🧫 Reclamation of Sodic Soils

This lesson explains key concepts in a structured way and connects them to practical agricultural applications and exam-oriented understanding.

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Reclamation of Sodic Soils

Principles of Reclamation

Unlike saline soils (where simple leaching removes soluble salts), sodic soil reclamation requires the replacement of exchangeable sodium on the clay complex with calcium ions. This is achieved by applying calcium-rich amendments, followed by leaching the displaced sodium out of the root zone. The process restores soil structure, improves infiltration, and lowers pH.

Chemical Amendments

Gypsum (CaSO4.2H2O)

Gypsum is the most widely used and economical amendment for sodic soil reclamation. When applied to soil, it dissolves slowly and provides calcium ions that replace sodium on the exchange complex:

Na-clay + CaSO4 --> Ca-clay + Na2SO4 (leached out)

Gypsum Requirement (GR)

The gypsum requirement is the amount of gypsum needed to replace a specified percentage of exchangeable sodium. It depends on the initial ESP, target ESP, CEC, and soil depth to be reclaimed.

GR (tonnes/ha) = (Initial ESP - Desired ESP) x CEC x 0.086 x bulk density x depth

CSSRI, Karnal recommends applying gypsum at 50% of the calculated GR for the top 15 cm soil depth for practical and economical reclamation.

Other Amendments

• Calcium chloride (CaCl2) — more soluble than gypsum and works faster; but more expensive
• Sulfuric acid (H2SO4) — directly lowers pH and reacts with native CaCO3 to release calcium; used in soils rich in free lime
• Eleite sulfur — microbially oxidized to sulfuric acid; slow-acting but effective in calcareous sodic soils
• Iron pyrites (FeS2) — oxidized to sulfuric acid; available as a byproduct of mining
• Press mud (sugarcane filter cake) — organic amendment that provides both calcium and organic acids; effective as a partial substitute for gypsum

Reclamation Procedure

The standard reclamation technology developed by CSSRI involves the following steps:

1. Land leveling — to ensure uniform water distribution
2. Bunding — construction of field bunds to retain ponded water
3. Gypsum application — broadcast gypsum uniformly over the soil surface
4. Mixing — incorporate gypsum into the top 10–15 cm by plowing
5. Ponding and leaching — flood the field and maintain standing water for 2–3 weeks; drain and repeat 2–3 times to leach displaced sodium
6. Rice cultivation — grow rice as the first crop (tolerant of residual sodicity and standing water aids further leaching)
7. Subsequent crops — wheat in rabi season, followed by gradual intensification of cropping

Phytoremediation

Phytoremediation is a biological approach to sodic soil reclamation using specific plant species whose root activity facilitates sodium displacement:

• Root respiration releases CO2, which forms carbonic acid (H2CO3) in soil solution, dissolving native CaCO3 to release Ca2+ that replaces Na+
• Root exudates (organic acids) further solubilize calcium from carbonates
• Improved structure — root channels improve macroporosity and water movement

Effective phytoremediation species include Sesbania (dhaincha), Leptochloa fusca (kallar grass), Sporobolus (salt grass), and Prosopis juliflora. Dhaincha (Sesbania aculeata) is particularly effective as it can be grown as a green manure crop, adding organic matter and nitrogen while reclaiming sodic soils.

Phytoremediation is slower than chemical reclamation but is cost-effective, environmentally sustainable, and particularly suitable for resource-poor farmers who cannot afford gypsum.

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Summary Cheat Sheet

Key Recall Points

• Gypsum is the main amendment to replace exchangeable sodium in sodic soils.
• Correct gypsum requirement and incorporation decide field-level reclamation speed.
• Sustained improvement needs drainage, cropping strategy, and organic inputs.

Exam Traps

• Reclamation is not complete just after amendment application; monitoring is essential.
• High amendment dose cannot compensate for poor water movement.
• Phytoremediation supports but does not instantly replace chemical reclamation.

References

3 sources • [1] [2] [3]

[1]

CSSRI Protocols on Gypsum Requirement and Sodic Soil Reclamation

Official

[2]

ICAR Recommendations for Amendment-Based Reclamation

Official

[3]

Soil Amelioration Chapters in Standard BSc Agriculture Texts

Book