Illinois Fertilizer & Chemical Association
Supply · Service · Stewardship

UI Nutrient Bulletins

N responses in 2018

Written by Emerson Nafziger, University of Illinois

The 2018 growing season began with very cool April weather; it was not a wet month, but with very slow drying, little planting was done before late April. Planting in Illinois began a little late, but progressed very rapidly, helped by the very warm temperatures in May. Temperatures were close to normal June through August, and rainfall was above average in June, somewhat below average in parts of northeastern-north central Illinois in July and August, and normal otherwise. With favorable weather in most parts of Illinois, corn yields are projected to be record-high, with the November estimate at 210 bushels per acre.
 
We think that mineralization of organic matter to provide N to the plants began slowly due to low soil temperatures in April, and that might have lowered yields at low N rates, and also may have produced N responses up to higher fertilizer rates in some cases. This can be seen in some of the N response curves for corn following soybeans in central and northern Illinois in Figure 1 below, but averaged across trials, the optimum N rate was only about 5 lb N/acre above the current MRTN, which is about 175 lb/acre. Yields were on average a little higher (240 bushels per acre) than we’ve found across trials in recent years.
 
Figure 1. N responses of 45 on-farm N rate trials with corn following soybean in central and northern Illinois in 2018. MTRTN values are 175 lb/acre for the 42 trials in central Illinois and 157 lb/acre for the three trials in northern Illinois.
 
We also had ten trials with corn following corn, and found again what we have noted in 2016 and 2017: optimum N rates measured in 2018 were lower in than the MRTN values in use for the 2018 season.
 

Southern Illinois

The story was somewhat different in the 13 trials conducted in southern Illinois in 2018 (Figure 2). There, an average of about 35 lb N per acre more than the MRTN was needed to produce optimum yields, and the average optimum yield was about 211 bushels per acre, higher than we have typically measured in these trials in southern Illinois.
 
Figure 1. N responses of 13 on-farm N rate trials with in southern Illinois in 2018. MTRTN values are 180lb/acre for the 12 corn following soybean trials and 193 lb/acre for the one corn-corn trial.
 

Updating the N calculator database

With optimum N rates averaging close to the MRTN (which was based on a large number of N responses through 2017), adding the data into the database underlying the N rate calculator will have minimal effect on the MRTN after the calculator is updated before spring 2019. Some older data will be removed during the update, though, and this could move the MRTN values by a few pounds.
In southern Illinois, with a smaller database and with some older data to be dropped from the N rate calculator database, adding in the 2018 data will increase the MRTN rates by some amount, perhaps 4 or 5 pounds. In the 25 trials conducted over 2017 and 2018, we have also noted that optimum N rates and yields at those rates show some correlation. This suggest that, especially if high yields can be predicted during vegetative growth, an additional application of N may be justified. Lower soil organic matter in many southern Illinois soils means lower potential N amounts coming from mineralization, and since high yields mean increase demand for N, the additional amount may need to come from fertilizer.
 

Managing Nitrogen for Corn in 2019

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

The fall of 2018 and so far in 2019, there have been limited opportunities to apply nitrogen fertilizer. Average rainfall through the first 25 days of March ranged from a little less than normal in the northern half of Illinois to an inch or more above normal in south-central Illinois. But temperatures have averaged 3 to 4 degrees below normal, which slowed drying. There were several days in the first week of March when it was frozen on the surface and a considerable amount of P and K went on. This was followed by an inch or more of rain (which had been forecast) in many areas, and it’s likely that some of the nutrients—those in MAP/DAP and potash are soluble—moved from higher to lower parts of fields, or off of fields altogether. While it’s good to get P and K applied before spring work starts, we really should consider holding off the next time soils are frozen and substantial rainfall is forecast before a thaw.
 

