Gypsum Works on Sodic Patches Only When Water Can Leave
Sodic soil is not merely salty. Excess sodium on clay particles makes the soil swell and seal. Water sits on the surface. Roots starve for air. A white or dark hard patch in the field often marks that chemistry. Gypsum can repair the patch. Drainage decides whether the repair lasts.
Sodium dominates the exchange sites in a sodic layer. Calcium from gypsum replaces that sodium. The displaced sodium must then leave with percolating water as sodium sulfate. If water cannot move down or out, the sodium stays in the root zone. The crust returns after the next irrigation.
Test before you spread. Measure pH, electrical conductivity, and exchangeable sodium percentage on the patch and on the good soil beside it. A gypsum requirement calculation uses those numbers and the depth you want to reclaim. A single bag rate copied from a neighbour wastes money on a mild patch and under-treats a severe one.
Apply gypsum to the wettable layer, then mix it into the soil. Surface dust that never meets the sodic clay does little. Follow with a leaching irrigation if quality water exists. Light, repeated leachings beat one drowning that waterlogs the profile. Rainy-season pulses can do part of this work on rainfed fields if the land is graded.
Drainage is the other half of the pair. Open drains, subsurface pipes, or even a deep furrow to a farm pond give the sodium-laden water a path. On flat black soils a small patch may only need a shallow interceptor on the upslope side. On a basin floor, gypsum without an outlet is cosmetic.
Amendments are not interchangeable. Gypsum supplies calcium without raising pH the way lime would. Elemental sulphur can help where calcium carbonate is already present and microbes can oxidise the sulphur. Pressmud and organic matter improve structure, but they do not replace the calcium math on a true sodic spot.
Watch the crop, not only the lab. Germination, early vigour, and standing water after a shower tell you if the seal has broken. Re-test after a season. A falling ESP and a falling pH in the treated layer mean the pair of techniques is working. A shiny crust after the first rain means drainage is still missing.
Common errors are predictable. Broadcasting gypsum on dry clods. Treating the whole field when only two corners are sodic. Blocking the outlet with a bund. Irrigating with high-SAR water after reclamation. Any one of those puts sodium back on the clay.
In short, gypsum is the chemical key. Drainage is the door. Use both on mapped patches, at a rate the soil test justifies. Then give water a way out, or the sodium will not leave.