Alluvial Soil in India: Khadar and Bhangar Difference, Features and Map

Alluvial Soil: Meaning, Formation and Where It Is Found

What Is Alluvial Soil?

Alluvium also includes gravel, and rivers lay it down in stream beds, on floodplains and in alluvial fans. The word comes from the Latin alluere, to wash against. Alluvium is geologically young and not consolidated into rock, which is why these soils are called azonal, meaning younger, transported soils rather than soils weathered in place.

Alluvial soil is the most widely spread and important soil of India. The entire northern plains are made of it, and the Ganga plain and the Ganga-Brahmaputra delta are among the world's major concentrations of alluvial soils.

How Is Alluvial Soil Formed? The Himalayan Rivers and the Plains

Alluvial soils are formed by the deposition of sand, silt and clay in layers. In India most of this alluvium was laid down by three Himalayan river systems, the Indus, the Ganga and the Brahmaputra. Their plains run for about 3,200 km from east to west, 150 to 300 km wide, and the alluvium beneath them reaches a maximum depth of 1,000 to 2,000 m. Of the three, the Brahmaputra brings most of its silt from India: In dry Tibet it carries little, but in India it crosses a region of high rainfall and carries a large volume of water and silt.

Particle size changes along the way. The particles are bigger towards the upper river valleys, and near the break of slope at the foot of the mountains the soils are coarse, as in the piedmont plains of the Duars, Chos and Terai. From west to east across the Ganga plain the sand content falls, and the soils of the lower and middle Ganga plain and the Brahmaputra valley are more loamy and clayey.

The plain itself began as a trough. The Indo-Gangetic Plain was originally a geosynclinal depression that the three river systems filled with sediment, which is why its alluvium is so deep.

Where Is Alluvial Soil Found in India? Map and States

Alluvial soils are widespread in the northern plains and the river valleys. Through a narrow corridor in Rajasthan they extend into the plains of Gujarat, and in the Peninsula they are found in the deltas of the east coast and in the river valleys. The Brahmaputra valley in Assam is alluvial too.

The plains carry the landforms of a mature river, such as sand bars, meanders, ox-bow lakes and braided channels. The Brahmaputra plains are known for their riverine islands and sand bars, and much of this land is flooded periodically as the rivers shift course and braid. Apart from these river forms, the plain is featureless, 50 to 150 m above mean sea level.

Khadar and Bhangar: Difference Between New and Old Alluvium

Khadar and Bhangar Compared: Age, Position, Kankar and Fertility

By age, alluvial soil is divided into two. Khadar is the new alluvium, deposited by floods almost every year, which keeps enriching it with fine silt. Bhangar is the older alluvium, deposited away from the present floodplains and lying above them as a terrace that floodwater cannot reach.

Kankar is nodular calcium carbonate that forms in soils of semi-arid regions; it is used to make lime and roads. In Haryana's bhangar the nodules range from less than 1 cm to more than 5 cm across. Where they lie well below the root zone the land is called nardak, and where they form a pan close to the roots it is called chhachhra; both tracts are less fertile than the rest of the bhangar.

Bhabar, Terai, Bhangar and Khadar: Belts of the Northern Plains

From the Shiwalik foothills southwards, the northern plains fall into four belts. Bhabar and terai lie at the foot of the hills, and the plain of fine alluvium beyond them is made of bhangar, with khadar along its rivers:

1. Bhabar: A narrow belt of pebbles and boulders at the break of slope, where the streams disappear; it is 8 to 16 km wide (NCERT Class XI gives 8 to 10 km).
2. Terai: South of the bhabar, 10 to 20 km wide, where the streams re-emerge without proper channels and create marshy, swampy land; once thickly forested, it was cleared for farming and for migrants after Partition.
3. Bhangar: The largest part of the plain, older alluvium forming terraces above the floodplains.
4. Khadar: The younger floodplain deposits along the rivers, within the bhangar, renewed almost every year and ideal for intensive agriculture.

Soils change with the belts. The bhabar soil is sandy, highly porous and holds little moisture, while terai soils are rich in nitrogen and organic matter. The Ganga plains also have local alluvial variants, such as bhur, a sandy soil of the Ramganga tract, and bhat, a whitish calcareous soil of the lower Gandak valley.

Khadar and Bhangar in the Doabs of the Ganga Plain

The Himalayan rivers divide the plains into doabs, the tracts between two rivers. Across each doab the land steps up from the river to low khadar and then to higher bhangar. The khadar near the rivers is low-lying and usually very fertile; the bhangar away from them floods less but is also less fertile on average. The best known is the Ganga-Yamuna Doab, the flat alluvial tract between the two rivers from the Shiwalik Hills to their confluence at Prayagraj.

The difference mattered enough to be taxed. For centuries villages in the doabs were officially classed as khadar, mixed khadar-bhangar or bhangar, with different agricultural tax rates on a scale of land productivity.

Haryana shows how many local names one alluvial plain can carry. Its Ghaggar-Yamuna plain, below 300 m, is made up of bhangar, khadar, nali and bet. Within it the narrow, low-lying floodplains are the khadar of the Yamuna, the nali of the Ghaggar and the bet of the Markanda. On the Yamuna near Faridabad the khadar is generally 3 to 5 km wide; when floods recede they leave fine silt that is easy to plough, and the soil keeps its moisture after the rains. There the bhangar soils are loams and the khadar soils silty loams. The older bhangar is covered by Pleistocene deposits with kankar at varying depths.

