Take a good soil sample to help make good decisions

By Dave Nicolai, University of Minnesota Extension

Soil tests can be no better than the sample. Therefore, proper collection of the soil sample is extremely important.

< Divide the field into uniform areas.
Each area should have the same soil color and texture, cropping history, and fertilizer, lime and manure treatments. The use of soil survey maps that indicate soil types, slope phase and erosion phase are useful because organic matter, nutrient levels and soil pH can vary following these map units. One sample should not represent more than 20 acres on level, uniform landscapes, or five acres on hilly or rolling land. Soil sampling using an intensive grid pattern of at least 2 1/2 acres in size is another option for growers working with a crop consultant or agriculture professional.

< Sample each area for composite samples.
Within each area collect 15 to 30 subsamples (cores, borings or spade slices) in a zig-zag pattern throughout the designated field area. The more variable the soil, the more subsamples should be combined per area sampled. Scrape off all surface residue from the subsample site. Sample to a depth of 6 to 8 inches (plow layer) for cultivated crops, or three inches for pasture or sod fields. Sample row crop fields between rows, except for ridgetill plantings. Where ridgetill is used, take the sample to a depth of 6 to 8 inches on the shoulder of the ridge, avoiding the starter fertilizer band. Also avoid sampling dead or back furrows, terraces, old fence rows, lime or fertilizer spill areas, headlands, eroded knolls, low spots or small saline areas. Sample at least 300 feet away from gravel or crushed limestone roads because their dust changes soil pH.

< Mix the subsamples.
Mix the subsamples thoroughly in a clean plastic pail, and fill the sample box or bag to the fill line (1 pint). If the samples must be taken wet, they should be dried before being mixed and submitted to the laboratory. Do not exceed a drying temperature of 97 degrees, and do not use a microwave unless only the nitrate test is requested.

While different soil testing laboratories may offer different soil test options, the procedures for conducting the actual tests typically tend to be similar. Listed below are the soil test options offered by the University of Minnesota Soil Testing Laboratory which can be used as a guide even if you are using the services of a private soil testing laboratory:

< Regular series: Includes phosphorus, potassium, pH and lime requirement, percent organic matter and texture. Sample the plow layer for cultivated land, or to three inches for permanent pastures or sod fields.

< Special test: Includes sulfur, zinc, copper, iron, manganese, boron, calcium, magnesium, percent of organic matter and soluble salts (electrical conductivity). Copper recommendations apply only for peat or muck soils. Research has shown that for Minnesota soils, tests for iron and manganese are not practical; they are included because of requests for the test. These tests are to be determined only on the plow layer sample.

< Nitrate test: This test requires soil be collected to a depth of 24 inches. There are two options. One is to submit two samples, 0- to 6-inch and 6- to 24-inch depths. The second is to collect the soil from 0- to 24-inches. The test applies to non-sandy soils in western Minnesota with an exception noted below. This test is preferred for making nitrogen recommendations for the counties west of and including Lake of the Woods, Beltrami, Becker, Otter Tail, Douglas, Pope, Kandiyohi, Renville, Redwood, Cottonwood and Jackson counties. In these counties, the test is used in making nitrogen recommendations for corn, small grains, potatoes and sugar beets.

If you’re in western Minnesota and you want the regular series test or any of the other special tests in addition to the nitrate test, take samples from 0- to 6-inches and 0- to 24-inches and enter on separate lines on the sheet.

For the counties east of those cited above, the soil nitrate test is used only if the sample is collected in the spring before or near planting (April 1 to June 15). Nitrogen fertilizer recommendations will not be based on the analysis of only plow-layer samples for nitrate-nitrogen. If only a plow-layer sample is submitted, nitrogen recommendations will be based on cropping history, intended crop, yield goal and soil organic matter level.

Samples collected for the nitrate test must be air dried immediately to slow down microbial activity and sent to the laboratory within 24 hours. Drying can be accomplished by spreading the soil in the sun or placing near a heat source. If only nitrate is to be determined, the samples can be dried in a microwave oven using several 2-minute power cycles, stirring between each cycle. Alternatively, samples can be frozen and sent to the laboratory in a well-insulated package.

(Dave Nicolai is a crops Extension educator with the University of Minnesota Regional Extension in Hutchinson.)