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UI Nutrient Bulletins

Managing when Planting is Delayed

Written by Emerson Nafziger, University of Illinois

Managing nitrogen fertilizer is one of the biggest challenges in establishing the 2019 corn crop. As I wrote in the Bulletin on March 26, fall N application was limited by wet soils; that is accurate regarding spring application as well, which lags far behind normal, and far behind what most Illinois producers had planned and hoped for.
 
I won’t rerun here the scenarios from that article—some are no longer relevant—but will emphasize only the fact that we need to avoid having corn plants emerge and start to grow with their nodal roots growing into soil with very low levels of N. We’ve seen in recent research a number of cases when waiting to apply N until plants are at V5 stage or later can lower yields, even when enough N is applied later. We have also found that applying low rates of N at planting with the bulk of N applied during sidedress tends to produce lower yields compared to applying most of the N at planting and the rest as sidedress.
 
With a lot of fields still without any fertilizer N, and the pressure to plant the crop increasing as time goes on, how do we get fertilizer N on the crop this year? With the inches of rainfall in recent days, we can be fairly sure that a lot of nitrate-N has moved down in the soil, at least to below the rooting zone for small plants. Fields that have not gotten any fertilizer probably didn’t have much nitrate present anyway, but April was warmer this year than in 2018 (even if it didn’t feel like it) and bare-soil temperatures at 4 inches deep have been ranging between 50 and 60 degrees for the past week. So some mineralization is taking place, and by the time planting can begin, there should be more soil N than there is now. That means that we should be able to get by with less of the fertilizer N applied by the time the crop emerges than if we were planting into colder soils.
 
Or those who plan to stay with planting-time applications of UAN as a carrier for herbicide, then coming back with ammonia, UAN, or urea to apply the rest in-season, it’s probably best to apply a third or so of the N fertilizer with the first application. That can include N from MAP or DAP, especially if that was applied this spring, and any N to be applied with the planter. For most people, the source of this N will UAN with herbicide, broadcast before planting and worked in, or broadcast after planting. With one-fourth of its N present as nitrate that can move readily down into the rooting zone, UAN may be a better source for getting N into the soil quickly than sources like urea. Ammonia can work for this, but we can’t place ammonia very close to the row without some risk of root damage in the event that soils dry out after application. And it will take the roots some time to get to the N in the ammonia band, especially if that band is small due to wet soils at the time of application, and if it’s 15 inches away from the row.
 
If it’s not possible to use broadcast UAN to supply the early needs of the crop, more creative ways might be considered. I responded to an email last week about a field where cereal rye cover crop had been sprayed right before planting corn. The main reasons to plant cereal rye is to take up soil N, and it does this very well; when allowed to grow up to planting time, it will strip nearly all of the nitrate from the top foot of soil. It’s most likely this—not allelopathy—that makes it so risky to plant corn into recently-killed cereal rye. In order to get some N to the roots, I suggested streaming or dribbling maybe 20 gallons of UAN right on top of the planted rows, if that can be done before emergence. Urea can be dropped onto the rows instead, and won’t injure plants if they’ve emerged, but it won’t get N down into the soil as quickly. If there’s no cover crop, a lower rate of UAN can be streamed or dribbled on top of planted rows, because it will concentrate N in the row.
 
Another way to get N on the crop is to use broadcast urea or urea-based N fertilizers. Those may be easier to get applied early, and unlike solution N, they are safe for the crop after it emerges. Whether or not to protect surface-applied urea with a urease inhibitor depends on the weather after application, but with moderate temperatures and a rainfall pattern that has been on the “abundant” side, inhibitor may not be needed. If urea is worked in after application, any ammonia released by urease activity will dissolve in soil water, so the inhibitor is unnecessary.
 
In-row application of UAN with a split-tube setup provides N closer to the plants than between-row injection, and can be used in larger corn than most injection implements. We have not found an advantage to repeated application—in fact, applying N once, near the time of planting, often produces the same yield as splitting the N into two applications. We’ll watch this year to see if we have conditions that suggest applying supplemental (extra) N, but with corn prices like it is now, this may not be the year for adding any expense that isn’t needed.
 

Dealing with very late planting

Written by Emerson Nafziger, University of Illinois    (View the U of I bulletin)

Despite the fact that the “active” weather pattern gave no signs of changing over the past month, few of us thought we’d see so little planting progress by now. But here we are, with only 35% of the Illinois corn crop and 14% of the soybean crop planted by May 26. With more rain this week, we will have less than half the corn and less than a fourth of the soybeans planted before June 1 in Illinois.
 
I’ll use the Q & A format to address some issues still outstanding as we try to get this crop planted.
 
Q: How bad is it?
A: The entire state of Illinois received above-normal rainfall in May, ranging from 1 to 2 inches above normal in the southeastern edge of the state to as much as 8 inches above normal (12+ inches total) in northwestern Illinois, from the Quad Cities to the south and east. That area had some planting days early—some of the corn at our Monmouth research center was planted in April—but that has been hammered by inches of rain. As bad as things are in Illinois, some areas west and southwest of Illinois are even worse, with some rainfall amounts more than 12 inches above normal for May.
 
Q: How soon can we expect progress?
A: We have a short break from rain in places the last few days of May, and some places that thunderstorms bypassed this week might dry up enough to plant by the weekend. The forecast isn’t promising a change to a drier pattern, though, and with soils as wet as they are now, widespread planting won’t start very soon and even after it begins, planting progress won’t set any speed records.
 
Q: Won’t driving on soils that are still wet 4-6 inches deep compact the soil so badly that yields will suffer?
A: We’ve consistently said that “mudding in” corn or soybeans in April does more harm than good. But that’s because planting in early April tends to produce yields no higher than planting in late April or early May; there’s no reward for aggressive planting that early. Once we get to late May, things change. We don’t want to get stuck in the field or to plant where the seedbed soil is too wet to crumble, but the need to get crops planted means that it makes sense to start even though we know that heavy equipment will (as always) cause compaction. Compaction from heavy equipment moving down the field (not directly on top of the row) typically does not lower yields appreciably in many of our more productive soils. While the formation of a physical barrier (if soils dry out after compaction) is a problem, pressing soil particles together as air is expelled can increase capillarity some, helping water to move to the roots from deeper in the soil. The most visible signs of compaction—plants stunted and with leaves showing drought stress symptoms—is typically on endrows, where equipment moves at right angles to rows and thus limits rooting ability more than it improves capillarity.
 
Q: Should we “open up” soils by tilling them when they’re wet so that the surface dries out faster?
A: Go to Farmdoc and read the useful articles posted there in the past week or so. One issue that isn’t yet there in detail (but may be coming soon) is what our yield expectations should be if the crop isn’t planted until after its last date for full crop insurance coverage: June 5 for corn; June 15 for soybean in the northern third of Illinois; and June 20 for soybeans in the rest of the state. We don’t have recent data on corn planted after early June, but in a recent paper from Iowa State [Baum et al., Agronomy Journal 111:303-313 (2018)], researchers reported yield losses of about 25% (55 bushels) by June 10, 40% (88 bushels) by June 20, and 61% (133 bushels) by June 30. So planting corn on the last insurable date (June 25) would be expected to produce about half the normal yield. I suspect it could do better than that, but for now that’s our best guess.
 
Based on experiences with doublecropped soybeans in the southern half of the state, we expect yield declines to be less steep for soybean than for corn after early June, reaching 50% only by early July in south-central and southern Illinois. There is a large effect of latitude with late-planted soybeans, though, and yields of soybeans planted in late June in central Illinois are very much affected by the growing season weather, and so can be expected to vary widely. Soybeans planted after mid- or late June in northern Illinois may not mature early enough to avoid frost.
 
Q: Should I change corn hybrids to ones with earlier maturity?
A: Maybe. Many people in northern Illinois have probably done this by now, or at least have lined up seed to do so if planting stretches out much longer. The Corn Growing Degree Day decision support tool can help with this question, but can’t answer it very precisely. That tool allows one to choose any Corn Belt county, enter the planting date and hybrid maturity, and generate a graph that shows projected GDD accumulations through the season, including the date on which you can expect that hybrid, planted on that date in that county, to mature.
 
As an example, a 108-day RM hybrid (which the tool estimates will need 2,600 GDD from planting to maturity) planted on June 1 in Woodford County, IL is projected to mature on October 10. Because October 10 is a 12 days before 50% chance of frost there, I would not hesitate to plant this hybrid if I were able to do so by June 5. That same hybrid, though, if planted on June 1 in DeKalb County, does not project maturity until the beginning of December, or about 6 weeks after the 50% frost date. Changing to a 100-day hybrid there would move projected maturity to October 9.
 
One important adjustment missing from this tool is the fact that planting corn late usually lowers the GDD needed to get a hybrid from planting to maturity. In an article on his website, Dr. Bob Nielsen at Purdue includes a calculator that uses data he and Dr. Peter Thomison at Ohio State University collected to adjust the GDD requirement downward based on how late planting actually is. This is not a trivial adjustment: planting a hybrid on June 1 lowers the GDD requirement by more than 200 GDD. So a hybrid that needs 2,650 GDD to mature if planted on May 1 will require an estimated 2,439 GDD if planted on June 1. The revised GDD number can be manually entered into the GDD tool instead of days RM for the hybrid.
 
