The wet weather continues and many may be concerned about the risk of nitrogen (N) loss. Some areas of Ohio have seen sizable rainfall amounts that can increase the risk of leaching or denitrification, but the question is: Should you be concerned about N loss?
At this point in the season, we would not be overly concerned if you applied anhydrous ammonia as your N source. Anhydrous ammonia is efficient because it is fairly resistant to microbial oxidation due to its fumigant properties; it eliminates the bacteria responsible for nitrification – which is the conversion of ammonium to nitrate – near the band of application. Thus, that material can be in the field for a week or two (or longer) prior to conversion to nitrate.
Additionally, the speed of microbial oxidation is a function of soil temperature. At this point in the growing season, soil temperatures are relatively cool, although a warm stretch in April caused our soils to be slightly warmer than usual compared to historic averages. We have computed the growing degree days for soil temperatures between April 1 and May 13 for the last 28 years, and we found that currently we are quite a little ahead compared to the long-term average.
For fields that may have received dry urea fertilizer, we would be a little more concerned, but only if the field was waterlogged for at least a day. Those few fields that may have received a sizable amount of urea-ammonium nitrate (UAN – liquid 28) would be at a little more risk of loss due to the application of nitrate especially if waterlogged for a day or more.
Our recommendation is to evaluate your crops over the next couple of weeks as soil and air temperatures increase and look for any visual symptoms of N deficiency (general chlorosis or yellowing). If you are still concerned, you can use the tool we developed a few years ago for evaluating the risk of N loss:
1. What N source was utilized?
- 1 point - Anhydrous ammonia with nitrification inhibitor
- 2 points - Anhydrous ammonia
- 3 points - Other fertilizer banded
- 4 points - Other fertilizer broadcast
2. When was the N applied?
- 2 points - After April 20
- 5 points - Before April 20
3. How much N has been applied?
- 1 point - >200 lbs./acre
- 2 points - 150-200 lbs./acre
- 3 points - 100-150 lbs./acre
- 6 points - <100 lbs./acre
4. What has been the predominant soil moisture status in the field this spring?
- 1 point - Normal
- 2 points - Wet
- 4 points - Excessively wet (saturated-standing water)
5. What is crop’s condition?
- 1 point - Green plants > 12 in. tall
- 2 points - Green plants < 12 in. tall
- 3 points - Chlorotic plants < 12 in. tall
- 5 points - Chlorotic plants > 12 in. tall
Total the score and use the following guidelines:
- Less than 13: Additional fertilizer not recommended
- 13-16: Evaluate again in four to seven days
- 17 or greater: Add an additional 40-70 lbs. N/acre
Some producers may consider the use of the pre-sidedress soil nitrate test (PSNT) to determine if additional N fertilizer is warranted. To attain a representative soil sample, collect 15, 1-ft.-deep random cores from a field and mix them thoroughly. Submit a grab sample from the composite to a reputable lab. You may want to contact the lab and find out the turn-around time (some may be able to complete analysis in a couple of days). If the nitrate level in the soil is between 25-30 ppm then additional N is probably not warranted. Nitrate levels lower than 25 ppm have an increased likelihood of response, but the rates should not be greater than 70 lbs. N/acre. Work out of Illinois reveals that application of only 50 lbs. N/acre results in maximum yield over a wide variety of growing conditions.