Magnesium Sulfate — Frequently Asked Questions

Because the water of crystallisation is inert weight. The heptahydrate carries seven water molecules per magnesium and is only about 9.8 percent magnesium; the monohydrate is around 17 percent. Buying on price per tonne without reference to hydration is a common and costly error, since the cheaper-looking heptahydrate delivers far less nutrient per tonne.

The heptahydrate, Epsom salt, because it dissolves considerably faster and more completely than the monohydrate. For soil application the monohydrate, kieserite, is usually more economic since it is more concentrated and cheaper to freight, and dissolution speed matters less when the material has a season to act.

Sulphur, in immediately available sulfate form, which has become agronomically important as atmospheric sulphur deposition has declined. Supplying both nutrients from one salt is efficient, though where sulphur is not wanted, or where sulphate would precipitate with calcium in a fertigation tank, magnesium nitrate is the alternative.

To adjust the mineral profile of the brewing liquor. Magnesium is a yeast nutrient and contributes to enzyme activity during the mash, and sulfate accentuates hop bitterness and gives a drier finish. Brewers adjust the sulfate to chloride ratio deliberately to shift a beer towards crispness or towards fullness, and magnesium sulfate is one of the salts used to do it.