The breakdown of urea by a process known as hydrolysis leads to production of ammonia (NH3, a gas) or ammonium (NH4+), a plant nutrient. When urea is top-dressed, any NH3 that is produced can be blown away from the site and this leads to poor use of applied nitrogen (N). A bit like tossing your money in the air and watching it blow away. The amount of NH3 that is lost from a site will depend upon many factors including;
· What the urea is applied to – pasture, bare cropping soil, trash-blanketed systems, etc. This is because the rate of NH3 loss depends on how quickly the urea is broken down and this depends on the amount of activity of an enzyme called urease, which is found in organic materials. This means in dairy pasture the risk of NH3 loss is high.
- Soil pH – with greater risk at higher pH (more alkaline soils). This influences whether the gas NH3 or the plant nutrient NH4+ is produced.
- The climate – Windy days have greater loss than still days. When rain falls (> 10 mm) urea is washed into the soil and so less NH3 is lost.
One way of minimising the risk of NH3 loss is to apply a urease inhibitor to your urea. This slows the activity of the urease enzyme and so slows the rate of urea breakdown. One product available in Australia is Green Urea 14 which offers 14 days protection. A field trial run by The University of Melbourne and Incitec Pivot tested the difference in NH3 loss from top-dressed urea and Green Urea 14 on a ryegrass seed crop in autumn and spring 2010 at Murroon in south west Victoria.
In autumn the NH3 loss from urea was 30 per cent of the applied N. Use of Green Urea 14 dropped this loss to nine per cent of applied N. After fertiliser application no rain fell for seven days but there was dew every morning. Wind speed over this time period ranged from <1 km per hour to 49 km per hour. In spring the NH3 loss from urea was 2 per cent of the applied N. When we used the Green Urea 14 this dropped to 1 per cent of applied N. Rain fell (4 mm) within 24 hours of application of the fertiliser and washed the urea into an already moist soil. So while the Green Urea 14 still reduced NH3 loss from urea in spring the benefit achieved was much less than in autumn.
Addition of fertiliser N increased dry matter (DM) production by 8 kg per unit of N (328 kg per hectare) for urea in autumn and 9 kg per unit of N (361 kg per hectare) for Green Urea 14. In spring the increase was 18 kg per unit of N (700 kg per hectare) for urea and 12 kg per unit of N (468 kg per hectare) for Green Urea. Biomass production did not reflect the savings in N achieved with the use of Green Urea and we think this was because N was not limiting in either treatment despite the losses that occurred. This might mean you can apply less N with Green Urea.
The results of the trial show, depending on the season, substantial benefits can be achieved from use of a urease inhibitor fertiliser such as Green Urea to reduce losses of NH3 from ryegrass. Then less N can be applied for the same production.
The project was partly funded by the Department of Agriculture Forestry and Fisheries and the Grains Research and Development Corporation.
For further information contact Helen Suter at Melbourne School of Land and Environment, The University of Melbourne; email@example.com