@Eetela, it is generally based on soil analysis, in general, 12-15 tonnes of gypsum per hectare (50% of gypsum requirement of 0-15 cm soil as per soil analysis) is sufficient enough to reclaim upper 15 cm soil layer of your soil. The amendment is uniformly spread in the whole field and thoroughly mixed within 10 cm top soil layer, followed by watering for about 7-10 days to promote leaching and create conducive environment for ionic reactions at soil exchange complex. Most soils only require one yearly application, you may find immediate improvement within seven days and continued improvement over time. Expect results to last up to two years, though a soil evaluation after a year is recommended.

Thank you very much sir