Soils of India – NDA Geography Notes

Introduction

Soil is not just dirt. It is a living, complex mixture of minerals, water, air, and organic matter. It is the foundation of all agriculture. Without healthy soil, crops cannot grow and people cannot eat.

India has eight major types of soils. Each type forms under specific conditions — depending on the parent rock, climate, rainfall, and vegetation. Each type supports different crops.

NDA tests this chapter through direct questions on soil types and crops, soil characteristics, the meaning of terms like Regur and laterite, soil conservation methods, and soil-forming processes like laterisation, calcification, and podsolisation.

The most important soils for NDA are: Alluvial, Black (Regur), Laterite, and Red and Yellow soils. Learn these four very well.

Understand the Topic

What is Soil Formation?

Soil forms when rocks break down over thousands of years. Several factors control what kind of soil forms.

Factors of soil formation:

• Parent material (parent rock) — the original rock from which soil forms
• Climate — temperature and rainfall determine how fast rock weathers
• Relief (topography) — slope affects erosion and water drainage
• Vegetation — plants add organic matter; roots break rocks
• Time — older soils are more developed

Human habitation is NOT a soil-forming factor. Humans use soil but do not create it. [NDA 2021-II]

Humus is the dark-coloured, organic part of soil formed by decomposition of plant and animal matter. It is amorphous (shapeless) and colloidal in nature. Because it is colloidal, it serves as a reservoir of nutrients for plants. [NDA tested via CDS]

Water in soil: The water most readily available to plant roots is capillary water — water held in tiny pores of the soil by surface tension. Gravitational water drains away too fast. Hygroscopic water is held too tightly to be absorbed by roots. [NDA tested via CDS]

Classification of Soils in India

The Indian Council of Agricultural Research (ICAR) classifies Indian soils into 8 major types:

1. Alluvial Soil
2. Black Soil (Regur)
3. Red and Yellow Soil
4. Laterite Soil
5. Arid and Desert Soil
6. Saline and Alkaline Soil
7. Forest and Mountain Soil
8. Peaty and Marshy Soil

---

Soil-Forming Processes (Important for NDA)

Laterisation:
Occurs in areas with high temperature and heavy rainfall. Heavy rain leaches (washes out) soluble minerals like silica, calcium, and potassium from the soil. What remains is a soil rich in iron and aluminium oxides. This process creates laterite soil. Leaching is maximum in laterite soil. [NDA 2016-II | NDA tested via CDS]

Calcification:
Occurs in areas where evapotranspiration exceeds precipitation — meaning more water evaporates than falls as rain. In dry conditions, water moves upward in the soil. It deposits calcium carbonate near the surface. This creates calcium-rich soils. Found in dry and semi-arid regions. [NDA 2023-I]

Podsolisation:
Occurs mainly in Taiga forests (boreal forests of cold temperate regions). Intense leaching under cold, humid conditions removes nutrients from the upper soil layers. Creates acidic, nutrient-poor soils called podzols. [NDA 2023-I]

Gleisation:
Occurs in waterlogged conditions where oxygen is absent. Creates grey or bluish soils.

The 8 Major Soil Types of India

---

1. Alluvial Soil

Where: Northern Plains (Indo-Gangetic Plain), river deltas on eastern coast, coastal plains

How it forms: Deposited by rivers — Ganga, Yamuna, Brahmaputra, and Peninsular rivers. Rivers carry fine particles of sand, silt, and clay and deposit them in the plains.

Most widespread soil in India — covers the largest area. [NDA tested via CDS]

Two types:

• Khadar = new alluvium; in active floodplains; renewed every year by floods; more fertile
• Bhangar = old alluvium; sits higher; contains Kankar (calcium carbonate nodules); less fertile

Characteristics:

• Made of sand, silt, and clay in various proportions
• Contains potash, phosphoric acid, and lime in adequate amounts
• Rich in potash; poor in nitrogen and phosphorus [NDA tested via CDS]
• Generally NOT rich in nitrogen — this is important. Statement “alluvial soils are rich in nitrogen” is WRONG. [NDA tested via CDS]
• Can be further classified based on age (Khadar vs Bhangar) [NDA tested via CDS]

Crops: Wheat, rice, sugarcane, cotton, jute, oilseeds

2. Black Soil (Regur Soil)

Where: Deccan Plateau — mainly Maharashtra, parts of MP, Gujarat, Karnataka, Andhra Pradesh

How it forms: Develops on Deccan basaltic lava under hot and humid conditions. The lava weathered over millions of years to form this rich black soil. [NDA 2024-I | NDA 2024-II]

Regur is the term used for Black Cotton Soil. [NDA 2015-II]

Characteristics:

• Dark/black colour due to titaniferous magnetite and humus
• Rich in: calcium carbonate, magnesium, iron, lime, alumina
• Deficient in: nitrogen, phosphorus, organic matter
• Self-ploughing — it swells when wet and cracks when dry. This is called self-tilling behaviour.
• Highly moisture retentive — holds water well; stays moist long after rain stops
• Sticky when wet — very difficult to plough during rains [NDA tested via CDS]
• Clayey, deep, and impermeable
• Black cotton soil has high clay factor — NOT less clay. Statement saying it has “less clay factor” is WRONG. [NDA 2024-II]

