How to handle urea

Urea nitrogen can be lost through volatilization, denitrification and leaching. It can also be immobilized (tied up) in the residue from a previous crop. Volatilization is not an issue with most other forms of commercial nitrogen.

Managed correctly, however, dry urea is a good source of nitrogen and one that many farmers will turn to out of desire or necessity in the years ahead.

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"In general, as long as nitrogen fertilizers are correctly applied, all are agronomically equal," says University of Nebraska agronomist Richard Ferguson.

Three I words are key to managing urea: Incorporation, injection and irrigation. If you don't incorporate dry urea with tillage, inject it beneath the surface residue or irrigate it in, you'd better hope for rain within a few days of application.

Other options are treating urea with Agrotain (a urease inhibitor) or using ESN, a controlled-release urea.

Ammonia volatilization poses the first risk for losing urea nitrogen. When urea is applied, an enzyme in soil and plant residue called urease quickly converts the urea into ammonia N. If this conversion occurs beneath the soil surface, the ammonia is converted to ammonium nitrogen and bound to soil particles. That's a good thing.

However, if the urea is converted to ammonia on the soil surface or on residue, as would be the case in a no-till system, ammonia gas can escape into the atmosphere. That process is called ammonia volatilization, and it's a bad thing.

"Ammonia loss can be significant where the producer surface-applies fertilizers containing urea without incorporation, particularly if significant amounts of residue are present and conditions are warm and moist," says Ferguson. "The amount of total nitrogen loss from fertilizers containing urea due to ammonia volatilization can vary considerably, from no loss to 50% or more of the applied nitrogen.

"Typical losses from urea broadcast to a moist silt loam soil in the spring, without rain for at least a week following the application, may be in the range of zero to 20% of the applied nitrogen," he adds.

Because of all the interacting factors, it's impossible to predict exactly how much nitrogen will be lost when urea is applied to the soil surface. Soil temperature, soil pH, soil texture, soil moisture, the amount of time between application and a significant rain and, of course, the amount of residue, all play a role.

University of Minnesota soil scientist George Rehm says that after 10 days on the soil surface, about 14% of the urea would be volatilized as ammonia at 75 degrees F vs. only about six percent at 45 degrees F.

Soil pH can make a big difference also, according to Rehm. After just two days on the surface, five percent of the urea can be volatilized as ammonia at a pH of 7.5 vs. only one percent at a pH of 7, and zero at a pH of 6.5. Never apply urea to fields that have recently had lime applied. Less urea is lost from dry soil surfaces or dry residue.

Opinions are mixed over how much rain is needed to incorporate urea and head off volatilization losses. Some researchers believe 1/4 inch is enough; others call for at least 1/2 inch. Nor is there a consensus over how soon after application rain is needed. Sooner is better than later, of course.

That is where Agrotain, the urease inhibitor, comes in. "Urease inhibitors reduce the activity of the urease enzyme for up to 14 days," says Lloyd Murdock of the University of Kentucky. "As long as it rains during this 14-day period, the urea will be moved into the soil where it can be converted to ammonium-N without the risk of volatilization."

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Opinions about late-fall applications are mixed. The potential exists for loss in late winter and spring. There is interest in using slow-release (poly-coated) forms of nitrogen for fall application.

The surest way to avoid volatilization losses is to ensure that the urea ends up in the ground and beneath surface residue as soon as possible. Pneumatic systems offer a way to inject dry urea in no-till systems.

Nitrogen is an escape artist. And dry urea knows more escape routes than any of the other common forms of nitrogen. If conditions are ripe for loss, some of the urea beats the applicator out of the field. Houdini would be jealous.

Half of the nitrogen in urea-ammonium nitrate (UAN), a liquid form of nitrogen commonly referred to as 28% or 32%, is also urea and subject to loss. Plus, the 25% that is ammonium is subject to volatilization loss immediately.