You are talking about Epsom Salts, and there are many opinions. They are supposed to be good for tomatos @ 1 tablespoon/month dug in around the base. Also they are good for bulbs. I use it with no problems. If you Google epson salts, you'll see a lot of discussions pro & con.

Epsom salts is really useful here, too, but I've noticed that it is used more in dryish climates, than in moist ones.