How Gypsum works
Each positively charged calcium molecule in gypsum attracts two clay particles and forms an aggregate. These aggregates form small spaces in the soil which allow air and water to penetrate. Clay particles are very fine and have almost no pore spaces in the soil.
In addition to this the sulfur molecules attract sodium and bind together. These are washed away when precipitation occurs.
The combination of these actions allows an improved soil structure to be formed and thus allows more root and shoot penetration of seedlings, hence increasing yield; and stops crusting of the surface of the soil.
Seedling root and shoots are not burnt off due to lack of calcium and sulfur because gypsum is a natural supplier of these essential soil nutrients.
Gypsum can also act to reduce soil pH and sodic levels over the long term. The addition of calcium in gypsum helps to restore the natural Ca:Mg ratios in soils. Austaralian soils can be high in Magnesium.