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Soil Sampling Tips (08/12/21)


Due to the large amount of residual nitrate left after the short crops in the state, there will be more soil sampling than usual. Here are a few tips to increase the practical use of the results from sampling.

  1. The soil may be hard where there has not been recent rain. It will be important for the sampler to make certain that the core is the correct depth and not to give up if the going gets tough. If a core comes out of the sampling tube incomplete, discard it and redo the sample a few feet over.
  2. Don’t work the land before the sampler arrives. The ability to take a 0-6 inch core required for P, K, organic matter, zinc, and surface nitrates (necessary for sugar beet recommendations) is much degraded if the soil is worked, particularly after chisel plowing or (God forbid) deep chiseling or plowing.
  3. When sampling, do not take samples from the headlands, or the turn rows around sloughs, because these areas have legacy overlap inherent with fertilizer/manure application. Make a mental note not to sample within 100 feet roughly of the edge of the field or around obstacles that would have resulted in applicator turning. The exception to this is in zone sampled fields with salty areas next to roads/sloughs. These should be sampled as a zone, because the NPK fertility in these areas will probably be vastly different than the rest of the field and fertilizer savings can be achieved by identifying these areas.
  4. To build zones, use multi-year yield mapping data, aerial imagery of growing crops, satellite imagery from growing crops, soil EC or EM sensor data if available, and topography if it can be identified and properly modeled (raw elevation data should not be used as it usually does not indicate a landscape position). The Web soil survey should not be used as a first zone development tool. It was not designed for this purpose and the boundaries are usually misleading.
  5. Know that nitrate sampling, or sampling for K for that matter, are moving targets and there is a plus-minus value at the end of analysis. Persistent dry weather after sampling will result in a spring value that is similar in nitrate to what is found now. Wetter weather may result in a bloom of nitrate, but this late in the season the increase in nitrate will be small, if seen at all. Nitrate may be immediately tied up in residue breakdown, with the awakened microorganisms that have been dormant all season. K values are at their lowest now through early September and if it stays dry, the K values will be low through fall. However, freeze-thaw and any moisture during winter/early spring will increase values, so next April the highest K values of the year are seen. It is best to analyze for K at about the same time during the year each time K is analyzed (it doesn’t have to be every year) to make sure that the relative values of K can be tracked and not be confusing.


What did the soil used to look like?

The annual 4-H/FFA Land Judging competition was held last week on Forest River Community land northwest of Inkster, ND. The competition consists of 4 practice soil pits, about 12 feet long, and about 4 feet deep that contestants can work with their teams on in the morning and 4 contest soil pits of similar dimensions in the afternoon. As one of the soil card scorers, I was particularly excited to see a buried ‘A’ horizon in one of the practice soil pits. It was located on the north side of a low ridge. I speculate that during one or a series of major dust storms about 100 years ago, the soil surface, protected from the destruction by the low ridge, was buried by particles streaming and churning by. The remaining A horizon was buried by 1 to 1.5 feet of wind-blown sediments and has retained much of its original organic matter.

I took a sample of the buried A material and had the NDSU soil testing lab analyze the organic matter. It came back at 5.9%. The soil all around it ranges from 2 to 2.5% organic matter today. But 100 years ago, this soil had organic matter at least a foot thick with at least 6% organic matter. There should be no mystery why, when weather was favorable, the 1902 soil survey of Grand Forks County stated that some fields averaged 40 bushels per acre of spring wheat (without any fertilizer).                                                                                                                                                                                                           


Soil pit northwest of Inkster showing a former ‘A’ horizon


Dave Franzen

Extension Soil Specialist