Magnesium Sulfate (MgSO4), AKA Epsom salts, is a very common vineyard amendment. It can be applied as a foliar spray during the growing season to provide vines with magnesium and sulfur. Magnesium, especially, is critical for photosynthesis. I talked to Hans Walter-Peterson, Viticultural Extension Specialist with Cornell University about the use of magnesium sulfate sprays in the vineyard and if we should be concerned about using them during the grape-growing year.
Basically, it’s fine to use during the growing year but you need to watch out as you approach harvest. As you allude to above, elemental sulfur is sometimes sprayed on vines to control powdery mildew and growers should be careful not to apply it too close to picking. Mr. Walter-Peterson tells me that MgSO4 will not break down into elemental sulfur, per se, but will break down into magnesium and sulfate ions. During fermentation, Saccharomyces cerevisiae can reduce environmental sulfate into sulfide. The yeast then incorporates some of the sulfide into amino acids, which can lead to the production of stinky volatile sulfur compounds (VSCs), with any excess converted to hydrogen sulfide (H2S). H2S can be a major sensory defect, stinking like rotten eggs. Therefore, the correct timing of MgSO4 sprays is just as important as elemental sulfur sprays.
Mr. Walter-Peterson shared some data from Cornell University that looked at the timing of sulfur sprays and its effects on hydrogen sulfide production in red and white wines. He summarized, “In a nutshell, for white winemaking, almost all sulfur residues settle out before fermentation, so there’s greater leeway there. For reds, if sulfur sprays stopped 6–8 weeks before harvest, very little elemental sulfur was found in must and thus little contribution to H2S formation.”
Since MgSO4 can release a sulfate ion, which just like elemental sulfur can continue down the sulfide reduction pathway to become H2S during fermentation, growers and winemakers need to be cognizant of when MgSO4 is applied. Mr. Walter-Peterson concluded, “So at a minimum and being super conservative, applying MgSO4 up to 5–6 weeks or so before harvest shouldn’t be a problem in reds. Applying on whites shouldn’t be a problem even closer to harvest.” I will leave it up to you to find your own comfort zone. Realistically, however, since MgSO4 is a foliar nutrient spray it is most often applied well before harvest, typically a few times around berry set. Mr. Walter-Peterson recommends a rate of 5–10 lbs. (2.3–4.5 kg) of MgSO4 to 100 gallons (380 L) of water per acre. Too close to harvest is too late to do your vines any good from a nutrient point of view so if you spray early, you don’t have to worry about MgSO4 contributing to sulfide and VSC defects in your wine.