In article , Phil L
writes

Call me an old cynic but water is water! - and I said I doubt if any plants
could care either way, how can dissolved calcium and magnesium be of any
detriment to a plant? - it either utilises them or it doesn't, unless they
are active poisons to a plant I can't see how they would suffer.

As Keith said, the calcium can lock up necessary minerals, (IIRC iron).
--
Kay
"Do not insult the crocodile until you have crossed the river"