Illinois Fertilizer & Chemical Association
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UI Nutrient Bulletins

Notes on fall fertilization

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

With harvest winding down in most of Illinois after another year with high to very high yields, it’s time to review some basics of fall fertilization. Neither fertilizer nor grain prices are historically high, so there’s reason to be aware of costs while making sure to cover the nutrient basics.


In a webinar on October 19 organized by the Illinois Fertilizer & Chemical Association, we looked at some of the nitrogen response data that have come in so far this fall and considered what this might mean in terms of fall N management. In some of the trials, modest N rates produced high yields, much like we’ve been seeing routinely in recent years. But in a few other trials, we found that the crop needed more N than we have seen most recent trials on productive soils. It’s too soon to call this a phenomenon for 2018. Even if this turns out to be more common this year, using previous research results to determine best N rates, which is what the N rate calculator does, means that unusual results get “diluted” by normal results from this and previous years. So adding data from this year will not move the MRTN (best) N rates by very much. We have no way to predict what next year will bring, and so using all of the data from recent years really is the best guess at what N rates we should use for 2019. We really can’t react to unusual responses in any trial in any year by making wholesale changes in how we manage N fertilizer.
The need for high N rates in some in some trials this year may be partly related to high yields, though yields are not much higher in most trials than we had in 2017, when moderate N rates were usually enough. One unusual feature of the 2018 growing season was the cool, wet April followed by unusually warm weather in May. This might have caused a delay in the start of mineralization of soil N, which, coupled with rapid early crop growth. might have meant that more of the N for the 2018 crop needed to be supplied with fertilizer. As more results come in we’ll be able to get a better handle on this, perhaps including some possible ways to improve N management if we see similar developments another year.
Soil temperatures dropped quickly in mid-October this year, and in central and northern by now they are at or a little below the 50 degrees F that we consider to be the maximum soil temperature at which to safely apply anhydrous ammonia fertilizer in the fall. The forecast calls for cool and cloudy weather most of the next week, and it appears likely that soil temperatures will stay below 50 degrees into November. Low temperatures lower microbial activity that converts ammonium to nitrate, which helps keep N mostly in the ammonium form; this is the form we hope to have present as soils freeze up this fall. Soil temperatures (we suggest 4 inches deep under bare soil) can be tracked at the WARM website maintained by the Illinois State Water Survey. Data are available there either as maps of Illinois or as soil temperatures at one of the individual weather-recording sites. Map options include daily maximum, daily minimum, or “average hourly” values, which has a default of 10 AM. I suggest changing 10 AM to 12 PM (noon) on that map; soil temperature at noon is usually a little higher than at 10 AM, so is a little more conservative. Nitrification inhibitors such as nitrapyrin (N-Serve® from Dow/Corteva) and pronitridine (CenturoTM, new from Koch) inhibit microbial activity responsible for conversion of ammonium to nitrate in the soil, and so have the same effect as do cool soil temperatures. Both an inhibitor and waiting for cool soils are suggested as ways to minimize nitrification activity following application of fall-applied ammonia.
If we enter the NH3 price as $525 per ton ($0.32 per pound of N) and the corn price of $3.50 per bushel into the N rate calculator, it calculates MRTN rates of 161 and 178 lb. N per acre for corn that follows soybeans, and 205 and 203 lb. N for corn that follows corn, in northern and central Illinois, respectively. The “profitable range” given by the calculator includes about 15 lb. N on either side of the MRTN. This range is based on our finding that the return to N stays near its maximum over a range of N rates when we combine a lot of N rate trial data. This provides some leeway for those who want to adjust the rates based on experience. But producers and advisers should have a good reason to move N rates above the upper end of the range. We are not seeing this year, nor have we seen in previous good years (like 2017), that high corn yields consistently require higher rates of N than do lower corn yields.
Conditions not conducive to fall N (as ammonia) application to a field include having soils warm or wet, very poorly-drained soils, and light-textured soils through which water moves quickly. The latter two—poorly-drained and light-textured soils—should not have fall-applied ammonia even if they are cool and dry enough, given the potential for loss before crop uptake begins next spring. Higher loss potential due to warmer fall and spring weather also makes it too risky to apply fall ammonia south of the terminal moraine that roughly follows IL Route 16. Applying NH3 to wet soils restricts the spread of NH3—ammonia moves away from its release point in the soil only until it’s dissolved in soil water, which in wet soils may only be an inch or two. Such a small band means less dispersion in the soil before next spring, which can limit root access to the N. Having the N so concentrated in the band can also increase the chances for NH3 to move up and out of the soil if the soil happens to dry out after application. Ammonia knives should be run deep enough to release NH3 about 6 inches deep on most soils, and perhaps a little deeper on loam soils or dry soils, where we expect NH3 to move farther from where it’s released. The old rule of thumb—if you can smell ammonia during application you need to check its placement—remains useful.