N rate

Results from the on-farm trials coordinated by Dan Schaefer of the IFCA and funded in part by the Illinois Nutrient Research & Education Council (NREC) showed that at about two-thirds of Illinois sites in 2018, the N rate needed to maximize the dollar return to N was higher than we have typically seen. We think that the slow start to mineralization during the cool weather in April 2018 might have meant more dependence on fertilizer N. Yields were also higher than we’ve usually found, which meant that the crop took up more N than usual.
 
Adding the 2018 data to the database used by the N rate calculator to calculate best (MRTN) rates for N in Illinois, and taking out some older data, resulted in an increase in the MRTN values compared to the previous (2018) version for corn following soybean. With N priced at $615 per ton of anhydrous ammonia ($0.375 per lb of N) and corn at $3.75 per bushel, the calculator gives MRTN rates for corn following soybean of 166, 180, and 192 lb N per acre for northern, central, and southern Illinois. These rates are 11 lb higher than the 2018 rates in northern and southern Illinois, and 5 lb higher in central Illinois. Those are modest changes because the 2018 data is added to a lot of existing data, but they illustrate how generating and adding new data keeps the guideline N rates responsive to research-based changes.
 
MRTN values for corn following corn did not change very much, in part because there weren’t as many trials in 2018, and in part because trials over the over the past several years have shown that corn following corn has required less fertilizer N than we found in many previous trials. It’s not clear why that is the case, but as we found with corn following corn, rates are responding to research findings to stay linked to what today’s corn crop needs.
 
Remember that the MRTN rates (and ranges) generated by the N rate calculator include all of the N that gets applied, not just to the main application. A 3-year, NREC-funded trial at two sites that we finished in 2018 showed that N rates from DAP applied in either the fall or in the spring produced the same yield response to N as spring-applied UAN. This means that we should be able to take full credit for N from MAP or DAP, providing these are applied after soils cool in the fall (about November 1 or later) or any time before planting in the spring. If these P fertilizers are applied before soils cool in the fall, some of the ammonium will convert to nitrate and be subject to loss. It’s reasonable to subtract maybe 20 to 30% of the N from P fertilizers that are applied in early October, before soils start to cool down.
 

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, we have found little or no difference in yield responses to N rate. That’s been the case in our N-tracking trials as well: we generally find nearly all of the fall-applied N still present in the soil at planting (although most of it is usually in the nitrate form by then), and we have rarely found a yield advantage to applying 200 lb N in the spring compared to 200 lb N in the fall. Tile-drainage studies do show a little more N loss from fall-applied compared to spring-applied N, though, and we have found in a few cases either higher yields with spring-applied N or similar yields produced with lower rates of spring-applied N.
We have also found in a few trials the opposite—that fall-applied N can sometimes give higher yields or need less N to produce the same yield as spring-applied N. This is more likely when N rates needed to maximize the return to N are not unusually high, and when spring-applied N is applied at or after planting, with some delay in how soon the crop’s roots can reach the application band to start taking up N. That is, having the N dispersed in the soil after application under better (drier) soil conditions in the fall may sometimes be an advantage compared to application into wetter soils in the spring. Wet fall weather like we had in 2018 likely means less chance of seeing such an advantage in 2019. We did not get many trials established last fall to make the comparison.
 
There is no reason to expect that the delay in N application in most Illinois fields so far this season will lower yield potential, but it will be important to keep a couple of things in mind as the planting season approaches. The main lesson we’ve learned from our N timing and N form studies over the past five years is that 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.
 
Table 1 shows yield averages from 15 site-years over the past four years (2015-2018) at four Illinois sites where corn followed soybean. The highest yields (those followed by an “a”) all came from treatments with 100 or 150 lb of N applied at planting, and applied in a way that we think made N available to the plants soon after emergence. Among the treatments with all 150 lb N applied at planting, broadcast SuperU (urea with both urease and nitrification inhibitors incorporated) and ESN (polymer-coated urea with extended release) produced the highest yields. Those that included N applied between the rows—especially NH3 with or without N-Serve, which would have been accessible to the roots only once the roots grew out to the band, yielded a little less. Adding nitrapyrin (Instinct) to UAN injected between the rows lowered yield a little, and those that had UAN surfaced-applied all yielded less: these may have lost some N or the N might have moved too slowly from the surface to the root zone to maximize yield.
 
