Tools of the trade: Soil sampling

November 3, 2016 / by MN Soybean Director of Research David Kee Categories: David Kee, Minnesota Soybean Research & Promotion Council, Soybean News

It is fall, time to dig out your sampling probe and go collect samples. Many of you will be sampling soils for nutrient analysis, others for soybean cyst nematode, some for both. In any situation, your goal is to collect a representative sample to provide a good estimate for whatever you need determined.

A field map divides your field into management units or areas you plan to manage similarly. This similar management will depend entirely on your specific farm. Some of you can change fertilizer rates or varieties on the fly, others of you can only plant one variety at a time, or do minimal changes.

Your field map should include soil associations, known abnormalities (old roads, fence lines, accidental over applications of manure, etc.) that might affect the measurements. These abnormal areas may not be sampled or sampled separately. It depends on what you consider the management unit, and what is important in that management unit. Generally, abnormal areas are noted, and then ignored.

Field maps are just the beginning of the project. Your samples should be placed in labeled containers and coded in such a manner that you know what sample goes to what area of the field. The implement used to collect a sample should be clean and contaminant free prior to collecting the sample. During the sample process, a cleaning regimen should be implemented so the sampling implement stays as containment free as possible. A good soil sample program requires almost as much time preparing to sample as collecting samples.

As you put together your field map, determine the sampling pattern. The sampling pattern depends on you, your system and your desired outcome. Random, zone or grid are all acceptable methods. Each has its strengths and weaknesses, the method chosen should be based on your unique situation.

Random means just that, random. The collector randomly covers the field, collect 5 – 25 subsamples, composite, prepare your sample, and send it off for analysis. With a zone pattern, the collector looks at the map and the field, divides the management area out into separate, distinct zones. These zones are usually associated with geography: the hill top, hill side or bottom ground. However, the zone can also be associated with cropping history, manure application or other man made boundaries. With a grid pattern, the field map is subdivided into grid points, commonly at uniform distances, and samples gathered within the grid unit.

Regardless of the sampling pattern, you should collect at least 5, preferably more, subsamples to composite for the analysis sample. Some people will recommend a Z, W, M or other pattern.   Others will use a random around that particular point. The whole goal of any sampling procedure is to have results that emulate the region around the sample point or zone. A single sampling point can greatly skew the results, consequently collecting a group of subsamples to composite into a single sample greatly enhances to probability of collecting a sample that accurately represents the region.

After the sample is collected, placed into a properly labeled sample container and then delivered to the appropriate lab. Once you get the results, sit down with your data and the real fun begins.

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