I have never had an OD of Epsom Salts...just start slowly with small amounts to test. Another way to get the Epsom salts to the plant is a light foliar spray of water and Epsom salts. Spray plants in the morning or evening when the lghts are off so the water doesn't give your plants a sunburn.
I usually use no more than 1 tblsp. per gallon in my garden and never ever dose more than once every 3 or 4 moss lightly. Don't soak the soil with the stuff just a light wetting will do. You can easily get out of balance the other way with this stuff.... Works great on making peppers produce.