Saline and Alkaline Soil of India

Start with the words. Three names appear again and again in books and in exam questions. Each name points to a different kind of trouble in the root zone.

1. What does "salt-affected soil" mean?

Salt-affected soil is an umbrella term. It covers soil with too many soluble salts, soil with too much sodium on its clay, or both.

Scientists test a paste made from the soil. Three numbers decide the class.

• ECe is the electrical conductivity of the water in the paste. It tells how salty the soil water is. It is measured in dS/m.
• pH tells how alkaline the paste is.
• ESP is the exchangeable sodium percentage. It tells what share of the clay’s holding spots is filled by sodium.

Using these numbers, soils are sorted into normal, saline, sodic and saline-sodic. The ICAR institute at Karnal adds a warning. No single set of limits is accepted in every country. So the limits you read depend on the book.

2. What is saline soil, in numbers?

Saline soil has so many neutral soluble salts that most crops suffer. The salts are mainly chlorides and sulphates of sodium, calcium and magnesium.

The accepted limits for India are simple.

• ECe is 4 dS/m or more, measured at 25 degrees Celsius.
• The pH of the saturated paste is below 8.2.
• ESP is below 15.

The salts keep the clay particles clumped together. So the soil keeps a stable structure. Water and air move through it much as they do in normal soil.

The damage comes from the salt itself. Plants find it hard to pull water out of salty soil water. Some saline soils also show a salt crust on the surface.

A simple way to remember this: saline means too much salt in the water of the soil. The problem sits in the soil solution. Washing the salt away with good water removes the problem.

3. What is sodic (alkali) soil, in numbers?

Sodic soil has too much sodium stuck on its clay. It has few neutral salts. Older books call it alkali soil.

The accepted limits are these.

• ESP is above 15.
• The pH of the saturated paste is above 8.2. In extreme cases it passes 10.5.
• ECe is usually below 4 dS/m.

Sodic soil often holds sodium carbonate. This salt makes the soil strongly alkaline. That is why the soil is called alkali soil.

In very sodic soil, dissolved organic matter rises with the water and dries on the surface. It leaves a dark patch. So such soils are also called black sodic soils.

Here the problem sits on the clay, not in the water. Sodium sticks to the clay particles. The clay swells and spreads apart when wet. When the soil dries, it sets hard. That is why simple washing does not help.

4. What is saline-sodic soil, and why do books show different limits?

Saline-sodic soil has both problems at once. In the older US Salinity Laboratory scheme, it has pH above 8.5, ECe above 4 dS/m and ESP above 15.

Here the sources do not agree on how many classes there are.

• In India, the CSSRI manual says only two classes are used. They are saline and sodic.
• In the USA, three classes are used. They are saline, sodic and saline-sodic.
• India uses a pH limit of 8.2 for sodic soil. The USA uses 8.5.

CSSRI chose 8.2 for a reason. In the sodic soils of the Indo-Gangetic Plain, pH 8.2 matches ESP 15 very closely. The match scores between 0.87 and 0.97 on a scale where 1 is perfect.

A 1987 scheme from the Soil Science Society of America uses ECe above 2 dS/m and a sodium adsorption ratio above 13. So two books can both be right. Each follows its own scheme.

Why does this matter for you? An exam may give a pH and an ESP and ask for the soil type. Check the ESP first. Then check the ECe. Then read the pH. Each number narrows the answer.

5. What are reh, usar and kallar?

These are old Indian names for salt-affected land. People used them long before soil labs existed.

• A slide set hosted by the FAO Global Soil Partnership says salt lands were called usara in Vedic times.
• It adds that medieval writings used usar, kallar, reh and other terms.
• J. W. Leather, an Imperial Chemist, led the first systematic study of the problem. In 1906 he advised gypsum with heavy manuring for reh.

The CSSRI manual links two of the names to one type. It says sodic soils are popularly known as usar or kallar soils.

So the three names are local words. They are not three different soil types. For exam work, treat them as names for saline and alkaline land.

Salt does not appear from nowhere. It starts in rocks and travels with water. Then something makes the salt stop in the topsoil. Several causes work together.

