LAISHA Explains
Soil science

Saline, sodic and saline-sodic soils, and their reclamation

How salt and sodium damage soils and crops, how EC, ESP, SAR and pH classify them, and how to reclaim each kind, with gypsum and leaching worked out.

Exam essential8 min readUpdated
On this page
  1. Two different problems
  2. Measuring salt and sodium
  3. Classifying salt-affected soils
  4. How soils become salt-affected
  5. Reclaiming saline soils: drain and leach
  6. Reclaiming sodic soils: replace the sodium first
  7. Key points
  8. What the exams ask
  9. Questions people ask
  10. Sources
  11. Related on Laisha
  12. Discussion

Salt-affected soils contain enough soluble salt, or enough exchangeable sodium, to reduce crop growth. They are a problem of dry climates and of irrigation: wherever water evaporates and leaves its salts behind faster than rain or drainage can wash them away.

FAO's 2021 global map estimated more than 400 million hectares of salt-affected topsoil worldwide, across every continent: the Indus and Tigris–Euphrates basins, Central Asia, the Murray–Darling basin, the western Americas, the Sahel and North Africa, and parts of China. A large share of the world's irrigated land is affected to some degree. Exams ask about their classification, their causes and, above all, the difference between reclaiming saline and sodic soils, which call for opposite first steps.

Two different problems

Salinity means too much soluble salt in the soil water: chlorides, sulphates and carbonates of sodium, calcium and magnesium. It harms plants mainly through osmotic stress: salty water holds on to its water molecules, so roots must work harder to take it up, and the crop behaves as if it were in drought even in a wet soil. Specific ions (sodium, chloride, boron) can also be toxic, especially to fruit trees and vines.

Sodicity means too much exchangeable sodium on the soil's clay and humus. Its main harm is to the soil itself. Sodium ions are large when hydrated and carry only one charge, so they cannot hold clay platelets together the way calcium can. When the soil is wetted with low-salt water, the clay disperses: aggregates fall apart, fine particles clog the pores, the surface seals into a hard crust, water stands on the surface, and roots and seedlings struggle. Sodic soils are also often strongly alkaline, which brings nutrient problems.

Measuring salt and sodium

Electrical conductivity (EC)

Dissolved salts conduct electricity, so salinity is measured as electrical conductivity, in decisiemens per metre (dS/m; 1 dS/m = 1 mmho/cm). The reference measurement is on the saturation paste extract (ECe): soil is wetted just to saturation, and the water drawn from it is measured. Quicker 1:2 or 1:5 soil–water extracts read lower and must be converted with a factor that depends on texture.

Exchangeable sodium percentage (ESP) and sodium adsorption ratio (SAR)

ESP is the share of the cation exchange capacity occupied by sodium:

Exchangeable sodium percentage
ESP = exchangeable Na ⁄ CEC × 100

Because ESP needs a full exchange analysis, the easier sodium adsorption ratio of the soil solution or irrigation water is often used. With concentrations in mmolc/L:

Sodium adsorption ratio
SAR = Na ⁄ √((Ca + Mg) ⁄ 2)

The two are closely related: an SAR of about 13 in the saturation extract corresponds roughly to an ESP of 15 in the soil.

Classifying salt-affected soils

The classification most textbooks still use was published by the United States Salinity Laboratory in 1954 and is used worldwide, with local variations.

Classification of salt-affected soils by electrical conductivity of the saturation extract and exchangeable sodium percentage: normal, saline, sodic and saline-sodic soils, with their typical pH and the main reclamation step for each Sodic pH usually above 8.5 clay dispersed, crusting, poor drainage Reclaim: gypsum (or acid-forming amendments), then leach Saline-sodic pH usually below 8.5 salts keep clay flocculated — for now Reclaim: gypsum first, then leach; leaching alone turns it sodic Normal no salt or sodium problem Saline pH usually below 8.5; white surface crusts plants suffer osmotic stress Reclaim: drain, then leach the salts with good-quality water 4 dS/m Salinity: EC of saturation extract (ECe) → ESP 15 Sodicity: ESP → 0
The four classes by salinity (EC of the saturation extract, threshold 4 dS/m) and sodicity (exchangeable sodium percentage, threshold 15). The pH ranges are typical, not defining. Some systems use lower sodicity thresholds, such as ESP 6 in parts of Australia.Diagram: Laisha, original.
ClassECe (dS/m)ESPTypical pHAppearance
Normalbelow 4below 15below 8.5—
Salineabove 4below 15below 8.5white salt crusts; good structure
Sodic (alkali)below 4above 15above 8.5, often 9–10dark surface from dispersed humus; hard crust; standing water
Saline-sodicabove 4above 15usually below 8.5looks saline; turns sodic if leached with plain water