The downward adjustment in GDD with late planting is not a chiseled-in-stone number; in fact, if growing season temperatures are on the cool side, planting late may not lower the GDD requirement at all. We saw this in 2009, when some corn in northern Illinois did not mature by November. There are two main reasons why late planting might lower GDD requirement. One is that the crop develops under higher temperatures, and the high-temperature “cutoff” of 86 degrees may actually be higher than 86; that is, corn may grow faster at 90 degrees than at 86 degrees. The other reason is that late-planted corn tends to have more limited root growth, which adds to any stress from periods of hot, dry weather. Late-planted corn also finishes in September or October, when days are shorter and temperatures are often lower than ideal during late grainfilling.
 
The outcome of all this is that reduction in GDD requirements when corn is planted late is almost always accompanied by a reduction in yield. Still, if the crop ends up needing fewer GDD to mature, it’s better that this come with a longer-season hybrid than with a shorter-season hybrid, which might mature earlier but at even more cost to yield. Adding to this is the fact that 100-day RM or shorter hybrids were not really developed to be planted in Illinois, and they may not bear up very well under Illinois conditions when planted late.
 
Because soybean flowering responds to photoperiod, varieties of typical maturities will flower at about the same time if planted in the second half of June. So there is little need to change soybean varieties to ones with earlier maturity, except that north of I-80 in Illinois, varieties no later than MG 2.8 or 2.9 should be used if planting is delayed past mid-June. If July temperatures are normal this might prove to be unnecessary, but cool weather (especially nights) in July could delay flower to late July, and that could substantially delay maturity and push seedfilling into less favorable conditions.
 
Q: What do we do about nitrogen fertilizer for corn?
A: This is one of the really tough challenges, with such a wet May following a wet fall with limited N application and limited ability to apply N this spring. As with other things, we’re in uncharted waters here: we have done a lot of N research over the past five years, but have had no conditions like this, and so we need to do a lot of guessing, of which only some will be “well-educated.”
 
If a producer was “lucky” enough to get be able to apply N last fall, how much is still in the soil to be available for this year’s crop, once it’s planted? We can be sure that nearly all of the N applied last fall or before May this spring is in the nitrate form by now, whether or not N-Serve was used. Nitrate moves with water, and with so much rainfall and no chance for the soil to dry to halt movement, there’s a good chance that most of the N has moved down into the soil, well below where the crop needs it to be once growth begins. While some of this N has likely reached the lines by now, we would expect that most of it is still in the soil, and capable of moving back up towards the roots zone when (if) water stops moving down. Where water has stood in fields for a week or more, we can expect that, at least by the time the soils there dry out, as much as half of it may be gone, lost to leaching and to denitrification. Some of the most persistent water holes won’t likely be planted, which will make the need for additional N there moot.
 
On the positive side, with soil temperatures in the 60s and 70s now, mineralization has kicked in, and this will be an important source of N as crop growth gets underway. That takes some of the urgency away from having to apply N right at planting. Even with the soil N supply kicking in now, it will be important to get fertilizer N on by the time growth gets underway, in order to avoid deficiency. Broadcast UAN used as herbicide carrier or N applied with the planter will boost the short-term N supply and postpone the need to apply in-season N. It will help a great deal if soils dry out at some point in order to stop the downward movement of N and to allow the roots to grow and take up N, and the plants to take on the darker green color that says they are not lacking in N. This won’t happen until the water coming into the plant carries with it more N than it is carrying today.
 
Any corn planted will likely benefit from some additional N, regardless of what’s been applied up to now. With so much water moving into and through the soil, we can’t count on full availability to the crop even of N applied a month ago in preparation for planting that didn’t happen then. How much N we should apply once the corn crop is planted, including how much to credit already-applied N, are tough questions.
 
If we applied 180 lb N in the fall following soybean harvest, let’s guess that 150 of it is still in the soil, and we’ll further guess (hope) that rainfall will slacken enough that, at some point before tasseling, soils will dry enough so that roots can take up some of this 150 lb of N. In such a case, we should still consider applying 10 gallons of UAN (about 30 lb N) at or after planting; if not before emergence, then dribbled near the row soon after emergence. The idea is to sustain the crop as it establishes itself, with additional N coming from mineralization until the roots begin to reach and take up the N from the fall application.
 
If less than the full amount of N planned for application was applied in the fall or early spring, then we might similarly discount that amount—by a fourth to a third—when determining how much more to apply. If corn was planted in mid-May with only 25 or 30 lb N applied then, we should apply the rest of the N as soon as we can reasonably do so in order to ensure a good N supply as growth gets underway. How we apply this N may not matter much, as long as we get it on in a way that keeps it relatively safe from immediate loss.
 
Although many may be shying away from anhydrous ammonia due to the need to apply before soils dry out, remember that ammonia injection provides better retention of applied N than does any other form or method of application. The addition of stabilizer to ammonia for in-season application is not recommended. If ammonia can be applied before the crop is more than a few inches tall, injecting it closer to the row than 15 inches can help improve access by the roots to the N. Once the crop has more than 4 leaves, application should be moved to the row middle to avoid damage to the roots.
 
If the main source of fertilizer N will be UAN, it should be applied using method other than surface broadcast. That means shallow injection; broadcast followed by incorporation by tillage; or dribbled, streamed, or surface-banded (all mean more or less the same thing) on the soil surface. The idea with streaming UAN is to concentrate the application so that some of the UAN moves down into the soil, where any ammonia released from urea (by urease enzyme) is captured by dissolving in soil water. Even so, applying a band of UAN in the surface leaves some of it without much protection from loss. Using a urease inhibitor may limit volatilization some, but when surface soils are warm, this won’t prevent all loss. Injection is a safer method, and dribbling UAN near the row, especially once the crop has 4 or more leaves, can get the N into the crop more quickly than if the UAN is midway between the rows. If rainfall continues, the concern will shift from urea volatilization loss to movement of the N deeper in the soil.
 
Broadcast urea can also be a good source of N, with the advantages of rapid application and the ability to broadcast by ground or by air onto the emerged crop without damage. A disadvantage is the potential for volatilization loss, as we discussed above. Urease inhibitors like Agrotain can be used to limit such loss, but rain within a few days effectively limits losses. Controlled-release forms should be used with caution, as we are interested in having the N from urea get into the soil and to the root quickly once the crop starts to grow.
 
If we get lucky and rainfall returns to more normal patterns in June, we should be OK using the MRTN rates for N, which in round numbers are about 180 lb N/acre for corn following soybean in southern and central Illinois, and about 170 lb N/acre in northern Illinois, with ammonia at $600 per ton and corn at $4.00 per bushel. Rates for corn following corn are 200 to 210 lb N per acre. If June is wet like May, then we may want to go back with another 30 to 50 lb N in the two weeks before tasseling. I don’t know of a good way to be sure whether this will be needed or not, but as a general guideline, having the crop take on a dark green color in mid-vegetative stages would tend to indicate that it isn’t needed, while getting a lot of rain during June but still having good crop growth and apparent yield potential might be a signal to go ahead.
 
Q: Should we change anything regarding other nutrients?
A: When we plant into warm soils, we tend not to see, and crop plants don’t experience, temporary nutrient deficiency symptoms related to having cool soils early. Any planned applications of P and K this spring that didn’t get done should be delated until fall.
 
Q: Should we adjust seeding rates or make other agronomic changes when planting this late?
A: Probably not for corn. For soybeans, there is a sense that narrow rows and higher seeding rates will help compensate for smaller plants when planting is late. That hasn’t been fully supported by research, but it certainly makes sense to avoid low stands and the potential for incomplete canopies during seedfill. Dr. Shaun Casteel at Purdue suggest increasing the seeding rate more the later the crop is planted. Our research has shown that we should probably have 115,000 to 120,000 plants already, and the important things is to make sure stands are at least that high.
 
Q: What about switching from corn to soybeans?
A: This is less an agronomic question than a matter of crop insurance, how much has already been invested in fields that were to be planted to corn, and current crop prices. For those with an optimistic mindset, corn prices relative to soybean prices, the fact that corn will not be planted as intended in some places (such as flooded river bottoms), and the fact that in some places in recent years (Ohio is one such place) corn planted in early June has yielded well will mean sticking with corn. It’s not an automatic decision to switch to soybeans when planting is so late like it might have been a few decades ago.
 
Q: Is there any possibility that we could end up with good yields despite the start?
A: Trendline yields for corn and soybean in Illinois in 2019 are about 190 and 67 bushels per acre, respectively. It’s too much even for an optimist to hope that actual yields will reach those levels, but if we get the best weather possible—no temperatures above normal (and not too many days much below normal, to get the crop to maturity)—and no lack of soil moisture on a single day during the growing season, we might hope to see yields in the vicinity of 90% of those trendline yields. Even that’s an audacious hope, but we haven’t tested today’s hybrids and varieties under such conditions in the past five years, and I’m hoping we’ll be surprised at what they can do in 2019.
 

