Magnesium sulfate is the salt of magnesium and sulfate. The form used in growing is the heptahydrate (MgSO4·7H2O), the same chemical as pharmacy Epsom salt, at about 9.9% Mg and 13% S. Anhydrous and monohydrate forms are stronger per gram, so check which you have.
In practice
It belongs in the B stock, away from concentrated calcium nitrate, because calcium and sulfate form gypsum. Growers using RO water or coco often add it to bring Mg into the 40-70 mg/L (ppm) range. Pharmacy grade works, but avoid scented bath products, which carry oils and dyes. It is sometimes used as a foliar spray, though root feeding is more reliable.
Units and formula
1 g/L of the heptahydrate gives about 99 mg/L (ppm) Mg and 130 mg/L S.
Don't confuse with
- Magnesium nitrate: Mg plus nitrogen, can share a stock with calcium.
- Cal-Mag: a blend, not the same as Epsom salt.
- Table salt: sodium chloride, harmful in a feed.
Sulfate (SO4 2-) is the form in which plants take up sulfur, delivered mainly by magnesium sulfate and potassium sulfate in mineral feeds.Calcium nitrate is the standard hydroponic calcium source, supplying about 19% calcium and 15.5% nitrogen, mostly as nitrate.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.Foliar feeding is spraying a dilute nutrient solution onto leaves for quick uptake through the leaf surface; it suits micronutrient fixes but cannot replace root feeding.Magnesium nitrate supplies about 9.5% magnesium and 11% nitrogen as nitrate, and unlike Epsom salt it can share a concentrated stock with calcium nitrate.Cal-Mag is a supplement supplying calcium and magnesium, usually as nitrates; need depends on cultivar, water source (RO vs tap) and medium (coco), not on LED lighting.Sodium (Na) is not essential for cannabis; it enters through water, softeners and some inputs, and at high levels causes salt stress and crowds out potassium and calcium.As a plant nutrient, chlorine is used as the chloride ion (Cl-) in tiny amounts for photosynthesis and osmotic balance; excess from water or salts is the usual problem.Magnesium (Mg) is a mobile secondary nutrient at the centre of every chlorophyll molecule; deficiency causes interveinal yellowing on older leaves.An A/B nutrient is a mineral feed split into two concentrates so that calcium (part A) is kept apart from sulfate and phosphate (part B), which would precipitate together.
