Nitrogen: Choosing source and timing for wheat
Topdressing urea on wheat or grass? Use Agrotain
Urea is a great fertilizer product but with one hitch: surface applications are prone to volatile loss of ammonia gas. Depending on complex interactions between a range of weather parameters, losses can range from 0 to 50% of applied N. A good number to pencil in for average loss is 25%. When urea first hit the world scene, this tendency to volatilize was almost universally managed by tilling it in following broadcast application. As we have moved to less tillage and more targeted goals for tillage, this option has become less attractive and less widely used.
Although ammonia volatilization from urea applied to summer crops like corn, rice, and cotton is widely acknowledged to be a problem requiring focused management, rumors have circulated through agricultural and fertilizer circles that ammonia loss is not an issue in cool-season crops like wheat and fescue. This was never based on any research that I am aware of, and recent research in Montana and Alaska has shown convincingly that ammonia volatilization from urea can be substantial even under cool temperatures, and even from frozen soil. Urea breakdown to ammonium/ammonia is accomplished by an enzyme, but an enzyme that is loose in the soil rather than only in living cells. This means that urea breakdown is a purely chemical reaction and is much less temperature-sensitive than other reactions (like the conversion of ammonium to nitrate) that take place only in living cells. Our research showed about a 5-bushel yield advantage for Agrotain-treated urea over regular urea, and this was true whether these products were applied in mid-January, mid-February, or mid-March. This clearly implies that ammonia was being lost from the urea, and that Agrotain was effective in reducing this loss. Measurements of ammonia volatilization following the March N applications confirmed this idea—12 lb N/acre was lost as ammonia from the urea treatment, while only 1 lb N/acre was lost when the urea was treated with Agrotain.
Agrotain treatment of urea applied to grass in March increased yield by an average of 330 lb N/acre over six experiments. This is only a bit better than break-even in most years, but still supports the use of Agrotain when urea is applied to tall fescue or other grasses, even in cool spring weather. In some of these experiments, rain within a few days of nitrogen application moved the urea into the soil and avoided volatile loss of ammonia. If rain is probable in the forecast within a day or two, it would be reasonable to omit the Agrotain.