Best crop: Cotton — this is why it is called Black Cotton Soil. It is the most suitable soil for cotton. [NDA 2008-I | NDA 2023-II]

Regur soil is NOT light-coloured — it is dark/black. Statement “It is a light coloured soil” is WRONG. [NDA 2024-I]

Regur soil is NOT fertile in the sense of being rich in all nutrients — it is specifically deficient in nitrogen and phosphorus. Black soil does NOT contain adequate nitrogen and phosphorus for plant growth. [NDA tested via CDS]

3. Red and Yellow Soil

Where: Eastern Deccan Plateau — Odisha, Chhattisgarh, parts of MP, AP, Tamil Nadu, northeastern states

How it forms: Develops from crystalline igneous rocks (granite, gneiss) under hot, dry conditions with low rainfall.

Characteristics:

• Red colour due to presence of iron oxide (ferric oxide, Fe₂O₃) — this is the reason red soils are red. [NDA tested via CDS]
• When hydrated (wet), iron oxide gives a yellow colour — hence “Red and Yellow”
• Porous and has a friable (crumbly) structure [NDA tested via CDS]
• Rich in: iron
• Deficient in: nitrogen, phosphorus, humus, lime, calcium, potash
• Statement “Red soils are rich in lime, humus and potash” is WRONG. [NDA tested via CDS]

Crops: Millets, pulses, groundnut, fruits and vegetables (not very good for demanding crops without fertiliser)

4. Laterite Soil

Where: Western Ghats, northeastern states (Assam, Meghalaya), parts of Odisha, West Bengal, Tamil Nadu

How it forms: Formed by laterisation — intense leaching in areas with high temperature and high rainfall. Heavy rain washes away silica and soluble minerals. Iron and aluminium oxides remain. [NDA 2016-II]

The word “laterite” comes from the Latin word “later” meaning brick. When laterite dries, it hardens like a brick. It was historically used as a building material.

Characteristics:

• Rich in iron and aluminium oxides
• Strongly acidic in nature
• Deficient in: nitrogen, potash, lime, organic matter
• Maximum leaching of all soil types — nutrients are washed away by heavy rain [NDA tested via CDS]
• Generally red in colour [NDA tested via CDS]
• Statement “Laterite soils are generally fertile” is FALSE — because of leaching, they lose fertility. [NDA tested via CDS]
• Statement “Laterite soils are rich in nitrogen and potash” is WRONG. [NDA tested via CDS]
• Laterite soils are NOT well developed in Rajasthan and UP — they are found in the Western Ghats and northeastern India. [NDA tested via CDS]

What is found in appreciable amounts in laterite soil? None of calcium, phosphate, potash, or nitrogen — all are leached away. The only thing found in appreciable amounts is iron and aluminium. Among NDA 2024-II options, the answer would be none of the listed minerals (trick question). [NDA 2024-II]

Crops: Tea, coffee, rubber, cashew, coconut, tapioca — crops that can grow in acidic, poor soil. [NDA tested via CDS]

Laterite soils develop in areas with high temperature AND high rainfall — Statement I is TRUE. Laterite soils are the result of intense leaching — Statement II is TRUE and it is the correct explanation. [NDA 2016-II]

5. Arid and Desert Soil

Where: Western Rajasthan, parts of Gujarat, southern Punjab and Haryana

Characteristics:

• Sandy texture; low moisture
• Saline in some areas — salt patches visible on surface
• High evaporation leads to accumulation of salt near the surface
• Low organic matter; poor fertility
• Red to brown colour
• Large accumulation of carbonates at depth
• Plants are scarce — mainly desert shrubs
• Aridisol (soil order): Lack of water for most of the year; absence of deep wide cracks; large accumulation of carbonates at depth. NOT high in organic matter. [NDA tested via CDS]

6. Saline and Alkaline Soil

Where: Over-irrigated areas (Punjab, Haryana, UP), coastal areas, arid and semi-arid regions, waterlogged and swampy areas

Local names: These soils are known by different names in different regions:

• Usar — in UP
• Reh — in Punjab and Haryana
• Kallar — in other areas

Characteristics:

• High salt content (sodium, calcium, magnesium)
• pH is more than 7 — alkaline in nature
• Soils having very high content of sodium and calcium with pH more than 7 are alkaline soils — TRUE. [NDA 2024-II]
• Not rich in nitrogen — saline soils are NOT rich in nitrogen. [NDA tested via CDS]
• Occur in arid and semi-arid regions, and also in waterlogged and swampy areas [NDA tested via CDS]
• Formed by over-irrigation — irrigation water brings salts; water evaporates; salts stay
• Treatment: Adding gypsum reduces alkalinity. Adding lime reduces acidity.

