Joseph, i think you're on the right track there with the gypsum idea. Perhaps you should try pure gypsum?
Check out the quote below from Noah and Forbes on their visit to Mexico:
Also, the plant habit was much more similar to the P. medusina plants we had seen than to our previous experience with P. heterophylla. But this plant couldn't be P. medusina! After all, P. medusina, as we all know, grows in Gypsum soils and is red in colour!