Alluvial soil

Topic: Soils of India: Types, Degradation and Conservation · NCERT: Class 7, Ch 1 "The Story of Indian Farming"; Class 10, Ch 1 "Resources and Development"

Meaning

Alluvial soil is soil made from alluvium: the sand, silt and clay that rivers carry and then drop on flat plains and in deltas. It is the most widespread and most important soil of India. It covers the whole Northern Plains and the east-coast deltas, and it feeds India's most crowded farming regions.

Explanation

How it forms: cause → process → result

• A river slows down on flat land.
• It loses the energy it needs to carry its load.
• The load settles layer by layer on the plain and in the delta.

• Over thousands of years these layers become deep, soft soil.

• Grain size changes from the hills to the plains.

• A river leaves the Himalaya at the break of slope (the point where steep hills suddenly meet flat land).
• It slows sharply and drops its heaviest load first: boulders, pebbles and coarse sand.

• Lighter silt and clay travel further. They settle far inland and in the deltas.

• Result: the soil is coarse in the piedmont plains (plains at the foot of mountains) and finer further inland.

• The piedmont belts named in NCERT are the duars (foothill plains of north Bengal and Assam), the chos (seasonal streams and their gravelly fans at the foot of the Shiwaliks in Punjab) and the terai (marshy foothill belt).

Types by age: bhangar and khadar

Feature	Bhangar (old alluvium)	Khadar (new alluvium)
Position	Higher, terraced land above flood level	Low floodplains next to the river
Renewal	Not flooded, so it gets no fresh silt	Renewed by floods every year
Texture	Coarser, with kankar nodules	Finer particles
Fertility	Less fertile	More fertile

• Bhangar is the slightly raised, terraced land made of older alluvium [3][4].
• Khadar is the more fertile, fresh alluvium on the low floodplain [3][4].
• The share of bhangar grows as you move upstream [3][4].
• Upstream, the river cuts deep into old terraces, so most of the land is bhangar.

• Downstream, the floodplains are wider, so khadar covers more ground.

• Kankar (small lumps of calcium carbonate, or lime) forms like this:

• Rainwater soaks down and dissolves lime from the upper soil.
• The water does not drain fully away. It dries out at some depth.

• The lime comes out of the water and builds up into hard nodules.

• Order from the mountains to the river: Shiwalik → bhabar (a belt of pebbles about 8–16 km wide where streams sink underground) → terai (marshy belt where the streams come back to the surface) → bhangar → khadar.

Properties and what they mean for farming

• Rich in: potash, phosphoric acid and lime.
• Poor in: nitrogen and humus (rotted plant and animal matter), so farms need fertiliser.
• Example: a rice–wheat farm in Nalanda district, Bihar, was tested under the Soil Health Card scheme. Its soil lacked organic carbon, nitrogen, phosphate and boron and had high electrical conductivity (a sign of salt build-up) [2].

• The farmer added compost and cow-dung manure along with the advised DAP and urea doses. The yield rose to 32 quintals/ha, against 27.5 quintals/ha on the farmer's usual field. That is over 16% more [2].

• In drier areas the alluvium is more alkaline. It needs treatment and irrigation before it gives good crops.

• Low rain does not wash the salts out.
• Water rises through the soil and evaporates, leaving the salts on top.

• The result is white salt crusts called reh or kallar. Gypsum and good drainage are used to treat them.

• Crops: sugarcane, paddy (rice), wheat, other cereals and pulses.

• Fertile soil, flat land and easy canal and well irrigation lead to intensive cultivation (farming the same land heavily, often with 2–3 crops a year). This is why these regions are among the most densely populated parts of India.

Where it occurs

• Northern Plains (the whole belt): laid down by the Indus, Ganga and Brahmaputra and their tributaries. It covers Punjab, Haryana, Uttar Pradesh, Bihar, West Bengal and Assam.
• Indus: rises in Tibet near Lake Mansarovar and flows to the Arabian Sea. In India its plain is the Punjab–Haryana tract.
• Ganga: rises at the Gangotri glacier in Uttarakhand and flows into the Bay of Bengal. With tributaries such as the Yamuna, Ghaghara, Gandak and Kosi, it builds the largest part of the plain.

