Granular urea is produced by building up successive layers of urea melt onto seed particles in a fluid-bed or drum granulator, with each layer solidified before the next is applied. The layered structure gives a larger, harder, denser particle than a prill. At 46 % nitrogen it is the most concentrated solid nitrogen fertiliser in common use, and the granular form is the standard choice for bulk blending and for wide-swath mechanical spreading.
Substance identity
| Chemical name | Urea |
| Synonyms | Carbamide, carbonyl diamide, carbonic acid diamide |
| Formula | CH₄N₂O (also written CO(NH₂)₂) |
| CAS No. | 57-13-6 |
| PubChem CID | 1176 |
| Molar mass | 60.06 g/mol |
| Appearance | White, near-spherical granules, 2–4 mm predominant, free-flowing |
Specification
— typical commercial ranges; not a guaranteed specification
| Parameter | Typical range | Note |
|---|---|---|
| Total nitrogen (N), dry basis | 46.0 % min | stoichiometric 46.6 % |
| Biuret | ≤ 1.0 % | ≤ 0.5 % specified for foliar and horticultural use |
| Moisture (H₂O) | ≤ 0.5 % | |
| Particle size, 2 – 4 mm | ≥ 90 % | median around 2.5 – 3.0 mm |
| Crushing strength | 2.5 – 3.5 kgf typical | the key advantage over prills |
| Bulk density | ≈ 0.74 – 0.78 t/m³ | |
| Formaldehyde-based granulation aid | ≤ ~0.5 % where used | processing and anticaking aid; declare if the end use is sensitive |
Properties
Very soluble in water — roughly 108 g per 100 g water at 20 °C, far higher than ammonium sulphate — and dissolution is markedly endothermic, so a dissolving tank cools noticeably and dissolution slows as it does. Melts at approximately 133 °C, and above the melting point it condenses toward biuret and then decomposes; this is why process temperature control determines the biuret figure. Hygroscopic: critical relative humidity approximately 72–75 % at 30 °C, lower than ammonium sulphate, so urea requires more careful humidity management in store.
In soil, urea is hydrolysed by the enzyme urease — present in soil and abundant in crop residue — to ammonium carbamate, which decomposes to ammonium, carbon dioxide and water. The reaction raises pH sharply in the immediate reaction zone, and if the urea is on the soil surface, ammonia can be lost to the atmosphere. Losses are greatest on warm, moist, high-residue, high-pH surfaces and after light rain that wets without moving the urea into the soil. Incorporation, irrigation of at least a few millimetres, or a urease inhibitor such as NBPT are the recognised mitigations. Once ammonium is formed, nitrification proceeds and releases acidity — roughly 1.8 kg CaCO₃ per kg N as a textbook approximation, less than half the acidification of ammonium sulphate per unit of nitrogen.
Applications
Bulk blending. A 2–4 mm granular urea is size-compatible with granular DAP, MAP, MOP and granular ammonium sulphate, so blends resist segregation during handling and spreading. Prilled urea is not size-compatible with these partners.
Wide-swath mechanical spreading. Granule mass, roundness and strength give predictable ballistics from a spinning disc, which is what makes wide working widths practical. Weak particles that shatter in the spreader change the size distribution mid-application and distort the pattern.
Long handling chains. Higher crushing strength means fewer fines from repeated transfer, which matters for imported product handled through port, silo, truck and blender.
Urea also has substantial non-fertiliser use, including urea-formaldehyde and melamine resins, and non-protein nitrogen in ruminant feed.
Handling and storage
Store dry, covered, below the critical relative humidity where possible; keep bags sealed and stacks moderate. Do not store blended with ammonium nitrate — the urea/ammonium nitrate mixture has a published critical relative humidity of roughly 18 % at 30 °C and will liquefy in almost any storage environment. Blends with ammonium sulphate reach roughly 56 % at 30 °C, which is workable for prompt spreading but not for storage. Away from strong alkalis and from strong oxidisers. Not classified as dangerous goods for transport.
Notes
Fertiliser-grade urea is not diesel exhaust fluid grade. Automotive urea solution (AUS 32 / DEF) is specified under ISO 22241 and requires much tighter limits on biuret, metals and insolubles than any fertiliser specification; fertiliser urea must not be substituted. Where urea is intended for foliar application, specify low-biuret material — biuret accumulates in plant tissue and causes leaf injury at concentrations that are harmless in soil-applied product.
Frequently asked questions
The specification says 46% N minimum — what does that figure actually cover, and why do offers quote 46.0, 46.2 or 46.4?
The theoretical nitrogen content of CH4N2O is about 46.6% (28.01 g of nitrogen in a molar mass of 60.06). Commercial specifications quote a guaranteed minimum, usually 46.0% min, with typical assays a little higher; the gap to theoretical covers moisture, biuret and analytical tolerance. Nitrogen is normally reported as total N, and biuret nitrogen is counted inside that figure, so a high-biuret lot can still meet 46% while being less useful agronomically. Read three things together: the guaranteed minimum, the test method, and whether the result is stated as-received or on a dry basis. Offers quoting 46.0, 46.2 and 46.4 may describe the same material analysed and expressed differently.
