Fall soil sampling: Another challenging prospect?

Carrie A.M. Laboski, Extension Soil Fertility/Nutrient Management Specialist

The dry conditions over the past four plus weeks may be giving you flash backs to 2012 and leave you wondering when you should soil sample this fall. Keep in mind that sampling very dry soil may provide erroneous soil test results for several reasons:

  1. It is difficult to sample to the desired depth consistently.
  2. The soil core does not stay intact, particularly when the surface soil is very dry, and some of the soil is lost between taking the probe out of the ground and placing the sample in the bucket.
  3. Soil test P and K may be lower with smaller differences for P and larger differences for K.
  4. pH may be slightly lower because of salt build up from a lack of rain.

Once rainfall has occurred, soils will begin to re-equilibrate and the effects of dry conditions on soil test P, K and pH will diminish. It is hard to provide an exact amount of rainfall that is needed to alleviate the effects of dry conditions on soil test results because it depends upon how dry the soil was, soil mineralogy, and likely other site specific conditions. Wait to soil sample until fields are moist enough to easily insert a probe 2 inches below your desired sampling depth. Doing so should help ensure a consistent depth of sampling and adequate re-equilibration.

Last year the guidance was to wait to sample until the probe could be inserted to the desired sampling depth. Feedback from some agronomists using this guidance suggests that waiting a little longer to sample may be prudent. Thus, the new guidance to wait to sample until the probe can be inserted 2 inches below the desired sampling depth.