Ammonia Volatility from Urea

Ammonia (NH3) volatilization losses from urea were quite variable, ranging from 1.1 to 44% (Table 1) over 21 trials conducted. Susceptibility of urea to ammonia volatility cannot be linked to soil or air temperatures, a specific calendar period (i.e. spring, fall or winter), or soil pH. Volatility losses were closely linked to surface moisture conditions at the time of fertilization and the size and distribution of precipitation events following fertilization.

Moist cold soil

The largest cumulative ammonia loss, 44% of the applied N, occurred during Trial 10, which was conducted on a silt loam (pH 8.4) soil. The total cumulative ammonia loss exceeded 30% of the applied N in three trials conducted on acidic soils (Trials 3, 4, 5).  A commonality of these large ammonia loss trials was that urea applications were made to moist cold soil surfaces.  Dissolution of the urea granules occurred at the soil surface and precipitation events that followed were less than 0.20” and scattered at least through the first 30 days. Volatilization from these large ammonia loss trials occurred over a long time, often lasting more than 42 days and at times when average daily soil temperatures were 28 to 41 °F. This tells us that the practice by winter wheat growers of surface applying urea to cold soils does not guarantee protection against large ammonia losses. Photographs of the soil surface immediately following fertilization for the Trials 3, 4, 5, and 10 (Figure 1) illustrate the conditions at the soil surface at fertilization.

Snow covered soil

Broadcasting urea onto modest snow packs (less than 6”) is sometimes practiced in Montana as growers believe it prevents ammonia loss. We conducted two such applications near Willow Creek and Denton. Cumulative losses were 24% and 21% of the application rate . The period of greatest volatilization followed the disappearance of the snowpack when the surface was drying, and soil temperatures were still cold. In Trial 9 this occurred during the 4th and 5th week post-fertilization when soil temperatures averaged 26 and 33°F. In Trial 16, volatilization peaked in the 2nd week post-fertilization when the snow pack melted and soil temperatures averaged 34 °F. More details are presented in Fertilizer Facts No. 70 and 71. Results from this portion of the study further validate our observation that applying urea to cold soils does not mean volatility losses will be small.

Dry cold soil

We found that when urea is applied to dry soil surface, losses will be considerably lower than when applied to wet or damp surfaces. An example of contrasting ammonia losses from a dry vs. wet soil is provided by Trial 2 and 5 (Figure 2). These trials were conducted at the identical field site. Ammonia losses were 3% in Trial 2 and 40% in Trial 5. During Trial 2, urea granules remained undissolved at the soil surface until 24 days post-fertilization when the site received nearly 1" of precipitation over a 3 day period. In Trial 5 the urea was applied to a light snow covered soil with 0.25" of rain on day 21 and another 0.8" over days 31-35 after fertilization. Essentially no other precipitation fell in these five weeks.

If the soil surface is dry then urea granules will remain intact or undissolved until the first measurable precipitation event(s) occurs (Figure 2). We have conducted several trials where urea prills remained visible at the soil surface for several weeks. During this period no measurable losses of ammonia were observed. Ammonia volatility problems do not begin until there is sufficient moisture for the urea granules to dissolve and convert to ammonium. Under dry surface application conditions the size of precipitation events following fertilization will have a great impact on the magnitude of ammonia loss. If the precipitation events that follow fertilization are light (less than 0.3") and scattered, then losses will typically be in the 10 to 16% range (Table 1, Trials 6, 7, 8, 12, 18, 19, and 20). If the precipitation events are heavy, e.g. greater than 0.7", then losses may be less than 10% (Table 1, Trials 1, 2, 11, and 21).

Mitigating Ammonia Loss

Adding Agrotain®

The addition of the urease inhibitor Agrotain® to urea always reduced ammonia loss. Total cumulative loss from Agrotain® treated urea averaged 6% of the applied N rate, a 64% reduction in volatilization compared with untreated urea (Table 1). In summer 2012, the cost of treating urea with Agrotain® was approximately $55 per ton of fertilizer, an approximate 10% price premium to urea fertilizer, which is somewhat less than the potential savings of N lost to volatilization.

Agrotain® mitigates ammonia volatilization from urea by a number of factors including a moderation of the soil pH rise that results with hydrolysis (urea’s conversion to ammonium); reduced concentration of ammonium in the soil solution around the fertilizer granule, thereby reducing amount of ammonia that is available to volatilize; and inhibition of hydrolysis. This provides more opportunities for precipitation to infiltrate urea deeper in the soil where N is less susceptible to volatilization. The benefit of Agrotain® was typically limited to two weeks on acidic soils; two weeks after fertilization ammonia losses was similar for the Agrotain® treated and untreated urea. However, the benefit of Agrotain® persisted longer at one site with calcareous soil suggesting that degradation of Agrotain® and/or its metabolites may have occurred more slowly at high pH. Please see Fertilizer Facts No. 60 and 71 for more details.

Seeding after fertilizing

To minimize ammonia volatilization, broadcast urea should be incorporated within a couple of days by at least 0.5" of water in one event, or mechanically. Unfortunately, applying urea immediately in front of air-drills was not sufficient incorporation to reduce volatilization losses (Trials 13, 14, 15 and 17; data not shown).

