With continued wet weather, growers are sharing concerns about how rains may have impacted pre-plant or at-plant nitrogen in fields designated for corn. Most producers have applied only about one third of their total corn nitrogen amount which means less is potentially exposed to heavy rains or flooding. In most cases, fields that drain fairly well with some breaks between rains do not lose significant amounts of nitrogen quickly. The amount of nitrogen loss depends on several factors such as the fertilizer source used, time between application and wet conditions, as well as the duration of waterlogging or saturated conditions. A rough estimate of nitrogen loss in medium textured soils using the information below may help producers determine if or how much fertilizer should be added back at layby timing.
Nitrogen in the nitrate form is most prone to loss via leaching or through denitrification in saturated soils. When fertilizer is applied to the soil, urea and ammonium eventually convert to nitrate over several days. The closer fertilizer is applied to wet conditions, the less nitrate conversion has taken place and the less potential loss would be expected. For Nitrogen applied as urea, UAN, ammonium nitrate, or anhydrous ammonia two weeks or more prior to very heavy rain events, we can assume the urea and ammonium have mostly converted to nitrate:
- In well drained fields that are saturated for 2 days or less we can assume minimal loss.
- In poorly drained fields, with the majority waterlogged for longer than 2 days, there is approximately 4% loss of nitrate nitrogen per day due to denitrifcation. For example, if 70 pounds of N were applied 3 weeks ago in a field waterlogged for 4 days, denitrification has occurred for 2 days, therefore 8% or about 6 pounds of the total nitrate nitrogen may have been lost from the field. The layby fertilizer rate can be adjusted by this amount.
The Pre-Sidedress Nitrate Nitrogen Test (PSNT) was designed to measure soil nitrate levels where manure is used as a fertilizer source. It is not suited to fields where N is banded or variable rate applied and it’s value in predicting nitrogen levels in broadcast N fields is widely debated. Tissue testing (6″ or taller) corn plants may be a more accurate predictor of nitrogen deficiency than the PSNT if nitrogen loss is suspected later in the season.