A practical guide to magnesium sulfate, why the heptahydrate and monohydrate suit different jobs, and how it is supplied, from a US-based distributor.
Magnesium sulfate (MgSO4, CAS 10034-99-8 heptahydrate) is the soluble magnesium and sulfur source of choice where nutrients have to be available immediately. The heptahydrate, Epsom salt, carries roughly 9.8% magnesium and dissolves fast; the monohydrate, kieserite, carries roughly 17% and suits dry blending. Stratus Chemical supplies both in bags and supersacks across the United States.
| Property | Value |
|---|---|
| Chemical name | Magnesium sulfate |
| Synonyms | Epsom salt (heptahydrate), kieserite (monohydrate), magnesium sulphate, bitter salt |
| CAS number | 10034-99-8 (heptahydrate), 14168-73-1 (monohydrate), 7487-88-9 (anhydrous) |
| Molecular formula | MgSO4·7H2O, MgSO4·H2O |
| Molecular weight | 246.47 g/mol (heptahydrate), 138.38 g/mol (monohydrate) |
| EC / EINECS | 231-298-2 |
| Appearance | White crystal |
| Water solubility | Freely soluble |
| Grades supplied | Heptahydrate, monohydrate; agricultural / feed / technical |
| Packaging | Bags, supersacks |
| Documentation | COA & SDS provided on request |
The figures below are representative of feed and fertilizer grade magnesium sulfate heptahydrate. They are indicative rather than guaranteed: specifications vary by source and by lot, and the certificate of analysis for your material governs.
| Parameter | Specification |
|---|---|
| Purity | 99.5% min |
| Water soluble MgO | 16.26% min |
| Water soluble SO3 | 32.46% max |
| Magnesium (Mg) | 9.80% min |
| Sulfur (S) | 12.90% min |
| pH | 5.5 – 7.5 |
| Loss on ignition | 49 – 50% |
| Grain size | 35 mesh |
| Appearance | White crystal |
| Arsenic (As) | 1 mg/kg max |
| Cadmium (Cd) | 2 mg/kg max |
| Fluorine (F) | 150 mg/kg max |
| Lead (Pb) | 10 mg/kg max |
| Mercury (Hg) | 0.1 mg/kg max |
| Nitrites (as sodium nitrite) | 15 mg/kg max |
| Dioxins | 0.75 ng WHO-PCDD/F-TEQ/kg max |
| Dioxins + PCBs | 1.0 ng WHO-PCDD/F-PCB-TEQ/kg max |
| Non-dioxin-like PCBs | 10 µg/kg max |
| Packing options | 50 lb bags, big bags / totes |
The loss on ignition figure of 49–50% is not a contaminant reading. It is the water of crystallisation being driven off, and for a heptahydrate that is close to half the weight of the product. Seeing it on a certificate and mistaking it for volatiles or impurity is a common misreading.
Both are magnesium sulfate and both are soluble. The choice is between concentration and dissolution speed, and it is the main specification decision.
The heptahydrate, long known as Epsom salt, carries seven waters of crystallisation and about 9.8% magnesium. It dissolves quickly and completely in cold water without agitation, which is exactly what a grower mixing a foliar spray in a tank in the field needs, and what a fertigation system requires to avoid residue in the lines.
The monohydrate, kieserite, carries one water and roughly 17% magnesium. Nearly twice the nutrient per ton means substantially less freight and storage for the same magnesium delivered, and it handles better as a dry, free-flowing material in bulk blends. It dissolves more slowly, which is fine for soil application and irrelevant in a dry blend.
The rule of thumb is simple: if it has to dissolve on site, buy the heptahydrate; if it goes into a dry blend or onto soil, the monohydrate usually wins on delivered cost.
The primary use for the heptahydrate. Magnesium sits at the centre of the chlorophyll molecule, so deficiency shows up directly as interveinal chlorosis on older leaves and as lost yield. Magnesium sulfate corrects it fast because both nutrients are immediately plant-available, which is what separates it from magnesium oxide.
Monohydrate goes into bulk NPK blends and direct soil application, supplying magnesium and sulfur together without altering soil pH. That neutrality matters where the ground is already at target pH and a liming magnesium source would push it too far.
Magnesium supports energy metabolism, nerve function, and muscle health in livestock. Feed grade magnesium sulfate is used in mineral premixes and is valued for its high bioavailability and quick dissolution, particularly in drinking water and liquid supplement systems.
