It does depend upon the plant, of course. Epsom salts is magnesium sulphate and is very soluble. Magnesium is a component of fertilizer as a trace element, so it's a supplement, not a substitute for fertilizer. But some plants (like brugs) use a lot more of it than the 'trace' they get in fertilizer. Big, fast growing plants that really like it want a continuous supply, so you can sprinkle it on the soil (but a heavy rain will dissolve it fast) or dissolve it in a watering can every week or two.
I dissolve it in water and water it in, for my big plants in pots and for shrubs in the spring when they are really growing fast. About 3 tablespoons to a gallon. For my orchids, I add it every time I spray them with orchid fertilizer. They love it, especially in cold weather. But I only use a couple of teaspoons per gallon on them as they are foliar feeders, and soluble salts left sitting on the leaves can be a problem in hot weather here.
Palms really like it as well. You can sprinkle it around the palm on the ground, but I buy a specially formulated palm fertilizer that has extra magnesium in it, encapsulated to release slowly so I don't have to remember to water the palms with the watering can.
I'm not absolutely sure, but I think it would be pretty hard to harm a plant with too much Epsom salts, since it is so very soluble. But in high doses it could alter the pH of the soil temporarily, so I still wouldn't put a whole bunch of straight crystals on a small plant. For small potted plants, dissolve it and water it in.
