A study in Illinois on the effects of N-Serve, a nitrification inhibitor, showed that 90 percent of the ammonium in Drummer soil (high organic matter content) and 85 percent in Cisne soil (low organic matter content) was not transformed to nitrate after 90 days at an incubation temperature of 39 degrees.

When N-Serve was not used, the numbers were 60 percent and 25 percent after 90 days for Drummer soil and Cisne soil, respectively. The study also showed that N-Serve degradation increased as temperatures rose, which could increase nitrification.

In addition to nitrification, possible nitrogen loss must be considered. Nitrogen is not lost when ammonium is transformed to nitrate, but the potential for loss is greater.

"While it is possible that some of the nitrogen that transformed to nitrate over the winter was lost, I suspect that this is not a large amount," said Fernandez.

"First, if the fall application was done carefully, a small amount of the total nitrogen, 10 to 20 percent, is likely to be in the nitrate form. Second, there has not been a lot of water in the soil to move nitrate down the soil profile or to create saturated conditions for denitrification."

At the start of last fall, soils were very dry and needed to be replenished before water could move down the soil profile. Moreover, while most soil surfaces were not frozen and water was able to enter the soil, rainfall since last fall has been moderate.

Warm temperatures have also created some water evaporation in the soil surface. This evaporation creates an upward suction force that moves water and nitrate surface of the soil. 

"While I am not too concerned about nitrogen loss at present, I am concerned about the potential for loss later in the spring," Fernandez said. "Nitrogen losses usually occur in the spring when there is too much water, too little evapotranspiration, low nitrogen uptake by the crop, and nitrogen is present in nitrate form."

If the spring is wet, Fernandez said, there might be substantial loss from nitrogen applied in the fall. However, he advises against applying any additional nitrogen "until we have a better idea on how the crop is growing."  

Current soil conditions in many fields are fit for applying anhydrous ammonia. Nitrogen applied now would be considered an early-N application, and the potential for wet conditions and chance for high N-loss is still ahead of us if the spring is wet.

Fernandez suggests using the N-rate calculator at http://extension.agron.iastate.edu/soilfertility/nrate.aspx to determine the total amount of nitrogen in the field.

"Then I would apply only a portion of the total nitrogen, reserving the remainder for an application closer to planting or during sidedress," he explained. "At this time, I would consider using only anhydrous ammonia or a slow-release nitrogen source."

His final recommendation is to consider using N-Serve for anhydrous ammonia applications done this early in the spring, even though N-Serve for pre-plant applications is not usually applied close to normal planting time.