August 2006

Volatilization Losses From Surface Applied Nitrogen

Concern over volatilization losses from surface applied nitrogen are justified with the present combination of high temperatures, high winds and little rain in the foreseeable future.

In the case of urea, hydrolysis converts the urea to ammonia, and if the urea is not incorporated, the ammonia is lost to the air.

Conditions favouring high volatilization potential are:

high soil temperatures
moist conditions, followed by rapid drying
windy conditions
high soil pH (>pH 7.5)
high lime content in surface soil
coarse soil texture (sandy)
low organic matter content
high amounts of surface residue (eg. Zero tillage)
nitrogen source: urea>UAN solution > ammonium nitrate

University of Manitoba studies indicate potential losses of 38%-46% of urea N during 5 days at 25C versus less than 7% loss when temperatures are 15C (U of M, Toews). (Also see Table 2.) More recent studies under zero tillage conditions at Brandon found 40% and 88% loss of urea N after 7 days in May and July, respectively (Table 3).

Our high soil pH increases volatilization losses. As soil pH increases from 6.5 to 7.5 volatilization losses double from 10% to 20% fro urea left on the surface for 4 days. (See Table 4.)

The best options for farmers wishing to topdress additional N on crops are:

• Broadcasting ammonium nitrate (34-0-0). Fewer dealers are handling this source in the Red River Valley, but it still is popular in areas of zero till and pasture production. Present prices are about $0.39/lb N for ammonium nitrate vs $0.35/lb N for urea and $0.36/lb for UAN (28-0-0).
• Spoke wheel injection of UAN solution (28-0-0).
• Surface strip banding of UAN solution. In theory this is better than surface broadcast UAN, but little difference has been observed in Brandon studies. (A 3 year study at Penn State found average NH3 volatilization losses within 10 to 20 days of 40, 22 and 17 kg N/ha from broadcast urea, sprayed UAN and dribbled UAN, respectively. Losses from sprayed and dribbled UAN were not different, but both were significantly less than urea.)

More Volatilization Facts

a) How much rain is requited to incorporate surface applied urea?

Quoted values vary form 0.1 inch (Michigan) to 0.25 inch (Minnesota, North Dakota).

The longer rainfall is delayed after application, the greater the volatilization losses.

(Fox and Hoffman, Pennsylvania – N on no-till corn)

b) The influence of temperature on urea volatilizations.

Table 2. Percent of surface-added urea volatilized as ammonia as influenced by soil temperature and days urea was left on the soil surface (from Overdahl, et al., 1987).

Table 3. Percent of surface – applied urea volatilized in 7 days as influenced by time and N source (Grant et al, 1996.)

 NBPT is an urease inhibitor not currently available in Canada.

Fertilizer was applied under zero till conditions in a nested application (similar to a dribble band).

c) The influence of Soil pH on urea volatilization

Table 4. Percent of surface-applied urea volatilized as ammonia as influenced by soil pH and days urea was left on the soil surface (from Overdahl, et.al., 1987)