Heptahydrate vs Monohydrate vs Anhydrous: Which Magnesium Sulphate Do You Need?

All three forms are magnesium sulphate. The difference is how much water is locked into the crystal, and that single variable changes nutrient content, solubility, storage behaviour and landed cost.

Heptahydrate — MgSO₄·7H₂O

Roughly half the weight of heptahydrate is water. It dissolves fast and completely in cold water, which makes it the standard choice for fertigation, foliar sprays, bath products and most food and pharma uses. It is available in a wide range of crystal sizes, from fine powder to large 4–7 mm crystals. Its main drawback is that you pay freight on the water.

Monohydrate — MgSO₄·H₂O

Monohydrate carries much more magnesium per tonne, so it suits fertilizer blending, granulation and feed premixes where nutrient density and freight efficiency matter. It dissolves more slowly than heptahydrate, which is an advantage for some slower-release soil applications and a disadvantage for drip systems.

Anhydrous — MgSO₄

Anhydrous material is the most concentrated. It readily takes up moisture, which is exactly why it is used as a drying agent in laboratories and industrial processes. It needs dry storage and well-sealed packing.

A quick way to choose

Compare on delivered magnesium

Ask every supplier for MgO and assay figures, then divide the delivered price by the magnesium content. That gives a fair comparison across forms. Tell us your application and destination port and we will quote the form that gives you the lowest cost per unit of magnesium actually delivered.