Magnesium sulphate monohydrate — mineralogically kieserite — is the lowest-hydration commercially traded magnesium sulphate. It carries magnesium and sulphur together in water-soluble sulphate form, and because it holds only one molecule of water of crystallisation instead of seven, it delivers substantially more nutrient per tonne than Epsom salt. That single difference governs almost everything that follows: freight cost per unit of magnesium, blending behaviour, storage stability, dissolution rate and the application methods for which each hydrate is suited.
This page covers the material generically. Two companion pages address the granular and powder/fine forms, whose specifications and end uses differ meaningfully from one another.
Substance identity
| Field | Value |
|---|---|
| Chemical name | Magnesium sulphate monohydrate |
| Mineral name | Kieserite |
| Formula | MgSO4·H2O |
| CAS number | 14168-73-1 |
| PubChem CID | 16211255 |
| Formula weight | 138.38 g/mol |
| Related hydrate | Heptahydrate, MgSO4·7H2O (Epsom salt), CAS 10034-99-8 |
| Anhydrous form | MgSO4, formula weight 120.37 g/mol |
Kieserite occurs naturally in marine evaporite deposits, generally in association with other potassium and magnesium salts, and is also produced synthetically by controlled dehydration of higher hydrates. Naturally sourced and synthetic material can differ in colour and in the level of accompanying chloride and insoluble matter, so the certificate of analysis rather than the mineral name is the governing document.
Specification
Two columns are given deliberately. The first is arithmetic — what the pure compound must contain. The second is what commercial fertiliser grades typically declare. Any given consignment is defined by its own analysis, not by either column.
| Parameter | Theoretical (pure MgSO4·H2O) | Typical commercial declared range |
|---|---|---|
| Magnesium (Mg) | 17.6 % | 15 – 17 % |
| Magnesium oxide (MgO) | 29.1 % | 25 – 27 % min |
| Sulphur (S) | 23.2 % | 20 – 22 % |
| Sulphur trioxide (SO3) | 57.9 % | 50 – 55 % |
| Water of crystallisation | 13.0 % | — |
| Free moisture | 0 % | typically ≤ 1 – 2 % |
| Water-insoluble matter | 0 % | varies by source; natural grades higher than synthetic |
| Chloride (Cl) | 0 % | trace to low percent in some natural grades |
| Appearance | white | white to greyish-beige (natural) or white (synthetic) |
Grades are usually sold against a minimum MgO figure. Because MgO and Mg express the same element, always confirm which basis a specification uses before comparing two offers: 25 % MgO and 15 % Mg are the same material.
Properties
Nutrient density. One tonne of the monohydrate contains roughly 176 kg of magnesium at theoretical purity; one tonne of the heptahydrate contains roughly 99 kg. The monohydrate therefore carries about 1.8 times the magnesium per tonne, and roughly 1.8 times the sulphur, because more than half the mass of Epsom salt is water of crystallisation.
Solubility and dissolution rate. Magnesium sulphate is freely soluble in water in all its hydrates — this is what separates it from magnesium oxide, magnesium carbonate and dolomite, which are only sparingly soluble and depend on soil acidity to release magnesium. However, solubility and rate of dissolution are not the same property. The monohydrate dissolves more slowly than the heptahydrate at the same particle size, because the crystal lattice must rehydrate before it disperses. Coarse granules dissolve slowly by design; finely ground powder dissolves quickly. For soil-applied use the slower rate is irrelevant or even useful; for tank mixing it is the single most important practical variable.
Physical stability. The monohydrate is markedly less hygroscopic than the heptahydrate and does not readily lose water of crystallisation under ambient conditions. Epsom salt, by contrast, effloresces, softens and cakes in warm storage. This stability is the reason the monohydrate dominates bulk shipping, long-distance trade and dry blending, while the heptahydrate remains common where fast dissolution matters more than storage life.
Reaction. Aqueous solutions are approximately neutral to slightly acidic. Kieserite is not a liming material and does not raise soil pH — an important distinction from dolomitic limestone, which supplies magnesium but only alongside a substantial liming effect and only slowly in acid soils.
