Powder and fine-crystal magnesium sulphate monohydrate is chosen when surface area matters. Dissolution rate scales with exposed surface, so the same compound that dissolves over days as a 4 mm granule dissolves in minutes as a fine powder under agitation. That makes the fine grade the form used where the fertiliser must enter solution — fertigation stock tanks, foliar sprays, suspension products — and the form used as a raw material where the salt is a feedstock rather than an end product. It also makes it the grade that demands the most care in dust control and in solution-compatibility management.
Substance identity
| Field | Value |
|---|---|
| Chemical name | Magnesium sulphate monohydrate |
| Mineral name | Kieserite (fine / ground) |
| Formula | MgSO4·H2O |
| CAS number | 14168-73-1 |
| PubChem CID | 16211255 |
| Formula weight | 138.38 g/mol |
| Physical form | Free-flowing powder or fine crystal |
Specification
| Parameter | Theoretical (pure MgSO4·H2O) | Typical commercial declared range |
|---|---|---|
| Magnesium oxide (MgO) | 29.1 % | 25 – 27 % min; higher-purity solution grades sit at the upper end |
| Magnesium (Mg) | 17.6 % | 15 – 17 % |
| Sulphur (S) | 23.2 % | 20 – 22 % |
| Sulphur trioxide (SO3) | 57.9 % | 50 – 55 % |
| Free moisture | — | typically ≤ 1 % for solution grades |
| Water-insoluble matter | 0 % | the critical parameter for solution use; low limits should be specified explicitly |
| Particle size | — | commonly specified as a percentage passing a stated mesh |
| pH, aqueous solution | — | approximately neutral to slightly acidic |
| Appearance | white | white to off-white powder |
For any solution application, water-insoluble matter and particle size are the two specification lines that decide whether the product works. Nutrient content is largely a given; filterability is not.
Properties
Dissolution. Fine kieserite disperses and dissolves readily in water with agitation. Dissolution is faster in warm water than cold — a practical point in early-season fertigation, when mains or reservoir water may be near freezing and an operator can mistake slow dissolution for insoluble residue. Adding powder gradually to circulating water, rather than dumping it into a still tank, avoids the clumping that traps dry powder inside a wetted shell.
Insoluble residue. All magnesium sulphate hydrates are freely soluble, so residue in the tank comes from accompanying insoluble minerals in the raw material, not from the salt itself. Naturally mined grades typically carry more insoluble matter than synthetic grades. Where the product feeds drip emitters, specify a low insolubles limit and filter the stock solution regardless.
Solution reaction. Solutions are approximately neutral to slightly acidic, so kieserite does not by itself drive spray-tank pH into a range that destabilises pH-sensitive tank-mix partners — but it also provides no buffering.
Physical stability. The monohydrate does not effloresce as the heptahydrate does, so the powder retains flowability far better in storage than fine Epsom salt. It will still cake if it takes up atmospheric moisture, and the finer the grind, the more readily bridging occurs in silos and hoppers.
Nutrient density. As with all monohydrate grades, roughly 176 kg Mg and 232 kg S per tonne at theoretical purity, against roughly 99 kg Mg per tonne for the heptahydrate — the reason bulk buyers of solution feedstock increasingly specify monohydrate and dissolve it, rather than shipping the water in Epsom salt.
Applications
Fertigation. Supplies magnesium and sulphate through drip, micro-sprinkler and sprinkler systems in protected cropping, orchards, plantations and open-field vegetables. Typically dosed from a dedicated stock tank so that it is diluted before meeting calcium-containing stock.
Foliar application. Magnesium sulphate is one of the standard foliar magnesium sources. Foliar sprays correct acute, visible deficiency quickly and are useful where root uptake is constrained — cold, waterlogged or compacted soil, or strong cation antagonism from high potassium. Foliar feeding is a corrective measure, not a substitute for the soil-applied programme: leaf surfaces cannot take up the quantities a crop’s total magnesium demand requires.
Soilless and hydroponic systems. Used in nutrient solution formulation where all magnesium and much of the sulphur must be delivered in soluble salt form.
Formulation feedstock. Used as the magnesium and sulphur source in the manufacture of water-soluble NPK blends, suspension fertilisers and micronutrient formulations, and as a raw material in non-fertiliser industries where a soluble magnesium salt is required.
Handling and storage
Keep the material dry. Fine grades take up atmospheric moisture faster than granules because of their surface area, and a caked powder is difficult to redisperse. Store sealed, under cover, off the floor, out of direct sun.
Control dust at every transfer point. Use local extraction where practicable, and provide dust masks or respirators, eye protection and gloves for operators handling open bags. Magnesium sulphate dust is irritating to eyes and airways rather than acutely toxic, but repeated exposure is avoidable and should be avoided. Consult the current safety data sheet for the specific grade.
Mixing discipline for solution use:
- Never combine concentrated magnesium sulphate stock with concentrated calcium nitrate or other soluble calcium sources — calcium sulphate precipitates and will block filters and emitters. Use separate stock tanks (conventionally the “A” and “B” tank convention) and let them meet only in the diluted irrigation line.
- Add to circulating, not still, water; add gradually.
- Jar-test any unfamiliar tank mix at the intended field concentrations before mixing a full tank.
- Filter the stock solution and flush lines after dosing.
- Where water is hard or high in bicarbonate, check the whole solution recipe rather than the single product.
Notes
-
Specify insolubles and mesh for any solution grade; the nutrient analysis alone will not tell you whether the material is fit for a drip system.
-
Powder is not suitable for broadcast spreading — it will not throw evenly and is subject to substantial wind loss. Use the granular grade for that.
- MgO and Mg express the same element; check the declaration basis before comparing offers.
- Foliar concentrations should follow the label and be validated on a small area first; magnesium sulphate sprays applied too concentrated, in hot conditions or on stressed foliage can cause leaf scorch.