I asked a sponsored grower about his nutes and feeding and this is what he posted.....
......Epsom salt is the household name. It's also known as magnesium sulfate which is a little more revealing.
Here's a copy, and paste from Wikipedia.
"In gardening and other agriculture, magnesium sulfate is used to correct a magnesium or sulfur deficiency in soil; magnesium is an essential element in the chlorophyll molecule, and sulfur is another important micronutrient. It is most commonly applied to potted plants, or to magnesium-hungry crops, such as potatoes, roses, tomatoes, lemon trees, carrots and peppers.'
The JR Peters mix is low in both magnesium, and sulfur. So epsom salt is added to complete that mix. I use what's called the 3-2-1 formula. Three parts 5-12-26 Hydroponic, two parts 15-0-0 Calcium Nitrate, and one part epsom salt. Mixing is pretty easy. I use measuring scoop spoons. For both 5-12-26 Hydroponic, and 15-0-0 Calcium Nitrate I use a 1/4 teaspoon scoop, and the epsom salt I use a 1/8 scoop. They mix easily with tap water. In fact the formula is made to use with tap water. They have another formula for RO water if you wanna go that route. I've never tried it tho.