Treatments with 150 lb N split into 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 (Table 1.) 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 very well, possibly because some of the N might have been lost, but more likely because there wasn’t enough N near the root system when it was needed, before sidedress. Otherwise, most of the treatments with 100 lb injected at planting followed by 50 lb as urea with Agrotain broadcast at V5 or V9, or as UAN dribbled in-row at V5, V9, or at tassel (VT) stage did well. Waiting until V9 and dribbling all 150 lb N as UAN at V9 was the lowest-yielding treatment, likely due to development of N deficiency (whether visible or not) that lowered yield potential in earlier stages. Injecting all of the N mid-row at V5 yielded as well as injecting 100 lb at planting and 50 at V5, which is counter to the idea that the crop needs more N early. We don’t know the reason for this.
 
Table 1. Corn yields with different forms and timing of 150 lb N/acre in corn following soybean. Data are averaged over 15 sites, with 3 or 4 sites per years for 4 years, 2015-2018. Averages followed by the same letter are not significantly different at the 10% (error) level.
Another piece of evidence that the crop needs a good supply of N relatively early to avoid lowering yield potential comes from a part of the same N form and timing study reported in Table 1. Averaged across 18 site-years, applying 100 lb N at planting yielded 13 bushels more (214 versus 201) than applying 50 lb at planting plus 50 lb (as injected UAN) at sidedress, stage V5-6. At rates of 150 and 200 lb N/acre, applying 50 lb at planting then the rest at sidedress yielded about 3 bushels less than applying all of the N at planting. At the 200-lb rate, applying all of the N early yielded significantly more than the 50+150 split at two sites, and the split N yielded significantly more than the all-early application at one site.
 
In another set of trials, 200 lb N as fall-applied ammonia with N-Serve yielded an average of 5 bushels more than 50 lb N as injected UAN at planting followed by 150 lb N as injected UAN at sidedress. The 50-150 split-sidedress treatment yielded more than fall-applied N at most sites in 2015, when June was very wet, but the 50-150 split yielded less than fall-applied N at most sites in both 2017 and 2018. This shows that applying some of the N at sidedress can bring yields up close to those from all-early application of the same rate, but keeping back most of the N to apply in-season is more likely to decrease yields than to increase yield compared to applying all of the N before planting, including in the fall. If we do sidedress, we need to apply at least half of the N where the roots of small plants can get access to it in order to prevent early-season deficiency that can result in lower yields.
 
We also noted that when we get really wet soil conditions in June after the crop has started to grow, like we had in 2015, split-sidedress N can outperform all-early N. Under these conditions, the crop may well need more N than we have (or would have) applied. In order to respond to added N under wet conditions, the crop needs to have its root system active, which won’t be the case if it’s still standing in water or saturated soils. Also, if the lower leaves have started to die off, the plant may not be able to take up and utilize added N. Even then, a period with the roots under low oxygen conditions may not yield fully, even if soil conditions improve. It’s important to get supplemental N applied as soon as possible so that the crop can take it up as soon as it’s able. Dropping urea (perhaps with urease inhibitor) from the air is expensive, but might be in order, especially if a planned sidedress application wasn’t made before it got wet.
 

N form and additives

Different forms of N fertilizer need to be applied in a way that assures crop safety and maximizes the chance that the N will be available to the crop when the crop needs it. Anhydrous ammonia is usually the N source with the lowest cost (per pound of N) and at 82% N, using ammonia means less volume to store and transport. But it requires injection to depth in the soil, and so is more costly to apply. It also needs to be handled very carefully to prevent accidental release into the air. Once in the soil, it spreads (in soils not too dry or too wet) several inches out onto the soil and, by desiccating soil microbes, it limits microbial activity that converts ammonium to nitrate; that is, ammonia partially sterilizes the soil, in the process limiting its own conversion to nitrate for a period of time. Conversion to nitrate makes N mobile in the soil, and nitrate is subject to loss by leaching and denitrification. This isn’t permanent: these microbes grow back quickly in the presence of so much N, and eventually reach levels higher than before the ammonia was applied.
 
