To try to get a handle on how much nitrate-nitrogen was left in the soil following last year's drought-reduced corn crop, ISU Extension field agronomists sampled the soil profile at various locations in Iowa last fall following the 2012 harvest. They cautioned that the amount of nitrate-nitrogen that might remain in the soil for use by the 2013 corn crop depended on the amount of rainfall this spring.
Unfortunately, much of Iowa has received considerable precipitation since soils thawed, especially the eastern two-thirds of Iowa. As of May 6, ISU Extension soil fertility specialist John Sawyer notes that tile lines are flowing again, and nitrate in the profile will move with percolating water. Not all of the precipitation entered the soil, but the amounts received and comments from ISU Extension field agronomists who have sampled soil profiles this spring for moisture content suggest the soil profiles in most of the state have been recharged with moisture.
"Therefore, we have lost the opportunity to use much of the profile nitrate that carried over from last year," says Sawyer. "Also, this spring's precipitation after the dry fall of 2012 reminds us why profile sampling for nitrate is not a routine practice in much of the Corn Belt."
Significant changes in soil profile nitrate have taken place since last fall
ISU Extension field agronomists have been collecting soil profile samples this spring at the same locations as last fall. Unfortunately, with the wet spring weather we've had, not all sites could be sampled by the time this article was published May 6. However, with samples that have been collected so far, a few things are clear, notes Sawyer.
* One, samples collected before the large spring rains still had high nitrate-N amounts.
* Two, samples collected after the large spring rains show nitrate movement deeper into the profile.
* Three, samples collected after the large spring rains generally have less total profile nitrate-N than last fall. The table which Sawyer has put together in the accompanying slideshow has several examples. In most, but not all cases, the amount of nitrate-N decreased from fall to the spring sampling. The southwest Iowa samples were collected in early April (before the largest rains), with the rest collected in late April.
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