Gypsum is applied to soil primarily to improve its structure, particularly in sodic soils where excess sodium ions are present. The core purpose of applying gypsum, as described in the reference, is to add large amounts Ca2+ ions that displace the Na+ ions from the exchange sites, and when flushed with clean water both salts and Na are removed from the soil.
Understanding Gypsum's Role in Soil
The application of gypsum (calcium sulfate, CaSO₄·2H₂O) facilitates a crucial chemical exchange process within the soil. Here's how it works based on the provided information:
- Adding Beneficial Ions: Gypsum introduces a high concentration of calcium ions (Ca²⁺) into the soil solution.
- Displacing Sodium: These introduced Ca²⁺ ions are more strongly held on the negatively charged soil exchange sites than sodium ions (Na⁺). Consequently, the Ca²⁺ ions displace the excess Na⁺ ions from these sites.
- Enabling Removal: Once displaced from the exchange sites, the Na⁺ ions become mobile in the soil water. The reference highlights that when flushed with clean water both salts and Na are removed from the soil. This flushing action is critical for carrying the now-soluble sodium out of the root zone, preventing its harmful effects on soil structure and plant growth.
This process is effective because of gypsum's properties. The reference notes that gypsum is used instead of limestone because of higher solubility and no increase in soil pH. Its solubility allows the calcium to become available in the soil water to initiate the displacement reaction, and it achieves the desired sodium removal without raising the soil pH, which can be beneficial in neutral to alkaline soils already struggling with sodicity.
Methods of Application: What the Reference Implies
While the provided reference clearly explains why gypsum is applied and how it works chemically once it's in the soil (adding Ca²⁺, displacing Na⁺, and enabling removal via flushing), it does not specify the exact physical methods used to get the gypsum onto or into the soil.
However, the description of the mechanism (adding Ca2+ ions that displace the Na+ ions... and when flushed with clean water both salts and Na are removed from the soil) implies that for this process to occur effectively, gypsum must be applied in a way that:
- Allows it to come into contact with the soil particles and soil water.
- Enables the dissolution of gypsum to release Ca²⁺ ions.
- Facilitates the passage of water through the soil profile for flushing displaced sodium and salts.
Therefore, while the specific application techniques (such as broadcasting, banding, or dissolving in irrigation water) are not detailed in the reference, the described function necessitates methods that ensure the gypsum interacts appropriately with the soil environment to initiate the chemical reactions and allow for subsequent leaching.
Summary Table: Gypsum's Action (Based on Reference)
| Action in Soil | Result | Benefit Over Limestone |
|---|---|---|
| Adds large amounts Ca²⁺ | Displaces Na⁺ from exchange sites | Higher solubility |
| Followed by flushing | Removes displaced Na⁺ and salts from the soil | No increase in soil pH |
| Chemical exchange | Improves soil structure (by removing dispersive Na⁺) | N/A |
In conclusion, the application of gypsum, as described in the reference, is a targeted approach using its chemical properties to amend sodic soils by replacing harmful sodium with beneficial calcium and allowing the sodium to be flushed away with water. The application method itself must support this chemical process.
