What Is Epsom Salt Made Of? | Magnesium Sulfate Facts

For the full breakdown, see our best Epsom Salt guide.

Despite the name, Epsom salt has nothing in common with table salt. Table salt is sodium chloride; Epsom salt is a naturally occurring mineral compound of magnesium, sulfur, and oxygen. The “salt” label sticks because the crystal structure looks similar, but the chemistry is entirely different.

The name comes from Epsom, England, where the bitter mineral spring water first yielded these crystals. The compound is also called magnesium sulfate heptahydrate or its mineral name, epsomite. If you’re wondering what Epsom salt is made of at the molecular level, the answer is one magnesium atom, one sulfur atom, four oxygen atoms, and seven water molecules locked into each crystal.

Chemical Composition: Magnesium Sulfate Heptahydrate

The standard household product is the heptahydrate form, meaning each molecule carries seven water molecules in its crystal structure. The chemical formula is MgSO4·7H2O. That water of crystallization is part of the crystal itself, not moisture on the surface. When Epsom salt dissolves in a warm bath, those bonds break and release the magnesium and sulfate ions.

Magnesium makes up roughly 10% of the compound’s weight.

Anhydrous magnesium sulfate without the water molecules exists, but it’s a laboratory reagent, not the product sold in garden centers and drugstores. The heptahydrate is what you’ll find in every bag and carton.

Nutrient Role for Plants and Soil

Gardeners value Epsom salt for two elements: magnesium and sulfur. Both are secondary nutrients that plants need in meaningful amounts, though less than the primary nitrogen-phosphorus-potassium trio.

Magnesium sits at the center of every chlorophyll molecule, which is why a deficiency shows up as yellowing between the leaf veins. Sulfur supports protein formation and enzyme function. When soil tests show a genuine shortage of either element, dissolved Epsom salt delivers both at once.

Most soils don’t need it, though. Routine application without a soil test can push magnesium levels too high, which interferes with calcium uptake. The smart approach is testing first; if the test confirms a magnesium or sulfur gap, Epsom salt is an effective remedy.

Epsom Salt vs. Regular Salt: What’s the Difference?

Is It the Same as What You Buy at the Drugstore?

The Epsom salt sold for soaking sore muscles is chemically identical to the product marketed for gardening. Both are magnesium sulfate heptahydrate. The difference is labeling, packaging, and purity standards for human consumption.

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