Fall Nitrogen
Harvest in Illinois continues to lag some, with 47% of the corn crop and 55% of the soybean crop harvested by October 16. The dry weather continues this week, which should allow harvest to progress. The projected Illinois corn yield for 2022 increased to 210 bushels per acre in the October crop production report from NASS. Projected soybean yield stayed at 64 bushels per acre.
As harvest progresses the focus to fall application of nitrogen fertilizer, at least in central and northern Illinois where fall ammonia application is considered acceptable practice. This interest is elevated some this fall due to the current price of anhydrous ammonia, which is running around $1,400 per ton ($0.85 per lb of N.) That’s not a low price for ammonia, but the high natural gas price may well move the ammonia higher by next spring. Soils are also dry across most of the state, which provides good conditions for application.
Did the 2022 corn crop get enough N?
While corn yields are coming in higher than expected in many areas this fall, dry weather this past summer continues to have some people thinking that the crop might have run out of N in 2022, so that rates need to be kept high for 2023 to make sure that doesn’t happen again. In my article two weeks ago, I noted that some of the corn crop was staying green up to maturity, which is a signal that the crop had plenty of N.
There are several other pieces of evidence that back up the observation that N was plentiful in most fields this year. Dan Schaefer and I sampled from a very green plot of corn here at the South Farm (this corn was in the photo in the article mentioned above), which was mature but still had grain moisture above 30%. Analysis showed that the grain was 1.37% N (0.65 lb N per bushel), and that the rest of the plant was 1.01% N, both on a dry weight basis. At a harvest index of 52% (52% of the total dry weight was grain, 48% was the rest of the plant) and a yield of 208 bushels per acre, the grain had 134.9 lb of N and the rest of the plant had 91.8 lb of N per acre, for a total of 226.6 lb of N per acre, or about 1.1 lb of N per bushel. The “N harvest index” – the amount of N in the grain as a percentage of total N in the crop – was 60%. That means that 40% of the N in the crop at maturity was left behind when grain was harvested. Some of this N will go help microbes as they break down the residue between now and next spring; it may not be “wasted” N, although we don’t know for sure that all of it will stay in the field.
We also ran a couple of two-N-rate trials here at South Farm, designed to test the adequacy of the MRTN rate. Two fields planted on May 12 had N applied near the MRTN rate (185 lb N/acre) as preplant UAN, and two strips in each field had an additional 64 lb N applied as sidedressed UAN on June 13, at stage V7. The table below shows the yields by strip, and averaged over the strips in each trial. To calculate returns, we used a corn price of $6.50 per bushel and an N price of $1.00 per lb.
We normally show treatment averages as results from such studies, but these are relatively small trials, and individual strip yields show that yields were very consistent. With small differences between treated and untreated strips (in each trial, at least one untreated strip yielded as much as one treated strip) we are unable to say that the small yield increases from adding additional N were due to anything other than random variability. More importantly, adding 64 lb of N to a normal rate lowered the net return in both trials; the added yield did not come close to covering the cost of the added N.
Finally, in an N rate trial at Monmouth this year, corn following soybean yielded 210 bushels per without N, and the maximum yield of 306 bushels per acre was reached at an N rate of only 137 lb N/acre. The field with the study has produced high yields before without N fertilizer, which may be a legacy from the time it was a feedlot 50 or more years ago. But it was a dry location this year – rainfall in June and July totaled 4.7 inches – and we are seeing the benefits of having soils never get wet enough to compromise the root system or the N supply in the soil.
N Rate for 2023
Given the above, there is no reason to believe that the 2023 corn crop will need extra N, or that we need to “load up” this fall even if we believe that the price of N will be higher next spring. With the dry weather and low tile flow in recent months, nitrate levels in rivers may rise if there are wet spells next spring with a lot of tile flow. We can’t prevent that, but we can assume that some of that N is coming from extra N added to the 2022 corn crop that the crop did not need and did not take up. In other words, nutrient loss reduction starts now.
While we don’t know what prices of either N or corn will be next spring, it makes sense to apply only part of the N this fall. The N rate calculator (http://cnrc.agron.iastate.edu/) was upgraded last spring with new data, and the MRTN rate for corn following soybean, using an N price of $0.85 per lb and a corn price of $6.50 per bushel, is 163 lb N/acre for northern Illinois and 171 lb N/acre for central Illinois. The first subtractions that should be made when applying fall N are: 1) N from MAP or DAP applied this fall or next spring; and 2) any N that will be applied next spring, including UAN as herbicide carrier, as N applied with the planter, or as sidedress. Beyond those subtractions, it makes a great deal of sense to leave room next spring to adjust the total N rate in case N prices really go up and or corn prices drop. Both are possible in this uncertain world. Total N rate should be based on the MRTN calculation made based on the price of the last N applied, and the price of corn projected at that time. The N applied as herbicide carrier or with the planter next spring is generally regarded as necessary, which means that this fall’s application needs to be low enough to leave room to adjust rates next spring if the price ratio changes.
N application this fall
Pressure to get N applied is coming from the fact that some anhydrous ammonia has been sold with the requirement that it be applied before the end of 2022. This so far is shaping up to be a good fall application season, with dry soils – no part of Illinois has received normal rainfall so far in October, and most of the state has gotten less than half the normal amount. There is some rain in the forecast towards the end of October, but not (at this point) for large amounts. Soils are dry enough that application is not likely to be disrupted for very long by rainfall amounts up to an inch or so. Soil temperatures are also dropping, with every WARM station north of I-70 showing bare-soil temperatures at the 4” depth at 10 AM on October 18 in the upper 30s or low 40s. The weather is predicted to warm by the end of the week, and soil temperatures will follow. But by the end of October soil temperatures should be low enough to start applying in earnest.
One thing that continues to puzzle us is the large amount of tillage that is being done now, especially following soybean harvest. Soils are dry and are falling apart well – many fields look “ready to plant” here in mid-October. Fields of tilled soybean stubble have little residue cover, and that could mean blowing soil this fall or (more likely) next spring. Everyone has their own reasons to do such tillage, but loose, dry soil after tillage may not do a great job of holding NH3 that’s injected this fall. Ammonia can continue to escape as long as soils stay dry, even if it the band was covered adequately at application. Applying a little deeper, into soil that’s a little more moist, will help.