At times potassium (K) can be the forgotten element when determining appropriate rates of fertilizer to apply. Nitrogen and phosphorus typically are of main concern due to the potential yield response for corn to nitrogen and many soils around the state historically being low in P but medium to high in K. Potassium should not be a forgotten nutrient as there are situations where K fertilizer can be profitable.

Taking a soil test is the best option for determining where K is needed. Soil testing for K can be problematic as K levels can vary over the growing season. The uptake of K in plant residue can well exceed the amount of K removed in the grain. Potassium in plant stover can play an important role in the nutrition of crops planted the next year. As plants mature and begin to decay, K taken up during the growing season can be leached out of drying or decaying residue.  The recovery of K can be rapid for plants that senesce leaves early, like soybean, but the recovery of K for crops which residue is slow to break down, such as corn, can be slow. In both cases the rate of recovery increases with the amount of rainfall.  Waiting as late as possible to sample the soil in the fall and being consistent with sampling timing in a rotation is the best way to try to mitigate potential impacts of seasonal variation in soil test K.

Rapid drying of soil samples can affect K extracted by soil tests. Soils vary in their potential for fixing or releasing K upon drying. This is important since the effect would either over- or under estimate the amount of K fertilizer that may need to be required. From a production standpoint, underestimating fertilizer requirements is a big issue. Considering economics, both under and over-estimating fertilizer requirements can result in lost profit.  

Current research funding by the Minnesota Agricultural Fertilizer Research and Education Council (AFREC) and Minnesota Soybean Research and Promotion Council is attempting to provide answers to questions on K fertilizer application timing and rates in corn and soybean rotations and the probability of response to K fertilizer based on soil test level for both corn and soybean.  As soil test for potassium increases the probability that applied fertilizer will return a significant increase in crop yield decreases.  The greatest potential for a response to K should occur when soils test less than 120 ppm.  We are currently working to better identify probability of response to K across Minnesota based on testing soils on both field moist and dried soil samples.  

In a field study conducted in 2013 on a clay loam soil near New Richland, there was a 9 bu increase in corn yield from applied K fertilizer.  Soil test K for an air dried test ranged from 107 to 195 ppm and averaged 144 ppm (High soil test classification). Analyzing the same samples on a moist basis found that soil test K ranged from 37 to 247 ppm with an average of 129 ppm.  While the average value was similar analysis of the field moist samples indicated that some field areas may be highly responsive to K and their response was not being correctly predicted.

The previous example is just one field where we are studying the use of field moist testing of K.  Soils with higher clay content (loams and clay loams) in central Minnesota appear to be at more risk for underestimation of K availability. This does not mean that the traditional method of soil testing does not work. Rather it is best to be mindful of where soil test levels are in your fields and issues that may arise when testing soils. Silt loam and sandy soils have lower clay content and drying soil these types of soils has resulted in similar or slightly lower soil test K values (availability in some of these soils may be underestimated). The fact of the matter is the impact drying on the amount of K extracted by routine soil tests varies greatly depending on the soil being sampled so there is not simple answer when attempting to recommend what kind of K test should be used.

Fields or field areas testing greater than 200 ppm according to an air dried ammonium acetate test represent a situation where the chance for an increase in the yield of most crops is low.  When making decisions on fertilizer to apply the amount of K needed to maximize yield potential may be small. When commodity prices are low reducing, but not eliminating, the rate of K applied may be warranted along with band applications that may increase the efficiency of the amount of K applied. Removal rates of K are not needed in situations where soils are testing medium to high in K. Current suggestions for P and K fertilizer application on corn and soybean are on the web. If money is tight for purchasing fertilizer, taking a soil sample makes sense to better target nutrients where they are needed.

U of M nutrient Management on the Web

Soil test Sentinel plots

We are again looking for field locations for soil test sentinel plots. The purpose of this work is to track soil test values over time at a given point within a field. One question we are trying to address is the effect that drying has on soil test values for contrasting soils across Minnesota and if differences change throughout the year. The project requires soil samples to be collected from a set (geo-referenced) point within a field and mailed off to Saint Paul for analysis. We are looking individuals willing to collect samples over a minimum of 5 sampling times (One sample collected Fall 2014 and then monthly starting May through August 2015). Field sites without fertilizer applied would be preferred. Samples need to be taken and stored refrigerated in a sealed bag prior to sending to Daniel Kaiser at the University of Minnesota in Saint Paul.  Shipping costs can be reimbursed for samples and there is no limit to the number of sampling areas per field. We would like to have some comparison among soil types if possible. If interested please contact Daniel Kaiser by email at or by phone at 612-624-3482.