GYPSUM/AGRICULTURAL LIME!!!! I took 2 semesters of "Soil Science" in college (in Ca.), and that's what helps to turn clay into soil, with the addition of plenty of organic matter. Unfortunately, it can take years of doing what you're doing (with the addition of ag. lime). It has to do with breaking apart the cations (+) in the clay and getting them to bind with other atoms (nitrogen, sulfur, phosphorous, etc). It CAN be done, and is all the time by farmers, but it just takes a long time!