Magnesium Sulphate is more commonly known as Epsom Salts and has uses in both beer and wine. Magnesium is necessary for some chemical reactions to take place during the fermentation process.
In wine, a small dose of magnesium sulphate acts as a yeast nutrient, providing the necessary food for a strong yeast growth. The dose rate is half teaspoon per 5 litres of wine together with the yeast. It can also be used together with other nutrients to ensure a healthy yeast colony. If you live in a soft water area, then a pinch of magnesium sulphate in your water will be enough to ensure the correct reactions take place in a healthy fermentation.
Adding 1 teaspoon of magnesium sulphate in 5 gallons of water will roughly increase the magnesium ion levels by about 50ppm and the sulphate levels by about 210ppm.
Mixing both Magnesium Sulphate and Calcium Sulphate together will make a mixture that when added to a beer will give the character of Burton beers. This mixture is usually referred to as Burton Water Crystals.