Background
Urea is the most used nitrogen (N) fertiliser by farmers. However, losses from urea can be high under certain conditions and there is considerable interest in improving the efficiency of its application.
When applied to soil, urea is hydrolysed in the presence of moisture to ammonium, facilitated by the action of the urease enzyme present in the soil. The hydrolysis process is usually completed within 1–4 days.
During hydrolysis, the area surrounding the granules become temporarily alkaline, even in acid soils. Under alkaline or high pH conditions, the ammonium (NH4 +) can be converted to ammonia gas (NH3) which can be lost through volatilisation from the soil, especially when urea is surface applied. Other loss pathways for applied urea include leaching of nitrate (NO3 -) and denitrification of nitrate, mainly as nitrous oxide (N2O—a potent greenhouse gas) and dinitrogen (N2) under very high soil moisture content (Figure 1).
Incitec Pivot Fertilisers Green Urea NV® is an enhanced efficiency fertiliser that contains a urease inhibitor which slows the conversion of urea to ammonium, which is subject to loss as ammonia gas. The Green Urea NV minimises nitrogen losses, leading to more retention in the system for a crop and pasture production.
Extensive research trials by Incitec Pivot Fertilisers, with the help of co-operators including Riverine Plains at Murchison East in 2024, measured ammonia gas volatilisation from Green Urea NV compared to urea at 11 sites across Australia, with varying soil types and over multiple top-dressing applications.
Figure 1 Nitrogen transformation and loss pathways for applied urea. The urease inhibitor in Green Urea NV slows the urease enzyme responsible for the rapid hydrolysis of urea to ammonium, which can be lost as ammonia gas under certain soil and climatic conditions.
Factors impacting ammonia volatilisation
Soil pH
High soil pH drives ammonia gas volatilisation. At high pH, the ammonia-ammonium equilibrium is shifted towards ammonia, leading to high volatilisation losses. Regardless of soil type, soil pH under the urea granule will increase, converting ammonia to ammonia gas and leading to nitrogen loss. Green Urea NV slows this pH rise, reducing volatilisation losses.
Cation exchange capacity
Soils with a high cation exchange capacity (CEC) can retain ammonium (NH4 +) on its exchange (negatively charged surfaces), leading to a reduction in the ammonium available for conversion to ammonia gas. This results in lower volatilisation losses in soil with high CEC, such as soils high in clay and organic matter. Sandy soils have a low cation exchange capacity, meaning they don’t hold nutrients well. Green Urea NV reduces nitrogen loss, keeping more available for crop uptake.
Organic material (pasture thatch, crop residues, stubble, trash)
Organic material can trap urea granules above the soil surface, exposing them to volatilisation. Green Urea NV slows urea breakdown, helping retain nitrogen until it is incorporated.
Low rainfall or dew conditions
When urea granules dissolve but are not incorporated deeply enough into the soil, volatilisation risk increases. The amount of rainfall needed to reduce losses varies by soil type: sandy soils require >10mm, loam soils >16mm, and clay soils >25mm.
Aim
This trial focused on the measurement of ammonia gas volatilisation from top-dressed or surface-applied urea, and the performance of Green Urea NV® in mitigating the volatilisation losses.
Method
At all the trial sites, ammonia gas measurement was achieved using 150 x 300 mm PVC chambers with a cap and an acid-treated foam. The enclosed chamber, which was driven into the soil over the top-dressed area, holds a foam which traps ammonia gas above the soil surface, whilst the cap excluded moisture for the duration of measurement. The foam was replaced at weekly intervals for two weeks for continuous trapping of ammonia gas from both the urea and Green Urea NV treatments. The ammonia gas in the foam was subsequently extracted for analysis at the Nutrient Advantage Laboratory.
Winter crops including wheat, barley and canola were sown across the 11 trial sites, with the Murchison East site sown to canola in 2024. The sites received three separate top dressings of nitrogen as either standard urea or Green Urea NV in mid-June, mid-July and mid-August, applied at a rate of 46 kg N/ha.
Results
Figure 2 shows how many kilograms of nitrogen was lost as ammonia gas at each topdressing (mid May, mid June and mid August) at the Murchison East site. Table 1 shows the percentage reduction in ammonia gas lost from each Green Urea NV application, relative to standard urea.
At the Murchison East site, between 8.2 and 10.1 kg N/ha out of the 46 kg N/ha applied at each top dressing was lost through ammonia gas volatilisation. This equated to a loss of between 17–21.9 percent for each application. In contrast, Green Urea NV losses were between 3.2–4.1 percent of the total applied. Compared to the standard urea, Green Urea NV reduced ammonia gas loss by 76, 82 and 83 percent for the June, July and August applications respectively.
An average of 72 percent of the total nitrogen lost occurred in the first week of urea application.
Results for the remainder of the trial sites can be found on the Incitec Pivot website.
Ammonia nitrogen lost from the cropping system can be accounted for in two ways; as a direct cost of the ammonia loss and as the opportunity cost of lost grain yield and/or protein.
Observations & comments
Trial results from Murchison East and other sites across Australia show the high volatility of surface-applied urea in winter crop applications. As farming scale has increased, along with the rate of nitrogen required in modern cropping systems, urea applications are often applied in situations that promote volatilisation losses.
Factors such as rainfall forecast accuracy and logistical demands can also affect the optimal timing of urea applications, increasing the risk of losses. Green Urea has shown to be highly effective in reducing the ammonia losses from surface applied urea, providing a potential management tool in reducing the risk of nitrogen loss.
Acknowledgements
This trial work was undertaken by Incitec Pivot Fertilisers. Riverine Plains provided in-kind support to the Murchison East trial site.
Author
Lee Menhennet Incitec Pivot Fertilisers
Author
10 July 2025