Persistent rain and saturated soils have generated concern about nitrogen fertilizer loss. Nitrogen fertilizer is subject to considerable loss from excessive rainfall and wet, saturated soils. The loss resulting from saturated conditions and flooding occurs primarily through denitrification, particularly in heavy textured soils. Denitrification occurs when nitrate nitrogen is converted into nitrogen gas by microorganisms and escapes into the air. Two to three days of saturation are required for the process to begin.
The amount of nitrogen loss is very difficult to predict, and depends upon many factors. The primary factors affecting loss are fertilizer application timing, nitrogen source, and quantity applied, relative to the duration of saturated conditions and soil temperatures during this time. Essentially, seasonal nitrogen loss can be quite high, if a lot of nitrogen is exposed to numerous days of saturated conditions, particularly when soil temperatures are warm – in other words, early nitrogen application increases potential loss. This is why we strongly advocate applying nitrogen fertilizer to corn using split application methodology.
Warm soil temperatures accelerate the denitrification process. Research indicates denitrification rates range from 2-3% per day at soil temperatures from 55-65 F or 4-5% per day if soil temperatures exceed 65 F. Fortunately, the cool temperatures thus far this spring, should have limited daily denitrification rates to the low range of this scale.
Timing of nitrogen application and nitrogen source affect the amount of applied nitrogen that will be in the nitrate form. This is important, because only the portion of nitrogen fertilizer present in the nitrate form is subject to denitrification. The following table estimates percentage of fertilizer present in the nitrate form for different nitrogen sources depending upon the time after application (adopted from "Evaluating Flood Damage in Corn, Lee et al., Univ. of Kentucky, AGR-193).
For example, if you applied 200 pounds/a of N as UAN (32%) three weeks before the field became saturated and soil temperatures were about 60 degrees F, about 120 pounds N (200 x 60%) are present in the nitrate form. Each day of saturation after the initial two days results in an approximate nitrogen loss of about 3.6 lbs. of N per day (120 x 3%). Thus, if this field has been saturated for about 12-14 days, then anticipated N loss would be about 40 lbs./a.