Older literature calls saline soils "white alkali" and sodic soils "black alkali", after the white salt crust of one and the dark, dispersed organic matter of the other. In the World Reference Base, strongly saline soils are Solonchaks and soils with a sodic subsoil horizon are Solonetz.

How soils become salt-affected

  • Primary salinity: natural weathering of saline rocks, old seabeds, wind-borne sea salt, and closed basins where water only leaves by evaporation.
  • Secondary (human-induced) salinity:
  • Irrigation without drainage: every irrigation adds salt, and if water tables rise to within a couple of metres of the surface, capillary rise brings salt up to the root zone (waterlogging and salinity together).
  • Irrigation with saline or high-sodium water, including some groundwater.
  • Clearing deep-rooted native vegetation, which raises water tables and mobilises stored salt (dryland salinity, seen widely in southern Australia).
  • Seawater intrusion into coastal aquifers as fresh groundwater is over-pumped, and flooding of coastal fields by storm surges and rising seas.

Reclaiming saline soils: drain and leach

Soluble salts can be washed out. Two conditions are essential.

  1. Drainage. Water must be able to leave the root zone. Where the water table is high, subsurface drains (tile or perforated pipe) or deep open drains come first; leaching a field with no drainage only raises the water table and brings salt back.
  2. Leaching with good-quality water. Enough water is applied to dissolve the salts and carry them below the roots. As a rule of thumb, ponding a depth of water roughly equal to the depth of soil to be reclaimed removes a large share of the salt, with intermittent ponding using less water than continuous flooding.

On land that stays irrigated, a leaching fraction is added to each irrigation to keep salts from building up again.

Worked example

How much yield does salinity cost?

Crop response to salinity is often described by a threshold and a slope (Maas and Hoffman, 1977): yield is unaffected up to a threshold ECe, then falls in a straight line.

Relative yield (%) = 100 − b × (ECe − a), where a is the threshold and b the slope.

For wheat, typical published values are a threshold a of about 6 dS/m and a slope b of about 7.1 per cent per dS/m. At ECe = 10 dS/m: 100 − 7.1 × (10 − 6) = about 72 per cent of full yield. For common bean (threshold about 1 dS/m, slope about 19 per cent), the same soil would give nothing at all. Thresholds vary with variety, climate and irrigation method; treat them as guides.

Salt tolerance varies widely. Barley, cotton, sugar beet, date palm and many halophytic forages are tolerant; wheat, sorghum and soybean moderately so; rice and maize moderately sensitive; beans, carrots, onions, citrus and most fruit trees sensitive.

Reclaiming sodic soils: replace the sodium first

Leaching alone does not help a sodic soil; the water hardly gets in, and the sodium is held on the exchange sites, not dissolved. The sodium must first be replaced by calcium, which also re-flocculates the clay, and only then leached away.

  • Gypsum (calcium sulphate, CaSO4·2H2O) is the most widely used amendment: moderately soluble, cheap, and it supplies calcium directly.
  • In calcareous sodic soils, which already contain calcium carbonate, acid-forming amendments release calcium from it: elemental sulphur (oxidised by bacteria to sulphuric acid), sulphuric acid itself, or iron sulphate.
  • Organic matter, green manures and sugar-factory press mud improve structure and, as they decompose, release CO2 that helps dissolve native calcium carbonate.
  • Tolerant crops grown during reclamation (rice in ponded fields, Sesbania, salt-tolerant grasses) keep the land productive and open up the soil with roots.
Worked example

Calculating a gypsum requirement

A sodic soil has a CEC of 20 cmolc/kg and an ESP of 40. The aim is to bring the ESP of the top 15 cm down to 10. Bulk density is 1.5 t/m3.

