Getting the timing right for topdressed N fertiliser to best match crop demand is more important for N use efficiency than choosing the right N fertiliser form.
Field experiments generally show that granular urea, urea solutions and UAN perform equally well in supplying N when used in-crop. The most common form of N fertiliser used in Australian cropping is urea [(NH2)2CO] as a prilled solid, applied with a broadcast spreader. An alternative is Urea – Ammonium Nitrate (UAN), a liquid mixture of urea and ammonium nitrate, applied with spray or streaming equipment (or liquid drills at sowing). Urea is also available as a liquid solution.
Granular urea is generally the cheapest source, and while the liquids can supply some N to the crop through the leaves, generally this amount is small relative to total crop demand. Irrespective of source, N that lands on the soil needs to move into the root zone for uptake by the plant.
N that doesn’t reach the crop
Without follow-up rainfall, some soil applied N can be lost to the atmosphere. Urea has to go through two chemical reactions before it becomes available to roots. The first step rapidly converts urea-N into ammonia-N, which can be lost to the atmosphere under certain conditions (volatilization). Highest losses of ammonia-N from urea occur on light soils with alkaline pH and little crop canopy. The second stage, which transforms ammonia-N into the more stable, plant available ammonium-N and then nitrate-N forms, depends on moisture.
Field research has shown losses of N as ammonia gas from granular urea or liquid products used in-crop are generally less than 10%, even if there is little follow-up rainfall. Poor responses to applied N can still occur. N can become stranded in the dry topsoil. The ammonium nitrate component of UAN can also suffer this fate even though it is not subject to volatilisation. The amount available later depends on other loss processes. Leaching and denitrification can reduce the carryover of N to the following crop.
Foliar uptake of N
When applied to leaves as a liquid, urea-N is taken up 10 times more rapidly than ammonium-N or nitrate-N. This can give a rapid boost to crops but the amount taken up is small. Most of the N applied as UAN is absorbed via roots, especially if the canopy is sparse. If too much fertiliser is on the foliage it can cause leaf damage from urea toxicity or salt damage.
Solids vs liquids
Potential benefits of using liquid N sources include:
- a small but immediate boost in canopy N status
- more even application patterns through spray equipment
- the opportunity to apply tank mixes with registered crop protection products and
- any logistic advantages that come from handling, transport and storage of the liquids.
eXtensionAUS: Three reasons why topdressing can fail
Photo courtesy of the GRDC