As that amazing self-sufficiency guru John Seymour said, you do not feed plants, you feed the soil. Adding a manufactured inorganic chemical to the soil is not feeding the soil. It will instantly kill soil organisms and will hinder the soil's job of sustaining your crop. Make your own compost or compost some stable manure, then add that to your soil. Epsom salts can add only magnesium. If your tomatoes can't manage without the magnesium in Epsom salts, you're not managing your soil properly. If you really must spray your plants with anything, make it organic liquid seaweed extract. That contains all the magnesium your plants need and a whole host of other micronutrients too. If you can get your hands on seaweed, hose away the salt thoroughly then lay the seaweed around your plants. No need to compost it first. Anything made from seaweed, as long as you've washed away the salt, has an amazing beneficial effect on soil organisms. And grow some of that comfrey.

I've been gardening this way for over forty years, over thirty on hungry, sandy soil, have always had big, healthy crops as long as I haven't succumbed to laziness, and have never resorted to Epsom salts or any other inorganic fertiliser. Oh, and I have a degree in botany...