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Corn+Soybean Digest

Determining Nitrogen Rates to Maximize Returns

Maximizing investment returns is at the forefront of every farmer’s mind. One of the most expensive investments farmers make is the application of nitrogen (N) for corn. Every year farmers determine how much N is needed to maximize profitability and reduce the potential for water quality degradation associated with N use in farming operations.

The Corn Nitrogen Rate Calculator was developed to help farmers determine the most profitable N application rate. This calculator was developed using a robust database of recent corn trials conducted under many environments in the state and throughout many years.

Fabián G. Fernández, University of Illinois Extension specialist in soil fertility and plant nutrition, says, “This is a very useful tool to help farmers decide the optimum economical N rate when corn is following corn or when corn is following soybean. However, this tool cannot be used to predict how much N may be available in your soil when manure or other N-fixing legumes besides soybean have been grown in the preceding years.”

Because of this, it’s important to first determine how much N is in the soil. Then use that information to adjust the rates provided in the Corn Nitrogen Rate Calculator.

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

“Just like other tools, this test can be useful,” Fernández says. “But it is important to use this test under the right conditions. 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 N content, such as alfalfa.”

The PSNT is not useful for fields with soybean 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 says. “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 N needs to sample the first 12 in. of the soil, Fernández says.

Samples should be collected when corn plants are 6-12 in. tall (V4-V6 growth stage). If the field had a history of broadcast applications, randomly collect 20-25 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 cornrows. The first core should be collected 3 in. to the right of the cornrow, the second core in the middle of the two rows, and the third core 2 in. to the left of the next cornrow.

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, Fernández recommends. Collecting a sample less than the full 12 in. or not collecting all the cores will produce unreliable results.

If the samples cannot be delivered to the laboratory that day, 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 N. 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 N is needed if PSNT test levels are above 25 parts per million (ppm),” Fernández says. “On the other hand, a full rate should be applied if nitrate N levels are less than 10 ppm. When test levels fall between 10 ppm and 25 ppm, N rates should be adjusted proportionally.”

When calculating the rate, subtract any N that was already applied in any way. The Corn Nitrogen Rate Calculator is available online.

For more information, check out The Bulletin, an online publication written by U of I Extension specialists in crop science.

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