Problem Soils & Reclamation

Saline & Sodic Soils

20.1 Definition & Extent of Problem Soils

• Definition: Problem soils are soils possessing specific physical, chemical, or biological constraints that severely limit crop growth unless special reclamation or management practices are applied.
• India's Extent: Approximately 120 million hectares in India are degraded in some form. Of this, roughly 56 to 60 million hectares face active production constraints, resulting in enormous annual economic losses.
• Major Types: The primary categories include Saline soils, Sodic soils, Saline-Sodic soils, Acid soils, Waterlogged soils, Eroded soils, and Mine spoils.
• How Soils Become Problematic (Natural Processes): They form naturally through the weathering of salt-bearing rocks combined with high evaporation in arid regions, coastal seawater inundation, or naturally shallow saline water tables.
• How Soils Become Problematic (Human-Induced): The most common cause in India is secondary salinization driven by over-irrigation without adequate drainage. Other human causes include deforestation, industrial effluent discharge, and continuous use of acid-forming fertilizers.

20.2 Saline Soils

A. Definition & Diagnostic Criteria

• Definition: A saline soil contains sufficient soluble salts in the root zone to actively and adversely affect plant growth.
• Electrical Conductivity (ECe): The primary diagnostic criterion is an EC of the saturated paste extract greater than 4 dS/m (decisiemens per meter) at 25°C.
• Exchangeable Sodium Percentage (ESP): The ESP is strictly less than 15%. This means the exchangeable cations are dominated by Ca²⁺ and Mg²⁺, not Na⁺, keeping the soil structure generally intact.
• Soil pH: The pH is less than 8.5. It is not strongly alkaline because Na₂CO₃ is absent; only neutral salts like NaCl, Na₂SO₄, CaCl₂, and MgSO₄ are present.
• Visual Appearance: As water evaporates, salts crystallize on the surface, leaving a classic white salt crust. This gives it the name "white alkali" in the USA, and "reh" or "usar" (partial) in India.
• Flocculation Status: Paradoxically, saline soils often have much better physical structure than sodic soils because the dominant Ca²⁺ and Mg²⁺ ions keep the clay particles firmly flocculated (clumped together).

B. Formation of Saline Soils

• Primary Salinization: Natural accumulation occurs in arid and semi-arid regions (like Rajasthan, Gujarat, and Haryana) where evapotranspiration vastly exceeds rainfall. A shallow water table allows water to rise via capillary action, evaporating at the surface and leaving salts behind.
• Secondary Salinization (Main Cause): Heavy canal irrigation combined with insufficient drainage causes waterlogging. The water table rises, bringing deep salts to the surface via capillary action. This is rampant in the canal command areas of the Indo-Gangetic Plains (UP, Haryana, and Punjab).
• Coastal Salinization: Driven by seawater intrusion into coastal aquifers due to groundwater over-extraction, tidal flooding, and storm surges (common in coastal Gujarat, Maharashtra, AP, Odisha, and the Sunderbans).

C. How Salinity Affects Crop Growth

• Osmotic Stress: High soluble salt concentrations artificially lower the osmotic potential of the soil solution. This creates a "physiological drought," meaning that even if the soil is soaking wet, the plant roots physically cannot extract the water.
• Specific Ion Toxicity: Excess Na⁺, Cl⁻, and SO₄²⁻ accumulate in leaf tissues, disrupting cell membranes and degrading chlorophyll. This leads to severe leaf tip and margin necrosis (scorch or burn symptoms).
• Nutritional Imbalance: Excess sodium directly competes with vital potassium, calcium, and magnesium at the root uptake sites, inducing severe nutrient deficiencies even if those nutrients are plentiful in the soil.
• Seed Germination Failure: High electrical conductivity surrounding the seed creates osmotic pressure too high for germination, leading to patchy fields and poor crop stand establishment.
• Salt Tolerance Threshold: Every crop has a threshold ECe. Barley is highly tolerant (surviving ECe > 8), rice is moderately tolerant (ECe > 3), while groundnuts and most vegetables are highly sensitive (ECe 1–3).

D. Reclamation of Saline Soils

• Primary Method (Leaching): The fundamental reclamation approach is to apply excess, good-quality water to dissolve the soluble salts, force them to percolate deep below the root zone, and physically drain them from the field.
• The Leaching Requirement (LR): This is the calculated fraction of extra irrigation water needed to wash out the salts. The formula is LR = ECw / ((5 × ECe target) - ECw). Sprinkler leaching is often more uniform than flood leaching on sandy soils.
• Prerequisite (Drainage): Leaching is completely futile without a functional drainage system. The salt-laden water must physically leave the field via open trenches or subsurface tile drains, otherwise the water table simply rises and brings the salts right back.
• Gypsum Application: If the soil borders on being saline-sodic (elevated Na⁺), gypsum is added to supply Ca²⁺, which improves soil structure and allows the leaching water to actually infiltrate.
• Organic Matter: Applying 10 to 15 t/ha of FYM vastly improves soil structure and water movement, making the physical leaching process much faster and more effective.
• Salt-Tolerant Crops: During the active reclamation years, farmers should grow tolerant crops. Highly tolerant options include date palms and Salicornia. Moderately tolerant crops include barley, sugarbeet, and cotton.
• Biodrainage: Planting deep-rooted, high-transpiration trees like Eucalyptus or Prosopis in buffer strips acts as biological water pumps. They extract water from the shallow water table, lowering it safely to prevent capillary rise and re-salinization.

