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Pros and cons of applying nitrogen in spring vs. fall
Iowa Correspondent

URBANA, Ill. – Nitrogen applied as anhydrous ammonia in the fall is an effective source of nitrogen for the corn crop, but it is also a little more subject to loss, compared to nitrogen applied in the spring, according to a University of Illinois (U of Ill.) agronomist.
“One of the main factors that determines how much of the fall-applied nitrogen is subject to loss is how much of the ammonia converts to nitrate (is nitrified) in the fall and early spring, before the next planting season,” said Emerson Nafziger, U of Ill. Extension crop production specialist.
Anhydrous ammonia – and ammonia in liquid swine manure – is converted to nitrate by bacteria, with nitrate being the major form of nitrogen taken up by corn, and other grasses and forbs, said Virgil Schmitt, Iowa State University Extension field agronomist in Muscatine. 
“Nitrates are extremely soluble, so nitrates are very easily leached from the soil through tile and other subsurface drainage,” he said, “so the less conversion that takes place by bacterial action, the fewer nitrates are at risk of being lost to the environment prior to uptake by plants. 
“In the ideal world, the conversion to nitrate would occur just as plants needed the nitrogen,” he added.
Schmitt said anhydrous ammonia creates a hostile environment for bacteria, so little bacterial action converting ammonia to nitrate occurs for the first few days. 
“Then bacterial action resumes,” he said. “The rate of conversion of ammonia to nitrate begins to slow rapidly as soil temperatures fall below 50 degrees F. Therefore, the general recommendation is to not apply ammonia sources until the soil temperature is below 50 degrees F, and falling. 
He said the idea is to minimize the amount of conversion that takes place in the fall so the rains before soils warm back up have less nitrate available that can be leached from the soil.
“However, bacterial action does not completely stop until the ground freezes, so an extended period of time when soils are above freezing can still result on considerable conversion of ammonia to nitrate,” he said.
Nafziger said one way to slow nitrification of fall-applied ammonia, which converts quickly to ammonium in the soil, is to add a nitrification inhibitor.
The other way is to wait to apply ammonia until soil temperatures are low enough to limit the activity of the bacteria responsible for the conversion to nitrate, he added.
“Scientists have studied the effect of soil temperature on nitrification rates for decades,” he said. “Estimating how quickly ammonium is converted to nitrate in the soil turns out to be complicated. But like most biological processes, it takes temperatures close to freezing to stop the nitrification process completely.
“Adding the ammonium as anhydrous ammonia also increases the pH and kills a large number of bacteria, effectively delaying the start of nitrification by days or weeks, with longer delays when soil temperatures are lower,” he added.
Once nitrification begins, soil temperatures in the upper 70s produce maximum rates of nitrification, he said. 
“Rates drop to about 50 percent of maximum at 60 degrees, to 10 percent of maximum at 50 degrees, and to about 5 percent of maximum at 40 degrees,” he said. “Because nitrification still takes place (slowly) at 50 degrees, why don’t we wait until soil temperature is less than this – say 45 or 40 degrees – before starting to apply ammonia?
“Waiting until 50 degrees to apply ammonia is a compromise that produces some opportunity to apply at all during seasons when harvest is delayed, soils are wet, or soils freeze early,” he added. “It takes advantage of the delay in nitrification caused by ammonia itself, and of the fact that soil temperatures continue, at least on average, to drop over time.”
On the other hand, he said soil temperatures typically rise and fall unpredictably during the fall, and a rise in soil temperature both shortens the delay in the start of nitrification, and speeds up the breakdown of nitrification inhibitors.
“Managing the risk of having substantial nitrification following application means taking into account both current and predicted soil temperatures when deciding when to apply fall nitrogen,” he said. “Because soil temperatures fluctuate during the day and are different at different depths, we also need to decide when and at what depth to measure soil temperature. 
“How much the soil temperatures fluctuate during the day depends on soil texture and color, soil moisture, air temperature and the amount of sunshine,” he added.
Nafziger said drier soils tend to limit nitrification some, and they also warm and cool more quickly as air temperature changes.
“We don’t want to use either maximum or minimum, so take soil temperature at 10 a.m. and 4 inches deep under bare soil (often) as an estimate of daily average conditions in the ammonia band after application,” he said.
But the good news is air temperatures averaged a few degrees below normal in October this year, he said.
“We know from records that soil temperatures can rise into the 40s and even into the 50s during the winter, so waiting until soils are cool before applying ammonia doesn’t rule out further nitrification,” he said.
“The fact that periods of heavy rainfall before soils are frozen or after they thaw can result in spikes in surface water nitrates tells us that some nitrification always takes place between fall and spring,” he added. 
However, he said some nitrate that moves from fields is present in the soil at crop maturity, either from unused fertilizer or nitrogen mineralized from soil organic matter after plant uptake stops. 
“But some also comes from fall-applied nitrogen,” he said. “We can minimize this by careful management, and can largely eliminate it by moving application to the spring.”