Which type of soil mainly forms the extensive northern plains of India in the Indo-Gangetic region?

Introduction / Context:
The northern plains of India, often called the Indo-Gangetic and Brahmaputra plains, are one of the most fertile and densely populated regions in the country. A common exam question tests whether students know which type of soil dominates these plains and why it is so suitable for agriculture. Understanding the origin and nature of this soil type helps in learning about Indian agriculture, population distribution, and river systems.

Given Data / Assumptions:

• The question specifically refers to the northern plain of India.
• The plain is associated with major rivers like the Indus, Ganga, and Brahmaputra and their tributaries.
• The focus is on the dominant soil type that forms this plain.
• Basic school level geography concepts about soil formation are assumed.

Concept / Approach:
To answer this question correctly, we need to recall how the northern plains were formed. Large rivers flowing from the Himalayas and the Peninsular plateau bring huge quantities of silt, sand, clay, and fine sediments. Over thousands of years, these sediments are deposited over a broad area, creating a thick layer of fertile soil. This river deposited soil is called alluvial soil. It is generally deep, fertile, and rich in minerals that support intensive agriculture. Other options like metamorphic soil, igneous rocks, and old crystalline rocks are associated with mountains and plateaus rather than low-lying river plains.

Step-by-Step Solution:
Step 1: Recall that the northern plains stretch from Punjab in the west through Haryana, Uttar Pradesh, Bihar, and West Bengal to Assam in the east. Step 2: Recognise that these plains were formed by the deposition of materials brought down by the Indus, Ganga, Brahmaputra, and their tributaries. Step 3: Understand that river deposited soil made of silt, sand, and clay is called alluvial soil. Step 4: Note that metamorphic soil, igneous rocks, and old crystalline rocks are associated with the Himalayas or Peninsular plateau, not with flat plains. Step 5: Therefore, the northern plain of India is mainly formed of alluvial soil deposited by rivers.

Verification / Alternative check:
If you look at any standard school atlas or geography textbook, the soil map of India clearly shows that the Indo-Gangetic plain is covered by alluvial soils. These soils are described as deep, fertile, and suitable for crops such as wheat, rice, sugarcane, and pulses. No other soil type has such a continuous spread along the northern plains. This cross check confirms that alluvial soil is the correct answer.

Why Other Options Are Wrong:

• Metamorphic soil: Metamorphic rocks are more common in Himalayan and some peninsular regions. They do not form the extensive flat northern plains.
• Igneous rocks and associated soils: Igneous rocks dominate the Deccan plateau, not the Indo-Gangetic plains.
• Old crystalline rocks and residual soils: These are typical of the Peninsular plateau and are not responsible for the northern plains.

Common Pitfalls:
Students sometimes confuse the northern plains with the Deccan plateau and may wrongly choose options linked with plateaus. Another mistake is to think that because the Himalayas are nearby, metamorphic materials dominate the plains. In reality, rivers break and carry material from various rocks but what matters in the plains is the fine river deposited alluvial soil, not the original rock type from the mountains.

Final Answer:
The northern plain of India is mainly formed of alluvial soil deposited by rivers in the Indo-Gangetic and Brahmaputra plains region.