Early Soil Sampling OK - Sort Of

Sampling for nitrogen and phosphorus can be done now, but tests for potassium should be done at about the same time every year.

Published on: Aug 8, 2012

Soil testing is usually done following small grain harvest. With the harvest starting three weeks earlier than usual this year, can you take soil samples early, too?

Dave Franzen, North Dakota State University Extension soils specialist, says it depends on what tests you expect to run.

"Soil phosphorus (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."

When you take soil sampling may affect some results. Photo: University of Kentucky.
When you take soil sampling may affect some results. Photo: University of Kentucky.

Sampling for potash (K), however, is trickier, however, Franzen says.

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  is at a peak. The K test then increases later in the fall when the soil usually wets up and there are frosts.

"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 it's best to pick a crop and sample after your latest crop in the rotation always and when you sample early, just run the nitrogen (N) and K," Franzen says.

N 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," Franzen concludes.

Source: ND Crop and Pest Report