High rainfall in parts of Iowa the past few weeks has produced another wet start to the growing season. A large portion of western and northern Iowa received 10-plus inches of rain in June. That’s about 200% or more of normal and has led to questions about nitrogen loss and need to apply supplemental N to corn.

“Unfortunately, this question has become almost the norm,” says John Sawyer, Iowa State University Extension soil fertility specialist. “I’ve written approximately 20 articles on the subject since 2007.”

Consideration of how much N loss has occurred in your fields should include losses from both the soil N supply and residual nitrate-N, says Sawyer. “There is usually tile drainage every spring and occasionally in the late fall, leading to N losses. Also, losses can be rapid if soils become saturated, are warm, and nitrate is present. These conditions lead to denitrification — biological conversion of nitrate to N gas.”

Some N loss from soils is typical, how much depends on many factors, he adds. In wet periods it’s difficult to predict the effect of these losses on the N supply for corn, and how much additional N fertilization is needed, etc. There are several approaches that can be used in making estimates of N status or loss.

Sawyer reminds farmers and crop advisers that the guidelines for N application rates for corn in Iowa take into account “normal” N losses as the N rate research trials upon which the ISU recommendations are based are conducted in the field. This is especially important as those N rate trials incorporate supply and loss of soil derived-N, not just applied N. This means that the accumulation of N rate research trial results, like those used in the Corn Nitrogen Rate Calculator, builds in some variation of soil N supply and climatic conditions.

Tips for N estimating
Nitrogen management and input decisions are difficult in a wet spring. There are a wide range of scenarios in the fields. Sawyer offers the following tips on ways to estimate the amount of N loss that has occurred in a field.

 • Use the Late Spring Soil Nitrate Test. Use of the LSNT test in Iowa corn production in described in an ISU Extension publication (CROP 3140). “Once you get into late June and then enter early July, in most fields, we are past the calendar time and growth stage calibrated for the LSNT test, and it should no longer be used,” says Sawyer.

• Modeling using computer programs. Use of computer models is relatively new for production agriculture. Several models are currently in the market place, including the ISU Extension FACTS website that supplies information on nitrate-N in the soil profile (the site was recently updated). You can pick a research site near you and look at the increase and decrease in soil nitrate-N concentration between soil depths.

“Remember, corn is rapidly taking up N now, so there is expectation of decreasing overall nitrate in the profile due to uptake,” says Sawyer. “However, very rapid changes would indicate leaching movement or denitrification, besides crop uptake.”

Nitrate-N production, loss
An example of this was discussed in a 2014 ICM News article (Estimating Nitrogen Loss in Wet Corn Fields), he notes. Important components are the estimation of how much nitrate-N has formed from applied N by the time of wet conditions, and the length of soil saturation (which can vary greatly across fields, for example ponded vs. not ponded areas, and runoff vs. infiltration).

When soils are warm, loss can be rapid and large, but slow when soils are cool or there is little nitrate, says Sawyer. Recent sidedress fertilizer N applications would be fully or partially “protected” from loss if the application included ammonium which is retained on the soil cation exchange complex and not subject to leaching or denitrification (such as urea, ammonium, anhydrous ammonia).

Anhydrous ammonia is all ammonium. Urea rapidly converts to ammonium, and urea-ammonium nitrate solution (UAN 28% or 32%) is one-quarter nitrate, one-quarter ammonium, and one-half urea. Early sidedress applications of N were likely converted to nitrate before the recent high rainfall events.

Effect of spring rain on N loss
Details of this approach were discussed in a 2016 ICM News article (Precipitation and Nitrogen This Spring). The amount of spring rainfall to trigger the need for additional N application was updated with research data from 2016. Those rainfall totals are now 17.8 inches from March 1 to June 30 for southeast Iowa, and 15.5 inches from April 1 to June 30 for the majority of Iowa.

“These rainfall totals have about a 76% chance for estimating correctly (adequate N or deficit N) if N loss is sufficient enough to consider additional N application,” says Sawyer. “You don’t need to wait until end of June to calculate total rainfall. That can be done on an on-going basis and if the total begins to approach those values, then you should be thinking about plans for applying additional N.”

Rainfall maps for this year
According to precipitation maps, Iowa has exceeded those rainfall totals in the central to northern part of Iowa, he says. Local rainfall measurements would provide more specific rainfall information. The more rainfall there is above those trigger totals, the more likely supplemental N would be needed for your field. A caveat to using the rainfall totals is if there are heavy, short duration, rainfall events.

“If water runs off the field, and doesn’t get into the soil profile, then there should be a discounting off the total,” notes Sawyer.

Sawyer says if rainfall reaches those totals in early spring, there should also be some discounting off the total due to less nitrate buildup and less denitrification with cool soils. For example, total rainfall amounts in just an individual month, like April or May, don’t provide the same level of success as when June rainfall is included.

The rainfall triggers are related to use of suggested economical N application rates (MRTN or Maximum Return to Nitrogen) from the Corn Nitrogen Rate Calculator. If higher or lower N rates were applied to fields, then the odds of needing additional N go up or down.

Resources for rate decisions:
• Use of the Late-Spring Soil Nitrate Test in Iowa Corn Production 
• Corn Nitrogen Rate Calculator
• ISU Extension and Outreach Soil Fertility
• Nitrogen Use in Iowa Corn Production