UPSC Prelims 2021 Q64: Geography – Indian Agriculture & Land Use
- (a) brown forest soil
- (b) fissure volcanic rock
- (c) granite and schist
- (d) shale and limestone
Answer: (b) fissure volcanic rock
Explanation
Black Cotton Soil (Regur) Formation
| Feature | Detail |
|---|---|
| Parent rock | Fissure/flood basalt (Deccan Trap basalts) |
| Region | Deccan Plateau: Maharashtra, MP, Gujarat, Karnataka, Andhra, Telangana |
| Formation | Weathering of basaltic lava flows under semi-arid climate creates montmorillonite clay minerals |
| Properties | High clay content, self-mulching (cracks when dry), highly retentive of moisture, rich in lime, iron, magnesium, calcium; low in N, P, humus |
| Ideal for | Cotton, sugarcane, jowar, tobacco, oilseeds |
Black cotton soil (regur) is formed by weathering of FISSURE VOLCANIC ROCK (Deccan Trap basalts). Answer: (b).
The Deccan Traps are one of the largest volcanic features on Earth, formed by fissure eruptions 65-67 million years ago (linked to Cretaceous-Paleogene mass extinction).
Answer: (b) fissure volcanic rock.
Why each option is right or wrong
- (a) Incorrect. Brown forest soils are formed in temperate mid-latitude forests (Europe, N America) - not related to black cotton soils.
- (b) CORRECT. Black cotton soil (regur) forms from weathering of FISSURE VOLCANIC ROCK - specifically the Deccan Trap basalts (65-67 million years old fissure eruptions). Rich in montmorillonite clay, ideal for cotton.
- (c) Incorrect. Granite and schist weathering produces red or yellow soils, not black cotton.
- (d) Incorrect. Shale and limestone weathering produces different soil types (calcareous, alluvial-like), not regur.
Key point
Black cotton soil (regur) = weathering product of fissure volcanic rock (Deccan Trap basalts). Deccan basalts erupted 65-67 mya (fissure eruptions). Rich in montmorillonite clay, moisture-retentive, self-mulching. Ideal for cotton, sugarcane.
Remember
The black cotton soil (regur) of India has been formed due to the weathering of FISSURE VOLCANIC ROCK - specifically the Deccan Trap basalts from massive fissure eruptions 65-67 million years ago. Correct answer: (b).
Common mistakes
- Confusing regur (black cotton, Deccan basalts) with alluvial soils (Indo-Gangetic plain).
- Not knowing the Deccan Traps are a Large Igneous Province (LIP) from fissure eruptions, linked to K-Pg extinction.
- Assuming all Deccan soils are red. The Deccan has both black (basalt weathering) and red (older gneiss weathering) soils.
Exam tip
Indian soil-parent rock pairs: Regur/Black cotton (Deccan basalt fissure eruptions), Alluvial (Himalayan sediments), Red soil (Precambrian gneiss/granite), Laterite (heavy rainfall leaching), Desert (aeolian sand), Mountain soil (weathering + organic).
Sources
- NCERT Class XI India: Physical Environment - Chapter on Soils
- Geological Survey of India - Deccan Volcanic Province