Continued saturated soils in some parts of the state have created concern with producers that N fertilizers that were already applied may be partially lost. Nitrogen loss through denitrification occurs when nitrogen in the nitrate (NO3) form is converted to nitrogen gas by bacteria in the soil. The amount of denitrification increases with the amount of time a field is water logged (soil needs to be saturated for at least 2 days in order for the bacteria to start the denitrification process).
Soil scientists estimate you lose 3 to 4 % of the nitrate in the soil per day of saturation, after the first two days. In fields where some or all nitrogen fertilizer has already been applied, estimating the amount of N loss depends on the 1) nitrogen source used, 2) how long the nitrogen has been on the field, and 3) duration of flooding if water is standing in the field. Below is a table we borrowed from the University of Kentucky with some commonly used N sources and the percentage of fertilizer that is likely available as nitrate nitrogen at different times after application denitrification loss will occur.
|N Source||Weeks After Application of N Fertilizer|
|% of Fertilizer as Nitrate Nitrogen|
Example: ammonium nitrate is applied 3 weeks before heavy rains. At 3 weeks ammonium nitrate will be at 80% nitrate form (180 lbs applied N/acre x 80% = 144 lbs nitrogen as nitrate/acre). With 5 days saturated soils minus 2 days for bacteria to begin causing denitrification= 3 days that denitrification loss will occur. At 4% loss/day x 3 days = 12% loss has occurred (144 lbs nitrogen/acre X 12% loss = 17.3 lbs nitrogen/acre lost).
Where layby N was missed due to weather, obviously rescue N application is needed in order to make a crop. Where all N has been applied consider the rate applied and estimate potential loss using the above information. Consider the realistic yield level of the field. Growers who may have applied more N than required for adequate yields may actually have enough buffer to get by without applying supplemental N.