Soil Test Values Can Vary Across Years

This article was originally published by the Noble Foundation's  Ag News and Views, Ardmore, OK and was written by Eddie Funderburg. A routine call we get involves a person who takes a soil sample this year and submits it for analysis. The data do not match the results of the last sample that was taken from the field three years ago. The logical question is, "why?" The usual reason is that soils vary in pH and nutrient content across the field, so the results will be different if subsamples were taken from different parts of the field in each sample. However, there are other possible reasons, and those are the ones I want to analyze in this article. The depth at which the samples are taken is critical. Soil labs assume the sample is taken from a depth of 0 to 6 inches unless they are told otherwise. Sometimes, real-world samples are not taken from a 0- to 6-inch depth. We primarily work in pasture and hayfield settings. When the soil is dry, it is difficult to get a probe in the ground deeper than 2 inches without breaking or bending it. If the soil is wet, we may go considerably deeper than 6 inches simply because we can. Why is sampling depth critical? Nutrient levels will usually be higher in the upper part of the soil in pasture and hayfield situations, as well as in no-till situations. Fertilizer and manure are placed on top of the ground, and plant roots cycle nutrients from deeper in the soil profile to the soil surface when the plants decompose.

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