NPK & complex fertilisers
A blend can pass the analysis and still fail the field.
Bulk blends and chemically granulated compounds. The certificate describes the bag; what reaches the crop depends on whether the granules were matched — and a mismatched blend separates on the way there without breaking any specification.
The desk
Two products called the same thing, behaving completely differently.
"NPK 15-15-15" describes two manufacturing routes that share nothing except the label, and the difference decides almost every practical question about the cargo.
A compound — sometimes called a complex — is chemically granulated: the nutrients are combined in the process and every granule contains all three. Analyse one granule and you get the stated ratio. A bulk blend is a physical mixture of separately manufactured materials, typically urea or AN for nitrogen, MAP or DAP for phosphate and MOP for potassium. Analyse one granule and you get whichever component you picked up.
Blending is cheaper, faster and infinitely flexible on ratio, which is why most of the world's NPK is blended. The cost is that a blend is only a blend for as long as it stays mixed — and every transfer, every conveyor drop, every truck journey and every spreader disc is an opportunity for it to sort itself back out by particle size. That is segregation, and it is the single most consequential thing on this page.
So the question we ask first is not the ratio. It is how many times the product will be handled between the blender and the field, and whether the components were matched to survive it.
The mechanism
Two numbers that decide whether a blend holds together.
Segregation is not a quality defect in any component. It is a physical consequence of mixing particles that differ in size, and it is predictable, measurable and preventable — by arithmetic done before blending rather than by care afterwards.
| Measure | What it is | How it is used |
|---|---|---|
| SGN — size guide number | The median particle diameter in millimetres, multiplied by one hundred. An SGN of 280 means a median granule of 2.8 mm. | Components should share a similar SGN. A widely accepted working rule is that the SGN of the blend components should not differ by more than about ten percent; the wider the spread, the faster the blend separates. |
| UI — uniformity index | A measure of how tightly the particle sizes cluster around the median. A high UI means most granules are close to the same size; a low UI means a wide spread. | Components should also share a similar UI. A component with a wide size spread carries its own internal segregation into the blend regardless of how well the medians match. |
What segregation looks like
Larger particles migrate to the outside of a cone and to the edges of a spread pattern; fines concentrate in the middle. A blend that leaves the plant at 15-15-15 arrives as a bag that is nitrogen-rich at the top and potash-rich at the bottom, and reaches the field as strips of over- and under-application.
Why the certificate misses it
A composite sample drawn properly still averages back to 15-15-15, because the nutrient did not go anywhere — it just stopped being evenly distributed. Segregation is invisible to the analysis that most contracts rely on, which is why it has to be prevented rather than detected.
What actually prevents it
Matched SGN and UI across every component, minimum handling between blender and field, filling and discharging without long free-fall, and, where the chain is long, choosing a compound instead. Anti-segregation additives help at the margin; matching the particles is the fix.
Compatibility
Some materials should not be in the same bag.
Beyond particle size, blend components interact chemically. Most combinations are fine; a few are not, and the failures are physical and immediate rather than agronomic and slow.
- Urea and ammonium nitrate together
- A classic incompatibility. The two form a eutectic mixture that goes damp and then liquid at ordinary storage temperatures. A blend containing both can turn to slurry in a warehouse in a warm week. This is why UAN exists as a solution and not as a solid blend.
- Urea and superphosphates
- Urea reacts with the calcium in SSP and TSP, releasing water and causing the blend to go wet and set. Not universally prohibited, but it needs deliberate handling and short storage rather than being blended casually.
- Ammoniacal materials and alkaline components
- Ammonium salts blended with lime or other alkaline materials release ammonia — a nutrient loss and a workplace exposure at the same time.
- Moisture is the common thread
- Almost every incompatibility shows up as a hygroscopic blend picking up water and setting. The critical relative humidity of a blend is lower than that of its components, so a blend goes damp in conditions where each ingredient alone would be fine.
