Alkali / Sodic Soils And Saline-Alkali : characters, Reclamation and management - Agrobotany

Definition

Alkali soils, often referred to as sodic soils, are soils in which the saturation-extract electrical conductivity (ECe) is below 4 dS m-1 and the exchangeable sodium percentage (ESP) exceeds 15. These soils commonly show strongly alkaline reactions with pH values typically between 8.5 and 10.0. In many arid and semi-arid landscapes alkali soils also contain appreciable calcium carbonate (CaCO₃) deposits. Continuous hydrolysis of this CaCO₃ releases hydroxyl (OH^-) ions into the soil solution, helping to sustain the elevated pH observed in calcareous alkali soils. Excess sodium adversely affects soil structure, reduces permeability and impairs fertility.

Expected loss of soil productivity due to ESP in different soils

ESP	Loss in productivity (%)
	Alluvium derived soils (Inceptisols / Alfisols)	Black soils (Vertisols)
Up to 5	Nil	Up to 10
5–15	<10	10–25
15–40	10–25	25–50
>40	25–50	>50

Formation

Soil colloids (clays and organic colloids) carry negative electrical charges that attract and hold positively charged ions (cations). Under normal conditions in arid regions calcium (Ca²⁺) and magnesium (Mg²⁺) dominate the soil exchange complex. However, when salts build up due to factors such as poor drainage, use of saline irrigation water, or high water tables, sodium (Na⁺) may increasingly occupy the exchange sites. Over time this replacement of divalent cations by monovalent sodium causes clay dispersion, breakdown of aggregates and the development of sodic conditions with poor structure and low infiltration.

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Major Production Constraints

• Dispersion of soil colloids: Sodium causes clay particles to separate and swell, producing dense surface crusts and subsurface pans that restrict water movement and root growth.
• Specific-ion effects: Elevated Na⁺ can be directly toxic to sensitive crops and can disturb uptake of essential nutrients.
• Poor infiltration and aeration: Compacted or dispersed soil limits percolation and gas exchange, stressing roots and reducing biological activity.
• Nutrient imbalances: High ESP often coincides with deficiencies of Ca, Mg and certain micronutrients, reducing productive capacity.

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Reclamation Methods

Physical Amelioration

Physical measures improve soil structure and hydraulic properties but do not directly remove exchangeable sodium. Common practices include:

• Deep ploughing: Breaks compacted subsurface layers (hardpan), improving infiltration and root penetration.
• Improved drainage: Installation or restoration of subsurface or surface drains prevents salt accumulation in the root zone and lowers the water table.
• Sand filling: Adding sand or coarse material to the surface can reduce crusting and increase porosity and capillary movement.
• Profile inversion: Turning over surface and subsurface layers to dilute sodicity and improve physical conditions (similar effect to deep ripping).

Chemical Amelioration

Chemical reclamation aims to replace exchangeable sodium with calcium and then leach the displaced salts out of the root zone. This is normally achieved by applying Ca²⁺-supplying amendments followed by sufficient irrigation/leaching to remove the displaced Na⁺.

• Direct calcium sources: Gypsum (calcium sulfate) and phospho-gypsum are widely used because they are soluble and supply Ca²⁺ effectively. Calcium chloride is another direct source.
• Indirect calcium releasers: Elemental sulphur, sulphuric acid, pyrite (FeS₂) and iron sulphate help acidify the soil or solubilize CaCO₃ in calcareous sodic soils, freeing Ca for exchange.

Gypsum is generally preferred for its cost-effectiveness and predictable reaction. Calcium carbonate (lime) is ineffective in calcareous sodic soils because CaCO₃ is already present and largely insoluble under high pH; however, it may be useful in non-calcareous sodic soils where surface pH allows some dissolution.

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Alternative Organic & Industrial Amendments

Certain agro-industrial wastes and effluents can assist reclamation when used appropriately and safely:

• Distillery spent wash: Strongly acidic (pH ~3.8–4.2) and containing magnesium; applied in controlled doses during summer, allowed to oxidize with intermittent tillage, and followed by fresh water leaching — this treatment can lower pH and reduce ESP.
• Distillery effluent: Rich in macro- and micronutrients; one-time, uniform application on fallow land (20–40 t ha^-1) followed by thorough incorporation and controlled decomposition can improve soil organic matter and nutrient status. Avoid allowing the field to dry completely during decomposition phase.
• Pulp and paper mill effluents: When properly treated and combined with organic amendments (e.g., pressmud at ~5 t ha^-1 or fortified pressmud at ~2.5 t ha^-1), these effluents can add organic matter and improve soil structure and fertility.

Note: use of industrial effluents must consider salinity, heavy metals and local regulations; application rates should be determined from laboratory analysis and field trials.

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Crop Selection and Management

Choosing appropriate crops and systems helps maintain production while reclamation proceeds:

• Rice: Often used on reclaimed alkali soils because its tolerance of submerged conditions reduces the harmful effects of high ESP, promotes beneficial microbial activity and can help leach salts when properly managed.
• Salt-tolerant grasses: Species such as Brachiaria mutica (Para grass) and Cynodon dactylon (Bermuda grass) can produce reasonable yields even at ESP levels above 30 and contribute to ground cover and organic matter.
• Agroforestry and silvopasture: Establishment of trees and shrubs in suitable systems improves soil cover, reduces erosion, enhances water relations and provides long-term returns while amelioration continues.
• Integrated management: Combine chemical reclamation (e.g., gypsum), physical measures (drainage, deep ripping), addition of organic matter, careful irrigation scheduling, and appropriate crop rotation to restore productivity sustainably.

