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Corn+Soybean Digest

Take Time To Soil Sample Correctly

Have you ever thought about what a soil sample is? More importantly, have you ever thought about what it represents? If you follow Ohio State University Extension guidelines and collect 15 cores to constitute a composite for every 20 acres, you will likely collect around a pound of soil. Were you aware that the weight of soil over an acre to a depth of 6 in. is around 2 million pounds? So you are collecting a very small sample to represent the average of a larger area.

Statistics reveal that 15 cores are adequate to accurately represent the average soil test level for this larger area, but you do need to make sure that the sample is representative of the bulk of your field. We often discuss the need to collect samples from “representative areas”. You as a producer know where those areas are within your fields, and more importantly you know where the “troubled” areas are. Those troubled areas should be sampled separately to ensure that they do not influence the average information you are collecting. This also allows you the opportunity to possibly identify why the “troubled” area is less productive, if it is related to soil fertility.

From a lab perspective, errors associated with soil test estimates can be minimized. Sample weighing, extraction time (if the sample requires chemical extraction) and instrumental analysis do have errors associated with them, but they can be minimized. The easiest way to introduce greater uncertainty is to do a poor job sampling.

Beyond what was discussed in the previous paragraph, sampling depth and moisture regime at the time of sampling is critical to the accuracy of the information you collect. Thus sampling to a consistent depth is critical to get an accurate representation of the soil’s actual nutrient supplying power. There is no preferred sampling time (comparing spring to fall), but be consistent about sampling time. Additionally, avoid moisture extremes (too dry or wet) when collecting soil samples.

After receiving the analysis from the lab, go the Tri-State Fertilizer Recommendations online to determine the rate needed to ensure maximum productivity. If soil analysis reveals the soil is well above the established critical level, consider not applying that nutrient this fall. If below the critical level, the nutrient should be applied to ensure it is not yield limiting.


  • Collect a minimum of 15 cores per composite.
  • Be consistent about sampling depth.
  • Be consistent about the timing of sample collection and avoid moisture extremes.
  • Avoid contamination. Use clean probes for sampling and clean buckets for mixing.
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