Magnesium sulfate is used in textile dyeing and sizing, in paper manufacture, as a drying agent, in cement and construction products, and as a nutrient in fermentation.
| Magnesium source | How it behaves | Trade-off |
|---|---|---|
| Magnesium sulfate heptahydrate | Approximately 9.8% Mg, dissolves fast in cold water, pH neutral | The right product for foliar and fertigation, but the most freight per unit magnesium |
| Magnesium sulfate monohydrate | Approximately 17% Mg, soluble but slower, pH neutral | Better delivered cost for soil and dry blends; not ideal for tank mixing |
| Magnesium oxide | Highest analysis at roughly 60% Mg, slightly soluble, alkaline | Cheapest per unit magnesium and limes as it feeds, but far too slow to correct a deficiency in-season |
| Dolomitic lime | Supplies magnesium and calcium with strong liming action | Very cheap per ton, but low analysis and unusable where pH is already adequate |
How to choose: work from soil pH and from how fast you need the magnesium. Where pH is already at target, a neutral sulfate is the only sensible option, because an alkaline source will overshoot. Where the crop is showing deficiency now, the sulfate is the answer regardless of pH, since the oxide releases over months. Where you are building soil magnesium ahead of a season on acidic ground, magnesium oxide or dolomitic lime does the job for far less money. See our crop micronutrients guide for how magnesium interacts with potassium and calcium uptake.
Store magnesium sulfate dry in closed original packaging, off damp floors. Both hydrates cake if they take on moisture, and caked material will not meter or blend evenly.
The heptahydrate additionally effloresces: in warm, dry air it slowly gives up water of crystallisation and shifts toward a lower hydrate. The material is still usable, but the analysis drifts upward as water is lost, so keep it sealed and out of heat where the assay matters to a formulation.
Magnesium sulfate presents a low hazard in normal industrial handling. Use the dust and eye protection specified in the Safety Data Sheet for the specific grade.
Magnesium sulfate is used chiefly as a soluble magnesium and sulfur source in agriculture, correcting magnesium deficiency through fertigation, foliar spray, and soil application. It is also used as a magnesium supplement in animal feed, and industrially in textile dyeing and sizing, paper manufacture, as a drying agent, and in construction products. Stratus Chemical supplies heptahydrate and monohydrate forms in agricultural, feed, and technical grades across the United States.
Magnesium sulfate has a different CAS number for each hydrate. The heptahydrate, commonly called Epsom salt, is 10034-99-8. The monohydrate, kieserite, is 14168-73-1, and the anhydrous form is 7487-88-9. All share EC number 231-298-2. Molecular weights are 246.47 g/mol for the heptahydrate, 138.38 g/mol for the monohydrate, and 120.37 g/mol anhydrous.
They are the same compound at different levels of hydration. Epsom salt is the heptahydrate, carrying seven waters of crystallisation and about 9.8 percent magnesium, and it dissolves quickly and completely in cold water. Kieserite is the monohydrate, carrying one water and roughly 17 percent magnesium, so it delivers nearly twice the nutrient per ton and costs far less to freight, but dissolves more slowly. Use the heptahydrate where it has to dissolve on site and the monohydrate for dry blends and soil application.
Because that is the water of crystallisation being driven off, not a contaminant or impurity reading. Magnesium sulfate heptahydrate carries seven water molecules, which account for close to half the weight of the product, so a loss on ignition of 49 to 50 percent is exactly what a correct heptahydrate should show. Mistaking it for volatiles or impurity is a common misreading of the certificate.
It depends on soil pH and on how quickly the magnesium is needed. Magnesium sulfate is fully soluble, pH neutral, and immediately available, which makes it the only real option for foliar spray, fertigation, and correcting a deficiency the crop is already showing. Magnesium oxide carries far more magnesium per ton and costs less to freight, but it is only slightly soluble, releases over months, and is alkaline, so it works as a soil amendment on acidic ground rather than as an in-season correction.
Store it dry in closed original packaging, off damp floors, since both hydrates cake if they take up moisture and caked material will not meter or blend evenly. The heptahydrate additionally effloresces, slowly giving up water of crystallisation in warm dry air and shifting toward a lower hydrate. The material remains usable, but the analysis drifts upward as water is lost, so keep it sealed and out of heat where the assay matters.
Stratus Chemical supplies magnesium sulfate in bags and supersacks, in heptahydrate and monohydrate forms and in agricultural, feed, and technical grades, with a certificate of analysis provided for each shipment. Tell us the hydrate form, grade, grain size, and volume and we will confirm pricing and availability within one business day.
Building a nutrition program? See magnesium oxide, zinc sulfate, and the crop micronutrients guide, or the full agriculture range. Looking for something else? Browse all products or request a quote.
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