Rain, late planting, and nitrogen

Written by Emerson Nafziger, University of Illinois    (View the U of I bulletin)

One of the most pressing questions as planting continues into June after a very wet May is whether or not the high rainfall amounts over the past month have affected the amount of nitrogen fertilizer needed for the corn crop this year. This is a complicated question, related both to concern about how much early-applied N might be lost and to decreased yield potential from late planting that might lower the need for N. The recent price increase in corn also provides an incentive to make sure the crop gets enough N.
 
Dr. John Sawyer of Iowa State University posted an article this week in which he described their finding that the crop is likely to respond to additional N if total rainfall from April through June exceeds 16 inches in most of Iowa, and if March-June rainfall exceeds 18 inches in southeastern Iowa. The MRTN rate from the N rate calculator in Iowa is 140 lb N per acre, less than the 180 lb N or so in southern and central Illinois, and 170 in northern Illinois, for corn following soybean.
 
The current map shows that rainfall from April 1 through June 4 this year has totaled 10 to 12 inches in east-central Illinois; 12 to 14 inches in most of the rest of central and southern Illinois; 14 to 16 inches in much of northern Illinois; and more than 16 inches in a small area near the Quad Cities. If June rainfall is close to average, totals for April through June will exceed 16 inches over most of the state.
 
It’s not clear how well this way of predicting the need for additional N applies to corn in Illinois, especially with so much of the crop planted late this year. The on-farm N responses produced by Dan Schaefer of IFCA (with funding from NREC) may give us a clue about this. The following table shows percentages of central and northern Illinois (where most of the trials included here were located) that received more than 16 inches of rain between April and June each of the last four years, and percentages of the on-farm N trials that showed the need for more than 180 lb N.
 
April-June rainfall and N rates from on-farm N rate trials (corn following soybean) in Illinois, 2014-2018. Percent of area with more than 16 inches of rainfall was estimated from maps generated by the Cli-Mate program within the Midwest Regional Climate Center.
While these numbers show that higher rainfall during the spring is associated with the need for more N, the fact that the numbers in 2014 and 2018 don’t follow this trend indicates that this is complicated. For one thing, when it rains might be as important as how much it rains: April and May rainfall were at or below normal in 2014 and most other years (2017 was an exception), while June rainfall was well above normal in 2014, 2015 and, except in western Illinois, in 2018—all years with more responses to higher amounts of N. Yield levels were also higher in 2018 than in other years, which likely increased N demand. Because June rainfall isn’t very predictable at the beginning of June, we will probably need to wait into at least the middle of this month to assess the need for more N based on rainfall amounts.
 
When the N was, or will be, applied is a factor in trying to pin down N rate this year. It’s not a stretch to guess that some of the N applied last fall or early this spring has moved (as nitrate) below the rooting zone, and some of that is likely to have left the field through tiles. In places where water has stood, some has also been lost to denitrification. So in well-tiled fields in the “wet triangle”—from about Quincy to Chicago then west to Carroll County—it is likely that some early-applied N has been lost to this year’s crop, and may need to be replaced. If the crop got (or will get) some N at planting in that area, that will probably maintain yield potential until we can better guess at final amounts of N.
 
As I wrote here last week, when and if rainfall pauses long enough to allow soils dry to dry out will be a critical factor is how soon the crop gets access to N already in the soil. Corn planted on May 16 here (with N applied as UAN before planting) reached stage V4 on June 4 (at 403 GDD after planting, very close to the 395 GDD to V4 given in the Illinois Agronomy Handbook, Chapter 2), and is beginning to take on the healthy, green color we’ve been hoping for. So as roots grow out into the soil, they’re bringing N into the plant with water. If there isn’t much rain over the next week, we can expect a rapid improvement in the color of the crop. That’s helped along by good mineralization rates, and by warm, dry weather to stop the downward movement of N.
 
Where N was applied in May followed by wet weather, and the crop has been planted only in the last week, will that N be enough N? We should wait to see: if we’re in a period of moderating rainfall now, and the crop develops good color by the time it has 2 or 3 leaves, we may not need more. If it turns wet again and the crop stays yellow or pale green for a week or more after emergence, we might consider that as a sign that the crop isn’t getting the N that it needs. If it dries out (again) and the crop greens up, the roots are likely back to taking up N again, and the need for more N will be averted.
 

Soil tests for N?

Some have considered using a soil test for nitrate or perhaps a tissue test to see if there’s enough N for the crop. Tissue tests can be expected change quickly over time this soon after planting, and they are probably no better than simply observing the crop to see how green it is. Soil N testing that we’ve done (funded by NREC) over the past four years shows that, following application of 200 lb N in the fall (spring-applied ammonia is similar), nitrate levels in the top foot of soil were about 30 ppm in mid-May (stage VE to V2); about 30 ppm in early June (stage V5-V6); and a little less than 20 ppm by mid-June (stage V8-V10) averaged across three sites and four years. In all cases this amount of N was enough for maximum corn yield, so we can’t say with precision what the minimum soil N amount should be.
 
Plants take up less than 15 lb of N per acre by stage V5-V6, but take up as much as 75 or 80 lb per acre between stages V5-V6 and V10. So it makes sense that soil N levels decline as plants take up N; 1 ppm N in the top foot of soil is roughly 4 lb N per acre. So if soil N drops by 10 ppm (from 30 to 20) as the crop takes up 75 lb of N between V5 and V10, only about half of the N taken up is accounted for by the (net) drop in soil N; the rest comes from mineralization. Because there are these two sources of N, as well as some N perhaps moving up from deeper in the soil as plants take up water, the amount of nitrate we find in the soil during vegetative growth is not a very good indicator of the N supply to the plant, especially in June when soils are warm.
 

Using the crop to indicate N status

Growing plants will often indicate soil N availability better than does the amount of N in the soil. So we can watch the crop to see when it begins to take on a better green color. One difficulty with this is that it can, during early growth, be hard to tell how dark green the plant should be: we can’t very well know if a pale color means that there’s N in the soil that’s not available to the plant or if there simply isn’t enough N. Those doing canopy color sensing address this problem by putting “high-N reference strips” at one or more places in a field, with the idea that crop color in that strip can only be limited by factors not related to N level. Canopy color in the rest of the field is then referenced to that in the high-N strip to give a relative value.
 
I’ll suggest here using this approach in fields recently planted, especially in fields where it’s not clear if enough N remains available to keep the crop green and growing well. A simple way to do this is to drop some urea alongside or on top of the row in a small area in a field, then to watch the crop there relative to the crop that doesn’t have the extra N, to see if there is a difference in color. What rate we use for this isn’t critical, but a half-cup of urea (46-0-0) applied to 20 ft. of row is roughly 90 lb of N per acre—that should be plenty. To do this, find a place past the endrows (or a few rows into the side of the field) that will be easy to see later, and dribble a half cup of urea down 20 ft (8 steps is close enough) of each of 4 rows next to each other, to form a little “high-N patch” (HNP) for comparison. Plant a flag in the center of the patch, or flags on the corners. If a field has both light- and dark-colored soils, it might be interesting to place an HNP in each soil.
 
Then, simply look at the HNP every few days, especially a day or two after rain moves the urea into the soil, to see if the corn there turns greener than the corn outside the patch. If it doesn’t, then the crop is likely not limited by the amount of N available to it. If it does turn darker green, then continue to watch it for a week or so longer. If the rest of the crop greens up so that the HNP is no longer visible, then no more N should be needed, providing that enough N was applied.
 
But if the corn in the HNP stays greener than corn outside for a week or more, that is a signal that applying more N would likely improve yield prospects. How much more N should be used in a supplemental application? For the trials in the table above that showed the need for more than 180 lb N, the average amount by which they exceeded 180 lb N was between 25 and 35 lb N except in 2015, when it was 51 lb N per acre. So unless June turns out to be wetter than normal, 30 to 40 lb of additional N should be enough. Apply in such a way that gets N to the plants quickly—dropping solution N (UAN) near the row is probably the best way to do this. Broadcast urea would be next-best, and might be better if it can be applied by air when field conditions don’t allow ground applications.
 
Where corn is planted but most or all of the N still needs to be applied, the MRTN rate, applied as UAN, urea, ammonia, or a combination should be enough N for the crop, unless June ends up being very wet. A lot of rain in June often causes root damage, and that plus the lack of soil oxygen under very wet (including flooded) soils lowers the ability of the roots to take up N. Because the crop looks deficient whether there’s not enough N or the roots can’t take it up, it can be difficult to know whether adding more N will boost yields. We’ll wait to see what June brings.
 
A final note: now that we have a crop that’s mostly been planted late and into less-than-ideal soil conditions, it will be better if rainfall and temperatures during the rest of the 2019 growing season remain in normal ranges, and above-normal rainfall in July and August would be even better. There won’t be very many days this year between our wishing it would stop raining and our hoping that it starts again.

Fall fertilizer considerations in 2019

Written by Emerson Nafziger, University of Illinois    (View the U of I bulletin)

The high number of prevented-planting fields in some areas, the late start to harvest, and the inability to apply P and K fertilizer as planned last fall or this past spring combine to raise a number of questions about fall application of P, K, and lime over the next few months.
 

