Cathy, > Cathy Craig asked, > > > >In much of Southern California we have clay soil. ... Soils in So. CA respond to gypsum only if they contain an excess of the sodium ion. Unlikely to be the case unless your soil has saltwater intrusion or has been dredged from a saltwater channel of whatever kind. Even if those conditions exist, one would be wise to have a professional soil test done to confirm, or not, the presence of excess sodium. As was said in another post, thorough amending with organic matter -- compost -- is the key to managing soils, clay or not. Jean in Portland, OR, but previously from Long Beach, CA.