Granular magnesium sulphate monohydrate is the same chemical compound as the fine material — MgSO4·H2O — presented in a hardened, closely sized particle intended for mechanical spreading and dry bulk blending. The chemistry is not what distinguishes this grade; the physics is. Granule size distribution, crushing strength, abrasion resistance and bulk density determine whether the product throws evenly from a spinning disc spreader, whether it survives handling without turning to dust, and whether it stays homogeneously mixed with urea, DAP or MOP in a blend rather than segregating in the hopper.
Substance identity
| Field | Value |
|---|---|
| Chemical name | Magnesium sulphate monohydrate |
| Mineral name | Kieserite (granulated / compacted) |
| Formula | MgSO4·H2O |
| CAS number | 14168-73-1 |
| PubChem CID | 16211255 |
| Formula weight | 138.38 g/mol |
| Physical form | Granules, produced by compaction-granulation or by granulation of the fine salt |
Granules are typically made by roll compaction of the fine crystal or powder, followed by crushing and screening to the target size band, with oversize and undersize recycled. Compaction rather than binder-based agglomeration means the granule remains chemically the same material, with no diluent added.
Specification
| Parameter | Theoretical (pure MgSO4·H2O) | Typical commercial declared range |
|---|---|---|
| Magnesium oxide (MgO) | 29.1 % | 25 – 27 % min |
| Magnesium (Mg) | 17.6 % | 15 – 17 % |
| Sulphur (S) | 23.2 % | 20 – 22 % |
| Sulphur trioxide (SO3) | 57.9 % | 50 – 55 % |
| Free moisture | — | typically ≤ 1 – 2 % |
| Particle size band | — | commonly specified between about 1 and 5 mm, with a stated percentage inside band |
| Undersize / dust | — | a maximum percentage below the lower cut is normally declared |
| Crushing strength | — | declared per granule, typical granular fertilisers fall in the low single-digit kgf range |
| Bulk density | — | typically around 1.0 – 1.3 t/m³ |
| Appearance | white | white to greyish-beige granules |
Two derived numbers matter more than the raw sieve table for blending customers:
- SGN (Size Guide Number) — the median particle diameter in millimetres multiplied by 100. A granule with a 2.5 mm median has an SGN of 250.
- UI (Uniformity Index) — a measure of how narrow the size distribution is; a higher UI means a tighter spread of particle sizes.
Properties
Blend compatibility. Physical segregation in a bulk blend is driven overwhelmingly by particle size mismatch, secondarily by density and shape. Two components with similar SGN and reasonable UI stay mixed through loading, transport and spreading; a component that is markedly finer than its partners migrates downward and concentrates at one end of the field. Granular kieserite exists precisely so that magnesium and sulphur can be added to a blend built around urea, ammonium sulphate, DAP/MAP or potash without creating that mismatch. When specifying, ask for the SGN of the intended blend partners and match it — a nominal size range alone is not sufficient information.
Spreading behaviour. Ballistic distance from a centrifugal spreader depends on particle mass, density and shape. Denser, rounder, harder granules fly further and more predictably; fines are carried by wind and drop short, producing streaking. Adequate crushing strength therefore has a direct agronomic consequence: a granule that degrades in the auger, elevator or spreader hopper becomes dust before it reaches the disc.
Dissolution in the field. Granular kieserite is fully water-soluble but dissolves comparatively slowly, because dissolution is limited by surface area. In practice the granule takes up soil moisture and releases magnesium and sulphate over a period following the first meaningful rainfall or irrigation, rather than instantly. For broadcast field application this is an advantage — the nutrient is delivered without an immediate high-concentration pulse, and sulphate leaching losses are moderated on light soils.
Dust and handling loss. Compared with the powder grade, the granular product generates far less airborne dust, which reduces product loss during transfer, improves operator conditions and keeps warehouses cleaner.
Applications
Broadcast and drilled solid application. The primary use. Applied pre-plant, at planting or as a top-dress to correct or maintain magnesium and sulphur status in arable crops, grassland, plantation crops and orchards.
Component of bulk blends. Added to custom blends at a rate set by soil analysis, to build a single-pass product carrying N, P, K, Mg and S.
Feedstock for compound granulation. Used as the magnesium and sulphur source in the production of NPK+Mg+S compound fertilisers.
Plantation and orchard use. Widely used where magnesium demand is high and offtake removes magnesium continuously — oil palm, rubber, citrus, banana, coffee and tea are classic magnesium-responsive systems, and granular material suits the manual or mechanical broadcast placement typical of those crops.
What this grade is not for. Granular kieserite is not intended for fertigation, drip systems or foliar spraying. Its dissolution rate is deliberately slow, and dissolving granules in a tank invites undissolved residue, filter blockage and emitter plugging. Use the powder or fine grade for solution applications.
Handling and storage
Store dry, under cover, off the ground, away from direct sunlight and rain ingress. Granules are physically robust but not indestructible: minimise the number of transfer points and drop heights, since every fall degrades granules into fines and every unit of fines worsens both spreading uniformity and dust generation.
Limit stack height to avoid crushing the lower layers, which compacts them and promotes caking. Keep bulk piles in separate bays from alkaline materials and from any product with which cross-contamination is undesirable. In humid climates, keep big bags closed between uses.
Clean spreader and blending equipment after use — sulphate salt residues combined with residual moisture are corrosive to unprotected steel over time.
Notes
-
Specify the size band and the SGN together. “Granular” is not a specification.
-
Where a blend will travel a long distance or be transferred repeatedly, ask for crushing strength and an abrasion/attrition figure alongside the sieve analysis.
- Because MgO and Mg express the same element, confirm the declaration basis before comparing grades.
- Kieserite has no liming effect. Where soil pH is also low, address pH separately.