Too Early to Know About Nitrogen Losses

Losses from fall-applied N may not have been that significant.

Published on: May 14, 2008

While there's no way to know how much nitrogen was applied for corn last fall, there are obviously some acres applied. Recently, Bob Nielsen and Jim Camberato, Purdue University agronomists, have received some calls from people wondering how much of that N might still be around for the crop.

While the west spell in March that lingered into April may have triggered the calls, Nielsen doesn't believe there's that much cause for alarm yet. His reasoning concerns the form of N that was applied in the fall and the temperature and rainfall patterns throughout the winter. That assumes, of course, that the N wasn't applied until Purdue would consider the application safe in the fall. That's typically when soil temperatures have dropped to 50 degrees F, slowing down microbial action within the soil. Applying N in the fall is still not recommended by Purdue over most of southern Indiana, since soil temperatures tend to remain warmer there well up into late fall.

Nielsen and Camberato have completed two seasons of using sensing equipment to determine possible N rates that still need to be applied to corn before or around sidedressing time or slightly afterwards. While he says there is definitely promise in what they're seeing, Nielsen is not ready to make broad-scale recommendations on investing in and doing this type of scouting until they have more data. They plan to test out more plots on this concept this year,.

However, the pair are fairly confident in the recommendations they've reached for N rates, based on intensive Purdue University Farm trials and farmer-field trials over the past two seasons. Those will continue this season as well.

Their conclusion is that top economic yield on many soils in Indiana comes somewhere around 145 pounds of N per acre following soybeans. Add an extra 30 pounds per acre for following corn in rotation in all of their studies. If you're after maximum agronomic yield after soybeans, kick the rate up another 25 pounds per acre or so. However, remember that it's maximum economic yields that typically pay the bills.

"These recommendations seem to be well received," Nielsen says. "They are based on an average year, and there are variations from west to east across the state. But we believe they provide a starting point from which people can figure rates."

If you haven't already applied N because you're waiting to sidedress it after corn emerges, these recommendations may prove helpful yet this year. Remember that sidedressing also typically boosts efficiency of N usage by 5 to 10% or more.

The preplant N vs. sidedress N concept will be tested at Corn Illustrated plots near Edinburgh this year, assuming the weather cooperates. So far the CI crew have planted a large population study, but have not yet been able to plant the high yield test or sidedress vs. preplant N test.