MICHIGAN'S SOIL NITRATE TEST FOR CORN
2005

MSU SOIL AND PLANT NUTRIENT LAB
Michigan State University Extension,
Crop and Soil Sciences Department, Michigan State University

WHY TEST SOIL FOR NITRATES
            Nitrate testing of soil is an excellent and inexpensive way of evaluating the available nitrogen (N) status of your soil.  Michigan State University research and demonstration studies have shown that many farmers could reduce their N fertilizer application rate on corn without risk of reducing yields if they used the nitrate soil test.  Nitrate testing provides credit for available nitrogen already in the soil and therefore helps to prevent over-use of N fertilizers.  Excess N fertilizer use adds cost and may increase nitrate contamination of groundwater.

WHAT DOES THE TEST MEASURE
            The soil nitrate test measures only nitrate N.  It does not measure ammonium N or organic N.  If samples are taken in late May or in June, much of the ammonium and some of the organic N will have been converted to nitrate and will show up in the test.

WHEN TO TAKE SOIL SAMPLES
            Soil samples may be taken any time, however, samples taken in late May or in  June after the soil has warmed-up usually contain the greatest amount of nitrate N.  At this time the test measures both residual nitrate N from the previous year and recently mineralized N from ammonium and organic matter (plant residues and manure).

            Soil samples taken in early spring (April or early May) measure primarily residual nitrate, therefore the amount of N credit will be smaller.  However, testing in early spring may still be well worth the effort.  Testing for ammonium in early spring where manure has been applied will provide a preliminary indication of available nitrogen release. 

            Samples taken just prior to sidedress time can be used to the greatest advantage to determine the appropriate rate of sidedress N.  When sampling fields where anhydrous ammonia or liquid N has been knifed in preplant, you will need to double the number of sub-sample cores to avoid problems associated with sampling N fertilizer bands.  The test provides the best information about the available soil N status when small amounts of nitrogen (less than 40 lb N/A)  have been broadcast preplant.

            Samples taken in June from fields where N has been broadcast prior to planting can be used as a guide to adding additional N through the irrigation system or for planning next years application.  If the soil test in June indicates more than 25 ppm, no additional N is needed.  Soil test levels in excess of 40 ppm at this time indicate excess soil N.

            Samples taken in the fall may be used to evaluate how much N is left at the end of the season.  Soil test levels in excess of 15 ppm at harvest indicate excess soil N.

            Growers who have excess soil N in June or at harvest time, should consider reducing next years fertilizer rate or use the presidedress soil nitrate test (PSNT) to determine the appropriate N rate.

WHAT FIELDS SHOULD BE SAMPLED
            Sample all fields where corn is to be planted.  Manured fields and legume fields sampled in June will likely contain the most nitrate.  Sampling these fields early, however will not result in the maximum N credit that should be taken because ammonium N and easily decomposed organic N will not be measured by the test.  See Extension Bulletin E-2344 and E-2567 for determining the appropriate N credit from manure and legumes if samples are taken in early spring.

            Other fields that show the most nitrate N are fields with fine textured soils (i.e. loam, clay loam and clay) that were heavily fertilized with N the previous year.  Sandy soils even though heavily fertilized the previous year may not show much N carryover.

WHERE TO GET SOIL SAMPLE BOXES
            Soil sample boxes for nitrate testing and information on taking soil samples for the nitrate test are available from your County Extension office or the MSU Soil and Plant Nutrient Laboratory, East Lansing, MI  48824-1325.  ph. 517-355-0218

HOW TO TAKE SOIL SAMPLES
            Each sample should be a composite of 15‑20 soil cores taken from a uniform field area no larger than 20 acres.  Use a soil sampling probe.  Take each core to a depth of 12 inches.  Place the 15-20 cores in a clean pail and mix thoroughly.  Save one quart of the soil sample for drying.

HOW TO HANDLE SOIL SAMPLES
            Air dry the sample immediately in a warm room.  Placing the sample near a hot air vent or space heater will speed up the drying process.  Do not hold wet samples in a plastic bag for any length of time.  Microbial action in wet samples can significantly change the nitrate test results and the sidedress N recommendation

WHERE TO GET SAMPLES ANALYZED
      See your County Extension office.  Dry soil samples may be mailed to the MSU Soil and Plant Nutrient Laboratory, East Lansing, MI  48824-1325.  If the samples are still moist, do not put them in the mail.  Moist samples should be transported directly to the County Extension office or to the MSU laboratory for drying.

NITROGEN RECOMMENDATIONS FOR CORN
      The following table gives N recommendations for corn at four yield goal levels and seven soil nitrate test levels.  The amount of N credit given is obtained by multiplying the concentration (ppm) of nitrate in the surface soil sample by a factor of 6.  EXAMPLE:  If the test shows 10 ppm of nitrate in the soil sample, the estimated N credit is 60 lb/A.  Reading across in the table we obtain the adjusted N recommendation of 100 lb N/A for 140 bu/A yield goal.

                        N Recommendation = - 27 ÷ 1.36*YG - (6*ppm NO₃-N).

ECONOMICS OF NITRATE TESTING
      The cost of a soil nitrate analysis by the Michigan State University Soil and Plant Nutrient Lab is $9.00 per sample ($12.00 for ammonium & nitrate).  Each sample should represent no more than 20 acres.  Assuming it costs $6.00 to take the sample, the total cost is $15.00 to $18.00 per 20 acres or 75 to 90 cents per acre (less than one-half bushel corn).  At 33 cents per pound of N, each 10 lbs per acre of N credit  is worth $3.30.  A 60 lbs N/A credit is worth $19.80 /A.  This is a good return on investment.  Using the correct amount of nitrogen also has value in terms of good water quality.  Results from research and on-farm demonstration studies indicate that nitrogen adjustments based on the soil nitrate test would return at least $5.00 per acre in either cost savings or improved yield.