Wet spring conditions typically raise questions about the status of applied nitrogen. With all this rain, how much of that N is still in the soil? Do I need to apply additional N to my cornfields?

“At this time, in early June, we don’t know what the weather or rainfall will be for the entire springtime, but the extended period of wet conditions this spring is of concern,” says John Sawyer, Iowa State University Extension soil fertility and nutrient management specialist.

Long-term N application rate trials with continuous corn and corn rotated with soybeans were conducted at seven ISU research and demonstration farms in Iowa from 1999 to 2016. A summary of the results of these trials is used as a basis for indicating the potential for needing a nitrogen application rate greater than the Maximum Return to Nitrogen rate suggested by the Corn Nitrogen Rate Calculator. The summary is a “tool” to use to adjust N applications when a greater-than-normal rate would be appropriate. The MRTN rate is considered to be the normal rate.

Regions of state differ

The tool developed from that long-term research is the amount of accumulated total springtime precipitation, Sawyer says. Either March through June in southeast Iowa, or April through June in the main area of Iowa.

“These areas are delineated within the Corn Nitrogen Rate Calculator, when you use that tool. The database from the long-term trials provides 216 crop rotation [or] site-years of data. The analysis indicates the southeast area of Iowa should use accumulated March through June precipitation, and the rest of Iowa, April through June precipitation.”

Precipitation totals during those periods were evaluated for each of the site-years against the occurrence of an economic optimum N rate (EONR) that is more than 20 pounds N per acre greater than a MRTN rate.

The MRTN rates used were 140 pounds N per acre for corn following soybeans and 188 pounds N per acre for continuous corn in the main area of Iowa. The MRTN rates are 154 pounds N per acre for corn following soybeans and 204 pounds N per acre for continuous corn in the southeast area of Iowa.

Why apply 20 pounds more N?

In what conditions would a farmer apply more than 20 pounds of additional N per acre? Sawyer says it’s assumed there’s a need of at least a 20-pound-N-per-acre additional application rate for it to be worthwhile to apply. Is potential for a yield response adequate to pay for the application and N fertilizer, and to provide some buffer due to uncertainty in the data analysis?

Sawyer explains it this way: Evaluation of the precipitation and N response data indicated that an accumulated April-to-June precipitation amount (like a trigger level) of more than approximately 15.5 inches (can round to 16 inches) in the main area of Iowa correctly estimated if additional N was or was not needed 76% of the time.

A March-June precipitation total of more than approximately 18 inches for the southeast area of Iowa correctly estimated if additional N was or was not needed 76% of the time.

This showed if the MRTN rate was adequate or not — not a perfect determination, but pretty good odds of a correct determination. Many things affect soil N supply and N loss, which is why the evaluation is not perfect.

Additional complications

Additional complications this year arise from delays in planting, late N applications and corn N response in wet conditions, Sawyer says. Ponded areas of fields would have high gaseous loss through denitrification, so short-term high rainfall events could cause more rapid N loss than indicated by the springtime total, especially when soils are warm in June. Or high rainfall events cause runoff and, thus, not all the precipitation would affect N within the soil.

“Therefore, variation in N loss, additional N need and potential for crop response will differ among fields,” he notes. “In addition, an evaluation is needed of corn plant health and the potential for plant response if N were to be applied.”

Unfortunately, this precipitation tool is not reliable enough to indicate how much additional N would be needed, Sawyer says. It is directionally correct (that is, the more that the precipitation is greater than the indicated precipitation trigger level, the more N needed). But it cannot be calibrated to a specific rate. Somewhat depending on the already applied N rate, a suggestion would be to limit a supplemental N application to no more than 50 pounds N per acre, he adds. This suggestion assumes that an N rate was already applied close to the MRTN rate.

How much additional N?

“If the rate applied was much less than the MRTN rate, then more than 50 pounds N per acre could be considered as an additional amount to apply,” Sawyer says. “However, if an N rate applied was well above a CNRC suggested rate, then the chance of being short of N is much less than indicated by the precipitation tool.”

If you don’t have a rain gauge at your location or at your fields, you can access precipitation totals for weather stations across Iowa from two networks. One is the National Weather Service Cooperative Observer Program and the other is ISU Soil Moisture network. Both precipitation data sources are available from the ISU site Iowa Environmental Mesonet.