Growers are advised to resist the temptation to apply additional Urea onto saturated soils, as the potential for Nitrogen loss from denitrification is increased.
Denitrification is the process by which Nitrate Nitrogen is converted to gaseous forms and lost to the atmosphere. This process occurs when soil Oxygen levels are low as a result of waterlogging. It is important to note that soils are considered waterlogged when 60-70% of pore spaces are filled, regardless of whether water can be seen on the soil surface.
The two most important factors associated with denitrification are the availability of Nitrogen and conditions of low Oxygen.
Availability of Nitrogen
- The more Nitrogen available, the more there is to lose.
- Nitrogen applied as Urea is much more prone to loss as it is more easily converted to the gaseous form.
- Nitrogen from pulse stubbles or legume pastures must first be mineralised from organic Nitrogen, which makes it less prone to loss.
Availability of Oxygen
- The process only occurs under waterlogged conditions. This is much more likely on heavy clay soils than well-drained loams and sands.
Other factors which influence denitrification include:
- The rate of Nitrogen loss increases with increasing temperature, peaking at 25-30˚C. However the process starts at temperatures as low as 5˚C.
- Losses are higher on alkaline soils than acid soils.
While it may be tempting to apply additional Urea during the 2016 winter to capture potentially higher yield, applying Urea onto waterlogged soils significantly increases the risk of Nitrogen loss from denitrification. This should be considered when assessing topdressing requirements for crops and timing of Urea application.