Make Sense of Soil Test Variation
Soil test values are important indicators of soil chemistry. When properly sampled, soil test values will naturally vary by up to 10% from the “true” value. When greater year to year variation is observed, it is often caused by differences in sampling location, timing, or depth.
Sampling point location is often the biggest factor in soil test level variance. Because both soil type and past management practices (e.g. tile, manure spreading, pasturing, etc.) can have significant impacts on soil chemistry, Ag Spectrum emphasize ST/MZ-based soil sampling when implementing the Maximum Farming System. While improved georeferencing has largely eliminated this source of variation, the soil conditions at each location around the time of sampling are also important.
Seasonality impacts extractable soil chemistry, especially as it relates to levels of saturation and biological activity. Generally speaking, soil sampling is best done in the fall immediately post-harvest when the soil is relatively dry and before any new inputs have been applied. This makes for the most consistent sample state. Still, weather and practical constraints push some to sample later in the fall or early spring. This is not generally a problem except when samples are taken through newly applied inputs (like manure or lime) or substantial amounts of actively growing roots of fall cover crops.
Vertical sampling variation also can contribute to year-over-year soil test variation. However, it is only a significant factor when a) root and surface residue levels differ substantially between sampling dates, b) inputs (e.g. lime, gypsum, dry fertilizer, manures, etc.) are soil applied < 90 days before sampling, and/or c) erosion limits topsoil depth to < 6-8 inches on average.
As for lab-based variation, it is not frequently the cause of high sample-to-sample test variance, especially at well-managed testing labs. Lab personnel are responsible for ensuring that samples are properly air dried and mixed prior to performing individual tests, so there is always the risk of human error. However, most of those errors are caught and corrected before reporting back to the customer, and fewer than 1% of soil test results seem dubious enough to justify resampling or retesting.
Find a Maximum Farming System Associate to ensure your soil sampling is done effectively.
-submitted by Dr. Brian Gardener, Technical Director