There is a discussion on the AHA Forum about adding magnesium to brewing liquor. I have wondered about this since my water is so low in magnesium (3 ppm). That is pretty low. I had heard, as had the poster, that you do not need to worry about the level because malt contained all of magnesium needed1. The poster points out that John Palmer cites a minimum of 10 ppm in How to Brew. Finally, the poster mentions his own experience in identical brews where the only deliberate difference was adding 10 ppm of magnesium — as magnesium sulfate, or Epson Salts — resulting in greatly improved fermentation.
Martin Brungard talks about seeing a paper that shows significantly enhanced yeast flocculation performance in wort with 5 ppm magnesium.
Denny Conn cites Tobias Fischborn’s 2009 NHC presentation, which mentions the importance of the calcium to magnesium ratio, though it gives no guidance on what it should be.
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According to Winning Homebrew it is from Greg Noonan’s book New Brewing Lager Beer, but it is also in George Fix’s book Principles of Brewing Science:
Magnesium ions also play an important role in yeast growth, primarily as a cofactor in metabolic reactions. Malt generally will provide sufficient magnesium for these purposes, even when the brewing water is low in this ion. Corrections with MgSO₄ additions are needed only with very high adjunct worts.