Prevented-planting fields

If P and K fertilizers were applied last fall or this past spring but no crop could be planted, there’s no reason not to count all of the applied P and K as available for the 2020 crop. The same goes for any lime applied over the past 12 months. Any nitrogen (N) that was applied with MAP or DAP is likely no longer available, and shouldn’t be counted in the 2020 supply.
 
If the plan was to sample soil last fall or this spring to determine how much P, K, and lime to apply but that didn’t get done, these fields can be sampled now in preparation for fall or spring application. If the plan was to sample after the 2020 crop, there’s no reason to move that up to this fall; these nutrients didn’t (and won’t) go anywhere. By the same token, there’s no reason not to apply after two years based on estimated removal using the same P and K rates set to be applied a year ago. Unless a cover crop has been or will be harvested from a prevented-planting field this fall, removal will be zero.
 
Our most recent numbers to use for estimating P and K removal (see my Bulletin article with details) are 0.37 lb P2O5 and 0.24 lb K2O per bushel of corn and 0.75 lb P2O5 and 1.17 lb K2O per bushel of soybean.
 
We mentioned last spring the concern about the “fallow syndrome” that’s been associated with having no crop in a field for an entire growing season. This problem, which appears as a phosphorus deficiency, has been more commonly seen in fields or parts of fields where water has stood for much of the season; it was reported in the Mississippi River bottomlands in 1994 following the flood of 1993, when water stood on parts of fields through much of the summer. If weeds or cover crops grew on prevented-planting fields for most of this summer, especially in August and September, the crop-friendly fungi (VA mycorrhizae, or VAM) that prevent this problem likely are still present, and there’s no cause for concern.
 
In low-lying spots where water stood into mid-summer, and in fields kept weed-free through the summer by tillage or herbicide, we can’t rule out a possible problem due to loss of VAM. There are commercial preparations of VAM that can be applied in-furrow to inoculate corn next spring. In most cases, it will be enough to make sure there’s adequate P close the seed so the crop can take it up as growth begin, after which VAM will start to regrow in the roots of the new crop. Growing a cover crop this fall will restart VAM growth this fall, and should rule out the need for any additional steps next spring.
 
A year without a crop is used deliberately in some dry regions to store water for the next crop, but is a novelty for most Illinois fields. So we don’t have much research to help predict what this might mean for the next crop: is “fallow” in 2019 more like soybean or more like corn in its effect on the 2020 crop? We think the answer is “neither” – that 2019 will instead be an “amnesty” year, in which any effects of the 2018 crop got canceled or at least minimized, leaving open the choice of crop in 2020. Wheat planted this fall can be expected to do well on fields where neither corn nor soybean grew in 2019, as long as we get rid of plants that can serve as a reservoir of insect-vectored diseases (see Nathan Kleczewski’s Bulletin article on this), take care not to plant too early, and provide enough P for the crop.
 
The extent to which weeds or cover crops grew and matured might influence how having no crop this year might affect next year’s crop. Any addition to the weed seed supply could complicate weed control going forward. Large quantities of mature (high-carbon, low nitrogen) residue produced this year may act much like corn crop residue, increasing the N requirement for a 2020 corn crop. Because weed or cover crop growth requires soil water, there may be a little less stored soil water next spring in fields where there was a lot of growth this year. But most fields that didn’t grow a crop this year are likely to have more water stored in the soil now, and should also have more mineralized N, both because less N was taken up by a crop, and because there is less residue whose breakdown ties up N. These increases may well diminish by next spring, but they still might be helpful to next year’s crop, whether that’s corn or soybean. In using the N rate calculator to set corn N rates in fields with no crop and minimal weed or cover crop growth this year, I suggest choosing soybean as the previous crop; in fact, with no removal of mineralized N from the soil by soybean this year, it might be appropriate to also set N rates for next year’s corn crop a little lower (within the MRTN range) than usual. In fields with a lot of residue present now, it might be more appropriate to select “corn” as the previous crop when using the calculator.
 

Fertilizer application

Soils are currently dry enough to allow application of dry fertilizer materials over much of Illinois; the wettest part of the state is northwestern Illinois, where the crop still has to mature. Harvest started slowly in Illinois, but with the warm weather this week, it will accelerate quickly as long as it stays dry. The development of wet conditions could slow both harvest and fertilizer application that follows harvest, but soils in the drier parts of Illinois can take in an inch or two of rainfall without turning muddy or forcing much delay. Most people are anxious to start applying fertilizer after the delays and frustration in getting this done over the past year.
 
There has been a considerable amount of discussion about whether or not placing P fertilizer beneath the soil surface is a sound practice. The main reason for doing this is to keep the P in MAP or DAP, which is highly soluble, from dissolving and running down slopes and into streams in the event of heavy rain. How much of this might occur is affected by slope, permeability of the surface soil, how dry the soil is, how much crop residue is present, and the intensity of rainfall. Soils following soybean harvest are generally more permeable than following corn harvest, but corn leaves more residue. Tillage increases surface permeability, but also loosens soil to make it move more readily with runoff water. Drier soils can take in more water before runoff begins than can wet soils.
 
October and November are drier months, on average, than spring months, crops growing into the fall extract a significant amount of water from the soil thus leaving it drier, and high-intensity rainfall events are less likely in the fall. So overall, chances of getting high-loss conditions are lower in the fall than in the spring, but they aren’t zero. Surface-applied P will move into the soil under normal weather conditions, and will end up safe from direct loss (it can still move if soil runs off the field) by December. Most research has shown no yield benefit to subsurface P and K placement in the fall, and it is not clear that the added cost of subsurface placement will provide a positive return in most years and on most fields. In strip-till systems, however, where subsurface placement doesn’t add to the amount of surface soil disturbance, applying P and K beneath the strip while strip-tilling in the fall may be a cost-effective way to apply these nutrients.
 
Although we’ve found that the N in DAP tends to be available to the next year’s crop if DAP is applied after soils cool down to 50 degrees, applying MAP or DAP when soils are warm will allow much of the ammonium from these materials to convert to nitrate in the fall; once it’s nitrate it can move down with water into and through the soil, including to tile lines if there’s a lot of rainfall. Even if the N doesn’t move too far down in the soil in the fall before the soil freezes, it will have a head start when water begins to move through the soil in the spring. There can also be direct movement of ammonium (along with P) in surface runoff during heavy rainfall before the MAP or DAP has had a chance to dissolve and move into the soil.
 
While it may not be practical to hold off on applying MAP or DAP until soil temperatures fall to below 50 degrees, we should recognize that even though the amount of N in these fertilizers is relatively small, it can add appreciably to the N that moves to surface waters through drainage tile. One solution that has been suggested is to switch from using MAP/DAP as the P source to using triple-super-phosphate (TSP, 0-46-0) which contains no N. If TSP is available at about the same cost per pound of P as MAP or DAP, it would be a good source to use, especially for applications made before mid-October. The “free” N that comes with MAP or DAP is more likely to reach tile lines than the roots of next year’s corn crop if it’s applied when soils are warm in the fall. If it’s applied after soil temperatures reach 50 degrees or if it’s applied next spring, the N in MAP or DAP does contribute to the N supply for next year’s crop.
 

Soil temperatures and fall ammonia application

Written by Emerson Nafziger, University of Illinois    (View the U of I bulletin)

According to NASS, Illinois producers harvested 36 percent of the corn crop and 52 percent of the soybean crop by October 20. That’s still behind the average pace of harvest, but harvest continues in many areas this week, and as it progresses, fields in many areas are becoming available for fall field work to begin.
 
Many producers in central and northern Illinois have fall anhydrous ammonia application high on their to-do list, especially after the fall of 2018 and the spring of 2019, when getting any nitrogen fertilizer applied was a challenge. As we have seen before, most people were able to work around the weather issues to get N applied, in some cases by changing to in-season applications, and sometimes changing the N form to one easier to apply in the narrow windows of opportunity last spring. While we hope not to see a repeat of such challenges very soon or very often, this past year reminds us that retailers and producers are up to the challenge of getting N applied even when the weather doesn’t cooperate.
 
Timing of anhydrous ammonia application in the fall is a major issue, and there is a considerable amount of anxiety related to having to wait until soil temperatures are low enough for safe application. Ammonia and ammonium-containing N fertilizers are the only fertilizer materials that are safer from loss when applied to cool soils than to warm soils. This is because the soil contains large populations of nitrifying bacteria that convert ammonium ions to nitrate ions—this process is called nitrification. As a cation, ammonium ions are attracted to the negative charges on clay and organic matter surfaces; this attraction means that ammonium ions don’t move with water as it moves downward through the soil. As an anion (negatively charged ion), nitrate is not attracted to negative charges in the soil, and so can move downward with water.
 
Nitrifying bacteria extract energy from ammonium while converting it to nitrite and then to nitrate; in the process, three oxygen atoms are added and hydrogen atoms and water are released. Because this is a biological process, nitrification is sensitive to temperature. The bacteria operate (and multiply) fastest at temperatures in the low to mid-80s, and the cooler it is the lower their activity; the rate of nitrification is close to zero at 40°, and is only about one-fourth of maximum at 50°. This means that waiting until soil temperatures are 50° or less will mean slow nitrification, and once soil temperatures reach 40°, almost all ammonia applied (which converts quickly to ammonium) will remain in the ammonium form—and so safe from loss—until soil temperatures rise.
 
