Heptahydrate vs monohydrate magnesium sulphate: why a tonne is not a tonne

The answer, up front

Magnesium sulphate heptahydrate — Epsom salt, MgSO₄·7H₂O — is 51.2% water of crystallisation by mass. It carries 9.86% magnesium. The monohydrate, MgSO₄·H₂O, sold as kieserite, carries 17.56%. The anhydrous salt carries 20.19%. Those are theoretical figures from the formula weights, and they are the only honest starting point for comparing two prices.

To deliver the same magnesium you need 1.78 tonnes of pure heptahydrate for every tonne of pure monohydrate. Freight, bagging and handling all scale with the tonnage you move, not with the magnesium in it — and in the heptahydrate, half of what you are paying to move is water.

So the first thing to do with a magnesium sulphate quote is convert it to rupees per kilogram of magnesium delivered — and then price the sulphur separately, because these materials carry two nutrients and the sulphur ratio is not the same as the magnesium ratio. The two grades are also not substitutes for each other in every application, which the rest of this article deals with. Until you have done the arithmetic you are comparing two different products that happen to share a name.

The arithmetic

Theoretical compositions calculated from standard atomic weights. Commercial material always assays below these figures — use the number on your certificate of analysis, not the number in this table.

Two conversions you will need, because the fertiliser trade quotes magnesium as MgO and sulphur as SO₃ while chemical specifications quote the elements:

• MgO → Mg: multiply by 0.603. So 16% MgO is 9.65% Mg, and 25% MgO is 15.1% Mg.
• SO₃ → S: multiply by 0.400. So 32.5% SO₃ is 13.0% S, and 50% SO₃ is 20.0% S.
• Going the other way, multiply Mg by 1.658 for MgO and S by 2.497 for SO₃.

A worked comparison. Suppose heptahydrate declared at 9.6% Mg is offered at ₹A per tonne delivered, and kieserite declared at 25% MgO at ₹B per tonne delivered. The heptahydrate carries 96 kg of magnesium per tonne and costs ₹A ÷ 96 per kg Mg. The kieserite carries 151 kg and costs ₹B ÷ 151. Those two numbers are comparable; ₹A and ₹B are not. If one of your quotes is ex-works and the other delivered, neither number is finished yet.

Then do it again for sulphur. On the same declared grades the heptahydrate carries about 120 kg of sulphur per tonne and the kieserite about 200 kg. If you are buying partly for sulphur — and in Indian oilseed and pulse crops many buyers are — a magnesium-only comparison undercounts what the kieserite delivers.

What the Fertiliser (Control) Order actually specifies

In India, magnesium sulphate sold as a fertiliser is governed by the Fertiliser (Inorganic, Organic or Mixed) (Control) Order, 1985. It appears in Schedule I, Part A, under the sub-heading 1(f) MICRONUTRIENTS, at serial 11 in the consolidations we could read. The specification is short, and most of what buyers believe about it is wrong:

Four things follow from that table, and each of them routinely catches people out.

• The entry does not name a hydrate. But a 9.6% Mg minimum is 97.3% of the theoretical magnesium in the heptahydrate, so in practice it is a heptahydrate specification. No separate kieserite or monohydrate entry appears in the consolidations we could read — see the section below, because that gap has a consequence.
• There is no moisture limit. None. The physical control is “free flowing crystalline form” plus the 1.0% insolubles cap. A specification sheet that claims an “FCO moisture maximum” is quoting something the Order does not contain.
• Lead is the only contaminant capped. There is no arsenic limit and no cadmium limit. Worth knowing, because the neighbouring FCO entries for zinc sulphate do cap both. If you need arsenic or cadmium controlled — and in a food, pharmaceutical or potable-water application you will — you have to write it into your purchase specification. “FCO grade” will not get it for you.
• The pH window is 5.0 to 8.0, so conforming material can be distinctly acidic in solution. If your process is pH-sensitive, specify a narrower band rather than relying on the Order.