We normally use a nitrification inhibitor such as N-Serve or CENTURO (new from Koch Agronomic Services) when applying ammonia in the fall. The later we apply ammonia in the spring the less likely it is that a nitrification inhibitor will be needed to help keep N in the (immobile) ammonium form, and thus in rooting zone. As a biological process, nitrification is slow in cool soils, which usually means it’s slow through most of March. Illinois State Water Survey data show that at Bondville, just west of Champaign, 4-inch bare soil temperatures have been in the low 40s for the past two weeks. On average over the past five years, 4-inch soil temperatures have reached and then stayed above 50 degrees by about April 14 at this site, and it’s averaged about May 10 before soil temperature reaches and stays above 60. Nitrification is slow when temperatures are in the 50s, and begins to speed up once soil temperatures reach 60 and above. If we are able to plant more or less on time this year so that N uptake begins to accelerate in late May, and if we add in the effect of the NH3 itself in suppressing microbial activity, it’s unlikely that applications of ammonia made after April 1 will need the further delay in nitrification provided by nitrification inhibitor.
 
Because cool soils are slow to dry, applications of ammonia in the spring are usually done when soils are wetter than ideal. That doesn’t mean we should abandon this form of N, but applying it 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 is high. If the soil dries out after application, there is some danger than NH3 will begin to move up in the soil, and may 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, but that means applying in the direction of the rows instead of on an angle. Tilling after ammonia application can also help disperse the band and will usually lower or eliminate the risk of ammonia injury on seedlings.
 
Dry urea has the advantage of being quick and easy to broadcast-apply with flotation equipment, and has the additional advantage of being safe to apply after crop emergence. If spread on the soil surface and worked in with a tillage pass before planting, it is relatively safe from loss by volatilization, which is breakdown into carbon dioxide and ammonia, which can be lost to the air. If surface-applied without incorporation, using a urease inhibitor will help decrease volatilization. Getting a half inch or more of rain will move most of the urea into the soil, where any volatilized ammonia will be quickly dissolved in soil water. Urea doesn’t “self-sterilize” the soil to limit nitrification like ammonia does, though, so with warm surface soil temperatures, nitrification will begin soon after the urea is dissolved and in the soil (as ammonium). In the results in Table 1, SuperU (from Koch), which has both urease and nitrification inhibitors, produced the highest yields of any of the forms and application methods used in that study. Urea with the urease inhibitor Agrotain yielded 5 bushels less, presumably because of some loss (or movement below the root zone) of N following nitrification; urease inhibitor has no effect on the nitrification process once the N is on the ammonium form.
 

N application

Application methods are discussed several places above for different N forms, so only a few additional things will be noted here. Most application methods are not new, but there have been some innovations in recent years that offer more options. One big issue 15 or 20 years ago, perhaps related to aerial imaging that showed colors patterns in corn fields, was that of uneven distribution of NH3 across the knives on toolbars. A number of engineering improvements since then have diminished, if not eliminated, this problem, and as long as older manifolds have been replaced, it’s not a major issue now.
 
Application depth of NH3 has some influence on back-pressure and distribution, and on how safe the ammonia is from release into the air. Under normal soil conditions (not too wet), releasing NH3 5 or 6 inches deep is a safe depth, but if it’s wet, that will place it deep enough so that roots of small plants may not get access to it as soon as they should. This can be overcome to some extent by application of some of the N in more-available forms, such as 2 x 2 placement with the planter. This should be done with enough of the N (50 lb or more) to support early growth. In-furrow application of starter fertilizer or broadcasting 10 to 15 gallons of UAN 28 (30 to 45 lb of N) with herbicide helps, but unless soils are warm enough so that mineralization has kicked in by the time plants are at the 2-leaf stage (normally 20 to 25 days after planting), these applications may not provide enough N in time to maximize yield potential.
 
