Gypsum is a common hydrated calcium sulfate mineral. One of its benefits comes from its ability to break up heavy, compacted clay soils, as it can effectively change the soil structure of soils that have been compacted due to flooding, over cropping and heavy traffic. It has the ability to penetrate the millions of fine clay particles in heavy or hardpan type soils and loosen the soil structure. This process then creates air and moisture spaces that eventually loosen and break-up the soil structure. The only problem is that this doesn't happen overnight, it takes yearly applications, over about a three-year period to get the job done. For most home gardeners however, reducing compaction and improving aeration is better accomplished by the application of an organic mulch in nearly any landscape.
Gypsum is neutral and does not change the soil pH, thus making it useful for adding calcium without moving your ph of the soil. However it is also valuable for reducing excess sodium in soils. Excess sodium can interfere with nutrient as well as water uptake in the plants.
Lime and gypsum are not equal. Lime is calcium carbonate, while gypsum is calcium sulfate. Gypsum will add calcium to your soil without moving the ph while lime will move your ph upwards. As gypsum is more soluble than lime, it's calcium will become available more quickly to your plants. Careful however, as excess calcium levels in your soil can tie up the magnesium uptake, even when the magnesium levels are high.
