterlogging |
Rising water tables |
Punjab, Haryana |
| Salinization |
Salt accumulation |
Indus Valley |
Important Facts for UPSC
- India loses 5,334 million tonnes of soil annually to erosion.
- Rajasthan has highest area under desertification (62%).
- Waterlogging affects 8.4 million hectares in India.
8. Soil Conservation Methods
Traditional Methods
- Contour Plowing: Plowing across slopes
- Terracing: Step-like cultivation on slopes
- Strip Cropping: Alternating crops in strips
- Shelterbelts: Tree rows to reduce wind erosion
Modern Techniques
| Technique |
Description |
Example |
| No-till Farming |
Minimal soil disturbance |
Punjab wheat fields |
| Biochar |
Charcoal to improve fertility |
Northeast India trials |
| Precision Agriculture |
GPS-guided nutrient application |
Karnataka pilot projects |
Government Initiatives
- Soil Health Card Scheme: Assess soil nutrients (2015)
- Pradhan Mantri Krishi Sinchayee Yojana: Micro-irrigation
- National Mission for Sustainable Agriculture: Climate-resilient practices
Important Facts for UPSC
- Contour bunding can reduce soil loss by 50%.
- India aims to restore 26 million hectares of degraded land by 2030.
- Jhum cultivation (shifting agriculture) causes soil degradation in NE India.
9. Economic and Ecological Significance
Economic Importance
- Agriculture contributes 16% to India's GDP (soil-dependent)
- 60% of Indian population depends on agriculture
- Soil minerals have industrial uses (kaolin, bauxite)
Ecological Services
- Carbon storage (3x more than atmosphere)
- Water filtration and storage
- Habitat for 25% of Earth's biodiversity
Important Facts for UPSC
- Indian soils store about 30 petagrams of carbon.
- 1cm of topsoil can hold 250,000 liters of water per hectare.
- Soil microorganisms contribute to nitrogen fixation.
10. UPSC Preparation Tips
Focus Areas
- ICAR classification of Indian soils
- Soil formation processes (laterization, podzolization)
- Soil conservation programs and schemes
- Linkages between soil types and agricultural patterns
Preparation Strategy
- Create a soil distribution map of India
- Compare Indian soils with global counterparts
- Relate soil types to crops (e.g., black soil - cotton)
- Study recent government initiatives on soil health
Important Facts for UPSC
- Previous questions have asked about black cotton soil, laterite, and soil erosion types.
- Understand the difference between alluvial Khadar and Bangar soils.
- Note how different soil conservation methods suit different regions.