A certificate of analysis is a technical document that also functions as a commercial one. Knowing which lines carry consequences, and which are informational, is the difference between a specification that protects a buyer and one that merely looks thorough.
Nitrogen content
The headline. Pure ammonium sulphate is 21.2 % N by stoichiometry, so commercial grades are commonly specified at a minimum of 20.5 % or 21 %. A figure materially below that indicates dilution by moisture, insolubles or free acid — nitrogen content is in practice a proxy for overall purity, which is why it is the first line on every specification.
Free acid
Residual sulphuric acid from the process. Free acid is the single most under-appreciated parameter on the sheet, because its consequences appear later and elsewhere:
- it is corrosive to storage structures, conveyors, vessels’ holds and spreaders
- it promotes caking during storage
- it is a hazard consideration for handlers
- in a compound or blend it can attack other components
Limits are typically specified as a low maximum percentage. A buyer with steel handling equipment should treat this line as more important than a marginal difference in nitrogen.
Moisture
Free moisture — water that is not chemically bound. High moisture means paying freight for water, and it drives caking, which converts a free-flowing product into a solid mass requiring mechanical breakout. Moisture is measured at the time and place of testing, so the sampling point matters: material can pick up moisture in humid storage after it left a compliant plant.
Water-insoluble matter
Anything that will not dissolve. Critical for anyone making solutions or feeding fertigation equipment, and an indicator of process cleanliness for everyone else.
Particle size distribution, SGN and UI
The full sieve analysis, and the two derived numbers taken from it: SGN (median diameter in mm × 100) and UI (uniformity of the distribution). These determine whether the product can be bulk-blended without segregating and whether it spreads evenly. Where blending is intended, the SGN of the intended blend partners is the reference point.
Crushing strength and attrition
How well a granule survives handling. Relevant to granular and compacted grades and to any product facing multiple transfer points, long transport or spreader augers. Weak granules become dust, and dust degrades spread pattern, generates loss and creates a dust hazard.
Caking tendency and anti-caking treatment
Whether the product has been coated or treated, and with what. Relevant for long storage, humid climates and bulk piles. Some coatings affect solubility or foam behaviour in solution — worth checking if the product will be dissolved.
Colour and odour
Cosmetic in agronomic terms, but genuine as a commercial specification. Direct-neutralisation material is typically white; co-product streams may be off-white, cream or grey and may carry a faint odour from residual organics. Buyers serving retail bagged markets often specify colour tightly because end customers use it as a proxy for quality.
Contaminants
Heavy metals and other regulated contaminants, with limits set by the destination market’s fertiliser legislation rather than by the seller. Requirements differ between jurisdictions, so a certificate that satisfies one market may not satisfy another. This is a compliance line, not a negotiating line.
Sampling and testing method
The most frequently omitted and most frequently disputed part of a specification. A specification should state which sampling protocol and which analytical method apply, because results differ between methods and a single grab sample from a heterogeneous bulk cargo is not representative. Agreeing the method in advance converts a potential dispute into an arithmetic check.
A workable minimum specification
Nitrogen minimum; free acid maximum; moisture maximum; water-insoluble maximum; size band with percentage inside, plus SGN and UI where blending is intended; crushing strength for granular grades; anti-caking treatment declared; colour range; contaminant limits referenced to the destination regulation; and the sampling and analytical methods named. Anything less leaves the definition of “conforming” to be settled after the goods arrive.