  1. Sodium to replace: (40 − 10) ⁄ 100 × 20 = 6.0 cmolc/kg.
  2. Gypsum's equivalent weight is 172 ⁄ 2 = 86 g, so 1 cmolc/kg needs 0.86 g of gypsum per kg of soil. 6.0 × 0.86 = 5.16 g/kg.
  3. Soil mass to 15 cm in one hectare: 10,000 × 0.15 × 1,500 = 2.25 million kg.
  4. Pure gypsum: 5.16 g/kg × 2.25 million kg ≈ 11.6 t/ha.
  5. Agricultural gypsum of 80 per cent purity: 11.6 ⁄ 0.8 ≈ 14.5 t/ha, applied before the leaching water. In practice the dose is often applied in stages, and an efficiency factor is added because not all the calcium exchanges.

Saline-sodic soils are reclaimed like sodic ones: apply gypsum first and then leach. If they are leached with good water before the sodium is replaced, the salts that were keeping the clay flocculated are washed out, the clay disperses, and the soil turns sodic.

Key points

  • Saline soils: too much soluble salt (ECe above 4 dS/m), osmotic stress, good structure; reclaim by drainage and leaching.
  • Sodic soils: too much exchangeable sodium (ESP above 15), dispersion and crusting, pH often 8.5–10; reclaim by replacing sodium with calcium (gypsum, or sulphur in calcareous soils), then leaching.
  • Saline-sodic soils: both; gypsum before leaching, or they become sodic.
  • SAR = Na ⁄ √((Ca + Mg) ⁄ 2); an SAR of about 13 corresponds to an ESP of about 15.
  • Gypsum requirement = (ESPinitial − ESPfinal) ⁄ 100 × CEC, at 0.86 g gypsum per cmolc per kg.
  • Most secondary salinity comes from irrigation without drainage and from rising water tables.

What the exams ask

  • School-leaving and entrance papers: the difference between saline and sodic soils, their pH, appearance and the amendment for each.
  • Degree and research-fellowship entrances: the classification thresholds, EC, ESP and SAR, clay dispersion, causes of secondary salinity, salt-tolerant crops.
  • Licensing and professional examinations for agronomists and irrigation engineers: gypsum requirement and leaching calculations, irrigation-water quality, drainage design and crop yield under salinity.

Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.

Questions people ask

Why not just flood a sodic soil to wash the sodium out?

Because the sodium is held on the clay, not dissolved, and the dispersed soil lets almost no water through. The calcium from gypsum must first displace the sodium; once the clay flocculates, water can move and carry the sodium away.

Does gypsum lower the pH of a sodic soil?

Indirectly. The high pH comes from sodium carbonate and exchangeable sodium. Gypsum converts sodium carbonate to calcium carbonate and sodium sulphate, which is leached, and the pH falls as a result. On a normal acid soil, gypsum does not change pH.

Can salty water ever be used for irrigation?

Often, with care: tolerant crops, a leaching fraction, good drainage, irrigation methods that keep the root zone moist (such as drip), and blending with better water. The water's EC and SAR together decide the risk.

Sources

  1. Richards, L. A. (ed.) Diagnosis and Improvement of Saline and Alkali Soils. USDA Agriculture Handbook 60, United States Salinity Laboratory, 1954.
  2. FAO. Global Map of Salt-affected Soils (GSASmap). Food and Agriculture Organization of the United Nations, 2021.
  3. Ayers, R. S. and Westcot, D. W. Water Quality for Agriculture. FAO Irrigation and Drainage Paper 29 Rev. 1, 1985.
  4. Maas, E. V. and Hoffman, G. J. "Crop salt tolerance: current assessment." Journal of the Irrigation and Drainage Division, ASCE 103, 115–134 (1977).
  5. Qadir, M. and Oster, J. D. "Crop and irrigation management strategies for saline-sodic soils and waters aimed at environmentally sustainable agriculture." Science of the Total Environment 323, 1–19 (2004).
  6. Brady, N. C. and Weil, R. R. The Nature and Properties of Soils, 15th edition. Pearson, 2017. Chapter 10.

More in soil science

Cation exchange capacity and base saturation7 min readThe essential plant nutrients and their deficiency symptoms7 min readThe nitrogen cycle in agricultural soils8 min readSoil erosion and the Universal Soil Loss Equation7 min read

Discussion

Questions, corrections and notes from readers on this article. Every comment is checked before it appears: keep it on the topic, and do not share phone numbers, links or contact details.