20.3 Sodic Soils

A. Definition & Diagnostic Criteria

• Definition: A sodic soil is one where exchangeable sodium constitutes more than 15% of the total Cation Exchange Capacity (CEC), fundamentally destroying the soil's physical properties and driving the pH up to toxic levels.
• Exchangeable Sodium Percentage (ESP): The primary diagnostic criterion is an ESP strictly greater than 15%. The USDA also uses a Sodium Adsorption Ratio (SAR) greater than 13 in the saturation extract.
• Electrical Conductivity (ECe): The ECe is less than 4 dS/m. Because the sodium is physically stuck to the exchange sites rather than floating freely in solution, soluble salts are low. This is precisely why there is no visible white salt crust.
• Soil pH: The pH is strictly greater than 8.5, frequently reaching 9.5 to 10 in severe cases. This is driven by the formation of sodium carbonate (Na₂CO₃), which undergoes alkaline hydrolysis in water to produce highly caustic hydroxyl (OH⁻) ions.
• Visual Appearance: The surface appears dark or black because the highly caustic conditions dissolve soil organic matter (humic acids), which then complexes with the sodium to form a black colloidal suspension. Hence, it is called "black alkali" in the USA, and "usar" (full) or "khar" in India.
• Soil Structure: The structure is completely destroyed. The massive influx of sodium causes the clay particles to violently repel each other (deflocculation/dispersion). It forms an impermeable, sticky mess when wet, and dries into a concrete-hard columnar structure with wide cracks.

B. How Sodic Soils Form

• Natural Weathering: The slow weathering of sodium-rich rocks (like Na-feldspars) and the geochemical concentration of sodium in enclosed drainage basins.
• Irrigation-Induced: Pumping and applying deep groundwater that is naturally high in sodium bicarbonate (SAR > 10). The Na⁺ slowly deposits onto the exchange sites, gradually displacing all the healthy calcium and magnesium over the years.
• Post-Leaching Failure: If a farmer attempts to leach a saline soil that also contains sodium without adding gypsum first, the soluble salts (NaCl) wash away easily, but the Na⁺ remains glued to the exchange sites. The salinity drops, but the sodicity spikes, permanently ruining the soil.

C. How Sodicity Damages Soil

• Clay Dispersion: Sodium is a large, monovalent cation. When it dominates the clay, it creates a massive electrical double layer that forces the clay plates to violently repel one another, completely destroying soil aggregation.
• Infiltration Failure: Because the dispersed clay particles clog every single pore, water infiltration drops to near-zero. Water pools completely on the surface during a rainstorm, even if the subsoil underneath is bone dry.
• Root Growth and Nutrition: The concrete-like hardness of dry sodic soil physically crushes roots. Furthermore, the extreme pH (> 8.5) chemically locks away iron, manganese, zinc, and copper, while totally suppressing microbial nitrogen mineralization.

D. Reclamation of Sodic Soils

• The Fundamental Principle: You cannot simply wash sodic soils. You must first chemically replace the Na⁺ on the exchange sites with Ca²⁺, and only then leach the displaced sodium out of the profile.
• Step 1 (Apply Gypsum): Gypsum (CaSO₄·2H₂O) is the ultimate reclamation agent. The calcium forcefully kicks the sodium off the clay complex (Na-clay + CaSO₄ → Ca-clay + Na₂SO₄). A practical dose is 5 to 15 tonnes per hectare, applied annually over 3 to 4 years.
• Step 2 (Leach with Water): Once the gypsum has reacted, heavy irrigation is applied. The newly formed, highly soluble Na₂SO₄ dissolves and leaches safely below the root zone through subsurface tile drains.
• Alternative Amendments: Elemental sulfur (which bacteria oxidize into sulfuric acid to dissolve native calcium carbonate), iron pyrites, acid-forming fertilizers like ammonium sulfate, or calcium-rich sugarcane press mud.
• Crop Sequencing: During reclamation, plant highly tolerant Karnal grass (Leptochloa fusca), followed by nitrogen-fixing berseem, then water-tolerant rice, and finally wheat as the soil physically recovers.

20.4 Saline-Sodic Soils

• Definition: The absolute worst-case scenario. These soils possess BOTH high soluble salts (ECe > 4 dS/m) AND high exchangeable sodium (ESP > 15%), combining severe osmotic stress with devastating structural collapse.
• Occurrence: This typically happens when saline soils are mismanaged or receive insufficient leaching water, allowing sodium to progressively build up.
• pH: Usually remains below 8.5 because the heavy presence of neutral salts buffers the pH and prevents the formation of highly caustic Na₂CO₃.
• Strict Reclamation Sequence: 1. Apply Gypsum FIRST to replace the exchangeable sodium while the soil still has enough structure to allow water penetration. 2. Leach heavily with good water to simultaneously wash away the original soluble salts and the newly displaced sodium sulfate. 3. The Fatal Error: Never leach a saline-sodic soil without applying gypsum first. If you wash away the salts without replacing the sodium, the EC drops, the ESP spikes, and the soil instantly collapses into a purely sodic, impermeable concrete slab.

