Soils of India – NDA Geography Notes

Soil is not just dirt. It is a living, complex mixture of minerals, water, air, and organic matter, 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.

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What is Soil Formation?

Soil forms when rocks break down over thousands of years. The factors that control what kind of soil forms are: parent material (the original rock), climate (temperature and rainfall determine weathering rate), relief/topography (slope affects erosion and drainage), vegetation (plants add organic matter; roots break rocks), and time (older soils are more developed).

Human habitation is NOT a soil-forming factor. Humans use and modify 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.

Water availability in soil: The water most readily available to plant roots is capillary water, water held in tiny soil pores by surface tension. Gravitational water drains away too fast. Hygroscopic water is held too tightly to be absorbed by roots.

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Classification of Soils in India

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

Alluvial Soil | Black Soil (Regur) | Red and Yellow Soil | Laterite Soil | Arid and Desert Soil | Saline and Alkaline Soil | Forest and Mountain Soil | Peaty and Marshy Soil

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Soil-Forming Processes

Understanding these four processes is essential. NDA asks them directly.

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]

Calcification occurs in areas where evapotranspiration exceeds precipitation: more water evaporates than falls as rain. In dry conditions, water moves upward in the soil and 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, creating acidic, nutrient-poor soils called podzols. [NDA 2023-I]

Gleisation occurs in waterlogged conditions where oxygen is absent, creating grey or bluish soils.

Process	Climate Condition	Dominant Region	Result
Laterisation	High temperature + heavy rainfall	Tropical humid	Iron/aluminium-rich laterite
Calcification	Evapotranspiration > rainfall	Dry / semi-arid	Calcium carbonate deposits near surface
Podsolisation	Cold + humid (Taiga)	Boreal forest zone	Acidic, nutrient-poor podzols
Gleisation	Waterlogged, no oxygen	Marshy / swampy	Grey or bluish soils

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The 8 Major Soil Types of India

Alluvial Soil

Alluvial soil is the most widespread soil in India, covering the largest area. It is deposited by rivers. The Ganga, Yamuna, Brahmaputra, and Peninsular rivers carry fine particles of sand, silt, and clay and deposit them in the plains and coastal deltas.

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

Alluvial soil comes in two types. Khadar is new alluvium, found in active floodplains, renewed every year by floods, and more fertile. Bhangar is old alluvium. It sits higher, contains Kankar (calcium carbonate nodules), and is less fertile.

Nutrients: Rich in potash and phosphoric acid. Deficient in nitrogen. The statement “alluvial soils are rich in nitrogen” is wrong.

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

Black Soil (Regur Soil)

Regur is the term used for Black Cotton Soil. [NDA 2015-II] It develops on Deccan basaltic lava under hot and humid conditions. The lava of the Deccan Plateau weathered over millions of years to form this rich, dark soil. [NDA 2024-I | NDA 2024-II]

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

Characteristics: The dark/black colour comes from titaniferous magnetite and humus. Black soil is highly moisture-retentive. It holds water well and stays moist long after rain stops. It swells when wet and cracks when dry, a behaviour called self-ploughing or self-tilling. It is sticky when wet, making it very difficult to plough during rains. It is clayey, deep, and impermeable.

Nutrients: Rich in calcium carbonate, magnesium, iron, lime, and alumina. Deficient in nitrogen, phosphorus, and organic matter.

Best crop: Cotton; hence the name Black Cotton Soil. [NDA 2008-I | NDA 2023-II]

Red and Yellow Soil

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

Red and yellow soil develops from crystalline igneous rocks (granite, gneiss) under hot, dry conditions with low rainfall. The red colour comes from the presence of ferric oxide (iron oxide, Fe₂O₃), not from humus. When hydrated (wet), iron oxide turns yellow. Hence the name “Red and Yellow.”

Characteristics: Porous and friable (crumbly) structure. Rich in iron. Deficient in nitrogen, phosphorus, humus, lime, calcium, and potash. The statement “red soils are rich in lime, humus and potash” is wrong.

Crops: Millets, pulses, groundnut, fruits and vegetables.

Laterite Soil

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

Laterite soil is 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 and was historically used as a building material.

Characteristics: Rich in iron and aluminium oxides. Strongly acidic. Deficient in nitrogen, potash, lime, and organic matter. Laterite soils have the maximum leaching of all soil types. Nutrients are washed away by heavy rain. Generally red in colour.

