Magnesium Sulphate Heptahydrate vs Monohydrate: Epsom Salt or Kieserite?

Same compound, different water of crystallisation

Both products are magnesium sulphate. The difference is how much water is locked into the crystal. The heptahydrate carries seven water molecules per unit (MgSO4.7H2O) and is the familiar Epsom salt. The monohydrate carries one (MgSO4.H2O) and is the kieserite-type material.

That bound water is dead weight from a nutrient standpoint. Because the heptahydrate is mostly water by mass, its magnesium concentration is lower per tonne; the monohydrate packs more magnesium into the same weight of product. Neither is "better" in the abstract — they are simply built for different jobs.

Magnesium content: what you actually pay for

The heptahydrate (Epsom salt) supplies roughly 9.9% magnesium — close to the 9.86% theoretical value for pure MgSO4.7H2O, and Michigan State University Extension cites about 10% actual Mg. The monohydrate is far more concentrated, typically around 17% Mg in our high-purity grade, near the ~17.6% theoretical maximum for pure MgSO4.H2O. For context, naturally mined/granulated kieserite such as K+S ESTA Kieserit is specified around 25% MgO (~15% Mg) plus ~20% S, while a high-purity manufactured monohydrate is more concentrated still at roughly 29% MgO (~17.5% Mg).

Both forms also deliver sulphur as sulphate, which is plant-available and useful where soils are sulphur-short. The two hydrates carry sulphur at different densities: the heptahydrate is about 13% S and the monohydrate about 23% S (typical), so the monohydrate is the higher-magnesium and higher-sulphur product. In either case you are buying a two-nutrient product (Mg + S), not magnesium alone.

• Heptahydrate: ~9.9% Mg, ~13% S (typical)
• Monohydrate: ~17% Mg, ~23% S (typical)

All assays here are typical values and are confirmed per batch on the Certificate of Analysis (COA).

Solubility and application method

This is the deciding factor for most buyers. The heptahydrate is fully and rapidly water-soluble, which makes it the standard choice for anything that goes through water: foliar sprays, drip and pivot fertigation, and hydroponic stock solutions. It dissolves cleanly without clogging emitters or leaving residue in the tank.

The monohydrate is fully water-soluble thermodynamically, but its dense crystal dissolves more slowly, especially in cold water — the limit is dissolution rate, not solubility. That slower dissolution is a drawback in a spray tank but is rarely a problem in soil, where moisture and time release the magnesium over the growing period. It is therefore the practical choice for broadcast soil application and for blending into dry NPK formulations, where its higher magnesium density and granular form are advantages.

• Choose heptahydrate for: foliar feeding, fertigation, hydroponics, fast correction of visible Mg deficiency
• Choose monohydrate for: soil/broadcast application, bulk-blend and granular NPK production, sulphur + magnesium maintenance

Cost per unit of magnesium and freight

Headline price per tonne can mislead. Because the monohydrate carries roughly 17% Mg versus about 9.9% for the heptahydrate, it delivers far more magnesium per tonne shipped. For soil programmes priced on units of Mg, the monohydrate usually wins on cost-per-unit-Mg and on freight, since you are not paying to ship bound water across an ocean.

The heptahydrate commands its value where solubility is non-negotiable. If the magnesium has to dissolve fast and move through an irrigation line or a sprayer, the extra cost-per-unit-Mg buys reliable performance that the monohydrate cannot match in cold-water systems. The right comparison is always application-specific, not price-per-tonne alone.

Handling, storage, and blending

Both materials are stable and non-hazardous to handle, but they have different habits. The heptahydrate can cake or lose surface water if stored hot or poorly sealed, so keep it dry and cool. The monohydrate is the more robust, free-flowing option for warehousing and for mechanical blending lines, and its granular form mixes well with other dry fertilizers.

For distributors, a common pattern is to stock both: monohydrate granules for soil and blend customers, and heptahydrate crystals for greenhouse, fertigation, and foliar customers. Packaging (25 kg bags, jumbo/FIBC bulk bags) can be matched to each channel.

Quick decision guide

• Spraying it on leaves or running it through irrigation? Buy heptahydrate (Epsom salt).
• Spreading it on soil or blending it into granular NPK? Buy monohydrate (kieserite-type).
• Need the most magnesium per tonne of freight? Monohydrate.
• Need fast, residue-free solubility? Heptahydrate.
• Want sulphur alongside magnesium? Either — both are sulphate-based.