ISSUE 14 August 6, 1998
SOIL SAMPLING SEASON
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, research in the past that supported the idea of a previous crop credit, and independent checks by private soil testing labs verify that the 0.5 lb/day adjustment is a reasonably good estimate of N release.
Last year, there was a large difference between N found in the fall and N found in the spring soil test. Most of these differences were following broadleaf crops, such as sugarbeets. Differences were rather small following small grain crops, with some exceptions following the mineralization of scabby wheat thrown out the back of the combine. Sampling after small grain harvest does not mean that growers should wade through soybeans or corn fields or other crops not yet harvested to get a jump on fall soil testing. In these crops, mineralization patterns may not be similar to small grains, so soil sampling for N analysis should begin only following harvest.
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 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.
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