One way to determine the amount of nitrogen (N) loss in a corn field following a stretch of wet weather is to calculate an estimate. Predicting the exact amount is quite difficult as many factors affect N losses. However, estimates can provide guidance for supplemental N applications.

That's how John Sawyer, an Iowa State University Extension soil fertility specialist, answers that question when farmers ask. He offers some observations and further explanation of what is happening in regard to the possibility of corn fields running out of nitrogen during the current growing season. 

Wet soils in June are more favorable for nitrogen loss, than in early spring

Many areas of Iowa, especially southern to southeastern Iowa, have experienced well above normal rainfall this spring, with several recent large rainfall events. This is now the fourth year in a row with excessively wet conditions in much of the state. "In the early spring, the cold soils helped reduce potential for nitrate loss due to slow accumulation of nitrate and slow denitrification," he says. "However, wet soils in June are much more conducive to nitrate loss as the soils are warm then, and with prolonged saturation and tile flow, the losses mount."

One way to determine the amount of nitrogen that you may have lost from a field is to calculate an estimate. "However, predicting the exact amount is quite difficult as many factors affect losses," says Sawyer. "However, estimates can provide guidance for supplemental N applications."

Researchers have measured how much nitrate is being lost

Research conducted in Illinois* indicated approximately 4% to 5% loss of nitrate-N by denitrification per day that soils were saturated. An all-nitrate fertilizer was applied when corn was in the V1 to V3 growth stage (late May to early June). Soils were brought to field capacity and then an excess 4 inches of water (above ambient rainfall) was applied by irrigation evenly over a three-day period (which maintained saturated soils for three to four days on the finer textured soils) or an excess of 6 inches of water was applied over an eight-day period (which saturated soils an additional three to four days).

The excess water application resulted in loss of 60 to 70 lb N/acre on silt loam and clay loam soils, due to denitrification loss. On a very coarse-textured, sandy soil, virtually all nitrate-N was moved out of the root zone by leaching. On the finer textured soils, an addition of 50 lb N/acre after the excess water was sufficient to increase corn yields to approximately the same level where no excess water was applied. This was not the case on the sandy soil because considerably more N was lost due to leaching.

Nitrate loss with tile drainage increases with above-normal rainfall

Nitrate loss via tile drainage does increase with above normal rainfall, notes Sawyer. At Gilmore City, which is between Humboldt and Pocahontas in northwest Iowa, there is an ag-drainage research site where Iowa State University has an extensive project that has been on-going for years. The tile-flow nitrate has been monitored since 1990, and the nitrate loss is greatest in years with higher precipitation and hence greater tile flow.

At N fertilization rates of 150 to 160 lb N/acre, the annual nitrate-N loss per acre was 52 lb in the 1990-1993 period, 9 lb in the 1994-1999 period, and 39 lb in the 2000-2004 period (average nitrate-N losses for the combined corn-soybean sequence). The range in yearly nitrate-N loss for the years studied was 1.0 lb nitrate-N/acre in 1997 to 75 lb nitrate-N/acre in 1990.

Typically, he says, a high portion of tile flow and associated nitrate-N loss occurs in the spring. The impact of excess precipitation on potential for nitrate remaining in the soil for crop use in wet springs is that more nitrate-N is lost via tile flow, and overall the annual loss would be in the range of perhaps twice the "normal" loss amount, increasing from around 15 to 25 lb N/acre to 40 to 50 lb N/acre.

Estimating nitrate loss involves several factors to consider

According to research at the University of Nebraska, the estimated denitrification loss of nitrate when the soil temperature is 55 to 60 degrees F is 10% when soil is saturated for five days and 25% when saturated for 10 days (2% to 2.5% per day). Loss increases with warmer soils.

Research conducted in Illinois with late May to early June (soil temperatures greater than 65 degrees F) with excess application of water on silt loam and clay loam soils indicated approximately 4% to 5% loss of nitrate present per day that soils were saturated.

"To estimate N loss, the first step is to estimate the amount of ammonium converted to nitrate-N," says Sawyer. "You could assume that with applications made late last fall, the anhydrous ammonia and manure ammonium would be nearly converted to nitrate by now. And with the early April preplant N applications made this spring, a majority would be converted to nitrate by now."

Less conversion to nitrate would occur with use of a nitrification inhibitor, he adds. Recent ammonium applications (within the last two weeks) would still be predominantly in the ammonium form, especially for anhydrous ammonia. Recent application of nitrate-containing fertilizers would result in more nitrate being present. Urea-ammonium nitrate solutions (28% or 32% UAN) contain one-quarter nitrate-N, and nitrify more rapidly.

Second step is to estimate the percentage of nitrate-N loss as described in the research above. The amount of N loss is calculated from these two estimates.

An example of how to do the calculation to make an estimate

Sawyer provides the following example, a situation with a spring preplant application of UAN solution and the wet conditions encountered this year. If 95% of a 120 lb N application is converted to nitrate, and soils were then saturated for 10 days when warm, the N loss estimate would be (120 lb N per acre x 95% nitrate/100) x (4% per day/100) x (10 days) = 45 lb N per acre. Add in increased tile flow on tile-drained fields, and the loss estimate could be 60 lb N per acre.

"Variation of lower or higher losses could easily occur depending on warmer or cooler conditions, different forms of applied N, more or less time from N application to wet conditions and more or less time and frequency soils are saturated," notes Sawyer. "The same will occur for different landscape positions and soils. With very coarse-textured/sandy soils, significant rainfall events (4 to 6 inches or more) in addition to already moist soils could easily result in all nitrate leaching out of the crop rooting zone."

* Footnote: Reported in the 1993 Iowa State University Integrated Crop Management Conference proceedings, pp. 75 to 89, and in Torbert et al., 1993, "Short-term excess water impact on corn yield and nitrogen recovery" reported in the Journal of Production Agriculture 6:337 344.