Since the conversion of ammonium to nitrate stops only at soil temperatures of 40 degrees or less, should we need to wait until soil temperatures are 40° before we apply ammonia in the fall? We probably would if we could, but soil temperatures in central Illinois don’t reach 40 until after mid-November, which in practical terms means that the opportunity to apply ammonia in the fall would be greatly restricted. Another reason is that anhydrous ammonia released into the soil acts as a powerful sterilizing agent: it spreads out into the soil as it changes from a liquid to a gas at the knife, and kills most living things in the soil into which it moves. This means that microbes need to grow back into the depleted application band, and that takes enough time at cool soil temperatures to allow the soil to cool even more before there are enough bacteria to get nitrification back on track.
 
Another method available to slow nitrification is the use of a nitrification inhibitor. The most common one in use over the past decades is nitrapyrin, which has been in use for more than four decades. The most common trade name for this chemical is N-Serve®, but nitrapyrin is also sold under other names. Centuro®, with the active ingredient pronitidine, is a relatively new nitrification inhibitor from Koch Agronomic Services. These act to decrease the activity of nitrifying bacteria, either by killing the bacteria or by chemically inhibiting their ability to convert ammonium to nitrate. These inhibitors break down in warm soil, and so they are effective for a longer time when applied in cool soils.
 
Using inhibitors with fall ammonia provides an extra measure of protection in the event soil temperatures rise (as they sometimes do) shortly after application, and when soil temperatures begin to rise in the spring. They do not “lock in” N to keep it perfectly safe from nitrification and movement; in fact, if soil temperatures drop to 40 degrees soon after application and stay down until spring, a nitrification inhibitor may not be necessary. We can’t know what will happen from fall until the next spring, though, so using a nitrification inhibitor provides insurance against unexpected increases in soil temperatures followed by wet weather that can move nitrate in the soil.
 
Our recommendation for fall ammonia application in Illinois is to wait until soil temperatures are below 50 degrees, and also to use a nitrification inhibitor. The most pressing question for most people is how to know for certain when soil temperature is low enough, and also to have an idea of whether soil will stay cool and cool down some more to help protect the N after application. Like air temperatures, soil temperatures change over the course of the day, so soil temperature is a moving target. Figure 1 shows how soil temperature changed over 24 hours on October 23, 2009. Low soil temperatures typically occur at about sunup, and the high temperatures at about sundown. The average soil temperature recorded for this date was 48.5, which is close to the value recorded at noon. So if we take a soil temperature at a particular time, noon will best represent the value for that day. Even so, the soil was above 50° for about 10 hours of the day, during which time nitrification would have proceeded.
 
Figure 1. Soil temperature (bare soil, 4” depth) at Peoria on October 23, 2019. Data are from https://www.isws.illinois.edu/warm
While these numbers show that higher rainfall during the spring is associated with the need for more N, the fact that the numbers in 2014 and 2018 don’t follow this trend indicates that this is complicated. For one thing, when it rains might be as important as how much it rains: April and May rainfall were at or below normal in 2014 and most other years (2017 was an exception), while June rainfall was well above normal in 2014, 2015 and, except in western Illinois, in 2018—all years with more responses to higher amounts of N. Yield levels were also higher in 2018 than in other years, which likely increased N demand. Because June rainfall isn’t very predictable at the beginning of June, we will probably need to wait into at least the middle of this month to assess the need for more N based on rainfall amounts.
 
Figure 2. Daily minimum, maximum, and average soil temperature averaged over six years (2013-2018) at Peoria. Data are from https://www.isws.illinois.edu/warm
Because we often have access to more information on air temperature than on soil temperature, it helps if we can predict soil temperature from air temperature. Figure 3 shows the daily average air and soil (bare, 4” depth) temperatures over the past six years at the Illinois State Water Survey WARM site (located at Illinois Central College) at Peoria. While these lines are close together when averaged across years, how air temperature changes in a given year can be quite different compared to how soil temperatures change. So while soil temperatures generally follow the drop in air temperatures in October and November, it’s helpful to follow air temperature trends, but to also track soil temperatures after mid-October each year in order to know when soils are cool enough.
 
Figure 3. Air and soil temperature (bare soil, 4” depth) at Peoria, averaged over 6 years, 2013-2018. Data are from https://www.isws.illinois.edu/warm
So, will we be ready to begin applying anhydrous ammonia in central Illinois soon? So far this October, soil temperatures at Peoria have, with some ups and down, trended cooler than over the past six years (Figure 4). Even more encouraging, the forecast is for below-normal temperatures to arrive this weekend, at to last into the beginning of November. We can’t know if they’ll rise after that, but if they drop to the mid- or lower 40s by early November, the N we apply should be as safe as we can make it. There’s no special need to rush—the wet weather we had a year ago doesn’t look likely to return soon—but in terms of soil temperatures, there’s no reason why application can’t start during this last week of October.
 
Figure 4. Average daily bare-soil (4” depth) temperatures at Peoria from 2013 through 2018, and so far in 2019. Data are from https://www.isws.illinois.edu/warm
There are a few basics to follow when applying anhydrous ammonia in the fall. Fall ammonia applications should include a nitrification inhibitor. Ammonia should not be applied in the fall south of IL Route 16 (which roughly follows the Shelbyville moraine) because soils there stay warm longer and warm up more quickly in the spring, increasing the chances of loss. Don’t apply fall ammonia on very heavy (clay) soils due to increased chances of N loss from denitrification next spring, or on light-textured soils (sandy loam or lighter) due to increased chances of leaching loss. The N rate calculator, which we will update with new data this winter, indicates that N rates of about 180 in central Illinois and about 160 in northern Illinois are appropriate. If you plan to apply N in any form—MAP or DAP this fall or next spring; planter-applied N next spring, or N solution as herbicide carrier—be sure to adjust the amount of N this fall by applying only the amount needed to produce the total for next year’s crop.
 
While some in the northern Corn Belt have begun to apply urea in the fall, we do not believe that this is a safe practice in Illinois. Even when urea is protected by polymer coating to slow its release or by inhibitors to slow the conversion of the ammonium that forms as urea breaks down in the soil, the N from urea usually ends up at shallow depths in the soil, and so is vulnerable to nitrification if surface soil temperatures rise. Urea also provides none of the sterilizing effect that ammonia release in the soil provides, and this means that nitrifying bacteria can go to work on the N from urea without delay. It's certainly possible that most of the N from fall-applied urea can carry over to be available in the spring, but that means that everything has to go just right. Under Illinois conditions, the chances of that happening are too low to make this a safe way to apply N.
 

Managing Nitrogen for Corn in 2020

Written by Emerson Nafziger, University of Illinois    (View the U of I bulletin)

As was the case a year ago, there have been limited opportunities to apply nitrogen fertilizer since last fall. Rainfall in Illinois through the first three weeks of March has been at or above average, and temperatures have been a few degrees above normal. Soils remain wet, and there is little in the current weather pattern to indicate that a drying period is on its way soon. Potential drying rates will increase as temperatures rise, though, and we will hope that rainfall remains at or below normal to allow soils to dry as we move into April.
 

N rate

Despite difficult conditions in 2019, Dan Schaefer of the IFCA and John Pike in southern Illinois, with funding from the Illinois Nutrient Research & Education Council (NREC), were able to conduct on-farm N rate trials that showed that responses in most regions, even with late planting, were similar to those found in recent years. Yields were generally not as high as in 2018, but in central and northern Illinois, the fact that responses were similar to those already in the database meant that adding the data from the 2019 trials didn’t change the guideline N rates (MRTN values) by very much for this part of the state.
 
The 2019 data in southern Illinois, however, continued the trend we saw in 2017 and 2018, in which higher yields required higher N rates to reach those yields. Such a correlation between optimum N rate and yield across trials does not exist in higher organic-matter soils in central and northern Illinois. We think this is because weather conditions (warm temperatures and plentiful moisture) that lead to high yields (and high N uptake) also increase the amount of N supplied by mineralization of soil organic matter, leaving the amount to be supplied by fertilizer unchanged, at least on average. In contrast, soils in southern Illinois have less organic nitrogen to mineralize, so high yield levels there make the crop more dependent on N from fertilizer.
 
This correlation between N rate and yield in southern Illinois supports the idea that we consider adding more fertilizer N to corn growing in lower organic-matter (<2% OM) soils in southern Illinois if the crop has high yield potential. I suggest using the MRTN rate for yields up to 190-200 bushels per acre, and for yield potentials above that (determined based on crop condition when corn is 2 to 4 ft tall), use a total of 1 lb of N for each bushel of expected yield. That may often mean applying N with high-clearance equipment, either as broadcast urea or as UAN dribbled near the row. Dribbled N often distributes more uniformly, and leaving UAN on the surface near the row moves it closer to the root system and may improve uptake.
 
Use the N rate calculator to calculate best (MRTN) N rates for corn in Illinois. We updated the database in early March, adding the 2019 data and removing some of the older data. Using a corn price of $3.50 per bushel and the current ammonia price of about $500 per ton ($0.30 per lb of N) produces the MRTN values and ranges shown on Table 1 below. Low and high ends of the range are those N rates at which the return to N ($ per acre) are $1.00 less than at the MRTN. MRTN values are also shown for N prices of $0.40 and $0.50 per lb, keeping the corn price at $3.50 per bushel. N prices for UAN and urea are currently around $0.43 per lb of N.
 
Remember that the MRTN rate (and ranges) generated by the N rate calculator includes all of the N applied to the field, not just to the main application. This means counting into the total any N applied with MAP or DAP in late fall or spring, any N applied with herbicide or with the planter. If N from several different sources is used, base the rate of the last application (adding in all previous amounts) on the price of fertilizer N that is used for the last N application.
Table 1. Current MRTN (guideline N rates) for corn in Illinois, after adding the data from the 2019 growing season.
 

N timing

In about 90 percent of on-farm trials comparing N rates applied as ammonia in both the fall (with N-Serve) and the spring, fall- and spring-applied N have produced virtually identical responses to N rate, at the same yield levels. Across 16 trials, including several in which spring-applied N performed better, and several in which fall-applied N performed a little better, the optimum N rate averaged about 12 lb higher—181 versus 169 lb/acre—and the yield at the optimum N rate 1 bushel less—235 versus 236 bushels per acre—for fall-applied N compared to spring-applied N. That meant an advantage of $9 per acre in return to N for spring-applied N, but since getting that added return would have required knowing when and by how much to decrease N rates for spring-applied N, it would have been difficult to realize this benefit. The average optimum N rate for fall-applied N was almost identical to the MRTN for central Illinois (Table 1): using the MRTN would meant using more than the optimum N rate in more of the spring-applied N trials, and so would have meant less advantage for spring-applied N across these trials.
 
One of the main lessons we’ve learned from our N timing and N form studies in recent years is that, in order to maximize yield potential, corn plants need to have a substantial amount of N available in the soil near the row after plants emerge and before their nodal (main) root system starts to develop. In one study in 2019, the crop was planted in late April but fertilizer rates couldn’t be applied until early June due to wet May weather. As a result, N responses rose in a straight line up to the maximum N rate used (250 lb of N), and did not reach a maximum. We also saw several instances in which cover crop rye was not controlled early, and probably because the rye roots had stripped the N from the upper soil, corn yield suffered even when high rates of N were applied after the crop emerged.
 
We don’t know exactly how much N needs to be present during early corn growth, but we believe that this N needs to be in the soil near the plants when the nodal roots begin to appear—at about growth stage V2. To have 40 to 50 ppm available N in the upper soil at V2 means incorporating 40-50 lb N in the top 3.5 inches of soil, and having most of the N stay there. If we incorporate N into a zone 7.5 inches wide by 3.5 inches deep centered on the row, only 10 to 12 lb N per acre will produce 40 to 50 ppm, if the N uniformly distributed and if it stays there for at least 3 weeks (300 GDD) after planting. That amount (but not much more than that) could be applied in-furrow, but any downward movement of that N would take it out of the rooting zone of small plants. Applying 30 to 50 lb N in a 2 x 2 placement, or dropping liquid or dry fertilizer over the row to provide 30 or 40 lb of N per acre would better assure having N when it’s needed early, if there’s equipment to do that. In-furrow placement of 10-12 lb of N as UAN is better than nothing. Seedling damage from such applications is rare, but split-tube placement with a seed firmer will protect a little better against this.
 
Even if planting is delayed and takes precedence over N application, some N really does need to be applied into or atop the row before the crop emerges: it is too risky to wait for several weeks to get the first N applied, especially if even that N is not placed near the row. If it stays wet this spring, some producers and retailers might need to get creative in order to get this done. Delayed planting means warmer soils at planting, and warmer soils mean more mineralization. This will boost the soil N supply some, but especially if rain moves some of the mineralized N down, there may still not be enough to maximize yield potential of the crop.
 

Splitting N

In one set of results from different forms and times of application of 150 lb N per acre, we found that a split with 100 lb at planting and 50 lb applied in-season generally yielded a little more than applying all of the N between the rows at planting. Applying 50 lb N as broadcast UAN at planting (to mimic the use of UAN as herbicide carrier at or after planting) then 100 lb as UAN injected at stage V5 did not yield as well, probably because there wasn’t enough N near the root system when it was needed, before sidedress. Most of the treatments with 100 lb N injected at planting followed by 50 lb as sidedressing worked about equally well. Waiting until sidedress time to apply all of the N was not an effective way to apply N, and placing UAN on the soil surface also produced lower yields, even when urease inhibitor was included. All of these point to the importance of having enough N in the soil early enough to maximize yield potential during early growth, and of applying all of the N in a way that results in less loss.
 
We also found in these studies that splitting N—with some at or before planting and the rest as sidedress—often produces yields no higher than applying the same rate (with appropriate placement) early. That does not mean we shouldn’t split-apply N, but we should do it more for logistical purposes than as a way to get higher yields with the same (or lower) rate of N, at least on productive soils. We have found no advantage to keeping back 50 lb N to dribble in-row at tassel, nor have we found an advantage to applying N several time (spoon-feeding) during the season. Very wet June weather, such as we had in 2015, in some cases meant a response to adding additional N. But getting N applied under such conditions is not easy, and every trip to apply N brings the added cost of application as well as the risk of not having the N get to the roots for uptake in time for the plant to respond.
 

Inhibitors

Despite the fact that inhibitors sold as N fertilizer additives have been around for decades, there remains a considerable amount of confusion about these products, including what they do, and when and how they should be used. Nitrification inhibitors slow the activity of bacteria in the soil that convert ammonium to nitrate. Both ammonium and nitrate can be taken up by plants, but the ammonium form is attracted to negative charges on clay and organic matter, and so stays in the soil, while nitrate is negatively charged, so moves readily with water as it moves down through the soil. So slowing the conversion of ammonium to nitrate (nitrification) is a way to keep more N in the soil and available to the crop under high-loss (wet) conditions. Chemicals sold as nitrification inhibitors include nitrapyrin (products include N-Serve® and Instinct II® by Corteva); pronitridine, a newer product developed and sold as Centuro® by Koch Ag; and dicyandiamide (DCD), a nitrification inhibitor sold by a number of companies under different trade names.
 
We normally add a nitrification inhibitor with anhydrous ammonia applied in the fall. The later we apply ammonia in the spring the less likely it is that a nitrification inhibitor will be needed to protect the N. As a biological process, nitrification is slow when soil temperatures are in the 50s (through early-, mid- and late April in southern, central, and northern Illinois), and begins to speed up once soil temperatures reach 60 and above, which usually occurs in late April in southern Illinois and mid-May in northern Illinois. If we add in the effect of the NH3 itself in suppressing microbial activity, it’s unlikely that applications of ammonia made after mid-April in southern Illinois or after early April in northern Illinois will need the further delay in nitrification provided by nitrification inhibitor. There are exceptions to this: May can be warm and wet, with rapid conversion to nitrate, in which case a nitrification inhibitor can be helpful. But if the crop is planted early and grows fast in May, uptake starts early as well. And if the weather is relatively dry, N is unlikely to move in the soil even if it’s all nitrate. This makes it difficult to know at the time of application whether we should add a nitrification inhibitor, and we should play the odds based on current conditions and expected planting time to help make this decision.
 
Because cool soils are slow to dry, early spring (preplant) applications of ammonia are usually done when soils are wetter than ideal. Ammonia application on wet soils means more soil compaction, and with the diameter of the ammonia band very small when application is into wet soil, its concentration in the band is high. If the soil dries out considerably after application (a rarity if it’s wet into April), NH3 can begin to leave the band where it’s been dissolved and to move up in the soil through the knife track, where it could damage seeds or roots. Using RTK to apply the band 6 to 8 inches away from where the row will be planted can eliminate such damage. Tilling after ammonia application can also help disperse the band and will usually lower or eliminate the risk of ammonia injury on seedlings. Deeper placement can also help prevent damage, but will leave the N farther from the roots.
 
The other type of inhibitor sold for adding to N fertilizer is urease inhibitor. Inhibitors that do this include NBPT (sold under different brand names), and mixtures of NBPT with duromide (ANVOL® from Koch Ag) and with NPPT (Limus® from BASF). Thiosulfate, which is also used as a sulfur source, is thought by some to inhibit urease, although lab studies tend to show that it’s less effective. As the name implies, urease inhibitors are effective only when added to urea or to other urea-containing fertilizers such as UAN solution. They do not slow the conversion from ammonium to nitrate; they only slow the breakdown of urea into ammonia and carbon dioxide. If this breakdown happens on or near the soil surface, ammonia can go off as ammonia gas into the air.
 
Ammonia is extremely soluble in water, so if urea breaks down in moist soil, the ammonia released will dissolve immediately, and hardly any of it will escape into the air. The urease enzyme that speeds up this breakdown is very common in soils, so if urea or UAN is broadcast on the soil and there is no rain for a week or more, a lot of ammonia can be lost into the air. Broadcast UAN, because it spreads the N in a thin layer over the soil surface, exposes more of the urea to urease activity. But only half the N in UAN is in the urea form—the other half is nitrate and ammonium, which aren’t affected by urease. UAN does contains some free ammonia in solution, and some of this may volatilize as the solution dries. Dry urea, once it dissolves in soil water, is all subject to urease, but urea granules that fall into cracks in the soil surface may gain some protection.
 
