Until the point of soil freeze up, late fall through early winter provides a window of opportunity to get out into fields and pastures and collect soil samples. The advantages of sampling at this time of year are that often there is more time for this task now as compared to the spring season, and there is time to analyze soil test results, make decisions and line up lime and/or fertilizer applications. By far the greatest issue with soil testing is collecting a representative sample to send in to the lab. An acre furrow slice of soil, generally defined as a depth of soil 6 to 7 inches deep across one acre (43,560 square feet), weighs in the neighborhood of two million pounds. A soil sample sent in to a lab is typically about one pint of soil. Now consider that we ask that one-pint soil sample to represent as much as 20 or 25 acres. Can you see where the confidence placed in soil test results may waver?
The accuracy of a soil test and the resulting confidence we place in the lab numbers depends upon representative sampling and multiple subsampling. The proper tool for soil sampling is a probe. The Wayne County Extension office has soil probes that can be borrowed to collect soil samples. Using a probe allows a uniform core of soil to be collected from each sampling location. Mark the probe to insure that each core sample is collected to the same depth. Push out the plug or core of soil into a clean plastic bucket. After all the subsample cores are collected, mix them up and from that mix fill the container that will be sent to the lab. A soil sample should be composed of at least fifteen randomly collected subsamples that represent a field area of similar crop/plant growth response. Soil test result accuracy is improved if 20 to 30 subsamples are taken. No individual sample should represent more than twenty-five acres.
The standard sampling depth is 8 inches and most lime and fertilizer recommendations are based on an 8-inch sample depth. However, in no-till situations nutrients such as phosphorus and potassium stratify over time and accumulate in the top two to four inches of the soil profile. Under continuous no-till management, sampling to a 4-inch depth is recommended.
Grid and zone management are the most commonly used soil sampling methodologies. Grid sampling involves sampling at regular intervals across a field, typically in patterns that represent 2.5 to 5 acres in a field. Generally, 8 to 10 cores are collected from the sample point within each grid. Sampling locations are mapped with GPS coordinates. Zone management works especially well when field yield history patterns are known. Use that knowledge to map out management zones that define individual soil samples, in areas up to twenty-five acres. Often those management zones also define sample areas by topography, soil type differences, drainage etc. Within each management zone, collect 15-20 core samples by zigzagging randomly through the area. If possible, record GPS coordinates for each subsampling location. This allows samples to be collected from the same or nearly the same locations in future years when submitting new samples. The trend line information provided is valuable.
An Ohio State University Extension fact sheet entitled “Soil Sampling to Develop a Nutrient Recommendation” is available online at https://ohioline.osu.edu/factsheet/AGF-513 or contact me at the Wayne County Extension office at 330-264-8722 and I can provide a copy of the fact sheet.
Rory Lewandowski is an OSU Extension Agriculture & Natural Resources Educator and may be reached at 330-264-8722.
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