April and May allowed for great planting progress, and warm temperatures in May helped crop development. Farmers were also able to get into fields and apply some post-emergent nitrogen to their corn. However, some heavy rains after that application have some farmers concerned about nitrogen.
Much of the rainfall has come with low or moderate intensity. There have been few multiple-inch downpours leading to standing water, and fears of crop damage and of loss of nitrogen, though rain on May 30 was heavy in some places, said Emerson Nafziger, University of Illinois crop scientist.
“Reapplying more nitrogen late or using higher rates at sidedress time due to expectations of nitrogen loss happen every year, and while that might be less common this year, it remains a consideration,” he said.
To find out if such adjustments of nitrogen rate are justified, Nafziger and colleagues initiated a new study this year, funded by the Illinois Nutrient and Education Council (NREC). The study aims to track nitrogen applied at different times and forms to see how much nitrogen remains in the soil and available to the crop through the vegetative growth period.
The researchers applied 200 lbs. of nitrogen in four different ways, including fall ammonia, early spring ammonia, planting time solution nitrogen, and some split applications. Beginning after the fall application and about every 10 days this spring, they have been sampling soil to 2 feet deep, with samples analyzed for both nitrate and ammonium.
“Sampling like this always finds a fair amount of variability,” he said. “But we did find more nitrogen in plots where we had applied nitrogen, and we are able to see changes in soil nitrogen as the soils have warmed up and mineralization has kicked in.”
Nafziger added that much of the applied nitrogen remains in the soil from both fall- and spring-applied ammonia. This confirms that losses of nitrogen have not been large, at least through most of May.
One surprise at the Urbana site has been the relatively large amount of nitrogen found in the soil where no fertilizer nitrogen was applied. ”We never expect to find zero nitrogen, but we would not have expected to find the 130 to 180 lb of nitrogen that we found in the last three (May) samples,” Nafziger said. “But knowing that we can often get yields of 125 to 150 bushels per acre in corn following soybean without nitrogen fertilizer, maybe it’s not surprising that we find such amounts as soils warm up.”
The majority of the soil nitrogen is now in the nitrate form, regardless of what form was applied, or when no nitrogen was applied.
“This is more or less as we expected for fall-applied NH3 and for spring-applied UAN, but we expected a little more of the nitrogen to still be in the ammonium form for spring-applied NH3,” Nafziger said.
Mineralization releases ammonium, but under the soil temperatures that increase mineralization rates, the ammonium converts to nitrate quickly.
Nafziger said the takeaway message so far is that there is no signal to adjust upward the total amount of nitrogen applied due to concerns about nitrogen loss. “The fertilizer nitrogen that was applied is mostly present as plant uptake begins, and we can add to that the nitrogen coming from soil organic matter,” he said.
“It’s also encouraging that the amount we’re finding has continued to increase as we move into the nitrogen uptake period. Last year, we found that the crop took up a total of less than 1 lb of nitrogen per bushel of yield so we don’t see a shortage for the crop coming anytime soon,” he said.
As part of this project, Nafziger added that he is working with Dan Schaefer of the Illinois Fertilizer and Chemical Association (IFCA) to sample several on-farm sites with similar treatments. They will also be sampling following spring nitrogen applications at two southern Illinois sites.
“We’ll keep you posted on what we find in the coming weeks,” he said.