Should you apply nitrogen fertilizer now or wait until spring?
By Emerson Nafziger, University of Illinois
With corn and soybean harvest nearly complete, the annual process of deciding when and how to supply nitrogen fertilizer for the 2014 corn crop is under way.
Lessons from This Past Year
Following the very dry first half of 2012 and low corn yields, soil sampling last fall revealed an average of about 80 lb N present as nitrate in the top foot of soil in Illinois. With a lot of rain in late winter and early spring, soil N levels by spring had dropped considerably. And early spring application in 2013 was often followed by a lot of rain before the crop started to take up much N.
The 2013 season was wet early then dry late, but with good root systems that took up water and nutrients well to produce good yields. So far overall N loss this past season does not seem to have been greater than normal. Evidence to support this comes from N trial data just coming in. In an on-farm study coordinated in Champaign County by Dan Schaefer of C-BMP, corn responded almost exactly the same, and with the same yield levels, to fall-applied NH3 as to UAN sidedressed in June. In a study with continuous corn at Perry, sampling in June showed that most of the N applied as UAN in April was still present after more than 12 inches of rain, and yields barely increased going from 180 to 240 lb N, at a yield level of about 200 bushels per acre. At Monmouth, supplementing 165 lb of N as UAN applied in early April with 60 more lb of N in early June, after some 18 inches of rain, did not significantly increase yield of about 240 bushels per acre.
High corn yields mean that large amounts of N were taken up. In the 2013 UI corn hybrid trials, the average yield among the three sites where we measure grain protein was 228 bushels per acre, and the average protein content (at 15% moisture) was 8.4 percent. That calculates to removal of 0.74 lb N per bushel, or 169 lb N per acre. We typically estimate that 2/3rds of the N in the plant at maturity is in the grain; that would calculate N uptake at more than 250 lb N/acre. When we can get 250-bushel yields with 180 lb of N as we did in some cases in 2013, it’s clear that the soil supplied a great deal of N to the crop.
It does not appear that much of the "extra" N needed for the high corn yields in 2013 came from N that was left over after the drought of 2012. As is typical, soybean fields had very low amounts of soil N after harvest in 2012, yet corn following soybeans is – again, as usual - showing less response to N rates than corn following corn in 2013. What data I have so far indicates that corn following corn may not be taking a big yield hit like we’ve seen in some areas in recent years. It’s possible that some of this is because of leftover N, but corn following corn is also showing a typical response to N rate, so it’s not clear that leftover N was a major contributor.
With the weather dry over much of Illinois since August (until rainfall this week), can we expect soil N levels to be high again this fall? I’ve seen numbers from only a few soil samples so far this fall, and they seem mixed, but generally lower than we saw in fall 2012. The crop clearly had the root system to tap into water that was deep in the soil in order to produce the yields that it did, and it’s likely that it brought a considerable amount of N up along with this water. We might expect that this meant removal of much of the N mineralized from soil organic matter, but with some rain now and soil temperatures still in the 50s, there is still some mineralization going on, at least for a few more weeks. But in general, we expect soil N levels to be more or less this fall; we have typically measured only 20 or 30 lb per acre of nitrate-N in the top foot, or even less.
The big question that many people have is whether or not to apply N fertilizer this fall or to wait until next spring. We know from nitrate levels in rivers that an appreciable amount of the N present last fall left fields through tiles lines last spring. Fall-applied N generally went out late enough last fall, and soils turned cold and stayed cold after application, so most of the N lost in tile lines last spring came from leftover N rather than from N fertilizer applied last fall. The fact that we’re seeing similar responses to fall- and spring-applied N would support that most fall-applied N stayed in the soil and available to this year’s crop. We are not seeing the unusually large responses to applied N this year that we would expect to see following high N losses.
The basics of fall N application have not changed: the form should be anhydrous ammonia (NH3); soil temperatures should be at or below 50 degree F at the time of application; using a stabilizer/nitrification will slow the activity of microorganisms that convert ammonium to nitrate (the form that can leach); soils should not be wet or very dry, but should have enough water to allow the ammonia to spread to a diameter of 4 to 6 inches as it is released in the soil; and application depth should be 6 inches or more so the NH3 gas doesn’t escape after application. Fall NH3 application should not be done in southern Illinois due to higher chances of soil warming in the fall and earlier warm-up in the spring, and N losses also go up on poorly-drained soils or very light soils, making fall application more risky.