Gypsum

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, overcropping 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. However it is also valuable for reducing excess sodium in soils, and for adding calcium.