Relative tolerance of crops to sodicity

ESP (range*)	Crop
2–10	Deciduous fruits, nuts, citrus, avocado
10–15	Safflower, black gram, peas, lentil, pigeon pea
16–20	Chickpea, soybean
20–25	Clover, groundnut, cowpea, pearl millet
25–30	Linseed, garlic, cluster bean
30–50	Oats, mustard, cotton, wheat, tomatoes
50–60	Beets, barley, sesbania
60–70	Rice

*Relative yields are only 50% of the potential in respective sodicity ranges.

Relative tolerance of fruit trees to sodicity

Tolerance to sodicity	ESP	Trees
High	40–50	Ber, tamarind, sapota, wood apple, date palm
Medium	30–40	Pomegranate
Low	20–30	Guava, lemon, grape
Sensitive	<20	Mango, jackfruit, banana

Summary: Sodic soils are created when sodium dominates the soil exchange complex, degrading structure and fertility. Effective reclamation requires a mix of physical, chemical and biological approaches — notably calcium amendment and leaching — accompanied by prudent crop choices and the safe use of organic or industrial by-products where appropriate.

Saline-Alkali / Sodic Soils — Overview & Management

Saline-alkali (sodic) soils are soils with high soluble salts and a high proportion of sodium on the exchange complex. They are chemically and physically degraded, limiting crop growth and soil biological activity unless properly managed.

What are saline-alkali / sodic soils?

These soils are typically defined by two laboratory measurements:

• Electrical conductivity (EC) of the saturation extract > 4 dS/m, and
• Exchangeable Sodium Percentage (ESP) > 15 percent.

The soil pH commonly exceeds 8.5 when exchangeable sodium dominates; however, pH can be lower when soluble salts are relatively more abundant. In short, the balance between soluble salts and exchangeable sodium determines exact chemical behavior and management needs.

How do these soils form?

Saline-alkali soils develop through a combination of salinization (build-up of soluble salts) and alkalization (increase in exchangeable sodium). Key contributing factors include:

• Poor natural drainage or blocked drains that prevent salts from moving out of the root zone;
• Use of saline irrigation water that brings salts into the soil;
• High evaporation rates that draw water upward and concentrate salts near the surface;
• Low rainfall that is insufficient to leach salts below the root zone.

If excess soluble salts are successfully leached downward (for example, by applying good-quality water in combination with drainage), the soil may lose much of its soluble salt load but still retain exchangeable sodium. In that case, the soil behavior may come to resemble a typical sodic soil.

Management & Reclamation

Reclaiming saline-alkali soils usually requires an integrated approach that reduces sodium, removes or redistributes salts, and restores soil structure and biology. Below are common, practical measures:

1. Chemical amendments

• Gypsum (calcium sulfate): A common amendment that supplies calcium to replace exchangeable sodium on soil particles. When calcium replaces sodium, sodium can be leached away with irrigation water.
• Other calcium sources: Lime or industrial by-products (where safe and recommended) can be used after soil testing and extension advice.

2. Drainage & leaching

• Surface drainage: Open ditches and graded fields allow leachate to leave the area.
• Subsurface drainage: Tile drains or perforated pipes lower the water table and carry dissolved salts away from the root zone.
• Leaching: Apply sufficient good-quality irrigation water (where available) to flush displaced salts below the root zone; drainage must be available to remove the leachate.

3. Irrigation management

• Use better-quality water if possible; if not, blend saline with better water to lower overall salinity.
• Practice controlled irrigation (alternate furrow, drip, micro-sprinkler) to reduce salt concentration near the soil surface and root zone.
• Follow a leaching schedule—periodic deeper irrigations to push salts below root depth—only when drainage allows removal of the leachate.

4. Organic matter and cropping

• Incorporate organic amendments such as farmyard manure, compost, or crop residues to improve soil structure, increase microbial activity, and help in ion exchange processes.
• Green manures and salt-tolerant cover crops (e.g., certain legumes or halophytes) improve soil health and can reduce erosion and surface evaporation.
• Choose salt-tolerant crops initially while reclamation proceeds; gradually shift to more sensitive crops as soil improves.

5. Cultural practices

• Avoid poor bed designs where salts can concentrate—use sloped beds and sow seeds above likely waterlines.
• Mulching can significantly reduce surface evaporation and slow salt accumulation in the root zone.
• Maintain regular monitoring of EC, ESP, and pH to track reclamation progress and adjust management.

Important: Reclamation is usually a multi-season process. Success depends on good planning, reliable water and drainage, correct amendment rates (based on soil testing), and monitoring. Local extension services or soil laboratories can provide site-specific recommendations.

Practical sequence for farmers (summary)

1. Obtain a soil test (EC, ESP, pH, texture) to assess the problem precisely.
2. Install or improve drainage where needed.
3. Apply gypsum or other recommended calcium amendments as per soil test.
4. Leach salts with good-quality water, following a planned schedule.
5. Incorporate organic matter and adopt salt-tolerant crops during reclamation.
6. Monitor soil parameters and adjust practices seasonally.

Sources: Soil science textbooks, ICAR/CSSRI guidance documents, and land reclamation manuals — consult local agricultural extension or soil testing labs for field-specific advice.