Role of Amendments & Drainage in Reclamation of Problem Soils

Short summary first: Amendments correct the chemical defects of the soil (pH, exchangeable ions, nutrient availability). Drainage corrects the hydrological defects (excess water, movement/leaching of salts) and is often required for amendments to be effective. Together they convert unusable soil into a productive root zone.

1) Why both are needed

• Many problem soils (saline, sodic, saline-sodic, waterlogged) involve both chemical and hydraulic problems.
• An amendment (e.g., gypsum) may displace Na⁺ from exchange sites, but unless excess Na⁺ and soluble salts are physically removed by drainage + leaching, sodicity remains a problem.

2) Key soil-status parameters to measure (facts & thresholds)

• pH (water): acidity if < 6.5; strongly acid < ~5.5.
• Electrical Conductivity of saturation extract (ECe): saline if ECe > 4 dS m⁻¹.
• Exchangeable Sodium Percentage (ESP): sodic if ESP > 15%.
• Sodium Absorption Ratio (SAR) — indicator of sodium hazard in irrigation water:
• Always base amendment rates on a soil laboratory analysis (surface & subsoil) and water quality test.

3) Role of Amendments; materials, mechanisms & practical points

a) Lime (CaCO₃, Ca(OH)₂, CaO, dolomite) — for acid soils

Role / mechanism

• Supplies Ca²⁺ and carbonate/bicarbonate which neutralize H⁺ and raise pH.
• Precipitates toxic Al³⁺/Mn²⁺ as hydroxides (Al(OH)₃) → reduces Al toxicity.
• Improves Ca:Mg balance, stimulates microbial activity and N-fixation.

Practical points

• Lime requirement should be computed (SMP buffer or equivalent).
• Application methods: broadcast + incorporation to root zone; for surface acid conditions spreading before rainy season is usual.
• Dolomite (CaMg(CO₃)₂) is preferred when Mg is deficient.
• Lime reacts slowly in cool/dry soils — allow time (weeks–months) before sensitive crops.

b) Gypsum (CaSO₄·2H₂O) — for sodic & saline-sodic soils

Role / mechanism

• Supplies soluble Ca²⁺ → replaces exchangeable Na⁺ on clay surfaces:
  • 2Na – clay + Ca²⁺ →  Ca–clay + 2Na+
  • Displaced Na⁺ forms soluble salts (e.g., Na₂SO₄) that are leached out of the root zone by drainage/irrigation.
• Improves soil structure, reduces dispersion, increases infiltration & permeability.

Practical points

• Gypsum is effective only when followed by adequate leaching/drainage.
• Application form: broadcast and incorporate; in extreme cases gypsum bands at depth or amendments in furrows.
• Alternative Ca sources (sulphur + native CaCO₃, sulphuric acid, pyrite) convert native CaCO₃ to soluble Ca²⁺ but require careful handling.

c) Elemental sulphur, pyrite, sulphuric acid — acidifying amendments

Role / mechanism

• Microbial oxidation of S → H₂SO₄ which reacts with CaCO₃ releasing Ca²⁺ to replace Na⁺ in sodic soils or to reduce calcareousness locally.
• Used where direct gypsum supply is limited, but oxidation is slow and depends on good aeration and microbes.

Precautions; Acid produced can be strong — manage rates and monitor pH; do not apply blindly.

d) Acid-forming fertilizers — for calcareous soils Role; Ammonium fertilizers (NH₄⁺ salts) and single superphosphate create localized acidity that increases availability of P, Fe, Zn. Used with foliar/micronutrient applications to correct deficiency.

e) Organic amendments (FYM, compost, green manures, biochar)

Role

• Improve aggregation, porosity, infiltration and water-holding capacity.
• Provide organic acids and CO₂ that can mobilize Ca and P, improve microbial activity and buffer pH changes.
• Increase cation exchange capacity (CEC) and serve as slow release nutrient source.

Practical points; Organic matter is vital for long-term sustainability — often combined with chemical amendments for faster short-term results.

f) Industrial by-products (phosphogypsum, fly ash, etc.)

Role

• Can be Ca-sources (phosphogypsum) or structure-improving (fly ash) in constrained economies.
  Caveats Must be tested for contaminants (heavy metals, radioactivity, acids). Use only if certified safe.

Role of Drainage — types, principles & integration with amendments

Why drainage is essential

• Removes excess water (avoids anaerobiosis) and provides the physical pathway to remove soluble salts and displaced Na⁺ after amendment.
• Without drainage, salts/Na accumulate or simply move laterally — reclamation fails.

Types of drainage

• Surface drainage; Open ditches, graded beds, furrows, field channels. Rapid removal of ponded water and surface runoff.
• Subsurface drainage; Tile drains, perforated pipes, mole drains, gravel drains. Lowers water table and controls seepage; essential in areas with shallow water table.
• Pump/tubewell drainage; Active pumping of groundwater to lower water table where gravity drainage not possible.
• Bio-drainage; Use of deep or high water-use species (e.g., eucalyptus, poplar) to transpire excess groundwater over time — useful as a complementary, long-term measure.

Design and operational considerations

• Drainage intensity must match soil hydraulic conductivity, drainable porosity and inflow (seepage/irrigation).
• Effective drainage requires planned outlet, maintenance (desilting), disposal of saline drain water (avoid downstream salinization).
• Sequence matters: often build drainage channels before applying amendments; amendments plus drainage are the combined treatment.