6. Where does the salt come from in the first place?

The main source of all soil salt is rock. Weathering slowly breaks rock and sets its salts free. Water then carries the salts away.

As water moves from wet regions to dry ones, the salts get more concentrated. That is why dry regions hold more salt.

The FAO bulletin makes a key point. Salt-affected soils rarely form from salt that was already in that spot. Most of the salt arrives from somewhere else, carried by water.

An Indian example shows the idea. At Hissar, an unirrigated soil had ECe of 1.4 dS/m in the top 10 cm. A layer 208 to 228 cm deep had 11.0 dS/m.

Deep salt does little harm. If it lies below about 150 cm, crops rarely suffer. Trouble starts when something lifts the salt up.

7. How does canal irrigation raise salt to the surface?

Before irrigation, a river basin has a balance. Rain, river flow, groundwater and evaporation all match. Canals break that balance.

Canal water seeps from unlined channels. Farmers also give more water than crops use. This extra water goes down and raises the water table.

The FAO bulletin says that after canals arrive, the water table has risen to under 2 m within 10 years in many places. Once the water table is within 1 to 2 m, it feeds evaporation from the surface.

Think of a sponge. Water climbs from the shallow water table. It evaporates at the top. It leaves its salt behind.

The FAO bulletin says a critical depth exists. It lies between 1.5 and 3.0 m, depending on soil and water quality. Above that depth, evaporation and salting rise sharply.

So the real culprit is not the canal water itself. Canal water is usually fresh. The trouble is the extra water that has nowhere to go. Without drainage, the field behaves like a bathtub with a blocked drain.

8. Why do dry regions get salty?

Dry regions have two things working together. Little rain falls, so little salt is washed away. And strong sun keeps evaporating water from the soil.

The CSSRI manual lists “lack of natural leaching” as a cause. It names the arid and semi-arid regions where the land shape also stops drainage.

The FAO bulletin adds a case. In a closed basin, evaporation can exceed rainfall. Then highly sodic soil can form if the incoming water has residual sodicity.

FAO also gives a pattern. Saline soils tend to dominate in arid and semi-arid regions. Sodic soils tend to dominate in semi-arid and sub-humid regions.

Arid soils are usually sandy. They are generally saline in nature. In some places the salt is so strong that common salt is taken by evaporating the water.

9. How does the sea make coastal soil salty?

Coastal soil gets salt from the sea in several ways. The CSSRI manual lists two of them as causes. They are ingress of sea water along the coast and salt-laden sand blown by sea winds.

The FAO bulletin lists tidal waves, underground aquifers and wind-borne salt spray.

The coastal research station at Canning Town adds two more.

• Cyclones push salty water over farmland.
• In river deltas, brackish groundwater lies close to the surface. The Sundarbans, the Mahanadi and the Godavari are named.

The manual says the coastal agro-ecosystem covers about 10.8 million ha. It lies along a coastline 7,517 km long.

Some mangrove soils also turn very acidic. FAO calls them acid-sulphate soils. Their pH falls below 3.5 to 4.0 when pyrite oxidises after drainage.

10. Why does the Indo-Gangetic Plain get sodic soil?

Sodic soil forms where water brings carbonate and bicarbonate. The CSSRI manual names three causes on the Indo-Gangetic Plain.

• Irrigation with groundwater rich in carbonate and bicarbonate.
• A rise in groundwater after canals arrive.
• Salty run-off from nearby land and undrained basins.

The manual also defines sodic water. Water with residual sodium carbonate above 2.5 and a sodium adsorption ratio above 10 is called sodic water.

The FAO bulletin adds a natural cause. In the wet season, water collects in low spots. In the dry season, it evaporates. Repeated over years, this raises sodium and pH.

Canals made it worse in places. The manual says waterlogging and secondary sodicity have grown in the Sarda Sahayak canal command of Uttar Pradesh. The first survey of such land was by Leather in 1914, at Etah in Uttar Pradesh.

Now put numbers on the map. The main source is the national table kept by ICAR-CSSRI. Other pages give slightly different totals, and the next items say why.