Rainfall moves urea into the soil and also wets the soil and dissolves ammonia, greatly decreasing the loss of ammonia. This means uncertainty regarding whether or not to use urease inhibitors. If urea or UAN is incorporated into the soil at or soon after planting or at sidedress time, there is no need to add a urease inhibitor, since ammonia rarely escapes from soil. Dribbling or surfacebanding UAN exposes it a little less to urease and moves some into the soil a short distance. UAN dribbled on the surface near the row is a little less exposed to sunlight and wind, and water coming down the plant stems from light rain or dew can help move the N into the soil. Still, surface-applied UAN can never be considered completely safe from volatilization loss, so an inhibitor might be useful if the forecast is for warm conditions without rain for a week or more after surface-banding near the row.
 
With warm surface soil temperatures, nitrification will begin soon after the urea is dissolved and in the soil (as ammonium). SuperU® (from Koch), which has both urease and nitrification inhibitors, has performed well in trials when broadcast on the surface, and has yielded more than broadcast urea with the urease inhibitor Agrotain (NBPT). Assuming that both products inhibited urease equally, the difference must have been due to more rapid conversion of ammonium to nitrate, and movement of some of the N out of the rooting zone.
 

Novel products sold to increase microbial N fixation

There has been a recent upswing in advertising and products that are said to provide the microbes or to stimulate existing soil microbes that fix atmospheric nitrogen and make it available to the corn crop. Microbial N fixation is the way that soybeans get most of the N they need, but such fixation in legumes involves the pant producing nodules that are attached to the roots below the soil surface, and in which anaerobic (low-oxygen) conditions exist to aid in the fixation process. We’ve known for a long time that there are some “free-living” (not in nodules) bacteria in soils that can fix N for the atmosphere, but measured fixation rates by such microbes tend to be very low – on the order of a few pounds of N per acre. That’s in part because fixing atmospheric N requires a great deal of energy, and a soybean plant can pump sugars into nodules a lot faster than sugars leak out of (corn) roots to feed this process in microbes that live near the roots. There was once hope that corn plants could be genetically modified to produce nodules and house bacteria that could fix much of their own N, but the machinery the plant needs to form nodules and to transport fixed N in the plant is so complex that this seems to be unlikely, or at least a long ways off.
 
There are two types of these products, mostly developed and marketed by startup companies backed by venture capital. One type is a preparation of the microbes (bacteria) that fix N; these are usually applied in-furrow, with the idea that they’ll multiply and grow near the root to eventually get enough sugars from the roots to fix N that the plant can take up. The idea is that corn plants and such bacteria form a mutually beneficial (symbiotic) relationship, with the corn providing sugars and other growth substances and the bacteria giving back N. It’s not entirely clear that bacteria can act as little “N pumps” like this, and if they can, it’s not clear how such a symbiosis would benefit either the plant or the microbe.
 
The other type of product being marketed is a chemical product that is said to stimulate the growth and function of bacteria that go on to fix N for the corn plant. It appears that some of these can be applied as foliar sprays, presumably with the idea that they can be released by the roots into the soil, or that they stimulate the plant to release something on its own that in turn stimulates growth of bacteria that fix N.
 
Claims on websites for these products might say that they make the plant (and roots) grow faster, and often show photos to that effect. Some mention how much N fixation might be expected from using the product. I have not done any work with any of these, but will just observe that pinning down rates of N fixation by microbes when rates are low (25 lb N per acre per season seems to be a somewhat typical amount) is really difficult, and any such numbers should be viewed with caution. One way that some such studies have been done in the past is to use a relatively high rate (say 200 lb N per acre) and then a lower rate, say 160 or 175 lb N per acre, along with the product, and if the yields are about the same, to conclude that product provided the difference.
 
I’d suggest a wait-and-see approach to products like this. Some companies are asking producers to conduct on-farm trials, and if it’s possible to do a set of paired strips, assigning with and without treatments randomly within each pair, that might provide some information. Split-field trials are a lot less satisfactory, since the two halves of a field never yield exactly the same, and field variability is likely to be greater than any treatment effect. But most companies will control the data from such trials, and in most cases products “win” when such results are put up on websites.
 

Managing N this spring

One lesson we learned from the 2019 growing season is that we can get nitrogen applied even when conditions are not very good. That doesn’t mean that N was used to its best advantage in every field: there were examples of fields where N was not applied early enough to maximize yield. But with proper attention to applying the right rate at the right time, using a form that will protect against loss, Illinois farmers have the ability and flexibility to get N management done right, even when spring conditions are challenging.
 
While it’s wet over most of Illinois now, and the weather forecast doesn’t look very promising that it will turn warm and start to dry very soon, we can begin to plan our N management strategy based on principles discussed above. Instead of developing elaborate scenarios of what might happen this spring and how to respond, I’ll list here a number of things to keep in mind as we go forward:
  • Use the N rate calculator as the start to determining how much N to apply. Note the “profitable range” that extends on either side of the MRTN. For most fields the total N rate should be within this range, and results of hundreds of trials over the years in Illinois tell us that we can expect the return to N (increase in yield and gross income minus N cost) to be maximized at the MRTN.
  • While we have said in the past that we might consider moving N rates out of (above) the range given by the calculator, we have found a consistent advantage to doing only when it’s been very wet in June. Root damage from too much soil water and/or loss of N may in such cases mean that the crop can benefit from additional N, but only soils dry some to improve root function, and if N can be applied by or before the time of pollination.
  • In southern Illinois, apply rates within the MRTN range, and wait until V5 or V6 to decide whether yield potential is above 190 to 200 bushels per acre; if it is, consider adding some N later in vegetative growth to bring the total rate up to 1 lb N for each bushel of expected yield.
  • Rainfall from last October 1 through March 23 has ranged from a little below normal to normal in the northern half of Illinois, and from 3 to 6 inches above normal in the southern half of the state. There were a few spikes in temperature and rainfall over the winter, but we don’t think that more fall-applied N has been lost than usual; we can count on its being present for the 2020 crop.
  • If we get a break in the weather that allows ammonia to be applied before late April, we should consider taking advantage of that. Ammonia is currently cheaper, and is safer to apply, than any other form of N. We should take care to avoid applying it in such a way that planter units can drop into the application band, but otherwise the chances for seedling damage from ammonia are low.
  • If wet soils delay both planting and the application of N, it will pay to find a way to get some N (at least 40 to 50 lb N per acre; more may be better if it’s not concentrated close to the row) applied so that it is available to the nodal roots as they start to develop at about stage V2.
  • If cereal rye is present in fields where corn will be planted, try to spray it to kill it several weeks before planting. The large the rye is when killed, the more critical it is to kill it early. If the rye makes substantial (more than 8 inches) of growth before it’s killed, pay additional attention to getting N close to the row at planting in order to replenish when the rye removed from the soil.
  • If you plant corn where there was no crop in 2019 and where weeds were controlled by tillage or herbicide, the 2020 crop might benefit from planter-applied phosphorus in order to prevent “fallow syndrome.” If there’s a flush of spring weed growth, or if MAP or DAP is broadcast this spring, there will be les™s (or no) need for placing P close to the row.
 

Is Fall a Good Time to Apply Nitrogen?

Written by Emerson Nafziger, University of Illinois    (View the U of I bulletin)

Applying anhydrous ammonia in the fall to provide N to the corn crop the following year has a long history in Illinois and in other parts of the Corn Belt where rotation, tillage, and manure management practices allow it. Fall application means getting a major field operation done when soil conditions are generally more favorable than they are in the spring, and it helps to spread the work load across more months. Historically, the cost of ammonia has also been lower in the fall than in the following spring, which has provided an extra incentive for fall application.
 
Although fall application remains a common practice in Illinois, it has come under increasing scrutiny in recent decades. Fall application means having N in the soil about six months before the crop will begin to take it up, and soil conditions during these months are not predictable. Ammonia converts to ammonium in the soil after application, and as long as soil temperatures remain low, the microbial conversion of ammonium to nitrate is slow. Once N is in the nitrate form, however, it can move downward with water moving though the soil, and, in tile-drained fields, can leave the soil and move into surface water. Fall-applied ammonia usually has a nitrification inhibitor added to slow this conversion process, but if soils warm up and there’s substantial rainfall by May, some of the ammonium will convert to nitrate, and some this nitrate may be lost. Spikes in river and surface water nitrate usually come in the spring, and while much of this N is from residual left after the previous fall’s harvest, fall-applied N undoubtedly contributes some to this.
 
As we might expect, the amount of fall-applied N that is lost (moved out of the field, denitrified, or moved below the rooting zone) before plants can take it up varies widely over years and soils. Measured and modeled estimates of loss show this variability, but as long as guidelines are followed, we do not believe that losses reach the high levels that some models have shown. These guidelines include: 1) no fall application on light or very heavy-textured soil; 2) no application at soil temperatures above 50 degrees; and 3) no fall application in regions without cold winters, including southern Illinois. There can of course be loss of spring-applied N as well, especially if N fertilizer used in the spring contains nitrate (for example, UAN) which can move in the soil without delay. Surface-applied, urea-based N sources can also volatilize (lose N as ammonia gas), dissolve and run off the field with heavy rain, or convert to nitrate rapidly when soil temperatures are warm; such N can be lost to downward movement or to denitrification if water stands in the field.
 
