This question concerns the process of soil reclamation, specifically addressing issues caused by the presence of sodium carbonate (Na₂CO₃) in the soil.

Soil Reclamation with Sodium Carbonate Issues

Soils containing high levels of sodium carbonate are often referred to as sodic soils. Sodium carbonate makes the soil alkaline and can lead to poor soil structure, dispersion of soil particles, reduced water infiltration, and difficulty for plant growth. To reclaim these soils, amendments that can replace the excess sodium (Na⁺) ions adsorbed onto soil particles with more beneficial ions, like calcium (Ca²⁺), are added. This process typically involves leaching, where the replaced sodium is washed away by water.

Gypsum Addition for Soil Improvement

Gypsum, chemically known as calcium sulfate dihydrate (CaSO₄·2H₂O), is commonly used for reclaiming sodic soils. Gypsum provides calcium ions (Ca²⁺) which are essential for improving soil structure in the presence of sodium.

Here's how gypsum helps:

• Gypsum dissolves in the soil water to release calcium ions (Ca²⁺) and sulfate ions (SO₄^2-).
• The calcium ions then replace the sodium ions that are bound to the soil's clay particles.
• This exchange process helps to aggregate the soil particles, improving soil structure, permeability, and aeration.

Chemical Reaction in Reclamation

When gypsum is added to soil containing sodium carbonate, a chemical reaction occurs. The calcium ions from gypsum react with the sodium carbonate, leading to the precipitation of calcium carbonate and the formation of sodium sulfate:

The reaction can be represented using the following chemical equation:

$CaSO_4 \cdot 2H_2O (s) + Na_2CO_3 (aq) \rightarrow CaCO_3 (s) \downarrow + Na_2SO_4 (aq) + 2H_2O (l)$

• Calcium Sulfate (Gypsum): $CaSO_4 \cdot 2H_2O$
• Sodium Carbonate: $Na_2CO_3$
• Calcium Carbonate: $CaCO_3$ (insoluble precipitate)
• Sodium Sulfate: $Na_2SO_4$ (soluble salt)

The resulting sodium sulfate is soluble in water and can be readily leached (washed out) from the soil profile by applying irrigation water. This removal of excess sodium is crucial for successful soil reclamation.

Evaluating Other Options

• Sodium sulphate ($Na_2SO_4$): Adding sodium sulfate would increase the overall salt content and wouldn't provide the necessary calcium ions to counteract the harmful effects of sodium carbonate.
• Iodine: Iodine is a micronutrient needed by plants in trace amounts and has no role in reclaiming sodic soils.
• Potassium: Potassium (K⁺) is a major plant nutrient. While important for plant health, it does not directly address the issue of sodium toxicity and poor soil structure caused by sodium carbonate.

Therefore, gypsum is the correct amendment added to reclaim land containing sodium carbonate through leaching.