Reclamation Methods for Problematic Soils

Problematic soils are characterized by chemical, physical, or biological imbalances that severely restrict root proliferation, water infiltration, and nutrient uptake, leading to drastically reduced crop yields. In Indian agriculture, these soils are broadly categorized into saline, sodic (alkali), and acid soils based on their pH, Electrical Conductivity (EC), and Exchangeable Sodium Percentage (ESP).

• Saline Soils: Characterized by an Electrical Conductivity (EC) of saturation extract greater than 4 dS m^-1 at 25 degrees Celsius, Exchangeable Sodium Percentage (ESP) less than 15, and pH usually less than 8.5. They contain an excess of neutral soluble salts such as chlorides and sulfates of sodium, calcium, and magnesium, creating high osmotic pressure.
• Sodic / Alkali Soils: Characterized by an EC less than 4 dS m^-1, ESP greater than 15, and pH ranging between 8.5 and 10.0. The dominant cation on the exchange complex is sodium, leading to severe dispersion of clay particles, poor structure, and low permeability.
• Acid Soils: Characterized by a pH below 6.5, caused by the accumulation of exchangeable hydrogen and aluminum ions, coupled with deficiencies in basic cations like calcium, magnesium, and potassium, alongside toxic levels of active aluminum and manganese.

Saline soils, often referred to as 'White Alkali' due to the surface encrustation of white salts, do not require chemical amendments because they already contain sufficient calcium and magnesium salts. Their reclamation relies primarily on physical removal and leaching of soluble salts through the root zone via adequate drainage.

• Leaching Requirement (LR): The fraction of irrigation water that must be leached through the root zone to maintain soil salinity at a specified level. It is calculated as LR = (EC_iw / EC_e), where EC_iw is the electrical conductivity of irrigation water and EC_e is the electrical conductivity of the saturation extract tolerated by the crop.
• Drainage Installation: Sub-surface drainage (tile drains or perforated PVC pipes) is critical to lower the shallow water table below the critical depth (usually 1.5 to 2.0 meters) and prevent capillary rise of saline groundwater.
• Agronomic Practices: Practicing ridge-and-furrow planting helps protect germinating seeds from salt accumulation at the soil surface, as salts tend to accumulate on the ridge tops during evaporation. Mulching with organic crop residues reduces surface evaporation and subsequent salt deposition.
• Salt-Tolerant Cropping: Cultivating tolerant crops during initial phases, such as barley (Hordeum vulgare), cotton (Gossypium hirsutum), sugar beet (Beta vulgaris), and dhaincha (Sesbania aculeata), aids biological amelioration.

Sodic soils, commonly known as 'Black Alkali' or 'Usar' soils, exhibit high ESP and pH due to sodium carbonate and bicarbonate. Reclamation requires chemical amendments that supply soluble calcium to replace exchangeable sodium on the soil exchange complex, followed by leaching of the displaced sodium.

• Gypsum (Calcium Sulfate Dihydrate, CaSO₄.2H₂O): The most widely used, economical, and effective amendment. Gypsum application neutralizes alkalinity by releasing calcium ions, which replace sodium on the exchange sites: 2Na⁺-Clay + CaSO₄ -> Ca²⁺-Clay + Na₂SO₄. The soluble sodium sulfate is subsequently leached out.
• Gypsum Requirement (GR): The quantity of pure gypsum required to reduce the ESP of a specific depth of soil to the desired level. Gypsum requirement is typically calculated based on the Exchangeable Sodium to be replaced, where 1 milliequivalent of exchangeable sodium per 100 g of soil requires approximately 1.72 tonnes of 100% pure gypsum per hectare-15 cm soil depth.
• Alternative Amendments: For calcareous sodic soils or under flooded conditions, elemental sulfur (S), pyrites (FeS₂), sulfuric acid (H₂SO₄), and calcium chloride (CaCl₂) are utilized. Pyrites act via microbial oxidation to produce sulfuric acid, which subsequently reacts with calcium carbonate to form gypsum in situ.
• Green Manuring: Incorporating green manure crops such as Sesbania aculeata (Dhaincha) at 45 to 60 days after sowing adds organic acids, reduces pH, increases carbon dioxide partial pressure, and improves soil physical structure.

Acid soils, prevalent in high-rainfall regions and lateritic zones, restrict crop growth due to aluminum and manganese toxicity, phosphorus fixation by iron and aluminum oxides, and reduced microbial activity, especially symbiotic nitrogen fixation.

• Liming Materials: The primary method of ameliorating soil acidity is the application of agricultural lime. Common liming materials include calcitic limestone (CaCO₃), dolomitic limestone [CaMg(CO₃)₂], quicklime (CaO), and slaked lime [Ca(OH)₂].
• Neutralization Reaction: Liming materials dissolve in soil water and react with hydrogen and aluminum ions, raising the soil pH and precipitating toxic aluminum as insoluble aluminum hydroxide: Al³⁺ + 3OH^- -> Al(OH)₃.
• Lime Requirement (LR): Determined by buffer methods (such as Shoemaker, McLean, and Pratt buffer method) to neutralize active and exchangeable acidity to a target pH (typically 6.0 to 6.5). Application is generally recommended 3 to 4 weeks prior to sowing to allow complete chemical reaction.
• Phosphorus Management: Acid soils fix applied soluble phosphorus as insoluble iron and aluminum phosphates. Band placement of phosphatic fertilizers or co-application with farmyard manure (FYM) reduces P fixation and enhances availability.

Managing crops in remediated problematic soils requires tailored agronomic interventions, precise seed rates, specialized fertilizer schedules, and vigilant plant protection to ensure economic yields.

• Nutrient Adjustments: Sodic soils typically require 25% higher nitrogen doses than normal soils because nitrogen losses through ammonia volatilization are high under alkaline conditions. Conversely, phosphatic and potassic fertilizers should be applied based on soil test reports, with micronutrients like zinc sulfate (ZnSO₄ at 25 kg ha^-1) applied basal, as zinc deficiency is ubiquitous in sodic soils.
• Seed Rate and Spacing: In saline or sodic soils, seed rates are increased by 20% to 25% compared to normal soils to compensate for lower germination percentages. Spacing is kept slightly closer to ensure optimal plant population despite localized mortality.
• Irrigation Water Quality: Irrigation water quality is evaluated using three primary indices: Electrical Conductivity (EC), Sodium Adsorption Ratio [SAR = Na⁺ / √((Ca²⁺ + Mg²⁺) / 2)], and Residual Sodium Carbonate [RSC = (CO₃^2- + HCO₃^-) - (Ca²⁺ + Mg²⁺)]. High RSC water causes rapid sodication of soil and must be treated with gypsum before application.