Epsom salts?

It supplements them with magnesium.
It is good to use when plants are magnesium deficient.

 

I'd only use epsom for a lack of Mg/magnesium. Mg-deficiency is easily fixed with X teaspoon/gallon of Epsom salts (first powdered and dissolved in some hot water) or foliar feed at X teaspoon/quart. When mixing up soil, use 2 teaspoon dolomite lime per gallon of soil for Mg. Mg can get locked-up by too much Ca, Cl or ammonium nitrogen. Don't overdo Mg or you'll lock up other nutrients.