UPSC CSE Prelims
Soil Types of India Previous Year Questions (PYQs)

Detailed Explanation:

Answer: Option 2 — fissure volcanic rock

Black cotton soil (also known as Regur soil) in India is primarily formed by the weathering of basaltic lava flows from fissure volcanic eruptions. These soils are predominantly found in the Deccan Plateau region, covering parts of Maharashtra, Gujarat, Madhya Pradesh, and Karnataka, where the parent material is basaltic rock. The dark color comes from the presence of titaniferous magnetite and aluminum-rich minerals, while the high clay content gives it swelling and shrinking properties.

📝 Short Notes: Black Cotton Soil (Regur Soil)

• Parent Material: Formed from weathering of basaltic lava (Deccan Trap volcanic rocks)
• Geographical Distribution: Deccan Plateau, Malwa Plateau - mainly in Maharashtra, Gujarat, Madhya Pradesh, Karnataka, parts of Andhra Pradesh and Tamil Nadu
• Physical Properties: Deep black to dark gray color; rich in clay, lime, iron, magnesium, and aluminum but deficient in nitrogen, phosphorus, and organic matter
• Characteristic Feature: High clay content causes swelling when wet and shrinking when dry, developing deep cracks in summer
• Moisture Retention: Excellent moisture retention capacity due to high clay content, suitable for dry farming
• Major Crops: Ideal for cotton cultivation; also suitable for sugarcane, jowar, wheat, linseed, and citrus fruits
• Other Names: Regur soil, Black soil, or Tropical Black Earth

Detailed Explanation:

Answer: Option 2 — 3 only

Among the three statements about agricultural soils, only statement 3 is correct. Statement 1 incorrectly claims that organic matter reduces water-holding capacity, when it actually increases it. Statement 2 incorrectly suggests soil plays no role in the sulphur cycle, when in fact soil is crucial for sulphur transformations and storage.

❌ Statement 1 – Incorrect: High organic matter content actually increases water-holding capacity of soil, not reduces it. Organic matter acts like a sponge, creating pore spaces that retain moisture effectively.

❌ Statement 2 – Incorrect: Soil plays a vital role in the sulphur cycle through microbial transformations (mineralization, immobilization) and storage of sulphur compounds like sulfates that are essential plant nutrients.

✅ Statement 3 – Correct: Prolonged irrigation, especially with poor drainage and high-salt water, causes accumulation of salts in the soil profile, leading to salinization that adversely affects crop productivity.

📝 Short Notes: Soil Properties and Agricultural Impacts

• Organic Matter Benefits: Increases water retention, improves soil structure, enhances nutrient availability, promotes microbial activity, and increases cation exchange capacity (CEC).
• Soil in Nutrient Cycles: Soil is integral to carbon, nitrogen, phosphorus, and sulphur cycles through microbial decomposition, mineralization, and nutrient transformation processes.
• Salinization Process: Occurs when irrigation water brings dissolved salts that accumulate in root zones due to evaporation and inadequate drainage, particularly in arid and semi-arid regions.
• Prevention of Salinization: Proper drainage systems, use of quality irrigation water, crop rotation with salt-tolerant species, and leaching of accumulated salts.
• Indicators of Salinization: White salt crusts on soil surface, reduced crop growth, increased soil pH, and presence of salt-tolerant weeds.

Detailed Explanation:

✅ Statement 1 – Correct: Laterite soils are characterized by their distinctive rusty-red colour due to high iron oxide content formed through leaching in tropical conditions.

❌ Statement 2 – Incorrect: Laterite soils are deficient in nitrogen, potash, potassium, phosphorus, and lime, though rich in iron and aluminium oxides; humus content is also very low.

❌ Statement 3 – Incorrect: Laterite soils are well-developed in high rainfall areas of Western Ghats, Eastern Ghats (Karnataka, Kerala, Tamil Nadu, Maharashtra), Odisha, Assam, and Meghalaya — not in semi-arid Rajasthan or UP.

✅ Statement 4 – Correct: Despite low fertility, tapioca, cashew nuts, tea, coffee, and rubber grow well on laterite soils with adequate fertilization and irrigation.