Our yard, under about 4 inches of topsoil, is a very dense clay. We need to start amending the soil so we can better grow trees, plants, grass, etc. In areas where we do NOT have grass growing, I am going to be tilling in compost, gypsum, and topsoil. However, in areas where the grass is growing, can we fertilize with gypsum without damaging the grass (St. Augustine)? And can we do this in winter when the grass is dormant or will the gypsum act as a fertilizer?