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Friday, October 23, 2015

Applying nitrogen fertilizer in the fall—a lose-lose scenario in southeast MN

With Minnesota's corn harvest more than half complete, it's time to start thinking about other fall farm tasks.
 
To help you start planning, University of Minnesota Extension Nutrient Management Specialist Fabián Fernández has published a guide to fall nitrogen application, which explains where, when, how, and how much nitrogen to apply in the fall. Here in southeast Minnesota, including Dakota County, the answer to all those questions is simple: don't.
 
In southeast Minnesota—counties Dakota, Goodhue, Wabasha, Olmstead, Winona, Fillmore, and Houston—applying nitrogen fertilizer in the fall is not recommend based on established Best Management Practices (BMPs): http://z.umn.edu/SENBMPs
 
Why not?  As Dr. Fernández puts it, "the soil is not a very good place to store nitrogen." Other nutrients, like phosphorus and potassium, tend to bind to soil particles, but nitrogen doesn't stay put for long. It can be lost to the atmosphere or leached out of the soil by spring rains. Corn takes up most of the nitrogen it needs after the V8 stage in late spring. So applying nitrogen in the fall leaves a long window of time for that nitrogen to be lost from the soil before the crop's roots can get to it.
 
In southeast Minnesota, this problem is exacerbated by our relatively wet climate and coarse-textured soils. The nitrate form of nitrogen moves with water, and southeast Minnesota gets more rain than any other part of the state. We also have sandy or silty soils on top of fracture-prone limestone or sandstone bedrock, all of which allows water to leach nitrogen below the crop's root zone more quickly than in other parts of Minnesota. Once the nitrogen leaches below the root zone, it's of no use to the crop and at risk of moving deeper into groundwater.
 
That's a lose-lose scenario for the farm's bottom line and for water quality. Efficient use of inputs is crucial to maximizing return on investment. In addition, Dakota County's well-water testing campaign has found some wells with nitrate concentrations already above the EPA's limit for drinking water, which makes efficient use of fertilizers crucial for responsible stewardship.
 
Surveys show that most farmers are already on board with this recommendation. Only a small percentage of farmers in southeast MN still apply nitrogen in the fall, based on surveys done by U of MN Extension and the Minnesota Department of Agriculture. Most farmers report that they apply their nitrogen fertilizer in the spring and summer, which is the recommended practice.
 
There are several recommended times to apply nitrogen fertilizer in southeast MN: pre-plant, at planting, and side-dress applications can be used to split the recommended amount of nitrogen up into several "doses," which can reduce the risk of loss in any one rain event.
 
Splitting nitrogen applications is especially recommended on irrigated sandy soils, where both risk of nitrogen loss and ease of in-season fertilizer application are greater. On irrigated sands, Extension research has found that splitting results in equal or greater corn yields compared to applying all the nitrogen pre-plant, even with a nitrification inhibitor.
 
You can find all of U of MN Extension's nitrogen recommended BMPs online at http://z.umn.edu/Nrecs 
 
The recommendations described above focus on commercial nitrogen fertilizer. Manure is another important source of nutrients, including nitrogen, and organic matter. To learn about Extension's recommendations for applying manure as a fertility source, visit http://z.umn.edu/MNmanure

Source: Neith Grace Little, Dakota County Extension Educator in Agriculture Water Quality, U of M Extension Regional Center, Farmington, 651-480-7723, nglittle@umn.edu , http://z.umn.edu/dakota

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