9.6% is the minimum, and 9.5% is not a grade

You will see Indian producers declare 9.5% Mg. Schedule I Part B of the Order carries a permissible variation of 0.1 for magnesium sulphate — but that is an analytical tolerance applied when a sample is tested against the prescribed standard. It is not permission to declare a lower figure. The Order prohibits selling a fertiliser that is not of the prescribed standard, and separately requires the minimum guaranteed percentage of plant nutrient to be exhibited. The guarantee on the bag should read 9.6%. If a supplier offers you 9.5% as the declared minimum, that is a question to put to them in writing, not a variant grade to accept.

Micronutrient, not secondary nutrient — and there is a live mismatch

Agronomically, magnesium is a secondary nutrient, and Indian producers market it under that heading. The FCO classifies it as a micronutrient. That is not pedantry, because the concessional GST entry for micronutrient fertilisers is written by reference to the FCO micronutrients sub-heading.

Be aware of two things here. First, the rate changed: micronutrient fertilisers moved from 12% to 5% GST, announced in December 2025. A contract or an assessment covering an earlier period sits under the old rate, so always date the rate you are applying. Second, the tax notification cross-refers to sub-heading 1(g) while the Order’s own text places micronutrients at 1(f). That mismatch is real and unresolved, and it is the kind of thing an assessing officer can raise.

The concessional entry also carries a condition that buyers commonly get backwards: it attaches to the manufacturer being registered under the FCO, not to the buyer holding a licence. A Gujarat advance ruling has read it that way — though an advance ruling binds only the applicant who sought it and the officer concerned, so it is persuasive rather than settled law, and the industry is still pressing for a uniform rate. Material sold as a technical or industrial inorganic chemical falls under the general inorganic-chemicals entry instead, at a higher rate.

Rates, classifications and the effect of an advance ruling all turn on facts we cannot see from here. Confirm the position with your tax adviser against your own invoices and your own supplier’s registration.

The kieserite gap in the FCO, and why it matters

Granular kieserite comfortably clears the 9.6% Mg minimum. But the same FCO entry also prescribes “free flowing crystalline form”, and granular kieserite is a granulate, not a crystal. So material that passes on assay may not match the prescribed physical form, and no separate monohydrate entry appears in the consolidations we could read.

We are not going to tell you that settles anything — the Order also contains provisions for the sale of fertiliser that does not meet a prescribed standard, under conditions, and how all of this applies to a given consignment is a question for a regulatory adviser, not a web page. But it is a real question, and it is the buyer’s exposure as well as the seller’s. If you are buying kieserite into an agricultural channel in India, ask your supplier in writing which FCO entry they are selling it against. A supplier who cannot name one has told you something.

Solubility: the comparison that is usually made wrongly

You will read that kieserite is less soluble than Epsom salt, usually quoting something like 417 g/L against 710 g/L at 20 °C. Those are masses of two different salts, and the published figures frequently do not state their basis — grams per litre of finished solution and grams per litre of water are different quantities, and the heptahydrate formula weight carries six extra moles of crystal water either way. Producer literature for kieserite quotes a lower figure again, on an explicitly stated end-volume basis. Figures quoted on unstated or different bases cannot be compared, and we are not going to pretend otherwise by doing arithmetic on them.

In any case equilibrium solubility is not what decides the grade. Dissolution rate, particle size, residue and the application method are, and those differences are clear:

• Heptahydrate is the foliar and fertigation grade. It is the form producers supply for spraying and for injection into drip, and it is what Indian producers publish spray rates for.
• Kieserite is a soil grade. It is granular, it spreads evenly at width, and at 25% MgO against 16% MgO it puts far more magnesium on the field per tonne of freight. The IPNI nutrient source sheet is explicit that kieserite is not used for foliar application or in fertigation.
• Anhydrous is an industrial and blending material, not a field fertiliser. It is hygroscopic — it is sold as a desiccant precisely because it binds water — so it needs sealed, lined packaging in an Indian monsoon warehouse.

A claim to retire: kieserite is not a slow-release magnesium source. Its producers describe it as quickly soluble and immediately available. What is slow is soil correction of a magnesium deficiency by any source, which commonly takes seasons, against a foliar spray that acts in days. That is a soil-versus-foliar difference, not a kieserite-versus-Epsom one.

A second one: magnesium sulphate is not a liming material. Both hydrates are neutral salts and neither raises soil pH. If you are substituting magnesium sulphate for dolomite you will get the magnesium and none of the pH correction — which may be exactly what you want on an already-alkaline soil, and exactly what you do not want on an acid one.

