By Top Crop Manager
When managed properly, urea or UAN can replace AN.
When ammonium nitrate (AN) was taken off the market, farmers who depended on
the technology were left searching for alternatives. However, the transition
has not been as hard as many expected, and farmers do not necessarily have to
resort to fertilizer additives or formulations that delay volatilization.
|Liquid UAN has lower volatilization losses than urea.|
"As agronomists, we all just have to stop and think a little bit more,"
says Ray Dowbenko, senior agronomist with Agrium at Calgary, Alberta. "That's
not a bad thing. We have to think about how to use the products we recommend
so that we are getting the most efficiency with the least environmental impact."
To put the loss of AN into perspective, for the fertilizer year 2003/04, AN
represented about 5.5 percent of the total agricultural fertilizer N market
in Canada. In western Canada, Alberta was the single largest user of AN at 66
percent, with Manitoba at 15.7 percent and Saskatchewan at 14.3 percent. Dowbenko
says the high Alberta usage is likely the reason that most AN replacement inquiries
have come from Alberta.
The vast majority of AN was applied in the spring at 89.7 percent, with 3.6
percent applied in the summer and 6.7 percent applied in the fall. In Alberta,
the majority of the spring applied AN was applied to forages and winter wheat
as a top dress application.
Understanding the risks
Surface applied, unincorporated urea is subject to volatile loss. In the presence
of the urease enzyme, it is converted to ammonia and carbon dioxide, both gases
that are lost to the atmosphere. However, other N fertilizers are also at risk.
Urea ammonium nitrate (UAN 28-0-0 liquid) for example, is approximately one-half
urea and that portion is also susceptible to fairly rapid volatilization under
the right conditions. Ammonium sulphate and even AN also can suffer volatile
|Figure 1. Comparison of different N sources at 80 pounds N per acre
compared to AN. Source: MZTRA 2005.
Dowbenko provides a table (see Table 1) that illustrates when the risk of volatilization
loss is high, or low, so that farmers can make an informed decision on when
surface application of urea might be possible.
The conditions of low risk potential would suggest that urea or UAN could be
used in early spring, late summer or early fall, depending on environmental
and soil factors. Conversely, UAN or urea would not be a good choice of N fertilizer
for use during early to mid summer, or anytime that the high risk conditions
Dowbenko has looked at research across western Canada that compared urea, AN
and UAN. On the surface, the results appear to conflict and support the notion
that urea will suffer high volatilization losses. However, he says that where
urea or UAN performed poorly, the conditions were frequently conducive to high
For example, research at Agriculture and Agri-Food Canada at Brandon showed
that urea applied in June (20 to 25 degrees C) suffered a 40 percent loss and
an 88 percent loss in August (30 degrees C). UAN, on the other hand, suffered
a loss of seven percent in June and 50 percent in August. "The difference
is temperature. Even the June application had temperatures that are conducive
to volatilization losses for urea," explains Dowbenko.
|Table 1. Soil and environmental factors that affect N losses
from surface applied N sources.
|Low loss potential||High loss potential|
|Dry soil surface.||Moist soil surface.|
|Cold soils.||Warm soils.|
|No wind.||Windy conditions|
Greater than 1/2 inch rain within one to three days
of fertilization. Air temperatures below 10 degrees C.
Less than 1/2 inch of rain or precipitation occurs more
than three to five days after fertilization. Air temperatures above 10
|Fine textured soils.||Course textured soils.|
|High soil organic matter content.||Low soil organic matter content.|
Low lime contents.
Soil pH < 7.0.
High lime content.
Soil pH > 7.0.
Few plant residues on soil surface. Soil temperature
below 5 degrees C.
Abundant plant residues on soil surface. Soil temperature
above 5 degrees C.
|Source: Ray Dowbenko; Agrium.|
Conversely, research by Manitoba Agriculture, Food and Rural Initiatives (MAFRI)
found that surface broadcast urea, AN and UAN produced similar barley yields.
Research at AAFC Brandon also found similar results on winter wheat. There,
spring broadcast urea, spring surface banded UAN, and spring broadcast AN produced
similar wheat yields. Finally, research by the Manitoba Zero Tillage Research
Association (MZTRA) summarizes the issue of potential volatilization losses.
Under low risk conditions, yields were similar, but under high risk conditions,
urea yields suffered.
Dowbenko also cites Agrium research from 1996 to 2000 that compared forage
grass yields at 15 sites using AN, urea and UAN. Overall, AN produced forage
yields about two percent greater than urea.
"The data from all the research sources indicate that suitable alternative
N products exist for replacement of AN without causing economic penalty. If
you manage urea properly, there won't be much difference between urea and AN,"
says Dowbenko. "However, these N sources must be managed appropriately.
Lack of understanding with respect to potential volatile loss conditions can
lead to significant losses of surface applied urea and ammonium containing fertilizers."