Some producers may be a little concerned with their lower soil test potassium levels measured this past fall.  So what happened to cause these levels to be lower than expected?

As most of you noticed, late summer and early fall was quite dry across much of the state.  While this was good for harvest and caused fields to be open longer than normal, it was not ideal for collecting soil samples.

In some areas, it may have even been difficult to collected soil samples to the appropriate depth, and where soil samples were collected the information found in soil test reports did not match expectations based upon historical soil test information.

When we talk about soil testing, we often express concerns regarding conditions at the time of sampling.  Samples collect when soils are too wet or too dry can alter soil test values.

This is especially true for soil test potassium and pH. In some soils, dry soil conditions can result in lower soil test potassium levels due to potassium fixation between clay particles, and in other soils it can also result in higher soil test levels due to potassium release as clays dry. Thus some of you may have found lower soil test potassium due to fixation.

Drought conditions may have also resulted in more of the potassium being retained in the crop residue.  The dry conditions did not allow for the potassium to be washed out of the decaying plant residue so it would show up in the soil test level.  This is especially true if you are soil sampling after corn as stover holds a considerably higher amount of potassium than soybean stalks.

Take home message – if your soil test potassium was a little lower than normal (and you experienced dry conditions this fall), do not panic, this is not unexpected.  If your soil test levels typically hover around the established critical level (between 100 and 125 ppm for most Ohio soils) and this fall you fell a little below, dramatically altering your fertilizer plans is not likely warranted. 

For more information on soil test potassium and variability, check out the following links:

Jim Camberato, Purdue University http://www.purdue.edu/newsroom/outreach/2010/101101CamberatoSoil.html

Manjula Nathan, University of Missouri http://ppp.missouri.edu/newsletters/ipcm/archives/fullissue/v20n21.pdf

Carrie Laboski, University of Wisconsin http://www.soils.wisc.edu/extension/wcmc/2005/pap/Laboski2.pdf