Potential Nitrogen Loss When Wheat Was Top-dressed With Urea or UAN

Questions have been coming into the lab about potential nitrogen loss from urea-based wheat top-dress applications made this spring. Many of these questions are being raised due to weather patterns around application timing.  Early spring applications were often followed by heavy rain fall, while late applications were followed by warm dry soils. While nitrogen loss is always difficult to predict, there are some basics taking place in these situations. To start, both dry urea and liquid UAN, which is 50% urea-based nitrogen, are subject to three nitrogen loss mechanisms, volatilization, denitrification, and leaching.

The naturally occurring urease enzyme in the soil converts urea into ammonium, the ammonium can be lost to the atmosphere as a gas during this process if the ammonium is not captured by the soil. Conditions that promote this are surface application on very dry or moist soil with delayed or light rainfall (less than 0.2”-0.3”, following application), heavy residue, and high humidity. These conditions impact dry materials greater than liquid, especially when the liquid is concentrated in bands. These low moisture situations can lead to the dry granule dissolving and not having enough moisture to move into the soil. Dry urea requires about 0.5” rain to incorporate into the soil where the urease released ammonium can be captured by the soil. Liquid UAN takes less rain to incorporate and even less when branded/streamed. Steady winds, delays between light moisture events, high soil pHs, and warm air temperatures above 70 degrees Fahrenheit will accelerate urea nitrogen loss to urease volatilization.

The next potential loss of nitrogen is from the rapid conversion of ammonium-nitrogen to nitrate-nitrogen that can be lost through leaching or denitrification. For urea-based nitrogen sources, the conversion of urea to ammonium is slowed by cold soil temperatures reducing the overall amount of ammonium nitrogen subject to conversion to nitrate that can be lost. For UAN half of the nitrogen is in ammonium or nitrate forms to start and can be subject to loss quicker than pure urea in dry forms. Short spells of warm weather can lead to rapid conversion of ammonium to nitrate. Warm saturated soils are needed for denitrification and leaching to occur.

Volatilization can be significantly reduced with the use of Agrotain (NBPT), which is commonly used on dry urea. Denitrification can be slowed greatly with the use of Instinct (nitropyrin), usually used with UAN to delay the conversion of ammonium to nitrate. Research has shown that ammonium thiosulfate is not as effective as these products but appears to have significant activity in reducing nitrogen loss by both mechanisms.

This Spring early season wheat top-dress had warmer soils, but the time periods of saturated soil were very short. There was some denitrification, it was most likely limited. Late applications were made to moist soils followed by low humidity dry weather with warm temperatures. Volatilization losses without the use of Agrotian or ammonium thiosulfate could have occurred where rain was limited, dry urea would have been at greater risk than streamed UAN. While conditions indicate that the loss was not excessive the use of soil nitrate and ammonium testing where the condition favored accelerated volatilization, along with wheat tissue tests, may be needed to monitor the crops’ nitrogen needs.

The ALGL Agronomy staff took their own advice and pulled a couple of nitrate samples. Streamed UAN with ammonium thiosulfate and nitropyrin in late application made before a dry spell with low humidity on soil with an elevated pH resulted in approximately a 7% nitrogen loss two weeks after application in the Fort Wayne area. More information on making these determinations cab be found on our blog post "Making Sense of Soil Nitrate and Ammonium Values."