Organic Material, Ken. Tiffany this is so very true. My daughter's garden in Salt Lake City was nothing but a baked clay wasteland when they moved into their new house in 2007. On top of that (literally) the contractor had spread the excavated subsoil from the foundation over the whole back yard like a layer of icky peanut butter. We 'imported' 25 pickup loads of compost from the landfill there, planted what we could in the top layer, and, as you say, magic happened over the years and the clay became fertile soil as the organic material worked its way down into it. By 2013 when they moved, everything was growing so nicely . . .
Now they have a bigger house with lots of (ugghh) lawn and are learning the hard way how beautiful our perennial garden with no grass was. They got dogs, so we planted clover as a groundcover, but again, it improved the soil nicely.
Let's see if I can find 'before and after' pics for you:
