The dry soil sampling conditions this fall has led to a variety of questions focusing on when clients should stop pulling soil samples. The soils need to be very dry for an extended period of time before the impacts to soil sample data become noticeable. The first indication is when you are unable to collect a soil sample to the proper depth, especially with a probe sampler. Often an auger sampler will do a better job in dry soils. Incorrect sampling can have a bigger impact on soil test results than the dry soil itself.
A good way to make a determination on when to stop soil sampling based on drought conditions is to use the USDA drought monitor. Drought maps can be found at https://droughtmonitor.unl.edu/. The actual impact of drought on soil sample data will vary based on cropping system, region of the county, soils, etc. The following is a simplistic reference to help determine when drought conditions may bias soil test results. D0 drought should have little to no impact on soil test results providing a proper soil sample can be collected. However, D0 drought can begin to make sampling some fields with a probe challenging. Late D1 and into D2 drought is when you may consider stop sampling. This is about the point in the drought continuum when sampling and drought bias on soil test results can become noticeable. D3 is a time sampling should cease. If you have any questions reach out to your ALGL Regional Agronomist.