Soil salinisation and alkalisation

Also called: Salinisation, Alkalisation, Soil salinisation, Salinity and alkalinity of soil · Topic: Soils of India: Types, Degradation and Conservation · NCERT: Class 10, Ch 1 "Resources and Development"; Class 10, Ch 3 "Water Resources"; Class 12, Ch 3 "Land Resources and Agriculture"; Class 12, Ch 6 "Planning and Sustainable Development in Indian Context"; Class 12, Ch 9 "Geographical Perspective on Selected Issues and Problems"

Meaning

Soil salinisation is the build-up of salts in the topsoil. Alkalisation is the build-up of sodium carbonate, which makes the soil alkaline (raises its pH). In north-west India both usually follow waterlogging caused by over-irrigation and seepage from canals. They matter because they turn fertile, irrigated farmland into white, crusted, barren land. They are a hidden cost of the Green Revolution in Punjab, Haryana and western Uttar Pradesh.

Explanation

How it forms: cause → process → result

• Cause: the water table rises
• The water table is the top level of the groundwater.
• Over-irrigation: farmers give fields more water than crops need. The extra water sinks down and adds to the groundwater.
• Seepage from unlined canals: the canal bed is bare earth, not lined with concrete or brick. Water leaks out and raises the water table on both sides of the canal.

• Year after year, the water table rises into the root zone (the top part of the soil, where plant roots grow). This is waterlogging.

• Process: capillary rise and evaporation 1. The water table comes close to the surface. 2. Groundwater climbs up through tiny pores in the soil, the way oil climbs up a lamp wick. This is called capillary action. 3. In the hot, dry climate of north-west India, this water evaporates from the surface. 4. The salts dissolved in the water stay behind and form a white crust on the soil. 5. If the salt is mostly sodium carbonate, the soil also becomes alkaline.

• Result: the soil turns salty or alkaline, and crop yields fall. Badly affected land is often left fallow (unused).

Two types: saline and alkaline (sodic) soil

• Saline soil: many kinds of salts collect in the topsoil and leave a white crust on the surface.
• Alkaline soil:
• Sodium carbonate collects in the soil, and the soil's pH (a scale of how acidic or alkaline something is) goes up.
• It is also called sodic soil.

• It turns hard and sticky, and water cannot pass through it.

• Local names for these salty, barren soils: reh, kallar and usar. Usar usually refers to alkaline land.

Conditions that favour it

• A hot, dry climate: strong evaporation leaves salts behind at the surface.
• Poor drainage:
• flat land
• heavy soils that drain slowly

• hard layers under the soil that stop water from sinking further

• Heavy irrigation from canals and tube-wells, for example in the rice–wheat cropping system.

Remedies

• Stop the water table from rising:
• Canal lining: the canal bed and sides are covered with concrete or brick, so less water leaks out.

• Drip irrigation: water is given drop by drop at the roots. Less extra water sinks into the ground.

• Remove the salty water: sub-surface drainage (SSD)

• Perforated pipes (pipes with small holes) are laid underground. They collect the extra salty water and carry it away from the field.
• In waterlogged and saline areas of Haryana, SSD brought fallow land back under farming. Yields and farm incomes rose, and farmers moved to more profitable crops [3].

• SSD works only when there is a safe way to dispose of the salty drainage water and enough fresh water to wash salts out of the soil [3].

• Repair alkaline (usar) soil with gypsum (calcium sulphate):

• The calcium in gypsum takes the place of sodium on the soil particles.
• Irrigation water then washes the freed sodium down and out of the root zone.

• This is the main way to reclaim alkaline soil.

• Salt-tolerant crop varieties: crops bred to grow even in somewhat salty soil.

Where it occurs

• Indira Gandhi Canal command, western Rajasthan (Thar Desert):
• A command area is the land that a canal or dam project can irrigate.
• The canal starts at the Harike Barrage in Punjab, where the Sutlej and Beas meet.

• Desert soil sits on hard layers, so the extra water cannot drain away and the water table rises.

• Punjab–Haryana plains (north-west alluvial plain): heavy canal and tube-well irrigation for the rice–wheat system. Over-irrigation is the main cause of land degradation in Punjab, Haryana and western Uttar Pradesh.

• Sharda Sahayak command, central and eastern Uttar Pradesh: this canal system takes water from the Sharda river. Canal seepage and flat, poorly drained land raise the water table.
• Chambal command: the Kota region of Rajasthan and the Chambal area of Madhya Pradesh. Heavy black and alluvial soils drain slowly, so irrigation water collects in them.
• World: the same canal-irrigation problem affects the Indus basin of Pakistan and the Aral Sea basin of Central Asia.

Don't confuse with

• Waterlogging: the water table rises into the root zone. Waterlogging is the earlier stage and cause. Salinisation is the next stage, when capillary rise and evaporation leave salts behind.
• Salinisation vs alkalisation: salinisation is a build-up of salts in general. Alkalisation is specifically a build-up of sodium carbonate, which raises the pH and makes sodic (usar) soil. Gypsum is the remedy for alkaline soil.
• Naturally saline soils: some soils are salty because of dry climate or sea water, for example in the Rann of Kachchh. Salinisation in Punjab–Haryana is human-induced, caused by over-irrigation and canal seepage.
• Desertification: this is the wider land degradation of drylands (arid, semi-arid and dry sub-humid areas). It does not mean sand dunes moving forward. Salinisation is one way land can degrade, and it can happen even on well-watered farmland.

Prelims Hooks

• Correct order: over-irrigation or canal seepage → waterlogging → capillary rise → evaporation → salinisation. When the salt is mostly sodium carbonate, the result is alkalisation.
• Remedy pairs: gypsum for alkaline (usar) soil; sub-surface drainage for waterlogged saline land, shown to work in Haryana [3]; canal lining to stop seepage.
• Local names: reh, kallar and usar all mean salty or alkaline barren soil.
• Indira Gandhi Canal: starts at the Harike Barrage (Sutlej–Beas confluence, Punjab) and causes waterlogging and salinity in parts of western Rajasthan.
• Trap: the main cause of land degradation in Punjab is over-irrigation, not mining or overgrazing. Mining is the main cause in Jharkhand, Chhattisgarh, Madhya Pradesh and Odisha.
• Command area–state matches: Sharda Sahayak → Uttar Pradesh; Chambal → Kota (Rajasthan) and Madhya Pradesh.

Mains Points

• Green Revolution cost (GS-III):
• Cheap or free power and water, unlined canals and the rice–wheat cropping system in Punjab–Haryana raised the water table.

• This led to waterlogging, then salinity and alkalinity, and fertile land went out of use.

• A layered remedy package:

• Prevention: canal lining, drip and sprinkler irrigation, crop diversification and water-pricing reform.
• Cure: sub-surface drainage, proven in Haryana but dependent on safe disposal of salty water and enough fresh water [3], and gypsum for alkaline soil.

• Adaptation: salt-tolerant crop varieties.

• Regional and policy link: irrigation-induced degradation is concentrated in the north-west plains, so the answer there is water management, not mine reclamation or grazing control. Reclaiming salt-affected land supports India's UNCCD pledge to restore 26 million ha by 2030 [2] and the goal of land degradation neutrality (SDG target 15.3).

Related concepts

• Soil degradation
• Waterlogging
• UN Convention to Combat Desertification

Read more

• Land degradation: waterlogging, salinity and desertification