What are the concerns for fall applied nitrogen?
University of Missouri Extension agronomy Specialist
The unusual warm winter has producers wondering about their fall applied nitrogen. There are many factors that influence nitrogen transformations including soil temperature, time of application, use of a nitrogen inhibitor, rate of biological activity, drainage, amount and frequency of rain and soil type.
When anhydrous is applied, it reacts with soil water to convert to ammonium. This form of nitrogen is held by the soil. Once ammonium transforms to nitrate, it can be leached out or denitrified when soils are warm and saturated with water. The bacteria that mediate notification are most active above 50 degrees F. Nitrification does not occur when the soil temperature is below 32 degrees. Soil temperatures have been above average for this winter implying that nitrifying bacteria have remained active most of the fall and winter.
Most if not all producers used a nitrogen inhibitor when applying their anhydrous. Results from Illinois indicate that when N-serve was used in a high organic soil that 90 percent of the ammonium was not transferred to nitrate after 90 days at an incubation temperature of 39 degrees, compared to 60 percent transformed when N-serve was not used. In a low organic soil, 85 percent was not transformed when N-serve was used compared to 25 percent where N-serve was not used.
Other weather and soil conditions this fall and winter have been favorable for retaining nitrogen. First, there has not been a lot of water in the soil to move nitrate down or to create saturated soil conditions for denitrification. Secondly, warm temperatures have resulted in some water evapotranspiration in the soil surface. This creates an upward suction force that moves water and nitrate towards the surface. At this point in time there is probably not much real concern for significant nitrogen loss.
There is greater potential for nitrogen loss in the spring due to too much water, too little evapotranspiration, low nitrogen uptake by the crop and nitrogen being in the nitrate form. If you believe that you have significant nitrogen loss, it would be more efficient to apply any additional nitrogen closer to the time that the crop is able to use the nitrogen. Not only would this result in greater utilization of the additional nitrogen but would minimize the chances of additional nitrogen loss by applying additional nitrogen before planting. Applying additional nitrogen later in the season will result in a more efficient usage of the fertilizer producing the same result with less fertilizer.