Minnesota Corn, Soybeans: Fall Fertilizer Application Outlook

Post harvest crop stubble. Photo: University of Minnesota

Fall 2018 has brought late fertilizer applications followed by cold temperatures and freezing soils. Frozen soils can present problems for the application of commercial fertilizers. All commercial fertilizers are water-soluble. However, we see variation in how long it takes for the material to fully dissolve. It is important for reactions with the soil for the fertilizer to dissolve. Any material that has not had some reaction with the soil may be susceptible to loss should water move across the field.

Late fall fertilizer applications brings water quality concerns surrounding the loss of nitrogen and phosphorus. Potential loss of urea can occur until hydrolysis converts urea to ammonium which can be held by charges on soil clay. Urea is a neutral molecule and will readily move with water. Full conversion to nitrate also presents issues as nitrate is not retained by soil.

Phosphorus loss can occur when fertilizer is applied to the soil surface and not incorporated. Research in Iowa has shown that a ten-day delay of between application and rainfall is enough time to substantially reduce potential for phosphorus loss. Minnesota data also shows that a similar delay will reduce the risk for potential leaching losses of phosphorus. The simple fact is that the more time we can give phosphorus to react with the soil prior to a freeze the less risk for loss.

Just because fertilizer is applied to the soil it does not mean there will be loss. The potential for offsite movement depends on many factors. One being slope of the ground. The other is the potential for rainfall or snow melt. This can happen now or in the spring, should the soils not thaw enough to allow for some reaction of the material. Tillage can help reduce the risk for loss of P so far as it does not increase the risk for erosion loss.

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The greatest risk for runoff losses will be fields where water will rapidly run off. Avoid applying P on soybean stubble, in particular on frozen or partially frozen ground. Soybean stubble provides very little impedance to water flow and presents a high risk for loss with late fall application without tillage.

Tillage may not be an option at this point and delaying application into the spring is highly suggested to prevent loss of fertilizer. Loss of N or P not only has environmental consequences, but also is a waste of money.


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