- What follows for storage
- Blends have shorter practical storage lives than compounds, and the difference widens in humid climates. If a product will sit for a season before use, that is an argument for a compound — and we will make it even where the blend is the cheaper quote.
The label
What the three numbers actually mean, and what they leave out.
The N-P-K convention is older than most of the products it describes and it carries two quirks that cause real confusion in cross-border trade.
- Oxide, not element
- Nitrogen is declared as elemental N, but phosphorus is declared as P₂O₅ and potassium as K₂O — oxide equivalents of compounds that are not present in the product. A few markets declare elemental P and K instead, and the numbers are materially different. State the convention in the contract; the difference is not small.
- Which phosphate
- The P number should be qualified as total, available or water-soluble, exactly as on a straight phosphate. On a compound made from partially acidulated rock this is not a formality.
- The secondary nutrients
- Sulphur, calcium and magnesium are frequently present, sometimes in agronomically meaningful amounts, and are often left off the headline. A blend using ammonium sulphate or SSP may be delivering more sulphur than the buyer realises, which is either a benefit they are not counting or a reason the ratio is cheaper than it looks.
- Micronutrients
- Boron, zinc and the rest, where included, with the form stated — a chelated micronutrient and an oxide of the same element are not equivalent in availability, and the difference is most of the price.
- Tolerances
- Nutrient declarations carry permitted tolerances under the regulatory scheme of the destination. Those tolerances, and the rejection limits, belong in the contract as separate numbers from the guarantee.
Carriage
Nitrate grades travel under their own rules.
An NPK containing nitrate is a different carriage proposition from one that does not, and the classification is a property of the specific formulation rather than of the family.
| Classification | Group | What it means |
|---|---|---|
| Ammonium nitrate based fertilizer, UN 2067 | B — chemical hazard | Dangerous goods documentation, controls on contamination and ignition sources, and restrictions on stowage and on adjacent cargo. |
| Ammonium nitrate based fertilizer, UN 2071 | B — chemical hazard | The self-heating and self-sustaining decomposition classification. Temperature monitoring and separation requirements follow from it. |
| Ammonium nitrate based fertilizer (non-hazardous) | C | Formulations shown by test not to fall into the above. Carried as an ordinary dry bulk cargo — but the demonstration has to exist for this formulation. |
The failure mode
Self-sustaining decomposition.
Distinct from detonation, and the one that matters for NPK. A nitrate-containing fertiliser, once a decomposition front is started — typically by a local heat source such as a light fitting, a hot bearing or welding — can sustain that front through the pile without further heat input, travelling slowly and releasing toxic gases including nitrogen oxides. It does not explode; it produces a large volume of poisonous fume in an enclosed space, which in a ship's hold or a warehouse is the more likely way people are hurt.
Whether a given formulation does this is established by test, and the result depends on the chloride content, the ratio of nitrate to the other components, and the organic content. Small formulation changes move the answer, which is why the test certificate has to match the production, not the product family.
Our position: we establish the schedule before offering, we require the test evidence for the actual formulation, and we treat contamination controls, hold cleanliness and stowage separation as fixture terms rather than as operational detail. See also detonation resistance on straight ammonium nitrate.
Scope
Where our obligation ends.
- What we supply
- Chemically granulated NPK compounds and bulk-blended NPK to a stated ratio, in cargo lots, containers, big bags and small bags, FOB or CFR — and the straight components for buyers who blend themselves.
- What we do not do
- We do not manufacture or blend, we do not operate blending plants or terminals, and we hold no equity in production.
- On blends
- We will not supply a blend without SGN and UI figures for the components, and we will say when a proposed combination is chemically incompatible or will not survive the handling chain. If the honest answer is a compound at a higher price, that is the answer we give.
- Nutrient guarantees
- Written against a stated convention — oxide or elemental — with the phosphate basis named and the tolerances and rejection limits stated separately from the guarantee.
- Nitrate formulations
- Carriage classification established before offering, with test evidence for the actual formulation. We will not ship a nitrate-containing NPK on a certificate belonging to a different production.
