Determining nitrogen rates to maximize crop returns

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URBANA, Ill. — 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 for corn. Every year farmers determine how much nitrogen is needed to maximize profitability and reduce the potential for water quality degradation associated with nitrogen use in farming operations.

Most profitable application

The Corn Nitrogen Rate Calculator was developed 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 and throughout many years.

Fabian G. Fernandez, University of Illinois Extension specialist in soil fertility and plant nutrition, said, “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 soybean. 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 soybean have been grown in the preceding years.”

Because of this, 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. The most common test used to determine the amount of nitrogen present in the soil is the Pre-Sidedress Nitrate Test.

Taking samples

By sampling later in the season (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.

“Just like other tools, this test can be useful,” Fernandez said. “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 nitrogen 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 to say you need to apply the rate that we are already suggesting with the Corn Nitrogen Rate Calculator,” he said. “Doing the test only results in an expense to the farmer and they obtain information that is already available for free with the calculator.”

Test reliability

The reliability of the PSNT test depends on properly collected and processed samples. Some people suggest collecting samples that are two feet deep, but research shows that it’s more practical and just as useful at predicting nitrogen needs to sample the first foot depth (12 inches) of the soil, Fernandez said.

Samples should be collected when corn plants are 6-12 inches tall (V4 to V6 growth stage). If the field had a history of broadcast applications, 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 three inches to the right of the corn row, the second core in the middle of the two rows, and the third, three inches 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, Fernandez recommended. Collecting a sample less than the full 12 inches 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 them.

Caring for samples

To air-dry samples, spread them 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 parts per million,” Fernandez said. “On the other hand, a full rate should be applied if nitrate-nitrogen levels are less than 10 parts per million. When test levels fall between 10 and 25 parts per million, 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 http://extension.agron.iastate.edu/soilfertility/nrate.aspx.

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