Postpone Nitrogen Applications Until Soil Temperatures Cool

Rethink fall vs. spring application strategy.

Published on: Nov 1, 2007

Just because you normally apply nitrogen this week of the year, don't just assume that it's the right thing to do this year. Otherwise, you might set yourself up for rather significant N losses next spring. Losses could be high enough to easily offset any cost advantages you believe you're gaining by buying anhydrous ammonia now instead of gambling on possibly higher prices next spring.

"Since temperatures have been so much above normal, you need to let it cool off for a week or two before considering fall N applications of anhydrous," says Jim Camberato, Purdue University Extension soil fertility specialist. Remember that there is a delay between air temperatures dropping to more seasonal levels and soil temperatures reaching the same point. It takes a few extra days before soil temperatures drop to similar levels.

What counts on anhydrous applications in light of potential losses later are soil temperatures, not air temperature. Purdue's recommendations have typically been to not apply anhydrous until soil temperatures are consistently no higher than 50 degrees. Purdue has typically not recommended fall application at all in the southern part of Indiana.

Some farmers were set up to find out why unseasonably high fall temperatures can impact N losses a year ago, Camberato notes. But then spring weather patterns tended to bail them out. Few if nay significant loss situations were reported.

"It was much warmer than normal in early winter last year," the soil fertility specialist recalls. "Conversion processes kept occurring that took N to the nitrate form that is more subject to loss. The 50 degree recommendation is based upon the expectation that soil temperature will continue dropping toward 32 degrees. A year ago, it simply didn't get near that low for a long time.

"This spring was dry in most areas in May, after soil temperatures warmed up. So we didn't see widespread losses. You would have noticed it in plenty of fields that ran out of nitrogen where fall applications were made. The same situation pretty much applies to hog manure applied in the fall- if it stays warmer then expected and soil temperatures stay up, conversion of the N in manure to nitrate form can occur as well."

Some farmers are wondering how they can hold the line against escalating input costs for '08. Camberato says one option would be to move N application closer to the time plants need the nutrient. That makes for more efficient use of the product.

"If you've fall-applied, think about moving it to spring," he says. "If you spring-applied in the past, consider going to more sidedressing of N. These would be ways to improve efficiency of the input."