How much of the nitrogen that you applied early this spring or perhaps last fall has been lost due to wet weather this spring? That has become an annual question farmers and crop consultants ask this time of year. This spring is no exception, with the excessive rainfall and ponding that has occurred in some fields.

So, has there been nitrogen loss from your applied N? “That question should also include what has been the N loss from the soil’s nitrogen supply or residual nitrate-N,” says John Sawyer, Iowa State University Extension soil fertility specialist.

There is usually tile drainage every spring and sometimes but not usually in the late fall. Also, losses occur if soils become saturated (free water filling soil pores, standing water, anaerobic conditions) and soils are warm, and then denitrification happens. Denitrification is the biological conversion of nitrate to N gas.

N loss small or large
“So some N loss from soils is typically occurring and can be small or large depending on many factors,” says Sawyer. “Predictions of the effect on N supply, additional N fertilization need, etc., during wet periods are difficult. There are several approaches to take in making estimates of N status or loss.”

Remember that guidelines for N application rates for Iowa corn do take into account “normal” N losses, he explains. That’s because N application rate research trials are conducted in the field and with good management. 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 trials builds in the variation of soil N supply and climatic conditions.

Approaches to estimating loss
There are several ways to estimate the amount of nitrogen loss from your cornfields. Sawyer offers the following recommendations and guidelines:

• Use the Late Spring Soil Nitrate Test. Uses of the LSNT in Iowa corn production was recently described in a new ISU Extension publication CROP 3140. LSNT for estimating plant available N before sidedressing has been around for many years.

• Use modeling, relatively new for production ag. There are several in the marketplace, including the ISU Extension FACTS website that supplies information on nitrate-N in the soil profile. FACTS will soon be updated for 2017.

• Estimate nitrate-N production and loss during periods of soil saturation. An example of this was discussed in a 2014 ICM News article, Estimating Nitrogen Loss in Wet Corn Fields. 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 areas vs. not ponded areas of fields and runoff vs. infiltration. When soils are warm, this loss pathway can be rapid and large, but slow when soils are cool or there is little nitrate present.

• Use springtime rainfall total. Details of this approach were covered in a spring ICM News article, Precipitation and nitrogen this spring. Amount of spring rainfall to trigger the potential for additional N application need was updated with research data from 2016. Those rain 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% accuracy rate for estimating correctly (adequate N or deficit N) if N loss is sufficient enough to consider additional N application.

You don’t need to wait until the end of June to add up the total. That can be done on an ongoing basis, and if the total begins to approach those values, then be thinking about plans for applying additional N. According to precipitation maps into late May this year, we have not reached those rainfall totals, but localized amounts could be different.

A caveat to using the rainfall totals is if there are heavy, short-duration rainfall events. If water runs off the field and does not get into the soil profile, then there should be a discounting off the total. Also, if the rainfall reaches those totals in the 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, do not provide the same level of success as when June rainfall is included. The rainfall triggers are related to use of suggested economical N rates (MRTN) 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.

Other resources for nitrogen rate decisions:
• ISU Extension and Outreach Soil Fertility
• Nitrogen Use in Iowa Corn Production
• Concepts and Rationale for Regional Nitrogen Rate Guidelines for Corn