Granular or prilled urea — what is the practical difference, and which should I specify?
The chemistry is identical; the difference is how the solid was formed. Prills are made by solidifying melt droplets in a tower, are smaller (roughly 1–2.4 mm) and mechanically weaker. Granules are built up layer by layer in a drum or fluid bed, are larger (roughly 2–4 mm) and have higher crushing strength. Granular is normally specified for bulk blending with DAP or MOP and for wide-swath spinner spreading, because particle size and strength closer to the other components reduce segregation in the blend and in the hopper, and the material survives handling. Prilled is often adequate where the urea will be dissolved or further processed. Ask for the size distribution and crushing strength, not just the word granular.
What is biuret, and what limit should I put in the specification?
Biuret forms when urea is held hot during synthesis and finishing: two urea molecules condense and release ammonia. It is counted in total nitrogen but is phytotoxic at low concentration, mainly in foliar sprays and for sensitive crops such as citrus and some vegetables. Standard soil-application grade is normally offered at 1.0% biuret maximum; material intended for foliar spraying or fertigation is bought separately as low-biuret, commonly 0.3% maximum. Soil-applied urea is far more tolerant, because soil microorganisms degrade biuret over weeks. Two practical points: state the biuret limit explicitly, since it is not implied by 46% N; and note that agricultural grade carrying an anti-caking treatment is not interchangeable with technical grades, which have their own purity limits.
How should urea be stored, and what causes bags to set hard in the warehouse?
Urea is strongly hygroscopic. Its critical relative humidity is around 70–75% at normal warehouse temperatures; above that it takes up water from the air, then cakes as it dries again. In practice: keep bags sealed until use, store off the floor and away from external walls, avoid opening bulk stock in humid weather, and do not leave part-used bags open overnight. Caking also has a purely mechanical cause — pressure at the bottom of a tall stack — so limit stack height and rotate stock rather than blaming the product. Anti-caking treatment reduces the problem but does not remove it. Keep urea physically separated from ammonium nitrate stock, and away from heat: urea melts near 133 °C and decomposes, releasing ammonia.
What must urea not be blended with?
Do not make solid blends of urea with ammonium nitrate or calcium ammonium nitrate. The pair is deliquescent, so the mixture wets itself and collapses into a slurry, and the combination also raises decomposition and handling concerns. Urea and ammonium nitrate coexist safely only when co-dissolved as a solution (UAN). Avoid blends with single or triple superphosphate: free acid and water of crystallisation promote wetting, caking and ammonia loss. Avoid alkaline materials such as lime, basic slag and calcium cyanamide, which strip ammonia from urea. Urea with ammonium sulphate, MOP or DAP is common practice, but check particle size compatibility to limit segregation. Any freshly made blend containing urea should be spread promptly rather than stored.
How do I compare a urea offer against ammonium nitrate or ammonium sulphate on a like-for-like basis?
Compare on cost per unit of nitrogen, not per tonne of product. Divide the delivered cost per tonne by the guaranteed nitrogen fraction: urea at 46% N carries 460 kg N per tonne, ammonium nitrate at a nominal 34% carries 340 kg, ammonium sulphate at a nominal 21% carries 210 kg. Only after that conversion do the products compete on the same basis. Then adjust for what the figure hides: transport and handling per unit of nitrogen, which favour the most concentrated product; secondary nutrients you would otherwise buy separately, since ammonium sulphate also supplies sulphur; expected losses in your application method; and physical quality — size distribution, crushing strength, moisture, biuret — which decides whether the material handles as specified.
When is urea the wrong choice?
Several situations. Surface broadcasting without incorporation or following rainfall, on calcareous or high-pH soils or over heavy crop residue: urease hydrolyses urea quickly and a significant part of the nitrogen can escape as ammonia within days. Placement in or immediately beside the seed row, where released ammonia damages germinating seed; the risk is highest on sandy, low-CEC soils and with sensitive crops. Broadcasting into standing floodwater. Any case where nitrate must be available immediately, such as cold soils early in the season, since hydrolysis and nitrification take time. Foliar application with standard-grade material. And urea supplies nitrogen only: where sulphur, calcium or magnesium is also limiting, a sulphur- or calcium-bearing nitrogen source can be the better purchase despite a higher cost per unit N.
Related substances
- When urea is broadcast without incorporation and the surface is alkaline, ammonium sulphate is the usual alternative because its nitrogen is not exposed to urease at all.
- Ammonium chloride concentrates less nitrogen per tonne but skips the hydrolysis step entirely, which changes both the loss pathway and the handling requirements.
- Urea and DAP are the two products on this list most likely to release free ammonia beside a germinating seed, so read the DAP page before deciding what may go into the seed furrow.