Calcium Ammonium Nitrate: Caking Resistance; Granule Size and What the Bag Does Not Tell You

Calcium ammonium nitrate is one of the few fertilizers where the prill coating matters more than the nitrogen content printed on the bag. Two suppliers can deliver twenty-seven percent nitrogen to the same specification and behave completely differently in a spreader at eighty percent humidity.

This note is for agronomy buyers and procurement managers specifying granular calcium ammonium nitrate for field application. It covers what the coating actually does, how to test caking resistance before the season rather than during it, and the acceptance checks that catch a bad lot at the warehouse door. The segment structure behind the application is set out in this analysis of the fertilizer grade calcium ammonium nitrate market.

Caking resistance, not nitrogen content, is what determines whether a granular fertilizer can be spread at all

Nitrogen content is a certificate number. Caking resistance is a physical property that decides whether the bag empties into the hopper or has to be broken up with a bar. Buyers who negotiate on analysis alone discover the difference in the first humid week.

The mechanism is straightforward. Calcium ammonium nitrate is hygroscopic, and the prill surface absorbs moisture from the air until the crystal bridges between adjacent granules form a solid mass. Coating agents slow that absorption; they do not stop it. The question is how long the coating buys you under your storage conditions, not whether it works.

What the coating layer is actually doing

Most granular CAN is coated with a thin film that reduces surface wettability and limits moisture ingress at the granule boundary. The coating is a small fraction of the product mass and a large fraction of the handling behaviour.

Two failure modes appear when the coating is applied unevenly. The first is patchy coverage, where uncoated areas become the nucleation points for caking. The second is excess coating, which reduces the granule's ability to dissolve in the soil and can leave a residue on the spreader vanes.

Neither failure is visible from the bag. Both show up as a spread pattern complaint a month later, when the customer has already applied the product and cannot prove which lot caused it.

How to test caking before you buy

The usable test is a bag stack, not a laboratory instrument. Fill three bags from the lot, stack them under the load they will see in your warehouse, and hold them at the humidity and temperature of your worst storage week for fourteen days. Then drop the stack from one metre onto a hard floor and open the bags.

Record three outcomes: the proportion of the bag that remains as free-flowing granules, the size of the largest agglomerate, and whether the agglomerate breaks under hand pressure. Free-flowing above eighty-five percent with agglomerates that break by hand is a pass. A bag that has to be cut open and mechanically broken is a fail, whatever the certificate says.

Run this test on the first delivery and on every tenth delivery after. Coating suppliers change, and the change does not always reach the buyer as a specification revision.

Why granule size distribution changes the spread pattern

A spreader throws granules by mass and by aerodynamic drag. A lot with excess fines produces a pattern with heavy near-field deposition and thin coverage at the edge; a lot with oversize granules throws further than the tray test predicted.

Ask for the sieve analysis, then verify it. Take a one-kilogram sample, sieve it at the two boundary sizes your spreader settings assume, and weigh each fraction. A fines fraction above the agreed limit is a reject, not a discount negotiation, because the application error it causes cannot be corrected after the field is covered.

This is the check most often skipped, and it is the cheapest one to run. A set of test sieves and a balance costs less than a single resupply of a field that was under-applied.

Storage conditions you control, and the ones you do not

Bagged CAN stored on a concrete floor draws moisture through the bag base. A pallet or a vapour barrier under the stack is not a refinement, it is the difference between a season-long lot and a three-week lot.

Stack height matters for a different reason. The load compresses granules at the bottom of the stack and accelerates bridging where the coating is thin. If your warehouse is at capacity, rotate the stock rather than adding height.

The conditions you do not control are transit and the port. A container that sat in a humid terminal for two weeks before the truck arrived has already started the process, and no warehouse practice will reverse it. Ask for the loading date and the discharge date, and treat a long gap as a risk factor on the lot, not a neutral detail.

What the nitrate regulation changes for buyers

Calcium ammonium nitrate sits in a category where storage and transport rules are stricter than for most fertilizers, because of its oxidizing behaviour. Buyers in Europe operate under national rules that implement the same underlying concern: quantity limits, separation from combustible materials, and record keeping.

The procurement consequence is that the cheapest supply route is sometimes the one with a compliance step nobody costed. A supplier who cannot provide the documentation package your site needs has not delivered a compliant product, even if the chemistry is correct.

It also changes who you can buy from. Nitrate chemistry sits inside the dual-use framework, which means export documentation, end-use declarations and, in some destinations, licensing. Buyers who treat the category as ordinary fertilizer discover this at the customs stage, which is the most expensive place to discover anything. The supplier landscape for these chemistries is mapped in this overview of dual-use chemical intermediate sourcing, and it is worth reading before the tender rather than after the first shipment is held.

What to inspect on arrival

Sample three bags per lot: one from the top of the stack, one from the middle and one from the base layer. Sieve each separately and compare the fines fraction across the three. A base-layer fines fraction more than five percentage points above the top layer means the stack has been compressing and abrading the product, which points at coating durability rather than at the lot as a whole.

Weigh each sample and record it. Bag fill weight drift is the earliest visible sign of a filling line that has moved, and it is much easier to raise before the invoice is settled than after.

Keep a sealed sample of each lot for the season plus one year. When a customer reports a spreader problem, the retained sample is the only way to separate a product fault from an application fault.

Frequently asked questions

Is higher nitrogen content always better for calcium ammonium nitrate?

No. The nitrogen figure is largely fixed by the product type. Caking resistance, granule size distribution and coating uniformity determine whether the product can be applied accurately, and those are the properties that vary between suppliers.

How long should a bag stack test run?

Fourteen days at your worst storage conditions, with the stack loaded as it will be in the warehouse. Shorter tests do not reproduce the bridging that causes real caking.

What fines fraction should be rejected?

Set it against your spreader's settings and the pattern test it was calibrated with. Above the agreed limit the application error cannot be corrected in the field, so treat it as a reject rather than a price adjustment.

Does coating reduce nutrient availability in the soil?

The coating is a small fraction of granule mass and dissolves with the granule. Excess coating is a handling and residue problem rather than an agronomic one, which is why even coverage matters more than coating weight.

Why does the same supplier deliver different behaviour between lots?

Coating formulation and application rate can change without a specification revision reaching the buyer. That is the reason for re-testing every tenth delivery rather than only the first.

What to change in the next fertilizer tender

Add the fourteen-day stack test, the sieve limits and the retained-sample requirement to the acceptance terms. Ask for loading and discharge dates with each lot. And price the compliance documentation as part of the offer, because a product that cannot be stored legally on your site is not a delivered product.