What is biuret?
An impurity formed when urea is overheated during production. It matters because high biuret damages foliage in leaf-applied fertilizer — agricultural urea is normally capped at 1% maximum, with tighter limits for foliar use.
Key points
- Formed when urea overheats — a process signal, not a contaminant.
- Phytotoxic on leaves; soil application is more forgiving.
- Belongs on the CoA alongside moisture.
At a glance
| Formed by | Two urea molecules condensing under heat |
|---|---|
| Agricultural limit | 1.0% max |
| Foliar limit | 0.3% max or lower |
| Damage caused | Chlorosis and leaf tip burn |
| For AdBlue | Governed separately by ISO 22241 |
How it forms
Biuret is produced when two urea molecules condense under heat, releasing ammonia. It forms during the concentration and prilling or granulation stages, so its level is a direct signal of how tightly the plant controls temperature.
Because it is a process artefact rather than a contaminant introduced later, biuret content is characteristic of a producer and reasonably consistent between production runs.
Why it matters agronomically
Biuret is phytotoxic. Applied to leaves it causes chlorosis and tip burn, and citrus and other sensitive crops are damaged at low concentrations. Soil-applied urea is more forgiving because soil microorganisms break biuret down, though slowly.
The convention is a maximum of 1.0% for general agricultural urea, and 0.3% or lower where the product is intended for foliar application. Those are contract numbers, not guidance.
On the contract and the certificate
Biuret belongs in the specification and on the certificate of analysis for the shipped lot. A urea CoA without a biuret figure is missing one of the two numbers that determine whether the cargo is fit for its intended use, the other being moisture.
For technical urea destined for AdBlue production the relevant standard is different again — ISO 22241 sets its own purity requirements, and fertilizer-grade material cannot simply be substituted.
Frequently asked questions
- What is a normal biuret level in urea?
- Up to 1.0% for general agricultural use. Urea intended for foliar application is normally specified at 0.3% maximum or lower, because leaves are far more sensitive than soil.
- Why is biuret harmful to plants?
- It is phytotoxic on contact with foliage, causing chlorosis and leaf tip burn. Citrus and several horticultural crops are damaged at concentrations that soil-applied urea would tolerate.
- Does biuret matter for AdBlue?
- Yes, but under a different standard. Automotive urea solution is governed by ISO 22241, which sets its own purity requirements; fertilizer-grade urea cannot be substituted because its additives damage the SCR catalyst.
Related terms
The ISO 22241 specification for automotive urea solution — 32.5% high-purity urea in demineralised water, sold as AdBlue or diesel exhaust fluid. Fertilizer-grade urea cannot be substituted because its additives poison the SCR catalyst.
The laboratory document stating the measured composition of a specific parcel — purity, contaminants, moisture. It refers to the lot actually shipped, unlike a typical or datasheet analysis which describes what a plant usually produces.
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