Alkaline soils, also known as sodic or saline-sodic soils, pose significant challenges for agriculture due to their high pH and the presence of excessive sodium ($Na^+$) ions. These conditions can lead to poor soil structure, reduced water infiltration, and impaired plant growth. Reclamation aims to correct these issues, making the soil suitable for cultivation.

Alkaline Soil Reclamation Explained

The most effective method for reclaiming alkaline soils involves a two-step process: chemical amendment using a calcium source and physical removal of the displaced salts.

The Role of Gypsum in Alkaline Soils

Gypsum, chemically known as calcium sulfate dihydrate ($CaSO_4 \cdot 2H_2O$), is the most common and effective amendment for alkaline soils. Its primary function is to provide calcium ions ($Ca^{2+}$) to the soil.

• In alkaline soils, sodium ions ($Na^+$) often dominate the soil's cation exchange complex, leading to dispersion of soil particles and poor soil structure.
• When gypsum is added, the calcium ions ($Ca^{2+}$) from the gypsum replace the sodium ions ($Na^+$) adsorbed onto the soil colloids.
• This cation exchange process improves soil structure by promoting flocculation (aggregation) of soil particles. The basic reaction can be represented as:

  $$ \text{Soil-Na} + \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \rightarrow \text{Soil-Ca} + 2\text{Na}^+ + \text{SO}_4^{2-} + 2\text{H}_2\text{O} $$

  (Note: $Na^+$ and $SO_4^{2-}$ become soluble salts in the soil water)

The Importance of Leaching

While gypsum replaces problematic sodium ions, the displaced sodium, now combined with sulfate ions to form sodium sulfate ($Na_2SO_4$) or remaining as sodium ions ($Na^+$) with other anions, must be removed from the root zone. This removal process is called leaching.

• Leaching requires applying excess water (irrigation) to the soil surface.
• Effective drainage is crucial to allow the saline water containing the displaced sodium salts to move downwards and away from the root zone.
• Without leaching, the sodium salts would remain in the soil, and the benefits of adding gypsum would be limited.

Why Gypsum and Leaching Together are Crucial

The combination of gypsum addition and leaching is essential for successful alkaline soil reclamation:

• Gypsum addresses the chemical imbalance by swapping sodium for calcium on soil particles.
• Leaching removes the resulting soluble sodium salts, which are now less harmful due to the presence of calcium and the improved soil structure.

This combined approach restores the soil's physical properties (improving structure, permeability, and aeration) and chemical properties (reducing pH and sodium levels).

Analysis of Other Options

• Leaching only: While leaching helps remove soluble salts, it does not address the high sodium adsorption ratio (SAR) caused by sodium fixation on soil colloids. It is effective for saline soils but not sufficient for sodic soils on its own.
• Addition of gypsum only: Gypsum helps replace sodium, but without leaching, the displaced sodium salts remain in the soil profile, potentially hindering plant growth and preventing full soil structure improvement.
• Provision of drainage: Drainage is a necessary component for leaching, but it doesn't provide the chemical amendment (calcium) needed to counteract the effects of excess sodium in the soil. Drainage alone cannot reclaim sodic soils.

Therefore, the addition of gypsum followed by leaching is the standard and most effective procedure for reclaiming alkaline (sodic) soils.