20.5 Saline vs. Sodic Soils — Complete Differentiation

• Electrical Conductivity (ECe): Saline soils have an ECe > 4 dS/m. Sodic soils have an ECe < 4 dS/m.
• Exchangeable Sodium Percentage (ESP): Saline soils have an ESP < 15%. Sodic soils have an ESP > 15%.
• Soil pH: Saline soils generally sit between 7.0 and 8.5. Sodic soils are strictly > 8.5 and can reach 10.0.
• Visible Appearance: Saline soils exhibit a white salt crust ("white alkali"). Sodic soils have no crust and appear dark or black due to dissolved organic matter ("black alkali").
• Soil Structure: Saline soil structure remains generally intact due to calcium dominance. Sodic soil structure is completely destroyed, dispersed, and forms hard columns.
• Water Infiltration: Saline soils have moderate to good infiltration. Sodic soils have near-zero infiltration and severe crusting.
• Dominant Cations: Saline soils are dominated by soluble Ca²⁺, Mg²⁺, and Na⁺ floating in solution. Sodic soils are dominated by Na⁺ locked onto the exchange sites.
• Primary Plant Problem: Saline soils cause severe osmotic stress (drought) and ion toxicity. Sodic soils create physical concrete barriers to roots, block aeration, and lock away micronutrients.
• Reclamation Strategy: Saline soils simply require heavy leaching with good water and drainage. Sodic soils strictly require a chemical amendment (Gypsum) first, followed by leaching and drainage.
• Indian Terminology: Saline soils are called Reh or Usar (partial). Sodic soils are called Usar (full), Khar, or Kallar.

20.6 Agro-techniques for Management of Problem Soils

A. Crop Choices During Reclamation

• Highly Salt-Tolerant Options: Growing crops like Sesbania (which acts as a green manure and biologically reclaims the soil), Karnal grass, or saltbush. These crops build organic matter and drive root channels through compacted soil while tolerating the harsh chemistry.
• Ideal Sequence for Saline Soils: Grow Sesbania and plow it in, followed by flooded Rice (which dilutes salts), and eventually Wheat as the drainage improves.
• Ideal Sequence for Sodic Soils: Establish Karnal grass first to break the hardpan, follow with Berseem to fix nitrogen, transition to Rice, and finally Wheat.

B. Agronomic Practices

• Raised Bed Planting: In saline soils, crops should be planted on the slopes of raised beds rather than flat ground. As water evaporates, the salts are physically drawn to the very top ridge of the bed, leaving a lower-salt "safe zone" for the roots on the sides.
• Pre-Sowing Irrigation: Applying a heavy dose of water right before sowing pushes the salts temporarily below the immediate seed zone, granting the seeds a crucial window of low-salt conditions to successfully germinate.
• Mulching: Applying a heavy straw or plastic surface mulch halts surface evaporation. This physically breaks the capillary action that constantly pulls deep salts back up to the surface.
• Correct Fertilization: In saline soils, farmers must apply extra potassium to chemically counteract the massive influx of competing sodium ions. Potassium sulfate is preferred over MOP to avoid adding excess chloride, and Calcium Nitrate is preferred over urea to supply beneficial calcium.
• Deficit Irrigation: Over-irrigating is the primary driver of secondary salinization. Farmers must utilize ET-based (Evapotranspiration) scheduling, watering strictly to field capacity to prevent the water table from rising.

📝 Exam Focus / Past Year Question (PYQ) Hooks

• PYQ 2019 Q6(b) 20M: Differentiate saline vs sodic soils; agro-techniques for management. → Start your essay by clearly laying out the 10 bullet points from Section 20.5 (Differentiation). Dedicate the second half of the essay to Section 20.6, detailing crop choices, raised beds, and pre-sowing irrigation to secure the full 20 marks.

• PYQ 2018 Q5(c) 10M: Problems and reclamation of saline and sodic soils. → Summarize Section 20.2C & D for saline soils (Osmotic stress + Leaching), then summarize Section 20.3C & D for sodic soils (Structure collapse + Gypsum). Keep it tight to roughly 250 words.

• PYQ 2017 Q4(c) 10M: Describe soil salinity; methods of soil reclamation. → Use Section 20.2A to define the exact EC, ESP, and pH parameters. Follow up with Section 20.2D, explicitly detailing the Leaching Requirement, drainage necessity, and the use of tolerant crops.

• PYQ 2020 Q7(b) 20M: What is problematic soil; how soil becomes problematic; reclamation methods. → Start with Section 20.1 to define the scope and extent. Outline both natural and human-induced formation causes. Then dive deeply into the distinct reclamation methods for saline (leaching) vs. sodic (gypsum) from Sections 20.2D and 20.3D