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ISSUE 15   AUGUST 12, 1999


    It is more the rule than the exception that soil sampling begins in mid-September instead of immediately following small grain harvest. However, many growers miss an excellent window for soil sampling by waiting this long. The reason for waiting is the fear that additional N will be made available through mineralization (decomposition of plant tissues which results in release of inorganic nutrients to the soil). As a result of a number of studies NDSU uses a Sampling Date Adjustment to compensate for the small amounts of N released following small grain harvest. This is 0.5 lb N/day prior to September 15. This adjustment is very good at predicting the additional N released between harvest and freeze-up following small grains.

    There are a number of advantages in early sampling-

  1. Growers are more likely to actually use the test results to direct fall N application if tests are in hand and available soon enough to consider before hitting the field to prepare for next years crops.
  2. Soil sampling prior to fall tillage will result in a more consistent and reliable 0-6 inch sample core, particularly for P,K, soil pH and other nutrients exclusively analyzed on the surface core only.
  3. The fields are more likely to be sampled.
  4. Regrowth (germination and growth of a second "crop") will not hide available N from the test results.

    The reason growers are encouraged to sample early following small grains is that the risk of large amounts of N becoming available after harvest is very small. Small grain residue is very low in N. Breakdown of small grain residue by microorganisms actually results in a reduction in soil N levels which appears to offset and balance any release from additional organic matter mineralization.

    Sampling following early broadleaf crops should be delayed, however, until after September 15. Because broadleaf crop residue, such as field peas and canola are relatively rich in N, decomposition of these residues moves more rapidly than small grains and the activity of microorganisms is encouraged with these crops as opposed to being discouraged by the relatively small amount of N available following small grains. Therefore, it is unlikely that the 0.5 lb N/day sampling date adjustment that works so well in small grains can or should be used following short-season broadleaf crops.

    Of course, following long-season crops such as corn, soybeans and sunflowers the objective is to simply sample the fields before the snow flies, so early sampling is not an issue.

    For more information about sampling in general, refer to NDSU Extension Circular SF-990 (revised 1998).



    Newly published circulars on site-specific farming are available through Extension. There are four
circulars in the series which deals with different aspects of precision farming.

    1. SF-1176(1) What is site-specific farming?

    2. SF-1176(2) Soil sampling and variable-rate fertilizer application.

    3. SF-1176(3)Yield mapping

    4. SF-1176(4) Site-specific farming and the environment

    These circulars are in color and were funded entirely through a US-EPA 319 Water Quality project
which also helped to fund the research from which the information in the publications is based.


Dr. Dave Franzen
NDSU Extension Soil Specialist

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