The best way to measure the actual effect of fall versus spring N application is to compare timings in side-by-side N rate strips in field trials. We are fortunate in Illinois to have a set of such trials, managed by Dan Schaefer of IFCA and conducted, with funding support from the fertilizer tonnage fee through the Illinois Nutrient Research & Education Council, in farm fields from 2014 through 2018. To our considerable surprise, we found that in nearly every case, corn responded to N rate almost the same for fall-applied as for spring-applied N. Figure 1 below shows N responses in one of these trials in which both the optimum N rate and the yield at the optimum N rate were slightly higher for fall-applied N.
 
Figure 1. Comparison of fall- versus spring-applied ammonia in an on-farm N rate trial in 2018.
 
The results found in fifteen other trials were similar to those shown in Figure 1. Most of these trials were with corn following soybeans, and most were in central Illinois. Averaged over these 16 trials, the optimum N rate, using a corn price of $3.75/bushel and an N cost of $0.30/lb ($492 per ton of ammonia) was 186 lb N per acre with fall-applied ammonia and 176 lb N per acre with spring-applied ammonia. The yield at the optimum N rate averaged 235 bushels per acre for fall-applied N and 236 bushels per acre for spring-applied N. We can also use these responses to calculate a “return to N” (bushels added by N × corn price minus N rate × N price); the RTN averaged across these 16 sites was $406 per acre for fall-applied N and $414 per acre for spring-applied N.
 
We elected to drop one trial from this set of data due to its unusual response. This trial was in the river bottoms near the bluff, and it’s possible that the lighter-textured soil there received extra water that moved off the bluff. At that site in 2017, it took 193 lb of N as fall-applied ammonia to produce 227 bushels, while it took only 125 lb of N as spring-applied ammonia to produce 245 bushels. There was a considerable amount of variability in the trial, and using the MRTN rate of 180 lb N/acre for both would have produced yields only 12 bushels higher for spring-applied N. But this does show that when conditions for N loss are high, fall-applied N losses can be substantial. At the same time, these results show that the crop has nearly the same access to fall-applied N as to spring-applied N, and that applying N in the fall is not as economically or environmentally problematic as some have suggested.
 
So why not continue to apply N in the fall, or even move more application to the fall? In some areas this option has become limited by the fact that some fertilizer dealers no longer offer anhydrous ammonia, but instead offer other forms, typically UAN solution. Reasons for this often include safety—anhydrous ammonia is more hazardous to transport and apply than other forms of N. Ammonia needs to be injected into the soil, and this requires toolbars, large tractors, and labor when harvest and tillage may still be underway. Soil conditions also need to be dry enough, but not too dry, to allow application and proper sealing. Wet fall weather, the need to wait until about November 1 for soils to be cool enough for safe application, then having a short window to apply before soils freeze are all factors that have acted to nudge N application towards spring. We have found, in recent years when fall application has been limited, that moving a lot of N application to the spring is possible with existing equipment. Although this pushes a lot of extra work into the spring months, dealers and producers have been creative in finding ways to make application and very wet spring weather and late planting, producers managed to get N applied on the crop that got planted.
 
Despite its limitations, anhydrous ammonia typically remains the lowest-cost source per unit of N, although application costs and safety concerns may decrease that advantage some. More importantly, ammonia is the form of N most likely to stay in the soil and remain available to the plant. Uniformity of application has improved with development of better equipment, and at 82% N, using ammonia means transporting less weight. As I’ve written many times before, there needs to be some N fertilizer applied near the row at or very soon after planting; UAN is probably a better source than ammonia to place N in this way. A nitrogen management program that includes perhaps a third of the N as preplant or planting-time UAN, then the rest as sidedressed ammonia, may be worth considering.
 
Going into the fall, the main challenge with ammonia application is to wait until soil temperatures are low enough to keep the nitrification process to a minimum. In a typical fall that means waiting until the end of October. While going a few days early may not seem like a big deal, remember that soil microbial activity doesn’t stop completely until temperatures near freezing, and with soil temperatures continuing to fluctuate, soils will usually spend some time at temperatures above 50 even if they’re at or below 50 on the day of application.
 

Soil temperatures and fall ammonia application

Written by Emerson Nafziger, University of Illinois    (View the U of I bulletin)

As we indicated in a recent Bulletin article, nitrogen applied as anhydrous ammonia in the fall is an effective source of N for the corn crop, but is also a little more subject to loss compared to N applied in the spring. One of the main factors that determines how much of the fall-applied N 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.
 
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. Scientists have studied the effect of soil temperature on nitrification rates for decades. Most of this work has been done on soil brought indoors and kept at different temperatures, then adding ammonium and measuring the amount of nitrate that forms over time.
 
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. Once nitrification begins, soil temperatures in the upper 70s produce maximum rates of nitrification; 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.
 
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 in falls when harvest is delayed, soils are wet, or soils freeze early. 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, 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. 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 fluctuates during the day depends on soil texture and color, soil moisture, air temperature, and the amount of sunshine. We don't want to use either maximum or minimum, so often take soil temperature at 1 O AM and 4 inches deep under bare soil as an estimate of daily average conditions in the ammonia band after application. Drier soils tend to limit nitrification some, and they also warm and cool more quickly as air temperature changes.
 
As an example of the challenge of waiting until soil temperature are low enough and likely to stay low, soil temperatures (at 10 AM at a depth of 4 inches under bare soil) averaged over sites in central and northern Illinois as reported by the Illinois Climate Network from October 1 through 25, 2020, are shown in Figure 1 below.
 
Figure 1. Soil temperatures at 4 inches deep under bare soil, averaged over eight central and four northern Illinois sites from October 1 through October 25, 2020. Data are from the Illinois State Water Survey at https:llwww.isws.illinois.edu/warmlsoil/.
 
As an exercise, let's say that we applied 150 lb N as ammonia on the first day that soil temperatures were below 50 degrees (October 5). Let's further assume that it took 7 days before nitrification kicked in, that the maximum rate of nitrification at high soil temperatures is 10% of the amount applied (15 lb N) per day, and that the nitrification rate rises steadily, from 5% of maximum at 40 degrees to 50% of maximum at 60 degrees. Under these assumptions, 18 lb N per acre in central Illinois and 15 lb N per acre in northern Illinois would have been nitrified by October 25. That might not seem like a lot, but it is in the range of what we measure as tile line nitrate loss following fall (compared to spring) application, and even if temperatures are normal over the next six weeks, more nitrification will take place. With the lag period already over, early application of fall N also means that the nitrification process is primed and ready to go whenever soil temperatures allow.
 
The good news is that air temperatures have averaged a few degrees below normal in October, and the forecast is that they will remain somewhat below normal this week. Soil temperatures follow air temperatures, of course, and at this point we expect that soil temperatures will drop a little more this week, and that the drop will continue into November. This puts us into the same position we were in late October 2019, when I reported that starting ammonia application at the beginning of the last week of October would not pose an undue risk of nitrification.
 
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. 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. Some nitrate that moves from fields is that present in the soil at crop maturity, either from unused fertilizer or N mineralized from soil organic matter after plant uptake stops. But some also comes from fall­applied N; we can minimize this by careful management, and can largely eliminate it by moving application to the spring.
 
Keep in mind that managing N to minimize loss starts with choosing the best rate. In Illinois that is the MRTN rate, found by using the calculator at http://cnrc.agron.iastate.edu/. It is best to keep some N back to apply in the spring, both to provide flexibility (if prices change) and also to make sure that what is applied in the spring is included in the N rate. If the amount to be applied in the spring is already known-for example, if 45 lb N will go on between herbicide carrier (UAN) and through the planter-then use the expected price for next year's corn crop and the current price of ammonia in the calculator. At a corn price of $4.00 per bushel and ammonia at $475 per ton ($0.29 per lb N), the MRTN rates are 184 and 191 lb N per acre for corn following soybean in northern and central Illinois, respectively, and 216 and 21 O lb N per acre for corn following corn. Besides subtracting the amount to be applied in the spring, subtract also the N applied as MAP or DAP, if those were applied after October 15. If MAP or DAP was applied in September or early October, the N is by now mostly in nitrate form, and the amount subtracted can be decreased by perhaps a third to account for likely loss of some of this.
 
 

References:

Nafziger, E. "Is Fall a Good Time to Apply Nitrogen?" Department of Crop Sciences, University of Illinois, September 30, 2020. https://farmdoc.illinois.edu/field-crop-production/is-fall-a-good-time-to-apply­nitrogen.html
 
Nafziger, E. "Soil temperatures and fall ammonia application." Department of Crop Sciences, University of Illinois at Urbana-Champaign, October 25, 2019. https://farmdoc.illinois.edu/field-crop­production/uncategorized/soil-temperatures-and-fall-ammonia-application.html