Michael Bovis and Joe Touchton

Certain nitrogen-supplying fertilizers are more prone to lose nitrogen (N) into the atmosphere through ammonia volatilization than others. An AAES study examined the effect of different application methods and additives that may help make these fertilizers less volatile and found that these treatments had little effect on volatilization.

Surface-applied urea and urea-ammonium nitrate solutions (AN) tend to be more volatile than ammonium nitrate. Ammonia volatilization--the loss of free ammonia to the atmosphere--involves several steps. First, an enzyme (urease) present in soils and organic residue acts on urea and converts it to an unstable form. This unstable form can quickly change to ammonia and carbon dioxide. Under ideal conditions, the ammonia is converted to ammonium, ready for plant uptake. But, under less than ideal conditions, the ammonia can be lost to the atmosphere.

Many interrelating factors influence the amount of N lost through ammonia volatilization. These factors include urease activity, temperature, soil moisture, application method, soil pH, and soil cation exchange capacity. The greatest losses will most likely occur when urea or urea-containing fertilizer is surface-applied to a soil where high amounts of plant residue are present. Losses will likely be accelerated if the soil is moist at application, followed by five to seven hot and windy dry days. Urease activity is higher on plant surfaces and organic residues than in soils. Urea hydrolyzes rapidly when in contact with these residues, and conditions are created around the fertilizer that favor the formation of ammonia. This rapid hydrolysis of urea on the soil surface can cause the loss of ammonia to the atmosphere, especially when rapid soil drying conditions exist. Losses through ammonia volatilization can be essentially eliminated when urea is injected into the soil at application or incorporated into the soil within 48 hours after application either mechanically or by rainfall.

Band applications of urea-containing N solutions, such as UAN, will most likely result in less volatilization losses than broadcast applications. Banding the solution will give more opportunity for applied urea to contact the soil surface instead of being held by organic residue, which contains high levels of urease. Frequently, UAN solutions may be used as a carrier for herbicides to control weeds in winter cereal crops. This saves a trip across the field and provides a more potent herbicide mix than does a herbicide/water mix, but does not fit in with a band-application system. Although band application should reduce ammonia losses through volatilization, it is possible that a widely spaced band of fertilizer may not get the N close enough to some plants for them to use this nutrient.

Research has shown that the potential for N losses through ammonia volatilization can be reduced by using a urease inhibitor to slow or delay hydrolysis. Slowing or delaying this process will allow more time for rainfall to move the urea into the soil. One of the most effective inhibitors currently available is nBTPT, marketed under the trade name Agrotain^TM. The effectiveness of Agrotain depends upon soil and atmospheric conditions at and after the application of a urea-containing fertilizer. If conditions are not favorable for ammonia volatilization, the benefits of Agrotain will not be seen.

A study was conducted in 1996 and 1997 at the Wiregrass Substation in Headland on wheat and ryegrass to evaluate the effectiveness of the urease inhibitor Agrotain when applied with prilled urea and to compare band with broadcast applications of UAN. Agrotain was incorporated onto prilled urea at the labeled rate. Prilled urea, prilled urea plus Agrotain, and prilled ammonium nitrate were broadcast-applied in early March at the rates of 30, 60, and 90 pounds N per acre. In addition, UAN was applied at the same rates using broadcast and band (10-inch spacing between dribble points) methods. Rain-free periods after application were six days and five days in 1996 and 1997, respectively. The March application is probably too late for maximizing yields, but weather patterns were not favorable for ammonia volatilization until this time. Flag-leaves were sampled from wheat at early heading, and grain was combined at harvest. Ryegrass was clipped in May for dry matter yield and N content.

Wheat and ryegrass yields (averaged over N sources and years) as affected by N rates.

Wheat grain yields (see figure) followed a typical response to N rate. The response to N rate was the same for both years, so the data in the figure are averaged over both years. Band applications of UAN resulted in higher leaf-N levels than broadcast applications, which indicated that the band applications were more efficient. However, this efficiency was not reflected in grain yields (see the table). Yield response to N sources was consistent across N rates and years so the data in the figure are averaged over rates and years. Leaf-N levels were higher with urea-plus-Agrotain treatment than urea alone, but again this efficiency was not reflected in yields.

Ryegrass yields (see figure) also followed a typical response to N application rate. Like wheat, the responses were the same for each source in both years, so data in the figure were averaged over years and N sources. Although tissue N levels were higher when UAN was applied in a band, broadcast applications resulted in higher yields (table). Again, yield response to N sources was the same across N rates and years, so the data in the table are averaged for each source over all rates and both years. Nitrogen levels in the ryegrass tissue were higher with urea plus Agrotain over those from urea alone, indicating increased efficiency when Agrotain was used. However, higher yields were found when urea alone was used (see table).

In conclusion, the study did not show any yield advantage of using Agrotain applied with prilled urea. According to the data, broadcast applications of UAN to ryegrass were actually better than applying it in a 10-inch band. However, wheat yields did not vary according to the application method of UAN. Rain-free days after application certainly favored N loss through ammonia volatilization. Other soil and climatic conditions were obviously not favorable, because under favorable conditions, yields with prilled urea should have been lower than those with ammonium nitrate, especially at the lower rates.