Urea is hygroscopic. It absorbs atmospheric moisture readily, and when that moisture partially dissolves the granule surface and subsequently evaporates, crystal bridges form between granules. The result is a bag or a hold of solid material that has to be broken mechanically before it can be spread.
The conditions that cause it
- Relative humidity above the critical point for urea — roughly 72 % at 30 °C — during storage or transit.
- Temperature cycling, which drives repeated condensation and evaporation at the granule surface.
- Compression from stack height, which forces granules into closer contact where bridges can form.
- Residual moisture above specification at the point of bagging.
Note that only the last is a production variable. The other three are handled entirely by whoever specifies the packing and plans the route.
Specifying against it
Anti-caking treatment — typically a formaldehyde-based or amine coating applied at the plant — is the primary defence and should be requested explicitly on any consignment moving through a humid route or into monsoon-season storage. It is not universally applied by default.
Second, bag construction. A woven polypropylene bag with an inner polyethylene liner performs substantially better than an unlined bag on sea freight, and the cost difference is small relative to the value of the cargo. Third, stack height should be limited and stated on the packing instruction rather than left to the loading crew.
Anti-caking coating, a lined bag and a stated stack limit cost a few dollars a tonne. Breaking out a caked cargo at destination costs considerably more.
Granular versus prilled for export
Where the buyer’s application permits either form, granular urea is the better export product. The granule is harder and larger, which reduces attrition, dust generation and the surface area available for moisture absorption. Prilled material dissolves faster — genuinely useful in fertigation — but it is more vulnerable across a long sea movement, and that trade-off should be made deliberately rather than by default.