• Brahmaputra: rises in Tibet as the Tsangpo, enters India in Arunachal Pradesh and builds the Assam valley. It joins the Ganga to form the Ganga–Brahmaputra delta, the world's largest delta.

• Western corridor: a narrow strip that runs from the Indus–Ganga plain through north Rajasthan into the plains of Gujarat.

• East-coast deltas (north to south), all draining into the Bay of Bengal:
• Mahanadi: rises in Chhattisgarh; its delta is in Odisha.
• Godavari: rises at Trimbakeshwar, Nashik (Maharashtra); its delta is in Andhra Pradesh.
• Krishna: rises near Mahabaleshwar (Maharashtra); its delta is in Andhra Pradesh.

• Kaveri: rises at Talakaveri, Kodagu (Karnataka); its delta is in Tamil Nadu (Thanjavur area).

• Salt-affected alluvium (reh/kallar): drier parts of Uttar Pradesh, Haryana and Punjab.

• Soil testing: the Soil Health Card scheme was launched on 19 February 2015 at Suratgarh, Rajasthan [2]. It takes samples from 15–20 cm depth on a 2.5 ha grid in irrigated areas and a 10 ha grid in rain-fed areas [2]. Most alluvial plains are irrigated, so they get the finer 2.5 ha grid.

Don't confuse with

• Khadar vs bhangar: khadar is new alluvium on the low floodplain, renewed every year and more fertile. Bhangar is old alluvium on terraces above flood level. It has kankar and is less fertile.
• Bhabar vs terai: in the bhabar the streams sink underground through pebbles. In the terai they come back to the surface and make the land marshy. Neither belt is the fine, fertile alluvium of the plain.
• Kankar in alluvial soil vs in arid soil: in alluvial soil, kankar marks bhangar. In arid soil it lies in the lower layers and stops water from soaking down.
• Alluvial soil vs black soil: alluvial soil is carried and dropped by rivers (it is transported). Black soil forms in place from the rock below it. Alluvial soil is poor in nitrogen and humus but rich in potash.

Prelims Hooks

• Alluvial soil is India's most widespread soil group and the first of ICAR's 8 soil groups (ICAR is the Indian Council of Agricultural Research).
• Trap: questions often swap the two. Khadar = new, floodplain, finer, more fertile. Bhangar = old, terraced, kankar, less fertile.
• Moving upstream, the share of bhangar compared with khadar increases [3][4].
• Alluvial soil is rich in potash, phosphoric acid and lime and poor in nitrogen and humus.
• East-coast alluvial deltas from north to south: Mahanadi → Godavari → Krishna → Kaveri.
• Soil Health Card scheme: launched 19 February 2015 at Suratgarh (Rajasthan). It tests 12 parameters (N, P, K, S, Zn, Fe, Cu, Mn, B, pH, EC, OC). Since 2022–23 it is part of RKVY (Rashtriya Krishi Vikas Yojana) as "Soil Health and Fertility" [2].

Mains Points

• Why the plains are crowded (GS-I):
• Himalayan rivers keep bringing fresh silt, so the Indus–Ganga–Brahmaputra plain has deep, well-sorted alluvium.
• Grain size goes from coarse (bhabar, terai) to fine (khadar, deltas).

• Fine soil, flat land and easy irrigation make farming and population most intense in the middle and lower Ganga plain and the east-coast deltas.

• Hidden fertility problem (GS-III):

• Rice–wheat systems that run on heavy chemical fertiliser are using up organic carbon and micronutrients, as the Nalanda soil test shows [2].
• The policy answer is soil-test-based balanced and integrated nutrient management through Soil Health Cards. Over 25 crore cards were given out by July 2025, and there were 8,272 soil-testing labs by February 2025 [2].

• Salt build-up (reh/kallar) in the irrigated plains of Punjab, Haryana and western UP lowers yields. The fixes are gypsum, better drainage and careful canal irrigation.

• Floods vs soil renewal (GS-I/GS-III):

• The same yearly floods that renew khadar also wreck lives and property, for example on the Kosi and in Assam.
• Embankments that stop floods also stop the fresh silt. So flood control has to be weighed against keeping the soil fertile.

Related concepts

• Kankar

Read more

• Alluvial soils: khadar and bhangar