Soil sampling do’s and don’ts

crops and forages

Soil sampling do’s and don’ts

by Ev Thomas and Bill Mahanna
Thomas is retired from the William H. Miner Agricultural Research Institute and president of Oak Point Agronomics Ltd.; Mahanna is with DuPont Pioneer and an adjunct professor at Iowa State University.

A soil analysis is only as good as the sample on which it was based. Therefore, sampling should be done by either the farm’s crop consultant, the farm owner or a trusted employee who has been trained in proper sampling techniques.

There’s no magic number for how many cores (subsamples) should be taken to form the composite submitted to the soil testing laboratory; the more cores taken, the more representative the analysis. But as a suggestion, take 15 to 20 cores from fields of less than 20 acres, removing any vegetation or crop debris from the surface of the sampling site. Separate fields larger than about 20 acres into at least two units for sampling, especially if there are differences in topography, soil type or previous field history including manure applications.

Sample to the depth of the plow layer, usually 6 to 8 inches. When sampling no-till fields, take two samples, one from a 0 to 2-inch depth and the other from 2 to 6 inches. The 0 to 2-inch sample is primarily to check for any changes in pH due to nutrient applications.

Mix the cores thoroughly, breaking up any clods and discarding stones and debris. One acre of soil at plow layer depth weighs about 2 million pounds, so a soil sample from a 10-acre field represents 1 pound in 20 million. That’s why proper sampling is critical.

Be sure to label soil samples in such a way that, at a later date, you’ll be able to link the analysis with the correct field. This sounds simple, but too many times a farmer submitting five soil samples will label them one through five. This is fine until he samples five other fields the following year and also labels them one through five!

What time of year you sample is important, especially for soil potassium status. This is more important where soils freeze during the winter, but it’s a good idea to sample at about the same time each year regardless of where you farm.

A good time to sample is during the fall after harvest has been completed for the season; there’s not the pressure of spring work, no standing crops to deal with, and fields are usually dry enough to support a four-wheeler or pickup truck. This may also provide time for nutrient or lime applications after the results are back. Sample each field at least once every three years; more often if you’re attempting to correct serious pH or nutrient deficiencies or if yield (and therefore nutrient removal) is very high.

You’ll also need to decide on where you’ll submit the samples. Your crop consultant may have a preferred soil testing laboratory, but our preference is to choose a reputable lab and then use the same lab every year. That’s because not all labs use the same soil extraction chemicals, and different extractants can result in very different “numbers.”

Most commercial soil analysis laboratories do a good job of determining nutrient status: That is, whether a field tests high, medium or low in soil pH, P, K, and some secondary and micronutrients. However, soil analysis for some micronutrients is much less reliable, so before you spend a lot of money for micronutrients based on soil analysis you should discuss the results with your crop consultant or extension educator.

It may be necessary to back up the soil analysis with a tissue analysis. Also, a fall soil analysis may not be reliable for water-soluble nutrients such as sulfur — the analysis itself may be accurate but that may not equate to the availability of the nutrient by the following crop season.

There are two parts to a soil analysis report: The chemical analysis and the nutrient recommendations based on the analysis. How reliable the recommendations are is influenced by the data provided on the information sheet submitted with the sample.

Be very suspicious of the fertilizer recommendations if the information sheet didn’t include a section for you to indicate past, present and planned manure applications! The more information on field history, soil type and crop management provided to the soil test lab via the information sheet, the better the job the lab can do in making recommendations.

Don’t assume that university soil test labs routinely recommend less fertilizer than do commercial labs or even the soil test labs operated by fertilizer companies. We’ve seen cases where, with the same soil test values, the fertilizer company recommended less fertilizer than did the university lab.

An advantage of using the same lab year after year is that you can compare soil analyses over time. If, for instance, you’ve been topdressing 300 pounds of 0-0-60 per acre each year on established alfalfa but soil test K on these fields is declining, that’s an indication that you may need to raise your fertilizer rate. This assumes, of course, that you’ve done a good job of soil sampling each year — which brings us back to where we started.

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