WTH is "Gypsum"

[DaveW]

The only problem with cactus experiments is they can take years to get definitive results. Some only re-pot every three years or so, therefore they would probably need leaving for that long in the same compost for meaningful results. Constant re-potting often mitigates the effects of gradual toxic build up in the compost but plants in habitat do not get regular soil changes, which was why when I started they used to recommend re-potting every year in fresh compost for maximum growth and to avoid salts build up.

[adetheproducer]

The only problem with cactus experiments is they can take years to get definitive results. Some only re-pot every three years or so, therefore they would probably need leaving for that long in the same compost for meaningful results. Constant re-potting often mitigates the effects of gradual toxic build up in the compost but plants in habitat do not get regular soil changes, which was why when I started they used to recommend re-potting every year in fresh compost for maximum growth and to avoid salts build up.

Maybe I should make up extra large batches of soil and store them so any repotting required the can go back into the same soil make it a long term experiment.