NDSU Extension Service - Ramsey County

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July 23, 2012 Agriculture Column

Howdy!!!

The area Extension/Research days are now over and attendance was very good.  I attended Langdon field days and was in conjunction with Canola days.  Aster Yellows in Canola was the top of the conversation pieces during the discussion of the day.  The aster leaf hoppers were in and did their damage and there is no recommendation on timing as the leaf hopper can do their damage in a matter of 30 minutes.  For those of you not knowing what the result looks like, it is a pocket of material on the plant.  It could part or all of the plant.

Soil Sampling After Small Grain Harvest

Most years the small grain harvest, especially wheat, happens in mid-August. This week harvest will start probably the last full week of July- about 3 weeks early.

So the question is whether soil sampling can be done well that early. As an agronomist in Central Illinois for the first half of my career, I sampled standing corn and soybean fields starting in mid-June and kept going all the way past harvest. Soil P values are stable for a long time and are not influenced in this region hardly at all by soil moisture and other environmental and cultural factors. So early doesn’t affect them at all. In fact, sampling for P before the field is tilled would be especially helpful since after tillage the sampler has to guess what really is ‘the 6-inch depth’. Is it whatever gets into the probe? Does the sampler make an adjustment for the fluffiness or cloddiness of the worked soil to estimate what a 6-inch depth really is? Does the sampler take the core over the wheel track of the sampling vehicle, or take time to use a hand-probe and compact the soil with a foot before using the probe? There are lots of ifs and what-ifs after tillage for the 0-6 inch depth.

Sampling for potash, however, is trickier. Due to some work in Illinois where my former major professor, now deceased, Ted Peck, and his assistant Marilyn Sullivan sampled the same plots every two weeks throughout the year for many years, we know that K levels change with the season. The K levels are highest in the early spring after freezing weather and high soil moisture, and then lower to their lowest point in August when the soil is driest and the crop uptake (usually corn or soybean in Illinois) is at a peak. The K test then increases later in the fall when the soil usually wets up and there are frosts. The soil where most of the data survives came from a Flanagan or Drummer soil, which are high in smectitic clays, similar in composition to most soils in North Dakota. Therefore, I would expect that our soils would behave in a similar manner.

The take home lesson is that K sampling should happen at a similar time every year to be true to real trends. If you sample after wheat for K in August, you might need to be willing to walk out into a bean field in August next year to sample about the same time. If you decide to wait until September, you probably want to go into the corn field in September next time also. Since K doesn’t vary much between successive years, maybe its best to pick a crop and sample after your latest crop in the rotation always and when you sample early, just run the N and K.

Nitrogen would be just fine if you sampled now. We know that whenever you sample for N, there may be an increase in N later, a decrease in N later or it will stay the same. Sort of like the grain trade. So now is fine- just get it done.  Dave Franzen NDSU Extension Soil Specialist

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