Ammonium sulphate is chemically one product. Commercially it is four, distinguished purely by particle size and mechanical strength. The chemistry is constant at roughly 21 % N and 24 % S; the physics decides who can use it.
The four grades
Fine crystal. The smallest material, often a direct process output. It flows adequately but is dusty, prone to caking, and cannot be spread mechanically with any uniformity. It is used where the product is dissolved or further processed rather than applied as-is.
Standard crystal. The historic mainstream grade. Suitable for direct application by drill or by hand in some systems, and widely used as a raw material by compound fertiliser plants. Too fine to blend with modern granular products without segregating.
Compacted. Produced by squeezing crystal between rolls into a dense sheet, then crushing and screening it to a target size band, with off-size recycled. No binder is added and no chemistry changes; the result is an angular particle of granular dimensions.
Granular. Produced by true granulation, giving rounder, more uniform particles with generally higher crushing strength than compacted material.
Segregation: the reason the grades exist
Bulk blending — physically mixing finished granular products to a customer’s prescription — is only sound if the components stay mixed. They do not stay mixed if their particle sizes differ. Every time a blend is poured, augered, coned in a hopper or thrown from a spreader disc, differently sized particles sort themselves. The result is a fertiliser whose analysis varies across the load and across the field, so that some of the crop is over-applied and some under-applied while the delivery note remains entirely accurate.
The industry manages this with two derived measures:
- SGN (Size Guide Number): median particle diameter in millimetres × 100. A 2.4 mm median gives SGN 240.
- UI (Uniformity Index): how tightly the size distribution clusters around that median.
Blend components should have similar SGN and adequate UI. Standard crystalline ammonium sulphate, at an SGN far below that of granular urea, DAP or MOP, is fundamentally unblendable with them. Compaction and granulation exist to solve exactly this, converting an unblendable input into a blendable one.
Which buyer wants which grade
Compound fertiliser producers buy crystal. Their process re-forms the particle anyway, so paying for someone else’s granulation is pointless. They care about nitrogen content, moisture, free acidity and consistency of crystal habit.
Bulk blenders buy granular or compacted. They care about SGN, UI, crushing strength and abrasion resistance, because their product must survive the blender, the tender, the spreader and the throw.
Direct-application farmers buy whichever form suits their equipment. Broad-acre mechanised operations need granular; systems that band or incorporate by hand can use crystal.
Solution and industrial users buy fine or standard crystal, and care about solubility, insolubles and colour rather than particle mechanics.
The economics of upgrading
Compaction and granulation cost money — capital, energy, screening loss and recycle. That cost is only recovered if the buyer values blendability, so a granular premium is not arbitrary: it is the price of access to the bulk blend market. It also creates a persistent asymmetry, because the co-product routes that dominate supply naturally yield crystal, while the fastest-growing demand segments want granular. Bridging that gap is a processing decision, not a production one.
Specifying properly
“Granular ammonium sulphate” is not a specification. A usable one states the nutrient analysis, the moisture and free-acid limits, the size band with the percentage inside it, the SGN and UI, a crushing strength, and any anti-caking treatment. Where blending is intended, the SGN of the intended partners is the number that matters most — and it should be agreed before the order, not diagnosed afterwards in a field showing stripes.