Implications and Management Recommendations

Previous studies have observed large ammonia losses from urea fertilizers applied to initially wet surface soil followed by several days of slow drying with little or no precipitation; and that ammonia losses are mitigated when sufficient rain or irrigation, typically 0.5 to 1.0", occurred to move the N into the soil profile. In general, our results are consistent with these observations; however, they are unique in that we found cold soil temperatures did not provide protection against large ammonia losses if the surface water content was high at the time of fertilization with little or no precipitation after application.

It was long assumed that surface-applied urea was not susceptible to large volatility losses if applications were made during cold weather months. Our study indicates this assumption is not valid; significant ammonia can be lost from urea even though applications are made onto soils < 41 ˚F. In Montana's semiarid climate, fertilizer applications during the late fall, winter, or very early spring are often made to soil surfaces that are cold or frozen, high in water content, and sometimes covered with a modest snowpack. Urea applications under such conditions appear to be susceptible to volatility losses, particularly after the surface thaws and dries. To minimize volatility losses growers should wait until the soil surface is sufficiently dry to prevent dissolution of the urea granules. If time-constraints force growers to apply urea to soil surfaces with high water content, without subsequent incorporation, then addition of Agrotain® to urea may be warranted. Agrotain® will reduce volatilization losses by approximately 2/3 compared to untreated urea.

Current trials are evaluating the impact of volatilization amounts on grain yield and quality.

Some of the results presented here have been published or presented in:

Engel, R., C. Jones, and R. Wallander. 2011. Ammonia volatilization from urea and mitigation by NBPT following surface application to cold soils. Soil Science Society of America Journal. 75:2348-2357.

Ammonia Loss from Urea Surface-applied to Cold Soils(Fertilizer Facts No. 59)

Mitigation of Ammonia Loss from Urea Applied to Moist Soils by Agrotain®(Fertilizer Facts No. 60)

Ammonia Loss from Surface-applied Urea to Cold Soils: A second look (Fertilizer eFacts No. 70)

Nitrogen Recovery from Broadcast Urea is Affected by Application Timing and Agrotain® (Fertilizer eFacts No. 71)

WSARE Annual Report 2011 - Minimizing ammonia-N losses from no-till cropping systems (pdf)

 

 

Table 1. Percentage of applied N (90 lb N/ac) lost as ammonia-N following broadcast applications of urea and Agrotain* coated urea.

Season Trial Location Fertilization date % N lost
Urea Agrotain®

Fall

2

west Havre

Oct 8, 2008

3.1

1.4

3

north Havre

Nov 14, 2008

31.3

3.8

6

Kremlin

Oct 6, 2009

11.6

4.3

7

west Havre

Oct 13, 2009

10.4

4.8

8

north Havre

Oct 19, 2009

15.7

3.4

18

Denton

Nov 29, 2011

13.4

5.9

Winter

9

Willow Creek

Jan 27, 2010

24.3

9.3

10

Willow Creek

Feb 26, 2010

44.1

11.9

16

Denton

March 5, 2011

20.7

10.1

19

Kremlin

Dec 30, 2011

14.9

8.2

20

Coffee Creek

Feb 28, 2012

13.0

4.1

Spring

1

west Havre

April 3, 2008

8.4

4.4

4

north Havre

March 25, 2009

35.6

18.0

5

west Havre

March 26, 2009

39.9

18.1

11

west Havre

March 29, 2010

6.1

1.7

12

Kremlin

April 20, 2010

14.7

1.8

21

Coffee Creek

April 24, 2012

1.1

0.0

Average

 

 

 

18.1

6.5

 *Agrotain® was applied at a rate that equaled 2 lbs NBPT per ton of urea.

Trial 3 closeup

Trial 4 closeup

Trial 3 - November. Soil surface was wet from snow receive the previous day.

Trial 4 - March. Soil surface was frozen (19 F) on the day of fertilization, and moist when it thawed 3 days later.

Trial 10 close up

Trial 5 closeup

Trial 10 - February. Moist soil surface.

Trial 5 - March. A trace of snow was on the soil surface at fertilization. Note fertilizer granules are beginning to dissolve.

Figure 1. Trials conducted with wet or damp soil surfaces at the time of fertilization. Such soil conditions are conducive to high volatilization losses from urea fertilizers. Growers should avoid applying urea under these conditions unless the fertilizer has been treated with a urease inhibitor, or can be placed beneath the soil surface by at least 2 inches.

trial 2 trial 2 closeup

Trial 2 - Urea application to a dry soil surface during the fall of 2008. Note cloud of dust. Urea granules were still
visible at the soil surface 1 week post fertilization because of dry soil conditions at the time of application and the
absence of any precipitation.

trial 5 app on snow

Trial 5 - Urea application to a wet soils surface. This application occurred in the early spring (March 26, 2009) following a high snowfall winter.

Figure 2. Contrasting ammonia losses (Trial 2 = 3% N loss vs. Trial 5 =40% N loss) were observed at the identical field site in large part because of differences in soil moisture content at the time of fertilization.