Magnesium sulfate (or Epsom salt) is used to correct magnesium deficiency which can be common in the hydroponic production of vegetable crops.  It is perfect for use as a part of a hydroponic nutrient solution because it is inexpensive, highly soluble and naturally occurring.

This agricultural-grade magnesium sulfate heptahydrate (MgSO4•7H2O) is an extremely soluble and bioavailable source of magnesium and sulfur for the health of animals and plants.