The tank mistake that blocks a drip line

This belongs on the page because it is the costliest routine error in fertigation and it is not obvious. Do not put magnesium sulphate and calcium nitrate in the same stock tank. Sulphate and calcium precipitate calcium sulphate — gypsum — which blinds filters and plugs emitters, and a plugged drip line is a far bigger bill than the fertiliser in it. Phosphates in a concentrated stock tank raise the same problem.

• Keep calcium-containing fertilisers in a separate stock tank and inject them separately.
• Dissolve the magnesium sulphate first, into clean water, before anything else goes in.
• Run a jar test with your own irrigation water and your actual fertilisers at your actual concentrations before you commit a batch. If it stays clear, they are compatible in that water; if it goes cloudy, they are not.
• Your irrigation water matters as much as the fertiliser. Hardness, bicarbonate and the sulphate already in the water decide whether a solution precipitates at all, so test the water before designing the programme.

Published application rates vary widely between producers, which is a reason to follow the sheet for the material you actually bought rather than a general figure. Two large Indian producers publish foliar rates of 5 g/l and 20 g/l for different crops and timings, and soil rates that differ between them by a factor of five. European producers of branded foliar heptahydrate publish spray concentrations in the range of a few per cent by weight, which is an order of magnitude above the Indian foliar figures. These are not interchangeable numbers. Use your producer’s recommendation for your crop, and take agronomic advice for your soil.

“Dried magnesium sulphate” is not a grade

This is the trap that costs real money. Material is offered in India as “dried magnesium sulphate” or “partially dehydrated magnesium sulphate”, often with a purity figure such as 99.9% and no other number at all. There is no FCO entry, no Indian Standard grade and no agreed hydration state behind either phrase.

A purity figure without a stated basis is not a specification. The Indian Standard assays magnesium sulphate as MgSO₄ on material dried at 300 °C — that is, on the anhydrous basis. On that basis, pure monohydrate and pure anhydrous salt both assay close to 100%, while carrying 17.56% and 20.19% magnesium respectively. The same “99.9% pure” claim can describe material differing by a sixth in magnesium content.

The defence is simple and you should apply it to every offer: do not buy against a purity figure. Ask for magnesium as Mg or as MgO, and ask for loss on drying with the method stated. Two numbers, and the ambiguity disappears.

IS 2730, and what it does and does not promise

IS 2730:1977, Specification for Magnesium Sulphate (Epsom Salts) (First Revision, reaffirmed 2010), sets two grades, Pure and Technical. The assay is on the anhydrous basis — material dried at 300 °C — at 99.5–100.5% MgSO₄ for Pure and 98.0–100.5% for Technical.

There is a tighter sub-grade within Technical for the leather industry, at 0.6% chlorides and 0.001% iron. Note what is not in the standard: there is no pH limit. IS 2730 uses an acidity-and-alkalinity titration instead, so a purchase order reading “pH 5–8 as per IS 2730” is writing a term the standard does not contain — that pH window comes from the FCO.

No Quality Control Order makes IS 2730 mandatory. Magnesium sulphate is not among the chemicals brought under a QCO, so BIS certification for it is voluntary. It looks compulsory only because Indian public tenders habitually write “as per IS 2730” into their specifications. If your tender does, that is a contractual requirement your supplier has to meet; it is not a law. A second revision of the standard has been through public consultation, so cite the edition in your contract rather than the bare number.

The efflorescence clause

IS 2730 states that crystals becoming powdery through efflorescence does not render the material unacceptable unless the purchaser and supplier have agreed otherwise. Epsom salt loses water of crystallisation in dry air and goes to powder; that is physics, not adulteration, and the standard presumptively permits it.

So if you need free-flowing crystal on arrival — because it goes into a spreader, a dosing screw or a retail pack — write that into the contract, and say what your process needs it for. A bare IS 2730 purchase order gives you that clause and nothing more. Whether a particular consignment is nevertheless rejectable — for example because it was sold by description, or is unfit for a purpose you made known to the seller — is a question of contract and of sale-of-goods law, and it is one to take advice on rather than to settle from a supplier’s web page.

