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Soil Testing For K

By Daniel Kaiser, Extension Soil Fertility Specialist

With spring finally approaching it is a good time to address some questions on soil testing that came up of the winter concerning testing soils in a field moist state versus the standard dried samples that are run through soil testing labs.  First I would like to make it clear that the issue of drying of a soil sample mainly pertains to potassium. Most other tests routinely run through the lab are not affected by drying of the sample.  The reason why potassium is different is due to its chemistry in the soil.  We currently have finished the second year of potassium studies looking at both testing methods but will be continuing this work for the foreseeable future to gain a better understanding of what is going on within the soil.

Figure 1. Fall soil test K as affected by K application rate and soil sample treatment (air dried vs. field moist), Red Wing, MN, 2012.

Over the past two years we have been running samples both ways, field moist and air dried, to look at differences.  In Iowa, field research has noted some instances where the field moist test returns a lower value than the air dried sample.  For some soils this causes a serious issue as the air dried test would have the tendency to overestimate the amount of K available.  In our work beginning in 2011 we have not seen this effect to occur.  Figure 1 included from soil samples collected at a plot near Red Wing in the fall of 2012.  This typifies some of our locations in the past two years where the field moist test has actually returned higher values than the air dry tests.  With the dry soil conditions complicating matters, we really do not know whether one is actually correct in assessing the potential for a deficiency of K in out soils.  The fact is that we are too early in our work to tell the difference between the two tests.  If fact, our current calibration data shows that there is no difference in the assessment of crop response between a sample run field moist and air dry, even with the handful of locations testing higher with the field moist analysis.  We currently are trying to expand research into poorly drained fields as these should represent conditions where the field moist potassium test has a better chance of coming back lower than the air dry test.  The fact of the matter is that we know there are likely issues out there but have no concrete evidence to show where they are occurring.

Even with some of the issues noted I would like to make it clear that I have full confidence in the current analysis methods being used for testing K by soil testing labs in the state of Minnesota.  Again, we are looking at these issues closely but the development of the field moist test and if we do find some evidence that it better predicts K response in some soils it will become publically available.  The moist test is a different test than the air dried samples, and the value of the field moist cannot be directly converted to an air dry test.  Because of this all new field calibrations will have to be established prior to any recommended use of the field moist test for potassium.  This is very important for anyone thinking of having sample run at a lab using field moist testing for potassium.  A few labs are currently running samples on a field moist basis, but until it is clearly demonstrated that the field moist test better predicts potassium response it will not be recommended for use in the state of Minnesota.

We currently are in the process of establishing a soil test sentinel program to study changes in soil test over time across the state of Minnesota.  More information will be available in another e-news release on this program.  One of the factors we intend to study is the difference between field most and air dried samples for testing for potassium and how they change at a fixed point in space, over time.  What this program entails is taking soil samples from a fixed area of any field every 4 to 6 weeks and sending the sample to Daniel Kaiser at the Saint Paul campus.  What we are attempting to research is how soils from around the state vary in the difference between the two tests to get a better understanding if and where problem soils may be found.  With the diversity of soils this it is important to know how soils may differ.  Support for this program is coming through check-off dollars supported by AFREC.  Additional information will be available and can be obtained by contacting Daniel Kaiser at

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  1. Dan,
    Good explanation of the K test and the work you are doing. It should be mentioned that the best technique we have right now is reviewing the soil test history on a field to come up with more accurate recommendations. Thanks for all your work on this!!


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