University of Illinois (UI) Extension specialists developed the Corn Nitrogen Rate Calculator to help farmers determine the most profitable nitrogen application rate. This calculator was developed using a robust database of recent corn trials conducted under many environments in the state over the course of many years.

Fabián G. Fernández, UI Extension specialist in soil fertility and plant nutrition, explains, "This is a very useful tool to help farmers decide the optimum economical nitrogen rate when corn is following corn or when corn is following soybeans. However, this tool cannot be used to predict how much nitrogen may be available in your soil when manure or other nitrogen-fixing legumes, besides soybeans, have been grown in the preceding years." Therefore, it's important to first determine how much nitrogen is in the soil. Then use that information to adjust the rates provided in the Corn Nitrogen Rate Calculator, he adds.

The most common test used to determine the amount of nitrogen present in the soil is the Pre-Sidedress Nitrate Test (PSNT). By sampling in late May to early June, this test provides a measure of the amount of nitrogen mineralized into plant-available forms from organic nitrogen, plus the amount of carryover nitrogen still present in the soil.

“This test can be useful, but it is important to use it under the right conditions.” Fernández says. “The PSNT is often more accurate in high-yielding environments and in fields that have received manure or other organic fertilizers in the recent past, or that have had legume crops with high nitrogen content, such as alfalfa."

The PSNT is not useful for fields with soybeans or corn as the previous crop or where commercial inorganic fertilizers were applied, unless a substantial amount of carryover is suspected.

"If you sample a field that had soybeans last year, the test is likely going to say you need to apply the rate that we are already suggesting with the Corn Nitrogen Rate Calculator," he states. "Doing the test only results in an expense to the farmer and they obtain information that is already available for free with the calculator."

The reliability of the PSNT test depends on properly collected and processed samples. Some people suggest collecting samples that are 2-ft. deep, but research shows that it's more practical and just as useful at predicting nitrogen needs to sample the first 12 in. of the soil, Fernández says.

Samples should be collected when corn plants are 6- to 12-in. tall, at the V4 to V6 growth stage. If the field had a history of broadcast applications, producers are advised to randomly collect 20 to 25 samples from an area no greater than 10 acres.

If band applications of fertilizer or manure were used to fertilize previous crops, collect at least 10 sets of three cores each between two corn rows. The first core should be collected 3 in. to the right of the corn row, the second core in the middle of the two rows, and the third core 3 in. to the left of the next corn row.

In all cases, place the cores in a bucket and obtain a subsample after the cores have been mixed thoroughly. If mixing the entire sample to produce a representative subsample is too difficult, it is better to use large sample bags and keep the entire sample. Collecting a sample less than 12 in. or not collecting all the cores will produce unreliable results, Fernández explains.

If the samples cannot be delivered to the laboratory the same day, producers should freeze or air-dry the samples. To air-dry samples, spread them out on a paper, crushing the cores, and blowing air with a fan. Since drying can be difficult without proper facilities, freezing samples is likely the best option. Instruct the laboratory to measure nitrate—nitrogen. If the entire sample is sent, request that the whole sample be dried and ground before a subsample is taken.

"Once you have results back, you can be certain that no additional nitrogen is needed if PSNT test levels are above 25 ppm,” Fernández says. "On the other hand, a full rate should be applied if nitrate-nitrogen levels are less than 10 ppm. When test levels fall between 10 and 25 ppm, nitrogen rates should be adjusted proportionally."

When calculating the rate, subtract any nitrogen that was already applied in any way. The Corn Nitrogen Rate Calculator is available online at extension.agron.iastate.edu/.
For more information, check out The Bulletin, an online publication written by UI Extension specialists in crop science, at ipm.illinois.edu/bulletin/.