Laterite soils are generally NOT fertile. Because of leaching, they lose most plant nutrients. They are NOT found in Rajasthan and UP. They occur in the Western Ghats and northeastern India. None of the mineral’s calcium, phosphate, potash, or nitrogen is found in appreciable amounts. All are leached away. [NDA 2024-II]

Crops: Tea, coffee, rubber, cashew, coconut, tapioca: crops that tolerate acidic, nutrient-poor soil.

Arid and Desert Soil

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

Sandy texture, low moisture. Saline in some areas, as high evaporation leads to salt accumulation near the surface. Low organic matter and poor fertility. Red to brown colour. Large accumulation of carbonates at depth. Corresponds to Aridisol in USDA classification, characterised by lack of water for most of the year, absence of deep wide cracks, and large carbonate accumulation at depth. It is not high in organic matter.

Saline and Alkaline Soil

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

These soils are known by local names such as Usar (UP), Reh (Punjab and Haryana), and Kallar (other areas). They have high salt content (sodium, calcium, magnesium) and a pH greater than 7, alkaline in nature. Soils with very high sodium and calcium content and a pH above 7 are alkaline soils. TRUE. [NDA 2024-II]

Formed by over-irrigation: irrigation water brings salts, water evaporates, and salts accumulate. It is not rich in nitrogen. They occur in both arid regions and waterlogged areas.

Treatment: Adding gypsum reduces alkalinity. Adding lime reduces acidity (NOT increases it).

Application of lime makes soil less acidic (raises pH). The statement “lime makes soil acidic” is wrong. High acidity in soil is typical of humid climate. TRUE. Increasing soil acidity results in declining soil fertility. TRUE. Arid climate is characterised by alkaline soil. TRUE. [NDA 2015-II]

Forest and Mountain Soil

Where: Himalayan slopes, northeastern hills, Western Ghats forests.

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 soil zones develop on mountain slopes.

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

Peaty and Marshy Soil

Where: Kerala backwaters, coastal areas of Odisha, Sundarbans.

Very high organic matter content. Formed in waterlogged, marshy areas. Acidic in nature. Corresponds to Histosol in USDA classification, with very high organic matter in upper layers.

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Modern Classification of Soils (USDA Soil Orders)

NDA has begun testing the modern USDA soil classification. The key soil orders to know:

Soil Order	Characteristic	Indian Equivalent
Entisols	Weakly or quickly developed horizons; very little profile development	Young soils
Inceptisols	Weakly developed; some horizons but not fully formed	Alluvial soils (cover largest area in India)
Vertisols	Rich in clay; highly basic; self-ploughing (cracks when dry)	Black soil (Regur)
Histosols	Very high organic matter in upper layers	Peaty and marshy soil
Oxisols	Very old and highly weathered	Tropical lateritic soils
Aridisols	Dry; lack of water most of year; large carbonate accumulation at depth; NOT high in organic matter	Desert / arid soils

Inceptisols: It cover the largest geographical area of India. Alluvial soils of the Northern Plains fall under this category in modern classification.

Vertisols: self-ploughing = Black soil.

Histosols: very high organic matter = Peaty soil.

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Soil Erosion

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

Gully erosion occurs when 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; it is a region under intensive gully erosion.

Sheet erosion removes a thin layer of soil uniformly over a large area as flowing water moves across the surface. It is less visible but very damaging.

Wind erosion blows away topsoil in dry areas. It is most severe in Rajasthan.

Rill erosion forms small channels on slopes due to surface runoff.

Soil impoverishment, or the soil becoming very deficient in plant nutrients, differs from soil erosion. Erosion removes soil physically; impoverishment degrades its chemical quality.

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Soil Conservation Methods

Mulching means covering the soil with a layer of dry leaves, straw, or plastic sheeting. Mulching protects soil from sheet wash and wind erosion, and helps soil retain moisture and nutrients. However, mulching does NOT protect from gully erosion. It is a surface cover technique that cannot stop deep, concentrated water flow. [NDA 2014-I]

Contour ploughing means ploughing along the contours of a hill (horizontally). It prevents runoff from flowing straight downhill and carrying soil. Suitable for hilly regions.

Terrace farming means cutting steps into hillsides to create flat planting surfaces. This reduces runoff and soil loss.

Afforestation, planting trees, stabilises soil through root binding and reduces erosion.

Shelter belts are rows of trees planted to break the force of wind. This reduces wind erosion in dry areas.

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

Watershed management is a comprehensive approach to managing an entire drainage basin. The main aim is soil conservation.

Lime application reduces acidity in acidic soils (raises pH). Adding gypsum treats alkaline/saline soils. [NDA 2015-II]

Shifting cultivation is NOT a soil conservation method. It degrades soil. For hilly regions, terracing, contour bunding, and contour ploughing are the correct conservation methods.

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