- CBAM
- Nitrogen-containing NPK falls in CN 3105 and is inside CBAM; PK products under CN 3105 60 00 are carved out. We supply classification and producer data; the declarant obligation is the importer's.
How to specify
Seven lines, and the second one is the one nobody sends.
Ratio and tonnage arrive in every enquiry. The handling chain almost never does, and it is what decides whether a blend is the right product at all.
Ratio and convention
The N-P-K statement, whether P and K are oxide or elemental, and whether the P figure is total, available or water-soluble.
The handling chain
How many transfers between the blender and the field, how far, and whether it will be stored for a season. This decides blend versus compound.
Blend or compound
If a blend, the component materials you want used — and their SGN and UI, or our undertaking to match them.
Secondary nutrients and micros
Sulphur, calcium, magnesium; and any micronutrient with the chemical form stated, not just the element.
Climate and storage
Humidity and temperature at the storage site, and the expected residence time. Blends have a shorter practical life than compounds and the gap widens in the tropics.
Packing and basis
Bulk, big bags or small bags with the liner specified; FOB, CFR or delivered; vessel size and discharge rate.
Regulatory position
Destination conformity route, nutrient tolerances, and your CBAM declarant status where the product contains nitrogen.
Inspection
Sample for segregation, not just for analysis.
Independent inspection by an agency appointed for the cargo — SGS, Bureau Veritas or Intertek — instructed jointly. NPK is the one product on our book where the ordinary composite sample is the wrong test, because averaging is exactly what hides the defect.
- Composite analysis
- The conventional determination — N, P₂O₅ on the stated basis, K₂O, secondary nutrients and micros — against the guarantee and the tolerances.
- And spot samples, separately
- On a blend, individual samples from top, middle and bottom of a bag or a stockpile, analysed separately rather than combined. The spread between them is the segregation measurement, and it is the only way to see the problem before the field does.
- Sizing per component
- SGN and UI on each component before blending, and a sieve analysis on the finished blend. Retained samples of the components as well as of the blend.
- Physical condition
- Moisture, crushing strength, caking tendency and free-flow. Photographed at load and at discharge, with attention to lumps, damp patches and bag condition.
- Nitrate formulations
- Test evidence for the actual formulation supporting the carriage classification, holds inspected and certified with previous cargo declared, and the stowage separation confirmed.
- Documentation per cargo
- Certificate of analysis against the guarantee, spot-sample results on blends, component SGN and UI, sieve analysis, moisture certificate, hold cleanliness certificate with previous cargo declaration, classification test evidence and dangerous goods declaration where applicable, safety data sheet in the destination language, certificate of origin, bill of lading and the inspection report with photographs.
Straight answer
What we will tell you before you ask.
- That a blend and a compound are not the same product
- Same label, different behaviour. If the chain between blender and field is long, or the product will be stored for a season, the compound is the honest recommendation even when the blend is cheaper.
- That segregation passes every analysis
- A composite sample averages back to the stated ratio while the field gets strips of the wrong one. It is prevented by matching SGN and UI before blending, not detected afterwards.
- That urea and ammonium nitrate do not belong in one blend
- They form a mixture that goes damp and then liquid at ordinary storage temperatures. If someone has quoted you that combination as a solid, ask them about it.
- That N-P-K is an oxide convention
- Nitrogen elemental, phosphorus as P₂O₅, potassium as K₂O. Some markets declare elemental P and K and the numbers are a long way apart. It belongs in the contract.
- That nitrate grades need test evidence for the actual formulation
- Self-sustaining decomposition depends on chloride, ratio and organic content, and small changes move the answer. A certificate for a similar product is not evidence about this cargo.
Enquiries
Every enquiry is answered by the desk that handles it.
Send the product, quantity, delivery basis, destination and timing. We revert with availability, an indication and the documentation that accompanies it. Specifications are released once we know who we are speaking with.