11. How much land is salt-affected in India?

The CSSRI table gives a total of 6,744,968 ha. That is about 6.74 million ha.

The table splits the land into three parts.

• Alkali (sodic) soils: 3,788,159 ha.
• Inland saline soils: 1,710,673 ha.
• Coastal saline soils: 1,246,136 ha.

The two saline parts add up to 2,956,809 ha. This is the “2.95 million ha of saline soil” quoted in the CSSRI manual. So that figure includes the coast.

The table covers 16 states and union territories. A CSSRI paper from 2018 says about 2.1 million ha of this land has been reclaimed.

12. Why do books give 6.73, 6.74, 2.8 and 23.8 million ha?

Many totals float around. The sources do not always agree. Here is each one.

• 6,744,968 ha is the total on the CSSRI web table. It includes 17,500 ha of alkali soil in Jammu and Kashmir.
• 6,727,468 ha, or 6.73 million ha, is the total in Table 2 of the CSSRI manual. It has no Jammu and Kashmir row. The gap is exactly 17,500 ha.
• ICAR also says 6.73 million ha but adds that 2.8 million ha is sodic. The CSSRI table gives 3.77 to 3.79 million ha for sodic soil.
• Earlier estimates ran from 7.0 to 26.1 million ha. The manual blames different methods and no countrywide survey.
• The FAO bulletin of 1988 gave 23.8 million ha for India. It used the FAO/Unesco Soil Map of the World.

For exams, use “about 6.7 million ha” and the CSSRI mapping.

13. Which states have the most salt-affected land?

Five states hold about three-quarters of the land. The CSSRI manual says “about 75%”. Here are the totals from the CSSRI table.

• Gujarat: 2,222,000 ha.
• Uttar Pradesh: 1,368,960 ha.
• Maharashtra: 606,759 ha.
• West Bengal: 441,272 ha.
• Rajasthan: 374,942 ha.

Other states follow. Tamil Nadu has 368,015 ha. Andhra Pradesh has 274,207 ha. Haryana has 232,556 ha. Bihar has 153,153 ha. Punjab has 151,717 ha.

One small mismatch needs a note. The manual’s text gives 2.23 million ha for Gujarat. Its own table, and the CSSRI web table, give 2,222,000 ha.

14. Where is sodic soil common, and where is saline soil common?

The two types sit in different places. A good memory hook is “UP is sodic, Gujarat is saline”.

An AICRP report of 2022 gives the shares for sodic (alkali) land.

• Uttar Pradesh: 1.35 million ha, which is 35.75% of all alkali land.
• Gujarat: 14.36%.
• Maharashtra: 11.21%.
• Tamil Nadu: 9.41%.
• Haryana: 4.86%.
• Punjab: 4.02%.

These six states hold about 80% of India’s sodic land.

For saline land, the same report says Gujarat has 1.68 million ha. That is 56.84% of all saline soil. West Bengal has 14.92%. Rajasthan has 6.61%. Maharashtra has 6.23%.

The Indo-Gangetic Plain alone holds 2,347,658 ha across five states. Uttar Pradesh is the largest part, at 58%.

15. What are the Rann of Kachchh and Sambhar Lake?

The Rann of Kachchh is a large area of saline mudflats. Britannica says it lies in west-central India and southern Pakistan.

It has two parts.

• The Great Rann covers about 18,000 sq km, almost all in Gujarat.
• The Little Rann covers about 5,100 sq km in Gujarat.

The Rann was once a shallow part of the Arabian Sea. A geological shift cut it off. Now it fills with water in the monsoon. In the dry season the water evaporates and leaves a bed of white salt.

An NCERT geography chapter on natural vegetation also names “the vast saline expanses of Rajasthan, Gujarat and the Gulf of Kachchh”.

Sambhar Lake lies in east-central Rajasthan. Britannica calls it an ephemeral salt lake of about 230 sq km. Salt sheets often cover its bed, and salt pans work on its shore.

16. Where is coastal saline soil found?

The CSSRI manual covers coastal and deltaic soils in eleven states and union territories. They sit in three kinds of land. These are the coastal plain, the deltaic plain and mud flats with mangrove swamps.

