Changes were recently made to the University of Minnesota’s potassium (K) fertilizer guidelines for corn and soybean. These changes were made to reflect research evaluating critical soil test levels and primarily centers on medium and fine textured soils.
How are the new guidelines different?
The new guidelines follow the same fertilization strategy used in Minnesota for many years. The guidelines differ in that the ranges for the soil K classes have been adjusted to reflect current research which has shown the need for slightly higher critical soil test K levels for poorly drained medium to fine textured soils.
Changes in the ranges were made for both corn and soybean, putting the critical level at 200 parts per million (ppm). These changes reflect the use of the dried ammonium acetate K test, which is recommended in Minnesota.
In addition, suggested K application rates for soybean have been increased for the Medium and High soil test K classification.
If I applied K in fall do I need to make additional applications?
The answer to this question depends on how much was applied and what your current soil K status is. For low K soils, the suggested K application changes with the updates will not significantly change the recommended application rate enough to warrant an additional application. The biggest changes are in the soybean guidelines in the medium to high K classes.
I would not suggest making any major changes at this time if K was applied in fall, especially if the last soil test taken from the field is 160 ppm or higher. Planting this spring should be a top priority.
Are there any additional changes that will be made?
One issue which is not addressed with these updates are modified guidelines which reflect variations in soil physical or chemical properties, such as cation exchange capacity (CEC). Some of our recent studies have indicated that less K is required for sandy soils with low CEC (5 meq per 100g), which are difficult to build due to their low cation holding capacity.
For these soils, an alternative equation is available for both corn and soybean for use on a trial basis (see web links below). While the new modifications to the guidelines are complete, this is the first step in a major reevaluation of the potassium guidelines for Minnesota.
What is next for the potassium guidelines?
We are currently in the process of starting a larger evaluation of soil clay minerology across Minnesota and are seeking fields to establish on-farm K response trials, which have been funded by the Minnesota Corn Research and Promotion Council. These studies include a rate trial coupled with an evaluation of soil chemical properties. The end goal of this project is to determine whether clay species in the soil or factors such as CEC can be used to fine-tune potassium guidelines.
We are looking to establish pilot sites on farmers’ fields across Minnesota with soil K tests in the 150 ppm range this spring, with a larger effort this fall targeting areas we have traditionally not conducted K trials. In the end, we hope to better understand why some fields respond more than others and what tools are needed moving forward to better target potassium applications for maximum profitability.
Where can the new guidelines be found?
The new guidelines have been updated on Extension’s website: