After Drought, Wet Spring Has Changed Amount Of Nitrate In Soil

With wet conditions in parts of the Corn Belt this spring and a decrease in carryover nitrate, consider applying near-normal nitrogen rates for corn.

Published on: May 7, 2013

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.

RESIDUAL SOIL NITROGEN: How much carryover nitrate-nitrogen is left in the soil profile this spring? With lower yields in 2012 and lack of moisture to move nitrates in the soil, there was more nitrate than usual carrying over into 2013. That is, until the weather turned wet this spring. From soil nitrate samples collected this spring, its clear that carryover nitrate-N has moved deeper into the profile or was lost to tile flow.
RESIDUAL SOIL NITROGEN: How much carryover nitrate-nitrogen is left in the soil profile this spring? With lower yields in 2012 and lack of moisture to move nitrates in the soil, there was more nitrate than usual carrying over into 2013. That is, until the weather turned wet this spring. From soil nitrate samples collected this spring, it's clear that carryover nitrate-N has moved deeper into the profile or was lost to tile flow.

"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.

Post-Drought Soil And Fertility Issues. Just what has the drought left behind for the 2013 crop? And what steps do you need to take to get land ready for planting? Download our FREE report, 5 Post-Drought Strategies For A Better 2013.

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Spring preplant soil profile sampling near the time of crop planting can sometimes give the best indication of the amount of nitrate-N in the soil profile that might carry over to a corn crop, notes Sawyer. This certainly is the case this spring. If you still consider that there could be elevated nitrate-N levels in fields from last year's drought-damaged corn, and the field will be planted to corn again this year, sampling is the best approach to determine if a nitrogen amount can be subtracted from the normal application rate. 

Profile sampling is most viable for the northwest area of Iowa where rainfall has been about normal this spring, he adds. You can refer to the previous article published February 21, 2013, on ISU's ICM News website to find suggested sampling procedures. 

So the question now is, how much nitrogen should you apply for 2013 corn?
From the soil nitrate samples collected so far this spring, it is clear that the carryover nitrate-N moved deeper into the profile or was lost to tile flow, he explains. With the cold soil temperatures this spring, it is unlikely nitrate was lost by denitrification. Reports from stream monitoring are indicating increased nitrate-N concentrations, as would be expected where tile drainage contributes significantly to stream flow. In many fields, the change in soil profile nitrate-N from last fall is quite dramatic, and unfortunately this means less opportunity to account for nitrate-N carryover for 2013 corn crops. 

However, some fields still have carryover nitrate-N amounts that should be used to adjust nitrogen application rates, says Sawyer. "The precipitation maps accompanying this article are quite reminiscent of rainfall patterns of just a few years ago, and we know that in high rainfall years response to applied nitrogen is large, due to loss of soil derived mineralized N, carryover nitrate, and applied fertilizer and manure nitrogen. We'll see if this weather pattern continues for 2013."

CONCLUSION: With the overall wet spring and decrease in carryover profile nitrate, Sawyer suggests farmers consider applying near normal nitrogen application rates for 2013 corn. He recommends farmers use ISU's online Corn Nitrogen Rate Calculator for suggested rates.

Post-Drought Soil And Fertility Issues. Just what has the drought left behind for the 2013 crop? And what steps do you need to take to get land ready for planting? Download our FREE report, 5 Post-Drought Strategies For A Better 2013.



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