Dave Franzen,
Extension Soil Scientist
701-231-8884

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 nitrogen will be made available through mineralization (decomposition of plant tissue 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 nitrogen 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 nitrogen (N) released between harvest and September 15 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 soil test results are in hand and available soon enough to consider before hitting the field to prepare for next year's 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, % organic matter, zinc and other nutrients tested 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 sometimes discouraged from sampling right after small grain harvest is the risk of large amounts of N becoming available in the late fall. This risk is very small following small grain, because the 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, the decomposition of these residues progresses more rapidly than small grains. Plus, the activitiy 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 smapling date adjustment that works so well after 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 simply to 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).

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