By Ed Lentz and Steve Culman, The Ohio State University
Some parts of the country have recently experienced heavy rains. Producers in these areas may have concerns about nitrogen loss in corn fields. Nitrogen losses occur by two main pathways: denitrification (gaseous loss of N) and leaching of nitrate from soil through water leaving the tile line or into groundwater. There is no tool or test that can tell how much has been lost. An estimate can made on the loss potential, which is based on N source, time of application, soil temperature, and number of days that soils have remained saturated.
Most nitrogen that is lost from a field is in the nitrate form during wet conditions. Time of transformation to nitrate is dependent on the type of N fertilizer applied. Anhydrous ammonia is less susceptible to loss since it converts to nitrate rather slowly. Urea-ammonium nitrate (UAN) solution has about 25% as nitrate at application time has a greater risk for loss than anhydrous.
Soils have been warm enough that some transformation to nitrate may have already occurred this year depending on application date. However, the nitrate N will not be lost by denitrification unless soils have remained saturated long enough. Risk of loss is minimal for soils that remain saturated for one day, moderate risk for two days of saturation, and a substantial risk for three or more days of saturated conditions. Standing water is evidence of saturated soils, but even soils without standing water are considered saturated if an individual cannot walk across without making footprints.
Since there are no absolute tests that can tell the N status a point system developed years ago by the University of Minnesota and modified to Ohio conditions has been useful. This system asks a series of questions and assigns a point value depending upon the answer. The probability of a response to additional N increases with more points. The questions and points are given below:
FACTOR 1: What N product was used?
- Anhydrous ammonia with N-Serv 2 points
- Anhydrous Ammonia 3 points
- Other fertilizer banded 4 points
- Other fertilizer broadcasted 5 points
FACTOR 2: When was the majority of the fertilizer N applied?
- After April 20 3 points
- Before April 20 5 points
FACTOR 3: What has been the field soil moisture status the past month?
- Normal soil conditions 1 point
- Wet soils 3 points
- Standing water/saturated soils 4 points
FACTOR 4: What is the crop's current condition?
- Green plants > 12" tall 1 point
- Green plants < 12" tall 2 points
- Chlorotic plants < 12" tall 3 points
- Chlorotic plants > 12" tall 5 points
Total the score for the four factors and use the following guidelines:
- Less than 11 No supplemental N recommended
- 11‑16 Evaluate again in 4-7 days
- 17 or more Add an additional 40 or more lbs. N/acre
The "re‑evaluation" option is only viable until you no longer have side dressing options. Illinois research from the 1990’s found that 50 lb. N/acre as a supplemental N rate was satisfactory for a wide range of conditions. While a total score of 17‑18 would merit a 40 lb./acre N recommendation. A total score of more than 18 may require a higher N rate. Losing 100% of the N fertilizer applied via denitrification or leaching is extremely unlikely and so a reapplication of the total amount of N for the season is not recommended.
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