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Rainy Spring Could Be Culprit of Lost N

University of Illinois specialist says there are many factors to consider when wondering if your N is gone

Published on: May 9, 2013

If it isn't one thing, it's another. That's what many farmers may be thinking after the drought decimated crops last summer and rainy weather – in some cases significant flooding – came calling just a few months later.

That's why Fabian Fernandez, a nutrient management extension specialist in the Department of Crop Sciences at the University of Illinois, said evaluating your N will be more important than ever this growing season.

Fernandez explained that most of the fall-applied nitrogen is either ammonium (NH4+) or a form that transforms rapidly into ammonium. The conversion of ammonium to nitrate (NO3-), or nitrification, is a bacteria-mediated transformation.

University of Illinois specialist says there are many factors to consider when wondering if your N is gone
University of Illinois specialist says there are many factors to consider when wondering if your N is gone

This means that the amount of nitrification that occurs in the soil is dependent on soil temperatures and the amount of time between application and when the soil is saturated with water.

So, while wet soil conditions this spring may be a reason for concern that last fall's nitrogen has been lost, cold temperatures that followed the rain might have substantially reduced nitrification, Fernandez said. But, he added that warming temperatures will boost nitrification.

"When nitrate is present and soils warm up, nitrogen loss will start under saturated water conditions mostly through denitrification in fine-textured soils and through leaching in coarse-textured soils or intensively drained soils," he said.

Fernandez pointed out that an important aspect to keep in mind is that the portion of the applied nitrogen that is in nitrate form is only subject to denitrification or leaching. However, just because nitrogen is in the nitrate form does not mean that nitrogen is lost; it simply means that it is susceptible to loss.

After determining how much of the nitrogen is in the nitrate form, Fernandez said it is possible to estimate how much nitrogen is potentially lost through denitrification based on 4-inch soil temperature and the number of days the soil has been saturated.

If leaching is a greater concern than denitrification, for each inch of precipitation, nitrate moves approximately 5 to 6 inches in silt loam and clay loam soils and approximately 12 inches in coarser textured-soils, he said.

"This year an important concern is the potential for loss of the carryover nitrogen for fields going back to corn this year," Fernandez said. "Some fields had substantial amounts of nitrogen after last growing season. Most of this nitrogen was in the nitrate form, thus the potential for loss by leaching since last fall was not related to temperature, but by the amount of excess precipitation.

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Regardless of how much nitrogen may or may not be present, Fernandez said it will be more important to plant now and apply additional nitrogen later so planting is not delayed further. One way to determine the need for additional nitrogen in fields where substantial carryover nitrogen is suspected or where the soil has high potential for mineralization -- for example in fields where manure was applied within the last 2 to 3 years --  is to use the pre-sidedress nitrate test during late May to early June.

Fernandez said that this test provides a reliable measure of nitrogen because the potential for nitrogen loss of all the nitrogen present in the soil is low by the end of May.  Soil temperatures are also warm by then, and the test will measure mineralized nitrogen from the soil organic pool in addition to other sources such as carryover nitrogen.

"The sample needs to be collected from the top 12 inches of the soil," Fernandez said. "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 the previous crops, make a sample by collecting at least 10 sets of three cores each between two corn rows. The first core is collected 3 inches to the right of the corn row, the second core in the middle of the two rows, and the third core 3 inches to the left of the next corn row.

"Collecting a sample less than the full 12 inches or not collecting all the cores will produce unreliable results," he said. "If the test results comeback above 25 parts per million (ppm) no additional nitrogen will be necessary. If results are less than 10 ppm, a full rate will be needed. If test levels are between 25 and 10 ppm, the nitrogen rate should be adjusted proportionally."

Another method Fernandez described to determine if additional nitrogen will be needed is to establish a couple of strips with 10 to 25% more nitrogen than the full application amount planned, and then compare the color of the crop as the season progresses.

"If the strips are substantially greener than the rest of the field any time before tasseling, that would be an indication that additional nitrogen is needed," he said. "Some of the potential drawbacks to be aware of when using this approach are that color differences could develop too late for a timely application, or that there might not be sufficient rain to move the late-applied nitrogen into the root zone."

Source: University of Illinois