# Magnesium sulfate

> Magnesium sulfate heptahydrate, sold as Epsom salt, is the common hydro magnesium source, supplying about 9.9% magnesium and 13% sulfur.

- Canonical: https://genetixbay.com/knowledge/glossary/magnesium-sulfate.html
- Also written: Epsom salt, Epsom salts, MgSO4, magnesium sulphate, MgSO4·7H2O, epsom
- Glossary category: Nutrients · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Magnesium sulfate is the salt of magnesium and [sulfate](https://genetixbay.com/knowledge/glossary/sulfate.md). 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](https://genetixbay.com/knowledge/glossary/calcium-nitrate.md), because calcium and sulfate form gypsum. Growers using [RO water](https://genetixbay.com/knowledge/glossary/ro-water.md) 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](https://genetixbay.com/knowledge/glossary/foliar-feeding.md), 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](https://genetixbay.com/knowledge/glossary/sodium.md) [chloride](https://genetixbay.com/knowledge/glossary/chloride-nutrient.md), harmful in a feed.

## Related terms

[Magnesium](https://genetixbay.com/knowledge/glossary/magnesium.md), [Sulfate](https://genetixbay.com/knowledge/glossary/sulfate.md), [Cal-Mag](https://genetixbay.com/knowledge/glossary/cal-mag.md), [A/B nutrient](https://genetixbay.com/knowledge/glossary/a-b-nutrient.md), [Foliar feeding](https://genetixbay.com/knowledge/glossary/foliar-feeding.md)

## Read more

- [Hydroponic salts explained: what each fertiliser salt supplies](https://genetixbay.com/knowledge/nutrients/hydroponic-salts.md)
- [Magnesium (Mg) in cannabis: interveinal yellowing, targets and the K-Ca competition](https://genetixbay.com/knowledge/nutrients/magnesium.md)

## Sources

1. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6
2. Morad D, Bernstein N. 2023. Response of medical cannabis to magnesium (Mg) supply at the vegetative growth phase. Plants 12(14):2676. https://doi.org/10.3390/plants12142676