While most in-season (or at-planting) applications of UAN solution have traditionally been made by shallow injection, the recent advent of near-row dribble (Y-Drop®) technology and similar equipment, in at least one case with the ability to apply both between-row injection and near-row dribble at the same time, offers a different option for placement. One advantage promoted for nearrow dribble is the ability to apply N to corn of different sizes, from small plants to tassel stage or even beyond, using high-clearance equipment. This equipment has been promoted to some extent on the idea that “spoon-feeding”—applying N before and several times during the rapid growth/N uptake stages—can better match N to the crop’s needs, with less potential for loss, thereby maximizing yields. We have not found such an approach to be effective: as detailed above, we seldom (with some exceptions) find a yield advantage to keeping any of the N back for a single inseason application. And, we see no effective way to adjust N rates with later applications to end up lowering rates, thereby increasing N efficiency. We know beyond doubt that most soils are very effective as reservoirs for N, and this means that there is simply no yield advantage for breaking one or two N applications, including a major one at or soon after planting, into three or four applications. Without a yield advantage, the added application costs will lower returns.
 
As a way to apply in-season N, however, near-row dribble has some advantages over mid-row injection. Corn’s nodal roots grow down at an angle from the lower stem where they originate, so placement closer to the row means placement at less distance down to the root system. Dribbled UAN is also shaded a little more by the row so may be less prone to volatilization under high temperatures in direct sunlight. One question has been whether a near-row (or injected) UAN application made in-season will benefit from the addition of a urease inhibitor (like Agrotain) or even of a nitrification inhibitor (like Instinct). Injected UAN should never need a urease inhibitor, and if it’s dry and expected to continue to be dry, some consideration might be given to changing from dribbled to injected UAN. Until it rains, neither dribbled nor injected N will get to the roots for uptake, but at least the injected UAN-N won’t volatilize. It might make sense to use a urease inhibitor if surface application is the only option, but that won’t eliminate the risk that if it stays dry, the N won’t get into the crop in time to maximize its use no matter how we apply it. If it’s dry by early June and is projected to stay dry (as it did in 2012), it might make sense to skip the in-season N application altogether.
 
Dry urea can be applied across the top of emerged corn without concern for injury, although leaf edges sometimes show damage after application, especially when applied to larger plants, which catch more urea in the whorl. Some long-ago research showed that urea in the whorl didn’t decrease yield. Moderate wind tends to fold leaves over the whorl and may decrease urea capture, but of course won’t help uniformity of spread. Although urea applied to the soil surface is subject to loss by volatilization, enough rain to move the urea into the soil within a week after application will minimize losses. If it doesn’t rain, the urea may not do much good. Using Agrotain will help reduce volatilization and lower risk of loss, and might be appropriate if rain is likely to come late. Using polymer-coated urea (ESN) slows release of the urea into the soil, but in-season applications are usually made with the hope that N will get into the plants quickly, and slow-release will hinder that, and will lower effectiveness of the applied N. Polymer-coated urea can also move with surface flow of water following heavy rain, and in some cases might even leave the field.
 
While 2019 has so far presented some challenges in terms of applying N, a period of warm weather in April can greatly improve the prospect of getting N on this year’s crop without losing yield potential. We will need to retain flexibility, though, perhaps to the extent of changing form and timing of application to ensure that the crop gets enough N in time. One drawback to that, besides the challenges in equipment and timing, may be increased N costs that result from changing N form and application equipment. It would be good to enter revised N prices into the N rate calculator to see how this changes the amount of N to use. With expected corn prices and margins not very high, this might be the year to put off trying new and less-proven products and practices, and to focus instead on the basics of getting N to the crop in adequate amounts, both by choosing moderate rates (the MRTN should be the first option for most fields) and by applying N in a way that minimizes losses and maximizes crop access to this critically important nutrient.
 

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.