West Bengal has the most. It holds 441,272 ha, about 35% of the coastal total. Gujarat comes next, at about 33% in the manual’s text.

The sources do not agree on Gujarat’s coastal area. The manual’s text says 413,868 ha. The CSSRI web table says 462,315 ha. The sources do not explain the gap.

Other coastal shares in the manual are Orissa 12%, Andhra Pradesh 6% and the Andaman and Nicobar Islands 6%. The coastal total is 1,237,406 ha in the manual’s text and 1,246,136 ha in the web table.

The manual says soils on coastal plains, deltas and mud flats are commonly saline.

17. How do canals salt the black soils of the Deccan?

Black cotton soils are a clay type called Vertisols. They cover about 76 million ha of India. They are potentially fertile, but they drain poorly.

The CSSRI manual says new irrigation projects and unscientific use of water would further aggravate the problem.

The manual puts salt-affected Vertisols and linked soils at about 1.1 million ha. Maharashtra has 0.54 million ha. Karnataka has 0.29 million ha. Gujarat has 0.12 million ha.

Two examples come from the manual.

• In Karnataka, 45,527 ha in the Ghataprabha command and 17,153 ha in the Malaprabha command face waterlogging or salting.
• In Gujarat, the Bara tract in the Sardar Sarovar command shows early sodium build-up from careless canal use.

Both soil types cut yields, but in different ways. Knowing the way tells you why the cures differ. The numbers below come from field studies.

18. How does salt hurt a crop?

Salt makes water harder for roots to take up. The salt raises the osmotic pressure of the soil water. So a plant can stand in wet soil and still be short of water.

The CSSRI manual calls this “physiological drought”. Some salts also poison plants directly. Sodium, chloride and boron are examples.

The FAO bulletin says salt gives poor and spotty stands, and uneven, stunted growth. It also says no sharp limit exists. As salt rises, growth falls until plants die.

It gives these classes by ECe.

• 0 to 2 dS/m: non-saline. Effects are negligible.
• 2 to 4: slightly saline. Sensitive crops may lose yield.
• 4 to 8: moderately saline. Many crops lose yield.
• 8 to 16: strongly saline. Only tolerant crops do well.
• Above 16: very strongly saline. Only a few crops do well.

Cotton shows how this works. Its yield stays full up to 8 dS/m. At 10 dS/m it gives about 89%. At 27 dS/m it gives nothing.

19. How does sodium hurt a crop?

Sodium harms the soil structure first. The clay disperses and the soil clogs. Water and air move through it slowly.

The FAO bulletin lists three effects.

• A hard crust forms and stops seedlings from coming up.
• High pH locks up nutrients. Calcium and magnesium fall. The supply of nutrients such as phosphorus, iron, manganese and zinc is likely to be affected.
• Sodium, molybdenum and boron can build up to poisonous levels in plants.

In the field, a sodic soil may stay wet a few centimetres down while the top is dry and hard. After drying, cracks 1 to 2 cm wide open at the same spots each time.

Crops differ in how well they cope. At an ESP near 50, rice yield stays nearly unaffected while wheat almost fails. Rice and dhaincha are tolerant. Wheat and bajra are medium. Mash and lentil are sensitive.

Put simply, salt makes the plant thirsty. Sodium makes the soil shut its doors. One problem is in the plant’s water supply. The other is in the soil’s own structure.

20. How big are the losses, and what did one village show?

ICAR-CSSRI counts the loss in crop production: cereals, oilseeds, pulses and cash crops. Sodic land costs India about 11.18 million tonnes a year, worth about 15,000 crore rupees. Saline land costs 5.66 million tonnes, worth about 8,000 crore rupees. The saline value uses minimum support prices of 2015.

Together the two come to about 16.84 million tonnes. The CSSRI manual gives 16.84 million tonnes and 23,020 crore rupees. The AICRP report rounds the total to 17 million tonnes.

