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Manage broadcast urea to reduce volatilization

At Montana State University (MSU), soil scientist Dr. Rick Engel has heard questions about nitrogen (N) losses from broadcast urea (46-0-0) for many years, going back as far as 1983 when he joined MSU. “It was commonly thought that if you broadcast urea on cold soils, urea losses weren’t very large. Our research is showing that just because you apply urea to cold soil, it doesn’t necessarily give a whole lot of protection against volatilization losses,” says Engel.


April 30, 2010
By Bruce Barker

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At Montana State University (MSU), soil scientist Dr. Rick Engel has heard questions about nitrogen (N) losses from broadcast urea (46-0-0) for many years, going back as far as 1983 when he joined MSU. “It was commonly thought that if you broadcast urea on cold soils, urea losses weren’t very large. Our research is showing that just because you apply urea to cold soil, it doesn’t necessarily give a whole lot of protection against volatilization losses,” says Engel. 

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Even at cold temperatures, urea losses can be high, as shown in the Montana State University trial where 40 percent of N was lost in Campaign 5. (Photos courtesy of Rick Engel, Montana State University)
 
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The lowest risk application with broadcast urea is to cold, dry soils with low relative humidity, followed by a half inch of rain.


 

Urea has become the broadcast fertilizer of choice for many winter wheat growers on the Canadian Prairies, especially since ammonia nitrate fertilizer was removed from the market. The problem, though, with surface applications of urea is that the N is susceptible to ammonia volatilization. The urea molecule, in the presence of moisture and the soil enzyme urease, converts to ammonium carbonate, which can lead to the production of ammonia gas. 

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Engel explains that a number of environmental and soil-related factors interact together to affect this process and define the magnitude of loss. Most research shows that losses are enhanced by high soil pH, warm temperatures, and light rains sufficient to moisten the soil surface but insufficient to enable movement of urea below the soil surface.

In 2008 and 2009, Engel conducted research trails in Hill County of northern Montana, just across the border from Canada. He usea a micrometeorological system to measure ammonia gas that is released from broadcast ammonia. The results to date have convinced him that volatilization losses can be significant even if soil temperatures are near zero degrees C.

One of the farm sites provides the most relevant results for Canadian farmers, as it used an early spring broadcast application similar to practices used in Western Canada on winter wheat. Trials or gas sampling campaigns were conducted in 2008 and 2009 following fertilization to no-till winter wheat. Campaigns 1 and 5, show the risk and variability of volatilization, and how the N loss is dependent on soil moisture conditions, rainfall, and climatic conditions. During Campaign 1, urea was broadcast to very dry soil on April 3, 2008. Ammonia losses were not observed during the first two weeks because it never rained and there was insufficient humidity to dissolve the fertilizer prills. On the date of fertilization (April 3), the surface soil temperature was 0.9 degree C. The first significant rainfall event did not occur until April 19 (16 days post fertilization), when approximately 0.2 inches of precipitation fell as snow. This was sufficient to dissolve the urea prills and begin the hydrolysis process. However, during the eight-week measurement period, only 8.4 percent of the applied urea volatilized. 

Conversely, at the same site in 2009, Campaign 5 had large N losses, when urea was applied on March 26. A light dusting of snow was present at the soil surface at the time of fertilization from a snowfall event that occurred the evening before. Ammonia losses during the first week following fertilization were equivalent to 22.4 percent of the urea application rate. These were the highest losses observed during any of the trials.  Engel explains that the wet surface soil moisture conditions resulted in urea hydrolysis at the surface.  Because no rain occurred during the first two weeks following fertilization, urea was susceptible to volatilization. Overall, N losses totaled 39.9 percent for Campaign 5.

“The conditions that really seem to promote these ammonia losses from urea are prolonged damp conditions. In this part of the country, we are fairly dry, and the time of year when we are most likely to see prolonged damp conditions typically occurs in the late fall and early spring,” says Engel. “It’s really these prolonged damp conditions that seem to be most conducive to the highest losses of N from urea.”

Campaign 5: Calculated percentage of applied urea and Agrotain lost as NH3(g) during 8 weeks following fertilization on March 26, 2008.
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Research jibes with Canadian experiences

Alberta Agriculture and Rural Development (ARD) agronomy research scientist Dr. Ross McKenzie at Lethbridge, Alberta, says that broadcasting ammonium nitrate in the spring was the best choice for spring broadcast N application on to winter wheat. However, with the loss of commercially available 34-0-0, the next choices are granular urea or liquid urea-ammonium nitrate (28-0-0). McKenzie says that either product can be used but the urea is subject to volatilization losses. Either product can be used for spring application but must be managed with caution. “We have had some good luck with broadcast urea, but I am always the first one to also talk about the risks associated with the practice. For example, work in Saskatchewan in the 1980s showed that in the worst case situation 100 percent of the spring-applied urea can be lost,” says McKenzie.   

When broadcasting urea in the spring, McKenzie likes to see the soil temperature below five degrees C, air temperature less than 10 degrees C, and a dry soil surface. However, both McKenzie and Engel agree that even at temperatures below five degrees C, volatilization occurs. McKenzie indicates that it is necessary to apply urea at temperatures as cool as possible to try to keep losses to a minimum. He says that as the soil and air temperature warms up, and if there is enough moisture to dissolve the urea granule, then the risk of volatilization increases significantly. “In our experience if the weather is dry and cold, and you apply to dry soil, losses should be minimal, but once you get a little moisture, even at one or two degrees C at the soil surface, you can start to have loss,” explains McKenzie. “Ideally, you want to apply to a dry cold soil, and then hopefully get one-half inch of rain shortly after application. That would be the best scenario.”

McKenzie says another good scenario is after the ground has thawed out and a three- to four-inch wet snowfall occurs. Broadcasting urea on the snow allows the urea to burn down through the snow to the soil surface, where the N moves into the soil as the snow melts. 

Other possible options
“For spring applications, my preference was always ammonium nitrate, but since its availability is extremely limited, broadcast urea when managed properly, is the next option,” explains McKenzie. “You could also dribble-band 28-0-0 liquid fertilizer, but one-half of it is still in the urea form and susceptible to volatilization as well.”

Another option, says McKenzie, is to use Agrotain. Although it adds to the cost of the broadcast urea or 28-0-0, it can provide protection for 10 to 14 days against volatilization by inhibiting the urease enzyme. 

Engel included an Agrotain treatment in his MSU trials and did see suppression of volatilization. “What we’re finding with Agrotain is that it’s giving us two weeks of protection from volatilization losses,” Engel says.

For example, in Campaign 1, Agrotain cut N losses from 8.4 percent of broadcast applied N to 4.4 percent. In Campaign 2, losses were cut from 39.9 percent to 18.1 percent. The benefit of the reduced volatilization has to be balanced with the added cost of Agrotain.

McKenzie has examined spring broadcast application of ESN urea to winter wheat but found that the urea nitrogen releases too slowly to be effective. Therefore, he does not recommend ESN for broadcast application onto winter wheat.

McKenzie says the other option is to apply all of the N requirements for the winter wheat crop at seeding during the fall. His research that shows that this can be achieved with either a sideband urea application at seeding, or using ESN coated urea placed with the seed at rates up to 100 lbs of N per acre. As with Agrotain, ESN urea costs more than uncoated urea, and that cost needs to be weighed against the risks and benefits of other application methods. 

Complete results of the MSU study can be found at: landresources.montana.edu/ureavolatilization/ .