The same physics runs the other way and is worth understanding before you argue about an assay. In dry air at ordinary temperatures, heptahydrate gives up water to form the hexahydrate; that is efflorescence, and it is a humidity effect, not a heat effect. (The monohydrate and anhydrous forms are made by heating to well above anything a warehouse or a container reaches.) A consignment stored in dry conditions can therefore arrive assaying higher in magnesium than its label without anyone having adulterated anything — it has lost water and the magnesium has concentrated. Specify loss on drying alongside Mg%, test on an as-received basis, and this stops being an argument.

Handling and storage

• Magnesium sulphate is mildly irritating to the eyes and the respiratory tract. Wear safety spectacles and control dust when tipping bags; a harmful airborne concentration is reached quickly with powdered material. The safety data sheet carries the full first-aid and exposure detail.
• It decomposes on heating and gives off sulfur oxides, which are corrosive. That matters for a warehouse fire, not for normal storage.
• Heptahydrate has two opposite storage failures. It cakes under humidity and it effloresces to powder in dry heat. Store it cool, dry and humidity-stable, keep bags off the floor and away from a wall that sweats, and agree a stacking height with your supplier rather than guessing.
• Anhydrous and dried grades absorb moisture. They need sealed, lined packaging, and hydration releases heat — so a blend that picks up moisture in bagged storage can warm as well as cake.
• Agree a shelf life and a re-test interval. A certificate of analysis describes the material as it was at despatch. After a long storage in an Indian warehouse, the loss-on-drying figure on that certificate may no longer describe what is in the bag.

Where the material comes from

A claim worth correcting, because it gets repeated in procurement meetings: India is not a net importer of magnesium sulphate. In 2023 India exported roughly 44,000 tonnes under the manufactured magnesium sulphate tariff line against imports of roughly 1,700 tonnes — a net export surplus of about 42,000 tonnes.

Be careful how far you push that, and we will be careful too. The tariff line does not distinguish the hydrates, and the unit values inside that import figure range from under a dollar a kilogram to tens of dollars, which is the signature of bulk, reagent and pharmacopoeial material sitting in one line. So the import tonnage cannot be read as a monohydrate figure, and we are not going to read it as one. What can be said is that India manufactures and exports heptahydrate at scale, and that the kieserite offered in the Indian market is imported.

Tariff lines

• 2833 21 00 — sulphates, of magnesium. The manufactured product, whatever the hydrate, and whether it was synthesised or made by processing a mined mineral. There is no separate eight-digit line for heptahydrate, monohydrate or anhydrous, so Indian customs data cannot distinguish them — which is exactly why a contract has to.
• 2530 20 00 — kieserite, epsomite (natural magnesium sulphates). This is the crude mineral line. Mined kieserite that has been calcined, ground and granulated has been processed, and classifies under 2833 21 00 — so the tariff line tells you the degree of processing, not whether the material was mined.
• Straight magnesium sulphate does not classify in Chapter 31, because Chapter 31 Note 1(b) excludes separate chemically defined compounds. Two qualifications matter: heading 3104 does cover magnesium potassium sulphate, which is a different product; and once magnesium sulphate is blended into an NPK, the blend is a Chapter 31 good. Formulated or chelated micronutrient preparations have also been classified in Chapter 38 rather than Chapter 28.

Classification is the importer’s responsibility and turns on the actual goods. Confirm against the current tariff and your own documents.

Which grade for which job

What the certificate of analysis should tell you

• The hydrate, named.
• Magnesium, as Mg or MgO, with the basis stated.
• Sulphur, as S or SO₃, with the basis stated.
• Loss on drying, with the temperature and method. The Indian standard dries at 300 °C; a pharmacopoeial method uses a different condition and gives a different number. A loss-on-drying figure without its method is not a result.
• Matter insoluble in water.
• Heavy metals — lead always; arsenic and cadmium if your application needs them, because they are not automatic.
• The batch or lot number the certificate belongs to, and the date of analysis. A certificate that does not identify a batch is a sales document, not an analysis.

A certificate is only as good as the sample behind it. IS 2730 defines how a lot is sampled and provides for purchaser, supplier and referee sample sets. If you want a real right to reject, the sampling protocol and the referee sample belong in the contract alongside the specification — and so does the answer to the question that decides every assay dispute: whose laboratory result governs?