A village case shows the damage. At Santaraha in Hardoi, Uttar Pradesh, ICAR-CSSRI began a reclamation project in April 2011. Before it started, rice gave 2.95 t/ha on slightly sodic land and 1.22 t/ha on moderately sodic land. On sodic and severely sodic land, nothing grew.

After reclamation, rice averaged 4.42 t/ha on sodic land and 3.04 t/ha on severely sodic land. Normal soil gave 5.21 t/ha. Land that was once barren grew crops again.

The cure depends on the type. Saline soil needs the salt washed out. Sodic soil needs the sodium swapped for calcium first. Mixing the two cures up is the classic mistake.

21. How is saline soil reclaimed?

Saline soil is cured by leaching and drainage. Leaching means pouring good-quality water on the field so it carries the salt down below the roots.

The FAO bulletin ranks the methods.

• Leaching is by far the most effective.
• Scraping off the surface salt helps only for a short time. The salt still has to go somewhere.
• Flushing water over the surface removes little salt.

The salty water must leave the field. Subsurface drains can carry it away. Without drains, natural drainage must be good enough that the water table does not rise.

A handy rule says one unit depth of water removes about 80% of the salt from one unit depth of soil. So 30 cm of water clears about 80% of the salt in the top 30 cm.

Saline soil usually needs no amendment.

Notice the order. For saline soil, water comes first and drainage must follow. For sodic soil, gypsum comes first and water follows. Keep this order in mind.

22. How does gypsum fix sodic soil?

Sodic soil has sodium stuck on its clay. Leaching alone cannot wash it off. The sodium must first be replaced by calcium.

Gypsum is calcium sulphate. It supplies calcium. The calcium takes the sodium’s place. The sodium turns into sodium sulphate, which water can wash away.

The FAO bulletin says gypsum is the cheapest and most widely used amendment. It dissolves only a little in water, about one-fourth of 1 percent.

The CSSRI field steps are these.

• Level the land and raise strong bunds.
• Test the soil to find the gypsum requirement.
• Spread gypsum on the top layer. A thumb rule is 12 to 15 tonnes per hectare, which is half the requirement.
• Mix it into the top 10 cm of soil.
• Pond water on the field for 7 to 10 days.
• Start with rice as the first crop.

Gypsum is nearly absent in sodic soils. That is why the soil must be given gypsum before it can be washed.

Think of it as a swap. The soil holds sodium. Gypsum brings calcium. Calcium is held more tightly, so it pushes the sodium off the clay. Then irrigation water washes the sodium away.

23. What else can replace sodium, and what do green manures add?

Gypsum is not the only choice. The FAO bulletin groups amendments in three kinds.

• Soluble calcium salts, such as gypsum and calcium chloride.
• Acids or acid-forming materials, such as sulphuric acid, iron sulphate, sulphur and pyrite.
• Calcium salts with low solubility, such as ground limestone.

Limestone works mainly in soil with pH below about 7. Sodic soils have a pH above 8.2, so limestone is a poor fit.

Sulphur and pyrite must first be changed to sulphuric acid by soil microbes. So they work slowly. Calcium chloride is highly soluble and supplies calcium directly, but it costs a lot.

Organic matter helps too. Farmyard manure, straw and green manure help dissolve the calcium already in the soil. The process is slower than using gypsum. Dhaincha was tested as a green manure crop in an AICRP trial. Press mud, a sugar-mill waste, was used with gypsum at Santaraha.

24. What is subsurface drainage, and how much land has it cured?

Subsurface drainage is for waterlogged saline land. Plastic pipes are buried below the field. They carry away the salty water and lower the water table.

CSSRI says the cure takes 2 to 3 years. The soil ECe can fall from 10 to 52 dS/m down to 2 to 5 dS/m. Yields of paddy, wheat, cotton and sugarcane rise.

The cost is high. The manual gives about 122,000 to 132,500 rupees per hectare for alluvial soils in 2023. Disposal of the drained water is another problem.

Here the sources disagree on how much land has been cured.

• The manual’s first chapter says about 110,000 ha.
• The AICRP report of 2022 says about 66,500 ha.
• The manual’s fifth chapter says 75,335 ha up to June 2024.

None of the sources explains the gaps.

