Epsom salt is good for tomatoes as it is rich in both magnesium and sulfur, which are critical for healthy plant growth. Magnesium helps the plant to uptake nitrogen and phosphorus, which fuel both leaf and root growth, and sulfur helps produce vitamins and amino acids.
Sulfur is rarely deficient in soil, but magnesium is commonly lacking. If a tomato plant has yellow leaves or is showing signs of blossom end rot, then the plant is likely not getting enough magnesium from the soil. Epsom salt can be added to the soil directly at the roots when planting or sprayed on as a foliar spray on the leaves as the plant grows.