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Soil Sampling Following Early Crop Harvest (08/13/15)

Soil sampling is encouraged immediately following early crop harvest, including winter wheat, barley, spring wheat, any wheat, rye, canola and early flax. The following are concepts to keep in your head during interpretation for next years’ crop:

Soil Sampling Following Early Crop Harvest

Soil sampling is encouraged immediately following early crop harvest, including winter wheat, barley, spring wheat, any wheat, rye, canola and early flax. The following are concepts to keep in your head during interpretation for next years’ crop:

                -  Soil nitrate may or may not increase or decrease as the fall season continues. Work by Dahnke and Swenson in the 1970’s, where soil samples from the same area across ND were sampled from summer to major winter freeze-up and on into spring showed that sometimes nitrate increased, sometimes decreased, and sometimes stayed the same. I tried to relate it to soil moisture content and didn’t get anywhere. So anytime is a good time for nitrate sampling and earlier means it gets done.

                - The majority of our soil nutrient factors, P, K, pH, Zn, rely on a 0-6 inch soil test. Anyone care to tell me how a consistent 0-6 inch core can be taken following chisel plowing wheat stubble? Anyone? The best I can do is drive over the intended soil sample area, then probe into the track. Is this representative of the depth that the soil test is calibrated for? I think so, but sampling immediately after the combine is better. I believe that some of the variability in P tests is sometimes due to inconsistent soil core depth because of having to sample worked fields.

                - Potassium soil test values vary with monthly time of sampling. A number of potassium researchers, including myself, are helping to rewrite the text books on available potassium. Dr. Don Sparks at University of Delaware was the voice in the wilderness 20 years ago, followed by Dr. Mallarino at Iowa State recently, followed by me and others, including Dr. Kaiser from University of Minnesota. Potassium nutrition is complex. Clay chemistry plays a much larger role that taught in school, and minerals such as potassium feldspar, which are prevalent in many ND soils, contribute much more to seasonal plant nutrition than previously thought. Algorithms for K changes during the growing season are still being developed at NDSU and I will not be comfortable with any recommendations until after next season; but we know that soil K is highest in the early spring, with rapid or gradually decreasing K into June, holding relatively steady at a low level until fall when rain leaches K from standing stubble before and after grain harvest which causes soil K values to again start to increase. Work in Illinois by my former PhD mentor Dr. Ted Peck showed that winter freezing and thawing and the frosty mud characteristic of Illinois winter produced the highest K values.  So if one was planning to soil sample for K, choose a time period that would be consistent in the rotation. Potassium need only be sampled for in most soils (except low K sands) once every 3-4 years. Choose a time period and stick with it.

                - Soil P, Zn, and pH are relatively stable over a growing season (except pH after N application), so time of year is not very important for them.

Dave Franzen

NDSU Extension Soil Specialist


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