25. Which crops and trees can live on salty land?

Plants can help when the soil cannot yet be cured. CSSRI has bred salt-tolerant crops.

• Rice: CSR 36 tolerates sodicity up to pH 9.9 and salinity up to 10 dS/m. Basmati CSR 30 is described as the first salt-tolerant basmati in the world.
• Wheat: KRL 1-4, KRL 19, KRL 210, KRL 213 and KRL 283.
• Barley: it can now be grown on the saline coastal areas of the Sundarbans in West Bengal. It also grows on saline black soils of canal-irrigated areas in northern Karnataka.

Trees also help sodic land. Prosopis juliflora, Acacia nilotica and Casuarina equisetifolia lowered pH, EC and ESP in a CSSRI study. Karnal grass grows even at ESP 80 to 90.

Fruit trees are an option too. Ber, jamun, guava, aonla and karonda did well in a highly sodic soil.

Bio-drainage uses trees as pumps. Eucalyptus, poplar and bamboo pull water from the water table. The manual says bio-drainage is best as a preventive step, planted at the start.

26. What does the Central Soil Salinity Research Institute do, and how much land is reclaimed?

ICAR-CSSRI sits at Karnal in Haryana. It was set up in 1969 to find cures for salt-affected land. The All India Coordinated Research Project on salt-affected soils began on 14 April 1972.

The manual sorts CSSRI technologies into four groups.

• Blockbuster: the gypsum package.
• High impact: salt-tolerant crop varieties.
• Least adopted: subsurface drainage.
• Incubating: new methods still being tested.

Haryana’s gypsum-reclaimed sodic fields are fondly called “Sarkari Killa”. The name means an acre reclaimed by government agencies.

The reclaimed area is reported three ways. The CSSRI director says about 2.0 million ha. A 2018 CSSRI review says 2.1 million ha. The 2022 AICRP report says 1.95 million ha of alkali land.

No source says why these three figures differ.

Exam writers like this topic. They mix up the two soil types, or they hide the true statement among false ones. This group shows a real paper and a method.

27. What did the UPSC ask about soils and salinization in 2018?

The Civil Services Preliminary Examination of 2018, General Studies Paper I, asked this. It is Question 82 in the Series A booklet.

With reference to agricultural soils, consider these statements.

• 1. A high content of organic matter in soil drastically reduces its water holding capacity.
• 2. Soil does not play any role in the sulphur cycle.
• 3. Irrigation over a period of time can contribute to the salinization of some agricultural lands.

The options were (a) 1 and 2 only, (b) 3 only, (c) 1 and 3 only, and (d) 1, 2 and 3.

The official Series A key gives B for Question 82. In the paper, option (b) is “3 only”. The next item shows why statements 1 and 2 fail.

28. Why are statements 1 and 2 wrong, and what is the method?

Statement 3 is true. Over-irrigation raises the water table, and salt then rises with it. Items 7 and 10 show this.

Statement 1 is the organic-matter trap. Organic matter does the opposite of what the statement says. An FAO bulletin on soil organic matter says it increases moisture holding capacity, most in the topsoil.

Statement 2 says soil plays no role in the sulphur cycle. That is wrong. The FAO salt-affected soils bulletin describes soil microbes oxidising sulphur to sulphuric acid. So soil takes part in the sulphur cycle.

Use this three-step method.

• Step one: test each statement against a fact you know.
• Step two: strike out any option that holds a false statement.
• Step three: pick what is left.

Watch for these traps.

• Do not swap the definitions. Saline means high soluble salt. Sodic means high exchangeable sodium.
• Do not say gypsum cures saline soil. Gypsum is for sodic soil.
• Do not say leaching alone cures sodic soil. Gypsum must come first.
• Do not call the pH limit 8.5 for India. India uses 8.2.
• Do not give one total as final. Say “about 6.7 million ha”.

Last, always compare the two soil types side by side before you pick an option. Saline soil has high ECe, low ESP and a pH below 8.2. Sodic soil has high ESP, a pH above 8.2 and usually low ECe. Saline soil is washed. Sodic soil is first treated with gypsum.