Soil management in agronomy: for which soil problem is gypsum (CaSO4·2H2O) most beneficial as an amendment under field conditions?

Introduction / Context:
Soil amendments are chosen to correct specific chemical imbalances. Gypsum, a calcium sulfate dihydrate, is widely recommended to reclaim sodic (alkaline) soils by improving soil structure and permeability. Understanding what problem gypsum addresses prevents misapplication and wasted inputs.

Given Data / Assumptions:

• Gypsum supplies Ca2+ without raising pH significantly.
• Target scenario involves poor structure and dispersion due to excess Na+ on exchange sites.
• Irrigation and leaching are available to remove displaced sodium as Na2SO4.

Concept / Approach:
In sodic soils, Na+ saturates cation exchange sites, causing clay dispersion, crusting, and low infiltration. Gypsum provides Ca2+ that replaces Na+ on exchange sites. Subsequent leaching removes sodium salts, flocculating clay particles and restoring structure. Unlike lime, gypsum does not strongly increase pH and thus is suitable for alkaline soils.

Step-by-Step Solution:

Verification / Alternative check:
Soil science texts list gypsum as standard for sodic soil reclamation, often together with drainage and leaching plans.

Why Other Options Are Wrong:

• A: P problems are addressed by placement/liming; gypsum does not primarily solve P enrichment.
• B: Waterlogging is a drainage issue; gypsum cannot fix hydrology alone.
• D: Gypsum is not limited to “neutral saline”; it targets sodicity more than simple salinity.
• E: Acid soils are ameliorated by lime (CaCO3), not gypsum.

Common Pitfalls:
Confusing salinity (excess salts) with sodicity (excess Na+ on exchange sites); gypsum addresses sodicity.

Final Answer:
Alkaline (sodic) soils with high exchangeable sodium