Thermal behaviour. Loses water of crystallisation on strong heating to yield the anhydrous salt; decomposes only at high temperature.
Applications
Correcting magnesium deficiency. Magnesium is the central atom of the chlorophyll molecule and an activator of a large group of enzymes, including those handling ATP and those involved in carbon fixation. It is also required for loading sugars into the phloem, so magnesium-deficient plants often accumulate carbohydrate in the leaf. Magnesium is mobile within the plant, so deficiency appears first on older, lower leaves as interveinal chlorosis — yellowing between the veins while the veins themselves stay green — progressing to reddish or purple tints and marginal necrosis in severe cases. Deficiency is most common on light-textured, acid or heavily leached soils, and can be induced on soils with adequate total magnesium by cation antagonism where potassium, calcium or ammonium are present at high concentration.
Supplying sulphate sulphur. Sulphur is required for the amino acids cysteine and methionine and therefore for protein synthesis; crops cannot use nitrogen efficiently without it. Kieserite delivers sulphur already in the immediately plant-available sulphate form, unlike elemental sulphur, which must be oxidised by soil microorganisms before it becomes available.
Typical crop settings. Magnesium-responsive situations include oil palm, rubber, citrus, banana, potato, sugar beet, maize, grapevine, coffee, tea and protected vegetable crops; sulphur response is widespread in oilseeds, brassicas, alliums, legumes and cereals grown on soils with low organic matter.
Delivery routes. Broadcast or drilled solid application, inclusion in bulk blends and compound formulations, fertigation, and foliar spraying — each of which favours a different particle size, which is why granular and powder grades exist as separate products.
Handling and storage
Store under cover in a dry, well-ventilated space, off the floor and away from direct sun and rain. Although the monohydrate is far more stable than the heptahydrate, prolonged exposure to humid air still promotes surface caking. Keep bags closed; limit stack height in line with the packaging supplier’s guidance to avoid compaction of the bottom layers. Sweep up spillages — dissolved product is slippery underfoot and, in quantity, is a nutrient load that should not enter surface water.
Segregate from strongly alkaline materials. In concentrated solution, magnesium sulphate is incompatible with calcium-containing fertilisers such as calcium nitrate: mixing the two stock concentrates precipitates calcium sulphate, which blocks filters and emitters. Use separate stock tanks and combine only in the diluted line. Perform a small jar test before any unfamiliar tank mix.
Dust from fine grades is a nuisance dust rather than a highly toxic one, but respiratory protection, eye protection and gloves are appropriate where dust is generated. Consult the current safety data sheet for the specific grade before use.
Notes
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Compare offers on contained nutrient, not on tonnage. At equal price per tonne, monohydrate and heptahydrate are not equivalent purchases.
-
“Kieserite” in trade usage may denote either mined or synthetic monohydrate. Confirm origin if chloride or insoluble content matters to the application.
- Kieserite supplies magnesium without a liming effect. Where soil pH also needs raising, a liming magnesium source or a separate lime application should be considered.
- Rates should follow soil and tissue analysis. Visual symptoms confirm a deficiency but cannot quantify it, and interveinal chlorosis on older leaves has causes other than magnesium.
Frequently asked questions
One offer is quoted as “MgO 27% min” and another as “Mg 16% min” — which one is actually higher?
Convert both to the same basis before comparing. Mg × 1.658 = MgO, and MgO × 0.603 = Mg, so MgO 27% is the same as about 16.3% Mg — the two quotes are close, not far apart. The corresponding sulphur factor is S × 2.497 = SO3. Pure MgSO4·H2O is theoretically 17.6% Mg (29.1% MgO) and 23.2% S (57.9% SO3), so any commercial figure sits below theory because of free moisture, water-insoluble matter and, in mined kieserite, associated minerals. Also check whether each number is a guaranteed minimum or a typical analysis — those are different commitments.
What is the practical difference between the monohydrate (kieserite) and the heptahydrate (Epsom salt)?
