What's happening to nitrogen in fields after a wet, cool spring?

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ST. PAUL, Minn.—Many Minnesota fields have exhibited significant variation in plant growth and yellowing this spring. University of Minnesota Extension has received questions surrounding nitrogen loss caused by less than favorable conditions in May and June.

There are a few considerations when deciding if more nitrogen should be applied:

Heavy rainfall does not necessarily mean a heavy loss of nitrogen. Soils were cool enough in May, so denitrification as a result of water ponding on the soil surface should not have been a major issue up until recent rains. From now on, ponding water is of greater concern.

The Supplemental N Worksheet, a tool to judge if additional nitrogen fertilizer should be applied, is available on the nutrient management website: http://z.umn.edu/eis (PDF). Keep an eye of fields to determine if supplemental nitrogen is needed.

Jeff Vetsch at the University of Minnesota's Southern Research & Outreach Center in Waseca recently reported some movement of nitrate, based on soil samples collected from spring research trials following application of swine manure in October 2012. Any nitrogen applied early last fall would have the greatest chance for potential loss, especially nitrogen applied without a nitrification inhibitor.

Fields with early fall application of manure or nitrogen have the greatest potential for needing supplemental nitrogen. Data from Waseca, however, only showed movement of nitrate-nitrogen in the soil profile. If water is not flowing out of the tiles, any converted nitrate should still be within the soil profile. The question is whether the corn roots will be able to reach it or if recent rainfall events moved nitrogen too deep in the root zone to access.

Some issues with corn could be caused by a lack of oxygen to the roots. Oxygen is needed for normal root development and for efficient uptake of nutrients by the roots. When oxygen levels are depleted because of flooded soils, foliar symptoms of the plant may look similar to some nutrient deficiencies. If problems within fields are caused by the lack of oxygen, there is not much that can be done other than waiting until conditions improve. As temperatures increase, the appearance of the crop may improve. If soils remain saturated with water, additional nitrogen likely will not benefit the crop.

Consider other sources of nutrient deficiencies, especially sulfur. While there has been no direct evidence of widespread sulfur deficiencies, conditions are somewhat similar to early 2009, when soils were wet and temperatures cool. If sulfur was applied and crops still look deficient, the problem may be associated with lack of uptake. No fertilizer is likely needed. An exception would be elemental sulfur applications, which require higher soil temperatures for oxidization. It is likely that very little of the elemental sulfur applied last fall would have oxidized at this time.

Sulfur can still be applied as an early side dress, around the V5 growth stage, if a deficiency is expected. Dry fertilizer sources of sulfate sulfur can be broadcast applied at low rates. In most instances, 10 lbs. of sulfur per acre should be adequate for an in-season application.

Some leaf burning may occur, but that generally has not been found to reduce yields. Liquid sources containing thiosulfate should not be sprayed over the top of growing corn or severe crop injury may result. Coulter injection or dribbling ammonium thiosulfate is the best method for application.

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