Characteristics of Alluvial Soil and Its Crops

Characteristics of Alluvial Soil: Texture, Colour and Nutrients

Alluvial soil has these main characteristics:

• Texture: Varies from sandy loam to clay, in various proportions of sand, silt and clay.
• Colour: Light grey to ash grey; the shade depends on the depth of deposition, the texture and the time taken to mature.
• Nutrients: Most alluvial soils contain an adequate share of potash, phosphoric acid and lime.
• Profile: Young and layered; the horizons (the layers of a mature soil profile) are poorly developed.
• Alkalinity: Soils in drier areas are more alkaline, but can be productive after proper treatment and irrigation.

As a whole, alluvial soils are very fertile, and the khadar more so than the bhangar. Their youth is also their weakness: Being loose and close to the river, they flood and waterlog, so flood control, drainage or irrigation is usually needed to farm them well.

Other classifications use other names for the same soils. In the FAO's world classification they are Fluvisols, and in the USDA Soil Taxonomy Fluvents.

Alluvial Soil Crops and the Densely Populated Plains

Alluvial soils are ideal for sugarcane, paddy, wheat and other cereal and pulse crops. Because they are so fertile, the regions of alluvial soil are intensively cultivated and densely populated.

• Drier west: Wheat, barley and millet are the main crops of the western plain.
• Wetter east: Rice predominates in the wetter eastern parts of the plain.

The Indo-Gangetic Plain spans about 7,00,000 square km across northern South Asia and holds the world's largest expanse of uninterrupted alluvium. It is home to nearly one-seventh of the world's population and is one of the most densely populated and intensely farmed regions on Earth.

That fertility shaped history. The fertile terrain of the plain helped the rise and spread of empires such as the Mauryas, the Kushans and the Guptas, all of which had their demographic and political centres in the Indo-Gangetic Plain. The same alluvium was the base of the Green Revolution, and its canal-irrigated tracts now suffer from salinity.

Salinity in the Alluvial Plains: Irrigation, Gypsum and Deltas

Fertile alluvium can be spoiled. In areas of intensive cultivation with excessive irrigation, especially the Green Revolution areas, the fertile alluvial soils are becoming saline. Excessive irrigation under a dry climate promotes capillary action, which brings salt up and deposits it on the top layer of the soil.

• Where: Irrigated canal command tracts, especially in Punjab and Haryana, where salts form a white crust that is unfavourable for plant growth.
• Remedy: Farmers in such areas are advised to add gypsum, which treats the salt-affected soil.
• Deltas: Seawater intrusion makes saline soils common in the deltas of the east coast and the Sundarbans.

Difference Between Alluvial Soil and Black Soil

Black soil, the soil of the Deccan lava plateau, differs from alluvial soil first in origin. Alluvial soil is transported by rivers, while black soil forms in place from basalt.

The two also behave differently with water. Black soil is clayey and impermeable, swells when wet, cracks when dry and retains moisture for a long time, which helps rain-fed crops through the dry season. Alluvial soil depends on the river instead. The khadar is renewed by floods, and the plains are farmed with canal and tubewell irrigation.

Deltaic Alluvium: East Coast Deltas and West Coast Estuaries

Delta Soils of the East Coast

Alluvial soil is also found in the eastern coastal plains, particularly in the deltas of the Mahanadi, the Godavari, the Krishna and the Kaveri. The plain along the Bay of Bengal is wide and level, and these large rivers have formed extensive deltas on it. Where the Ganga and the Brahmaputra meet the sea they form one of the largest deltas in the world.

Delta soils are rich but exposed. The Sundarbans, on the seaward edge of the delta of the Ganga, the Brahmaputra and the Meghna, form the world's largest mangrove forest: It spreads over 10,277 square km across India and Bangladesh, about 4,260 square km of it in India, and is cut by tidal streams and channels. The thickly populated deltas of the Godavari, Krishna and Kaveri are often struck by the tropical cyclones of the east coast, and seawater intrusion can turn delta soils saline.

Why West Coast Rivers Form No Deltas

The west-flowing rivers of the Peninsula build no deltas, for two reasons of relief and structure:

1. A narrow coastal plain: The plain between the Western Ghats and the Arabian Sea is very narrow, so the coastal rivers are short.
2. Rift valleys: The Narmada and the Tapi flow in rift valleys, and rift valley rivers form estuaries rather than deltas; the Narmada meets the sea south of Bharuch in a broad estuary.

That does not mean these rivers carry no alluvium. Alluvial soils are found in the river valleys of the Peninsula, and the high banks of the lower Narmada are made of layers of old alluvial deposits, hardened mud, gravel of nodular limestone and sand.

Sources

• NCERT: Contemporary India I (Class IX), Physical Features of India
• NCERT: Contemporary India II (Class X), Resources and Development
• NCERT: India Physical Environment (Class XI), Structure and Physiography; Soils
• FAO: Lecture Notes on the Major Soils of the World, Fluvisols
• NATMO (National Atlas and Thematic Mapping Organisation): Soils
• Government of Haryana: Geography
• District Faridabad, Government of Haryana: About District
• Wikipedia: Alluvium
• Wikipedia: Indo-Gangetic Plain
• Wikipedia: Sundarbans
• Wikipedia: Doab
• Wikipedia: Kankar
• Wikipedia: Narmada River
• UPSC: Previous Year Question Papers, Civil Services Examination