Saline and Alkaline Soil

Saline and alkaline soils belong to the broader category of salt-affected soils, a degradation class recognized by the Food and Agriculture Organization (FAO) and, in India, mapped by the Indian Council of Agricultural Research–Central Soil Salinity Research Institute (ICAR-CSSRI), established at Karnal in 1969. The scientific basis for distinguishing them rests on diagnostic parameters codified by the United States Salinity Laboratory in its Agriculture Handbook No. 60 (1954): electrical conductivity of the saturation extract (ECe), exchangeable sodium percentage (ESP), and soil reaction (pH). A saline soil is defined by ECe exceeding 4 deciSiemens per metre, ESP below 15, and pH below 8.5; an alkaline (sodic) soil is defined by ESP above 15 and pH above 8.5, with comparatively low soluble-salt content. In the Indian subcontinent these soils carry vernacular names—reh, kallar, usar, rakar, and thur—and the National Bureau of Soil Survey and Land Use Planning (NBSS&LUP) classifies them within the order of salt-affected soils for resource mapping.

The mechanics of salinisation begin with the accumulation of soluble salts—chiefly chlorides and sulphates of sodium, calcium, and magnesium—in the root zone. In arid and semi-arid tracts where potential evapotranspiration exceeds precipitation, capillary rise draws saline groundwater upward, and surface evaporation deposits a visible white efflorescence. Over-irrigation without adequate drainage raises the water table, mobilises subsurface salts, and produces secondary salinisation in command areas of major canal systems. Alkalisation follows a different pathway: sodium ions progressively displace calcium and magnesium on the soil exchange complex, and in the presence of carbonate and bicarbonate anions the sodium hydrolyses to produce a strongly alkaline reaction. The dispersed sodium-saturated clay destroys soil aggregates, forming a dense, impervious hardpan that throttles infiltration and aeration.

Variants within this family are distinguished by the dominant salt chemistry and degree of sodium loading. Saline-sodic soils combine high ECe with high ESP and present a transitional hazard, because leaching salts without first replacing sodium can convert them into intractable sodic soils. Coastal saline soils, found along deltas and estuaries, derive their salts from seawater ingress and tidal inundation rather than from inland evaporative concentration. The reclamation prescriptions diverge accordingly: saline soils are corrected primarily by leaching with good-quality water and installing subsurface drainage, whereas alkaline soils require a chemical amendment—gypsum (calcium sulphate) being the standard—to supply soluble calcium that displaces exchangeable sodium, followed by leaching of the released sodium salts.

In India, salt-affected soils cover an estimated 6.7 million hectares, concentrated in the Indo-Gangetic plain, the canal-irrigated tracts of Punjab, Haryana, and western Uttar Pradesh, the black-soil region of the Deccan, the Rann of Kutch in Gujarat, and the coastal belts of Gujarat, West Bengal, and the Sundarbans. The reh tracts of the Ganga-Yamuna doab are textbook alkaline soils, while the deltaic Sundarbans exemplify coastal salinity. ICAR-CSSRI's gypsum-based reclamation programmes in Haryana and the cultivation of salt-tolerant rice and wheat varieties, alongside the planting of Prosopis juliflora and dhaincha (Sesbania) as green manure, have restored substantial acreage. Internationally, the Murray-Darling basin in Australia and the desiccated Aral Sea basin in Central Asia stand as cautionary examples of irrigation-induced salinisation.

The distinction from adjacent soil categories is precise and consequential. Saline and alkaline soils must not be conflated with laterite soils, which are leached, iron- and aluminium-rich, and acidic—the chemical opposite of alkaline tracts. They are also distinct from naturally fertile alluvial soils, even though salt-affected patches frequently occur as degraded zones within alluvial plains. The defining contrast lies in the governing variable: salinity is a function of soluble-salt concentration measured by electrical conductivity, whereas sodicity is a function of the sodium fraction on the exchange complex measured by ESP. Misdiagnosis carries a real cost, because applying a leaching regimen appropriate to saline soil onto a sodic soil aggravates dispersion and seals the profile.

Contemporary debate centres on the scale and reversibility of secondary salinisation. Critics of large gravity-flow irrigation projects argue that the absence of integrated drainage in command areas such as the Indira Gandhi Canal in Rajasthan has waterlogged and salinised land that was productive before irrigation arrived. Climate change intensifies the problem: rising sea levels accelerate saltwater intrusion into coastal aquifers, while higher evapotranspiration in already arid regions hastens surface salt accumulation. The Sustainable Development Goals, particularly SDG 15.3 on land-degradation neutrality, and India's commitments under the United Nations Convention to Combat Desertification have placed salt-affected-soil reclamation within the framework of national land-restoration targets, with bio-saline agriculture and halophyte cultivation emerging as adaptive strategies.

For the working practitioner—the civil-services aspirant, the agricultural-policy analyst, or the desk officer handling rural development—saline and alkaline soils are a recurring intersection of physical geography, agrarian economy, and food security. Mastery requires holding the diagnostic thresholds, the reclamation chemistry, and the regional distribution together, because policy interventions ranging from canal-lining subsidies to gypsum distribution schemes turn on correctly identifying which degradation process is at work. In the UPSC General Studies framework these soils recur in GS Paper I physical geography and in GS Paper III questions on land degradation and sustainable agriculture, making a firm command of the saline–sodic distinction an enduring analytical asset rather than a matter of rote recall.