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ISSUE 15   August 10, 2000



    The NDSU recommendations for soil sampling for N are that soil sampling may begin after August 1 following small grain harvest. A previous crop credit of 0.5 lb N per day prior to September 15 is either added onto the soil test N number, or subtracted from the N recommendation (either ending number is the same). So the maximum previous crop credit if a field is sampled on August 1 is 23 lb N.

    Some growers are reluctant to sample as early as August 1 because of fears of N release (mineralization) from biological activity beyond what is anticipated in the previous crop credit. However, NDSU research and independent checks by private soil testing labs verify that the 0.5 lb/day adjustment is a good estimate of N release.

    Early soil testing makes the most sense in small grain fields. Small grains, with a large carbon to nitrogen ratio in their residue tend to buffer changes in soil nitrate levels and tend to tie up N rather than release it into a free nitrate state during decomposition, which can take a very long time. Broadleaf crops, such as canola or field peas will tend to mineralize more quickly, so a late fall soil test will be more representative of spring soil test values.

    On a field to field basis, there were sometimes increases in soil nitrate levels between the fall and spring soil tests in 1999-2000. Our winter was not very cold, with relatively warm weather in the fall and early spring, so soil microorganisms were active longer. Most of these differences were following broadleaf crops, such as sugarbeets. Some of the larger differences were in fields with higher organic matter content (>3%). Differences were rather small following small grain crops and lighter textured soils. We currently do not have an organic matter adjustment in our soil test calculations, but some other states (Nebraska, for example) do.

    Although the uncertainty is small if soil sampling is conducted immediately following small grain harvest, the uncertainty may increase later in the fall, as fields are tilled and small grain seed germinates and results in significant regrowth in some years. When fields are tilled, it is more difficult to reach consistent depths with soil probes. It is also more difficult to sample for P (phosphorus), soil pH, K (potassium) and other nutrients important in the 0-6 inch depth because of inconsistencies of surface soil samples. A much better 0-6 inch soil core is obtained in unworked fields, so early soil testing is better in this regard.

    Sampling before significant regrowth is important because as much as 5-10 lb/acre of free N is taken up by seedlings and up to 100 lb N/acre has been reported in thick regrowth in some falls. Waiting to sample underestimates potential N when regrowth is present. Any regrowth (volunteer seeding following harvest) of small grains would be expected to be intermediate in N release the following spring, with much of the N available by mid to late June.

    Soil sampling is the most important tool available for estimating soil N levels in a field. Soil sampling is far more valuable than checking a neighbor to see what their level is or looking at county averages. In a year like many others when the farm economy is tight, soil sampling is more important than ever.

Dr. Dave Franzen,
NDSU Extension Soil Specialist

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