Agriculture & Rural India18 Concepts & Facts

What Is Soil Salinity? Salinization vs Sodicity, Osmotic Stress & Gypsum Reclamation

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Soil salinity refers to the excessive accumulation of water-soluble mineral salts—primarily chlorides, sulfates, and bicarbonates of sodium (Na+), calcium (Ca2+), and magnesium (Mg2+)—within the crop root zone at concentrations high enough to impair plant water uptake, seed germination, and agricultural yield. While primary salinization occurs naturally through the chemical weathering of salt-bearing parent rocks, marine inundation along coastal deltas, and wind-borne sea spray in arid basins such as the Rann of Kutch, secondary salinization is an anthropogenic land degradation crisis driven by faulty irrigation management. When unlined canal networks, flood irrigation, and poor subsurface drainage raise the underground water table to within 1.5 to 2 meters of the soil surface, intense solar evaporation in arid and semi-arid climates pulls groundwater upward through soil micropores via capillary action. As the water evaporates at the surface, dissolved salts precipitate behind as a white crust known locally across Northern and Western India as Reh, Kallar, Usar, Thur, Karl, or Chopan.

Plant physiologists classify salt stress as 'physiological drought.' Under normal soil conditions, the water potential inside plant root cells is lower (more negative) than the water potential of the surrounding soil solution, allowing water to enter roots passively via osmosis. When soluble salt levels rise in saline soil, the osmotic potential of the soil water drops sharply; even when the field appears visibly wet and irrigated, crop roots cannot extract water against the osmotic gradient, and water may even be drawn out of root cells into the soil, causing plasmolysis, leaf tip burn, and stunted growth.

Soil scientists strictly differentiate between Saline Soils ('white alkali') and Sodic/Alkali Soils ('black alkali') based on three laboratory metrics: Electrical Conductivity of the saturated paste extract (ECe), soil reaction (pH), and Exchangeable Sodium Percentage (ESP). Saline soils exhibit an ECe greater than 4 deciSiemens per meter (dS/m), a pH below 8.5, and an ESP below 15%, retaining good physical permeability that allows reclamation via freshwater leaching and subsurface tile drainage. Conversely, sodic soils exhibit an ESP above 15% and a pH between 8.5 and 10.0; excess sodium ions disperse clay micelles, sealing soil pores, causing waterlogging, and requiring chemical amendment with Gypsum (Calcium Sulfate Dihydrate, CaSO4.2H2O) to displace sodium from the clay exchange complex before leaching.

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According to the U.S. Salinity Laboratory and ICAR-Central Soil Salinity Research Institute (CSSRI, Karnal), a soil is classified as Saline when its saturated paste Electrical Conductivity (ECe) exceeds 4.0 dS/m at 25 degrees Celsius, its Exchangeable Sodium Percentage (ESP) is less than 15%, and its pH is below 8.5.

A soil is classified as Sodic (Alkali) when its Exchangeable Sodium Percentage (ESP) exceeds 15% (or Sodium Adsorption Ratio, SAR > 13), its pH lies between 8.5 and 10.0, and its ECe is below 4.0 dS/m.

A soil is classified as Saline-Sodic when both thresholds are exceeded simultaneously: ECe is greater than 4.0 dS/m and ESP is greater than 15%, while pH usually remains near or below 8.5 until soluble salts are leached.

Saline soils are traditionally called White Alkali soils because evaporating chlorides and sulfates of sodium, calcium, and magnesium leave a white powdery efflorescence on the topsoil.

Sodic soils are called Black Alkali soils because high pH (>8.5) dominated by sodium carbonate (Na2CO3) dissolves soil organic humus, depositing a dark, slick black film on the soil surface.

Across India, salt-affected soils are known by regional vernacular names: Reh or Usar in Uttar Pradesh, Kallar or Thur in Punjab and Haryana, Khar or Lona in Gujarat, Chari in Bihar, and Karl or Chopan in Maharashtra and Karnataka.

According to ICAR-CSSRI estimates, approximately 6.73 million hectares of agricultural land in India are salt-affected (roughly 2.96 million hectares saline and 3.77 million hectares sodic), concentrated in Gujarat, Uttar Pradesh, Maharashtra, West Bengal, Rajasthan, Haryana, and Punjab.