Chemically the same salt, differing in water of crystallisation. The heptahydrate, MgSO4·7H2O, carries seven waters and is about 9.86% Mg — roughly 56% of the magnesium per tonne of the monohydrate, with the balance being water you pay freight on. The heptahydrate dissolves quickly in cold water, cooling the solution as it does so, which is why it dominates foliar and fertigation use. The monohydrate carries more nutrient per tonne and per bag, resists efflorescence, and suits soil application, bulk blending and compaction. Standard granular monohydrate dissolves more slowly, so it is not a drop-in substitute for injection unless the grade was made for dissolution.
How should I compare two offers properly instead of just looking at price per tonne?
Price per tonne hides the differences that matter. Divide the delivered price — product plus freight, since freight is charged on weight, not on nutrient — by the guaranteed Mg (or MgO) percentage to get a cost per unit of magnesium, then repeat for sulphur and decide how much value you assign to the sulphur. After that, compare the physical qualities that change how much of the tonne is usable: water-insoluble matter, free moisture reported as loss on drying at low temperature (not total water, which includes crystal water), particle size distribution, dust content and, for granules, crush strength. A cheap tonne with high insolubles and heavy fines is rarely the cheaper tonne.
What should magnesium sulphate monohydrate not be mixed with?
In concentrated stock solutions, keep it separate from calcium nitrate and other soluble calcium sources: calcium sulphate has low solubility and precipitates as gypsum, which fouls filters, valves and emitters. Keep it separate from concentrated phosphate stocks (MAP, DAP, phosphoric acid blends) as well, since magnesium phosphates also precipitate. Standard practice is two stock tanks — calcium in one, sulphate and phosphate in the other — meeting only after dilution in the irrigation line. In dry blending, avoid combining fine monohydrate powder with granular products: mismatched particle size and density cause segregation in transport and uneven distribution across the spreader pass.
What do I need to watch for in storage?
The monohydrate is far more stable than the heptahydrate, which effloresces and cakes readily, but it is not indifferent to moisture. Store under cover, on pallets, away from walls and floors that sweat, and keep bags and big bags closed rather than opened and loosely resealed. Caking risk rises with high ambient humidity, with day–night temperature swings that drive condensation inside packaging, and with tall, long-standing stacks where pressure consolidates the fines. Powder grades cake more readily than compacted granules. Rotate stock rather than letting pallets sit, and check for lumps before charging a spreader or a dissolving tank, not after.
When is magnesium sulphate monohydrate the wrong choice?
Four common cases. First, on an acidic soil that is also low in magnesium: this is a neutral salt and will not raise pH — dolomitic limestone or magnesium oxide address both problems, this product only one. Second, where you need instant, complete dissolution in cold water for foliar spray or drip: the standard granular form is the wrong physical grade. Third, on saline soils, or where irrigation water is already high in sulphate, additional sulphate adds salt load without agronomic benefit; a nitrate-based or chelated magnesium source fits better. Fourth, where the deficiency is antagonism-induced by excess potassium, calcium or ammonium rather than a true shortfall — adding magnesium alone may not resolve it.
It is sold as a water-soluble fertilizer, so can I put any grade straight into a drip system?
“Water soluble” on a label is not a yes/no property — read the declared water-insoluble matter and the test method behind it. Mined kieserite carries residual gangue minerals; material produced from magnesium oxide or hydroxide and sulphuric acid is usually lower in insolubles. For broadcast soil application a small insoluble fraction is agronomically irrelevant. For injection it is not: undissolved residue accumulates on filters and inside lines, and dissolution rate falls with water temperature and rising concentration. Before committing a grade to fertigation, run a dissolution trial with your own water at your own temperature and target concentration, and check what is left on the filter.
Related substances
- Magnesium deficiency appears first on older leaves and zinc deficiency on the youngest ones, so the zinc page is the quickest way to confirm that interveinal chlorosis has been read correctly.
- Ammonium fertilizers suppress magnesium uptake through cation competition, so a sulphur programme built solely on ammonium sulphate can leave magnesium short even when soil magnesium tests adequate.
- Magnesium sulphate and MKP are compatible in the diluted line but not in the concentrate, which is the single most common cause of blocked drippers in a two-tank system.
- Comparing the monohydrate with an anhydrous sulphate shows how much of a delivered tonne is water of crystallisation and how that changes flow, caking and dissolution rate.