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Corn and soybean potassium fertilizer: What to know for fall 2019

corn potassium deficiency
Potassium deficiency symptoms in corn. (Image credit: Dan Kaiser/University of Minnesota Extension)

By: Dan Kaiser, Extension soil fertility specialist 

Changes were made to the corn and soybean potassium (K) guidelines in spring 2019. These changes reflect research funded through fertilizer checkoff dollars provided by Minnesota's Agricultural Fertilizer Research and Education Council (AFREC), the Minnesota Soybean Research and Promotion Council, and the Minnesota Corn Growers Association. Funds provided by each were instrumental in evaluating current soil test methods, current critical levels for corn and soybean, and a re-evaluation of rate recommendations. This research represents the beginning of a larger re-evaluation of potassium guidelines for Minnesota.

What should growers be aware of this fall?

The primary changes made to the guidelines were to the soil test classifications for corn and soybean, where the critical level for K was adjusted from 160 ppm to 200 ppm for both crops. Recommended K application rates were not changed for corn, as current data suggested no changes needed to be made beyond the adjustment of the critical level for corn. For soybean, recommended K application rates were increased for the new Medium and High soil test classifications. Updates to the corn and soybean guidelines can be found on the University of Minnesota Extension website at:
Also included were updated K removal values for both corn (0.19 lb. K2O per bushel) and soybean (1.10 lbs. K2O per bushel).

Next steps

Moving forward, there are two key areas of research we are focused on:
  1. Research is planned for 2020 to evaluate the current banded K guidelines, which currently suggest that the K rate can be reduced by up to 50% of suggested broadcast rates when banded.
  2. A more intense study began in 2019, funded by the Minnesota Corn Growers Association, to take a deeper look at the impact of soil properties and how they may impact corn K requirements and K fertilizer guidelines. 
Changes in K guidelines in Iowa and North Dakota have resulted in questions about K use in Minnesota. Minnesota has a diversity of soils and soil properties but little work has been done to tie soil properties to K fertilizer guidelines, in particular to see whether clay types can help to fine-tune K fertilizer needs. The map below shows collection points where data are available on clay species, but we need more sample sites to fill in the gaps and tie corn response to K to soil properties.

On-farm field trials
Minnesota soil survey

The new research needs more on-farm field sites across Minnesota. If you are interested in hosting field trials, please contact Extension specialist Dan Kaiser at or 612-624-3482. Research plots will have five rates of K applied with commercial equipment using variable rate prescription maps where the K rate can be varied independently from other nutrients. Our goal is to find sites in areas where K responses have traditionally not occurred and yield data can be collected using yield monitors. Gathering additional data is important to help us determine if K guidelines need to be tied to soil properties to provide a more accurate estimate of where K is needed and what the most economical K rates are.


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Support for this project was provided in part by the Agricultural Fertilizer Research & Education Council (AFREC).

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