What Is Magnesium Sulphate? A Buyer's Guide to MgSO₄

Magnesium sulphate is an inorganic salt made of magnesium, sulphur and oxygen, with the formula MgSO₄. In trade it is almost always sold in a hydrated form — bound to water molecules inside the crystal — and the number of those water molecules decides how it is named, priced and used.

The three commercial forms

Heptahydrate is the familiar Epsom salt: clear to white crystals that dissolve quickly in water. Monohydrate is produced by drying off most of that water, which concentrates the magnesium. Anhydrous material has essentially all of the water removed and is used where moisture must be kept to a minimum.

Why the water matters to a buyer

Because water adds weight, a tonne of heptahydrate delivers less magnesium than a tonne of monohydrate. When comparing offers, always compare on nutrient or assay content, not only on price per kilogram. A cheaper heptahydrate can be the more expensive option per unit of magnesium delivered, and the reverse is also true once freight is counted.

Grades

The same molecule is sold into very different markets, so suppliers specify it by grade: fertilizer or agricultural grade, feed grade, technical or industrial grade, food grade, bath and spa grade, and pharmacopoeia grades such as BP, USP, IP and EP. Higher grades carry tighter limits on purity, heavy metals and insoluble matter, and must be supported by a batch-specific Certificate of Analysis.

Where it is used

• Agriculture — correcting magnesium and sulphur deficiency in crops.
• Animal feed — as a magnesium source in premixes.
• Industry — textiles, detergents, water treatment, and as a drying agent.
• Wellness — bath salts and body-care products.
• Food and pharma — under the relevant food and pharmacopoeia standards.

If you are new to buying it, start by deciding the form and grade your process actually needs. Everything else — crystal size, packing and price — follows from that choice.