Soil Sampling: Spring or Fall?

When the best time of the year is for soil sampling, fall or spring? This is the question often asked. Personally, I prefer spring and there are some good reasons why. I realize that spring sampling does not provide much time for making decisions if they need to be made during the spring seeding season. But your results may be more indicative of what is available for the growing crop at hand; what the plant roots actually see.

Common comments are fall sampling is often recommended because labs may not be as busy, decisions can be made for fertilizer purchased before the end of the year, and it may be easier to collect samples (unless the soil is dry and hard). Much of the time in the dryland wheat areas of Eastern Washington we have very dry and hard soil. This can be a painful soil sampling experience.

Typically, we advise you collect soil samples at the same time of year each time. That way you have taken out one of the variables, meaning you should get a more accurate read on whether your fertility levels are truly rising or falling over time. Be consistent though, a key with soil testing is to be as consistent as possible for year over year comparisons. If you always soil sample in the fall, keep going with that practice. It is the same with spring sampling, be consistent.

If testing in the fall or in the spring, your fall vs. spring results could vary significantly. Soil test levels could actually rise in the spring, compared to fall. A study from the University of Kentucky (AGR-189) found that soil values vary seasonally and that they tend to be lowest in the fall. Of the commonly reported soil test measurements, nitrogen (N), phosphorus (P), potassium (K), and pH are the most affected by seasonal fluctuations.

Spring and early summer may the best times to sample your soils.  Here are reasons why:

  1. Soil recovery. In spring, soils have had a chance to recover from the previous season’s workload, such as:  harvest compaction, excessive or not enough rain, tiling events, waterway shaping, and fixing washouts.  Spring affords a much better environment to collect a good quality, representative soil sample.
  2. More accurate soil sampling. In spring, nutrients from residues have had the time and moisture to re-enter the soil rooting zone.  This is where samples will be taken, and therefore these nutrients will be reflected in your spring soil sample, showing you more accurately what nutrients are available for the upcoming crop.

Clain Jones, Extension soil fertility specialist and professor in the Department of Land Resources and Environmental Sciences, said MSU soil fertility guidelines are based on spring soil sample results. Spring sampling is ideal, he said, because spring levels are more indicative of growing season nitrogen availability, compared to fall nitrogen levels. I’ve become a big fan of spring sampling because soil moisture is much more consistent, and soils seem to have more time to equilibrate after the growing season.

Conclusion:

Soil sampling and testing can and should be highly informative for the agronomist and the farmer. Information from a well-conducted soil-sampling event can be useful in monitoring changes in soil fertility, developing fertilizer recommendations, and improving on-farm nutrient efficiency. In a dry climate that we farm in soil moisture can make a huge impact on the soil test results. Regardless of the soil-sampling program you employ, soil sampling using a consistent, well-conducted, and organized approach will lead to the most usable and informative soil test results.


For questions or comments, contact Paul Carter by email at cart@wsu.edu in the Department of Extension Ag and Natural Resources, Washington State University, located in Dayton WA.
Washington State University