Gypsum is often advertised to gardeners as a great source of calcium, but Rocky Mountain soils are naturally high in calcium. Adding gypsum to soils in the Rocky Mountain area is useless.
It’s easy for gardeners to become frustrated with hard, clay soils and to try anything that sounds like it may work. The belief persists that adding gypsum can break up compact clay soils, but this is not true. Loosening soils is a physical process, not a chemical one. Tight, clay soils are loosened by mixing in organic amendments, or organic materials. The amendments hold the clay particles apart, creating more space for air, which is critical to root growth.
Improve the aeration and drainage of clay soils with organic amendments such as compost, peat and manure. Add a two-inch layer of organic material to the surface of the soil and till or spade it in to a four to six inch depth. When purchasing organic soil amendments, a recommended amount to buy would be four cubic yards for every thousand square feet of area.
For more information, see the following Colorado State University Extension fact sheet(s).