When should I soil sample this fall?

When should I soil sample this fall?

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

This past summer’s drought has left some wondering when they should soil sample this fall. 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 very dry surface soil, 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 with 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. However, if the soil is moist enough to push a probe into the ground to the desired sampling depth consistently, it is likely that the soil has re-equilibrated. In some parts of the state, it is possible to take a good soil sample now. In others, it is difficult to insert the probe more than 4 to 5 inches; thus, waiting to sample these fields until more rain falls is appropriate.

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