KNOWLEDGE · SAFETY
Biuret: the hidden by-product in urea-based NPK
When urea is melted and granulated, a fraction of it converts to biuret. The compound is phytotoxic: it inhibits root growth and damages chlorophyll, and sensitive crops react even at concentrations that pass national standards. This article covers where biuret comes from, which crops are at risk, and how to specify a fertilizer that does not carry it.
Where biuret comes from
Biuret forms when two urea molecules condense at temperatures above roughly 130 °C — exactly the conditions inside a prilling or granulation tower. The hotter and longer the melt, the more biuret. Standard limits are usually ≤1.0% for urea and urea-based compound fertilizers, but the limit is a regulatory floor, not an agronomic safety line.
What biuret does to a plant
Biuret is absorbed but poorly metabolised. It accumulates at the growing point and in young leaves, where it:
- Inhibits cell division in root tips — the root system stays shallow and brown
- Breaks down chlorophyll, producing yellowing at the leaf tip and margin
- In severe cases kills the growing point, so the plant stalls for weeks
Symptoms are easily confused with salinity damage or ammonium burn, which is why biuret is often missed in diagnosis.
Crops that react first
Sensitivity is highest in crops with small root systems or high-value seedlings: banana, citrus and other fruit-tree nurseries, cucurbits, lettuce and leafy vegetables, tobacco, and pineapple. Cereals tolerate more, but the damage is still measurable at the seedling stage.
The alternative: nitrate-based NPK
A nitrate-sulphate NPK made from ammonium nitrate solution never passes through a urea melt, so it contains zero biuret by process, not by control. Two side benefits follow: nitrate nitrogen is taken up directly without nitrification, and the granule dissolves faster — both useful in cold weather and under drip irrigation.
| Property | Urea-based NPK | Nitrate-sulphate NPK |
|---|---|---|
| Biuret content | Typically ≤1%, present | Zero by process |
| Nitrogen form | Urea (needs conversion) | Nitrate + ammonium |
| Uptake in cold soil | Slow | Direct |
| Seedling safety | Risk at high rates | Low risk |
| Cost per unit N | Lower | Higher |
What to write in a specification
- Biuret content, maximum % (ask for the lot figure, not the standard)
- Nitrogen form breakdown: nitrate / ammonium / urea percentages
- For seedling or nursery use, specify a nitrate-based source instead
Frequently asked
Is biuret harmful in cereals?
At standard application rates cereals tolerate it, but seedling-stage damage is still measurable. The risk matters most for high-value and sensitive crops.
Can I remove biuret from urea?
No economically. The practical answer is to choose a nitrate-based nitrogen source for sensitive crops and stages.
Does a low biuret number mean safe for banana?
Not necessarily. Banana nurseries are sensitive enough that nitrate-based nitrogen is the safer specification regardless of the biuret figure.