Application of lime makes soil less acidic (increases pH). Statement “lime makes soil acidic” is WRONG. [NDA 2015-II | NDA tested via CDS]

High acidity in soil is typical of humid climate — TRUE. [NDA 2015-II]

Increasing soil acidity results in declining soil fertility — TRUE. [NDA 2015-II]

Arid climate is characterised by alkaline soil — TRUE. [NDA 2015-II]

7. Forest and Mountain Soil

Where: Himalayan slopes, northeastern hills, Western Ghats forests

Characteristics:

• Rich in organic matter in forested areas
• Acidic due to leaf litter decomposition
• Thin layer in high mountains; thicker in valley floors
• Changes with altitude — different zones on mountain slopes

Crops: Apple orchards (Himachal Pradesh, J&K), tea (Darjeeling, Assam), coffee

---

8. Peaty and Marshy Soil

Where: Kerala backwaters, coastal areas of Odisha, Sundarbans

Characteristics:

• Very high organic matter content
• Formed in waterlogged, marshy areas
• Acidic in nature
• Histosol (USDA soil order) = very high organic matter in upper layers — this corresponds to peaty soil. [NDA tested via CDS]

Modern Classification of Soils (USDA Soil Orders)

NDA and CDS have started testing modern soil classification. Important soil orders to know:

Soil Order	Characteristic
Entisols	Weakly or quickly developed horizons; young soils with very little profile development
Inceptisols	Weakly developed soils with some horizons but not fully developed
Vertisols	Rich in clay content; highly basic; self-ploughing (cracks when dry) = corresponds to Black soil
Histosols	Very high organic matter in upper layers = corresponds to Peaty/Marshy soil
Oxisols	Very old and highly weathered soils = found in tropical regions
Aridisols	Dry conditions; lack of water most of year; large carbonate accumulation at depth; NOT high in organic matter

Largest geographical area of India is covered by Inceptisols (alluvial soils of the Northern Plains fall under this category in modern classification). However, the PYQ answer (CDS 2020-II) says Inceptisols cover the largest area. [NDA tested via CDS]

Vertisols = rich in clay, highly basic, self-ploughing = Black cotton soil in modern classification.

Histosols = very high organic matter = Peaty soil. [NDA tested via CDS]

Soil characterised by very high content of organic matter = Histosol. [NDA tested via CDS]

Soil Erosion

Soil erosion is the removal of topsoil by water, wind, or human activity. It is one of India’s most serious environmental problems.

Types of soil erosion:

Gully erosion: Running water cuts deep channels (gullies) into the soil. The Chambal Valley in Madhya Pradesh is India’s most famous example of gully erosion — called badlands or ravines. This is the valley under the influence of intensive gully erosion. [NDA tested via CDS]

Sheet erosion: A thin layer of soil is removed uniformly over a large area by flowing water. Less visible but very damaging.

Wind erosion: Strong winds blow away topsoil in dry areas. Most severe in Rajasthan.

Rill erosion: Small channels (rills) form on slopes due to surface runoff.

Soil impoverishment = soil getting very deficient in plant nutrients. It is different from soil erosion. [NDA tested via CDS]

Soil Conservation Methods

Mulching:
Covering the soil with a layer of dry leaves, straw, or plastic sheeting. Mulching is effective because it: [NDA 2014-I]

• Protects soil from sheet wash and wind erosion
• Helps soil to retain moisture and nutrients
• Note: Mulching does NOT protect from gully erosion — it is a surface cover technique

Contour ploughing: Ploughing along the contours of a hill (horizontally) — prevents runoff from flowing straight downhill and carrying soil with it. Suitable for hilly regions.

Terrace farming: Cutting steps into hillsides to create flat planting surfaces. Reduces runoff and soil loss.

Afforestation: Planting trees stabilises soil. Tree roots bind soil and reduce erosion.

Shelter belts: Rows of trees planted to break the force of wind — reduces wind erosion in dry areas.

Check dams / Bunding: Small structures across streams or field boundaries to slow water flow and trap sediment.

Watershed management: Comprehensive approach to managing an entire drainage basin — the main aim is soil conservation. [NDA tested via CDS]

Lime application: Adding lime to acidic soils reduces acidity and improves fertility. Lime does NOT increase acidity — it decreases it. [NDA 2015-II | NDA tested via CDS]

Methods suitable for soil conservation in hilly regions: Terracing, contour bunding, and contour ploughing are suitable. Shifting cultivation is NOT a soil conservation method — it degrades soil. [NDA tested via CDS]

Key Summary of Soil Nutrients

Soil Type	Rich in	Deficient in
Alluvial	Potash, phosphoric acid, lime	Nitrogen
Black (Regur)	Calcium carbonate, magnesium, iron, lime, alumina	Nitrogen, phosphorus, organic matter
Red and Yellow	Iron	Nitrogen, phosphorus, humus, lime, potash
Laterite	Iron, aluminium oxides	Nitrogen, potash, lime, organic matter
Saline/Alkaline	Sodium, calcium (salts)	Nitrogen