Saline and Alkaline Soils

Saline and Alkaline soils—classified primarily under the orders Aridisols and Inceptisols in USDA Soil Taxonomy and known by various vernacular names like Reh, Usar, Kallar, Chopan, Rakar, and Thur—are intra-zonal soils characterized by an excess of soluble salts or exchangeable sodium. These soils are widespread across arid, semi-arid, and poorly drained agricultural tracts of India, posing a significant challenge to land resources and agrarian productivity.

Genesis and Structural Formation

The formation of saline and alkaline soils occurs through two distinct processes: natural (primary) salinization and anthropogenic (secondary) salinization.

Primary Pedogenic Processes

In arid and semi-arid regions of India, precipitation is insufficient to leach soluble salts out of the soil profile. The intense summer heat causes extreme evaporation, driving upward capillary action. Ground moisture holding dissolved salts moves to the surface; as the water evaporates, the minerals precipitate, leaving a dense, white efflorescent crust on the topsoil.

Secondary Anthropogenic Processes

The introduction of intensive canal irrigation under the Green Revolution in parts of Punjab, Haryana, and Western Uttar Pradesh altered the hydrological balance. Over-irrigation without adequate drainage networks caused the water table to rise. As the sub-surface water table rose to within a few meters of the surface, capillary action drew deep-seated salts up into the root zone and topsoil, converting fertile alluvial soils into barren tracts.

The Chemical Distinction: Saline vs. Alkaline vs. Saline-Alkaline

Though often grouped together, these soils are chemically distinct based on their Electrical Conductivity (EC), Exchangeable Sodium Percentage (ESP), and pH value.

Saline Soils (White Usar)

• Chemical Profile: These soils contain an excess of neutral soluble salts, primarily chlorides and sulfates of sodium (NaCl, Na₂SO₄), magnesium (MgCl₂, MgSO₄), and calcium (CaCl₂).
• Properties: The pH is generally less than 8.5. Electrical conductivity is high (greater than 4 dS/m), and the ESP is low (less than 15%). The white crust formed on the surface gives them the name “White Usar”. The structure remains flocculated and permeable to water.

Alkaline Soils (Black Usar / Sodic Soils)

• Chemical Profile: Dominated by sodium carbonate (Na₂CO₃) and sodium bicarbonate (NaHCO₃), with low amounts of neutral soluble salts.
• Properties: The pH is exceptionally high, ranging between 8.5 and 10.0. The ESP is greater than 15%, while the electrical conductivity is low (less than 4 dS/m). The high alkalinity dissolves soil organic matter (humus), which then deposits on the surface as a dark brown or black crust, hence the term “Black Usar”. The sodium ions cause clay particles to disperse, destroying soil structure and creating an impermeable, poorly aerated layer.

Geographical Distribution in India

Saline and alkaline soils cover approximately 6.74 million hectares (mha) across India, split between inland arid plains and coastal zones.

• The Indo-Gangetic Irrigation Belt: Extensive tracts across Punjab, Haryana, and Western Uttar Pradesh (specifically the Indo-Gangetic plains fed by the Western Yamuna Canal, Bhakra Canal, and Sharda Canal systems).
• Arid Western Plains: The arid and semi-arid regions of Rajasthan (specifically the Indira Gandhi Canal command areas) and northern Gujarat (including the Saurashtra and Kathiawar regions).
• The Marine Coastal Zone: The Rann of Kutch in Gujarat, where tidal wave incursions deposit massive amounts of sea salt. Coastal deltas of the Mahanadi (Odisha), Godavari and Krishna (Andhra Pradesh), Cauvery (Tamil Nadu), and the Sundarbans mangrove tracts of West Bengal, where sea-water intrusion contaminates the groundwater table.
• Peninsular Basins: Localized pockets in the rain-shadow basins of Maharashtra (Chopan soils of the upper Godavari and Bhima valleys) and northern Karnataka.

Diagnostic Matrix of Salt-Affected Soils

The operational parameters below outline the classification system established by the Central Soil Salinity Research Institute (CSSRI).

Reclamation and Management Techniques

Reclaiming salt-affected soils requires specific chemical, hydrological, and biological treatments based on the soil’s chemical composition.

Mechanical and Hydrological Remediation

• Flushing and Leaching: For pure saline soils, fields are flooded with fresh, high-quality water to dissolve the surface salts. The water is then drained out mechanically, or allowed to percolate deep down past the root zone through sub-surface drainage systems.
• Scraping: Physical removal of the white salt crust from the surface using mechanical scrapers before the onset of monsoons.

Chemical Amendments

• Gypsum Application (CaSO₄ · 2H₂O): Specifically required for alkaline/sodic soils. The calcium ions (Ca²⁺) in gypsum displace the toxic exchangeable sodium (Na^+) ions from the clay complex. The displaced sodium reacts to form soluble sodium sulfate (Na₂SO₄), which can then be safely leached out of the soil profile with water.
• Pyrites (FeS₂) and Organic Manures: Application of iron pyrites or green manuring (using crops like Dhaincha or Sunnhemp) produces organic acids during decomposition, which help neutralize high soil alkalinity.

Biological and Agronomic Interventions

• Cultivation of Halophytes and Salt-Tolerant Crops: Planting specific salt-resistant varieties of crops to gradually extract or tolerate soil salts.
• Agroforestry: Growing deep-rooted tree species like Acacia nilotica (Babool), Prosopis juliflora, and Casuarina helps lower the high water table, mitigating the drivers of capillary salinization.

Key Facts and Trivia for UPSC Prelims

The CSSRI Classification

The Central Soil Salinity Research Institute (CSSRI) is located in Karnal, Haryana. It is the premier national institute under ICAR dedicated to researching isolation and remediation mapping for salt-affected soils in India.

Salt-Tolerant Crop Varieties

Scientists in India have developed specific salt-tolerant and sodicity-resistant crop varieties to sustain agriculture in Usar tracts. Notable examples include Basmati CSR-30, Narendra Dhan-2064 (rice), and KRL-210 or KRL-213 (salt-tolerant wheat varieties).

The Chopan Soils

In Maharashtra, the local term Chopan refers to highly heavy, dark-colored, structureless sodic soils found along the canal tracts of the Deccan Trap area. They are highly problematic because their heavy basaltic clay base combined with high sodium makes them completely un-drainable.

Marine Wave Inversion

During the 1999 Odisha Super Cyclone and the 2004 Indian Ocean Tsunami, thousands of hectares of coastal alluvial soils in the Mahanadi delta and Tamil Nadu coast underwent sudden marine salinization. This was caused by marine wave inversion, which left toxic deposits of sodium chloride that took years of seasonal monsoon leaching to clear.