In Green Revolution canal command areas (such as the Indira Gandhi Canal in Rajasthan and Bhakra-Nangal command in Punjab-Haryana), over-irrigation without drainage raises the water table, triggering upward capillary rise and secondary salinization.

High soil salinity induces physiological drought: elevated salt ions lower the osmotic potential of the soil water, preventing plant roots from absorbing moisture via osmosis even in water-saturated fields.

#10

Excess sodium (Na+) on the clay cation exchange complex causes deflocculation (dispersion) of soil aggregates because sodium ions have a large hydrated radius and low single positive charge, destroying soil porosity and causing surface crusting and waterlogging.

#11

Purely saline soils (ECe > 4 dS/m, ESP < 15%) are reclaimed physically by flooding with good-quality irrigation water to dissolve salts and flushing them below the root zone through perforated subsurface PVC tile drainage pipes.

#12

Sodic soils (ESP > 15%, pH > 8.5) cannot be reclaimed by freshwater leaching alone because water cannot infiltrate dispersed clay; they require chemical amendment with agricultural Gypsum (CaSO4.2H2O).

#13

During gypsum reclamation, divalent calcium ions (Ca2+) replace monovalent sodium ions (Na+) on clay micelles via the cation exchange reaction: 2Na-Clay + CaSO4 -> Ca-Clay (flocculated) + Na2SO4 (leachable soluble salt).

#14

In calcareous sodic soils containing free calcium carbonate (kankar nodules), elemental sulfur (S) or iron pyrite (FeS2) can be applied; soil bacteria (Thiobacillus thiooxidans) oxidize them into sulfuric acid, which reacts with soil lime to generate in-situ gypsum.

#15

Halophytes are specialized salt-tolerant plants (such as Salicornia, Suaeda, and Atriplex) capable of completing their life cycle in substrate salt concentrations exceeding 200 millimolar NaCl through salt-excreting foliar glands or vacuolar compartmentalization.

#16

Among agricultural crops, barley (Hordeum vulgare), sugarbeet, cotton, date palm, and mustard exhibit high salt tolerance, whereas pulses (chickpea, lentil, mungbean), citrus fruits, and strawberries are highly salt-sensitive.

#17

Green manuring with Dhaincha (Sesbania aculeata) is widely practiced in Northern India to reclaim alkali soils because its deep roots improve hydraulic conductivity and its decomposition releases carbon dioxide and organic acids that lower soil pH.

#18

ICAR-Central Soil Salinity Research Institute (CSSRI), established in 1969 in Karnal, Haryana, has developed landmark salt-tolerant crop varieties including Basmati CSR-30, Rice CSR-36, CSR-43, and Wheat KRL-210 and KRL-213.

Subject Specialist Commentary

Analytical perspective & practical exam advice from the Master10 academic board

Educator's Insight

Soil salinity and alkalinity are core geography and agricultural science themes in UPSC Prelims, GS Paper I/III, and State PCS exams. Aspirants must memorize the exact quantitative boundary separating Saline from Sodic soils: an Electrical Conductivity (ECe) threshold of 4.0 dS/m, an Exchangeable Sodium Percentage (ESP) of 15%, and a pH boundary of 8.5. A frequent examination question asks why Gypsum (CaSO4.2H2O) is applied specifically to sodic/alkali soils rather than purely saline soils—the answer lies in replacing monovalent sodium ions on dispersed clay micelles with divalent calcium ions to restore soil flocculation and permeability.

In Mains answers, candidates should connect secondary salinization directly to ecological missteps of the Green Revolution in Punjab, Haryana, Western Uttar Pradesh, and the Indira Gandhi Canal command area in Rajasthan, while highlighting biological remediation through Dhaincha (Sesbania aculeata) green manuring and CSSRI Karnal's salt-tolerant CSR rice and KRL wheat cultivars.

What Is Soil Salinity? Salinization vs Sodicity, Osmotic Stress & Gypsum Reclamation

Key Concepts & Self-Assessment18 Key Facts

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