Gypsum/sheetrock and Dolomitic limestone would be more "permanent" than Alum.
Gypsum and Lime would allow constant leaching of what is needed for clearer water (positively charged Ions). Alum will stay in suspension, except for what attaches to it and settles out, and flow out with rain or during irrigation.
Alum is a GCS chemical...Generally Considered Safe....and should have no ill effects on plants.
There are some research articles that claim links between Alzheimers and Aluminum, but none I am aware offer any proof to the claims and are all ignored by the FDA.....No doubt it causes cancer in California though, but I think just living in Ca. may cause it.
