"Then the advertising about "fast-acting" makes sense. I shouldn;t have poked fun at their fancy-shmancy bag and price."
It depends what causes the benefit, because just lowering the pH makes some nutrients more available if it's a little high for them. I read it as the "fast-acting" referred to the pH lowering. Didn't the old matches have more sulfur than modern safety matches? When I looked up the latter they appeared to be mostly phosphorous and potassium with just a little sulfur.
I would think gypsum would be available at farm supply stores but if Daniel's current product does what he wants without costing more there's no reason to switch unless one wanted maybe to avoid the presumably small amount of polyacrylamide.