Corn plants that develop under high temperatures and with plenty of water tend to be taller than usual. In areas where the crop has been showing stress symptoms in the past week or two, though, we can expect plants to end up shorter than normal. Any water stress during rapid stem elongation – between V8 and tasseling – results in less elongation of cells in those internodes that are expanding during that time, and this results in shortened internodes and plants. As we saw in 2017, shorter plants can still yield very well, but that requires that they get adequate water by a week or so before tasseling to assure that the pollination process can proceed normally.
We have compared fall- versus spring-applied ammonia in a variety of experiments over the past five years in Illinois, and have generally found that both produce similar yields at similar N rates. We have found modestly higher yields, in some cases with less N, for spring-applied N, but we have also found, more rarely, fall N producing slightly more yield than spring N. Most measurements of tile-line N loss report loss of a little more nitrate-N from fall-applied compared to spring-applied N. In our soil N studies, we have found that spring-applied NH3 takes several weeks longer to convert from ammonium to nitrate compared to other forms and times of N. So when the spring weather is wet, more nitrate in the soil from fall-applied ammonia can mean more loss of N. In relatively dry springs, water doesn’t move through the soil or stand in parts of the field to cause denitrification, so there is little or no N loss even if all of the N is present as nitrate. If carefully applied, both fall- and spring-applied NH3 are very good sources of N. Wet soils in the spring typically make it more difficult to apply NH3 properly, and without a lot of soil compaction. In general, applying NH3 in the fall as long as conditions are good (soils cool and not wet), then waiting until spring to apply the rest (as NH3 or some other form or combination of forms and times) constitutes sound N management.
We just completed a three-year study designed to measure how much of the N in the common phosphorus fertilizer diammonium phosphate (DAP, which is 18% N and 46% P2O5) is available to the corn crop the following year. We did this by applying rates of N ranging from 0 to 80 lb. N per acre, as either fall-applied DAP, spring-applied DAP, or UAN (no P) applied at planting. These rates were in addition to a base rate of 100 lb. N applied as UAN, and we used triple-super-phosphate (TSP) fertilizer (which has no N) to make sure all plots got the same amount of P. The trial was run at both Monmouth and Urbana over the 2016-2018 seasons, for a total of six site-years. We found that the corn crop responded the same to N from all three sources (Figure 1.) While that’s a little surprising given that the N in fall-applied DAP is likely to be nitrate (and so subject to loss) earlier in the spring than spring-applied DAP or UAN, it tells us that we can count all of the N applied as MAP or DAP as part of the total fertilizer N rate we plan to use. One caution: we applied fall DAP in this study between mid-October and early November; applying it earlier in October when soils are warmer will increase the rate of conversion of the N to nitrate, thereby increasing the potential for loss.
Figure 1. Corn yield response to N applied as fall-applied DAP, spring-applied DAP, or UAN at planting, averaged over six site-years in Illinois, 2016-18.
It is important to include in the N rate to be applied for next year’s corn crop all of the N we apply— “main” applications in fall or early spring, fall or spring DAP, N applied with the planter, N used as herbicide carrier after planting, N applied as manure, and any sidedressed amounts. We sometimes tend not to count “minor” amounts, but unless we have some way to know that some N was lost, we can’t justify leaving any of the N out of the total we record as the rate we will apply.

P and K

Fall application of the dry fertilizer materials to supply P and K to the next year’s (or next two years’) crops is normal practice, although there has been some moving of P and K applications to the spring. That’s not a problem with timing—even though P and K are relatively immobile in the soil, applying them as surface broadcast well in advance of crop emergence tends to work well. But fall soil conditions are often better for driving application equipment over fields, and many producers don’t want to add fertilizer application to the list of spring tasks. Most P and K fertilizers are broadcast, but some now apply these materials as bands placed into the soil, in some cases beneath where rows will be planted. Research has shown limited if any yield response to banding P and K compared to broadcasting, especially on productive soils with adequate P and K test levels already present. An advantage to placing P into the soil is that it is less prone to running off with rainfall. But this requires special equipment, and application of dry fertilizer in bands is substantially slower and more costly than broadcast application.
While most P and K fertilizer is applied to soybean stubble in preparation for corn the next year and then soybean the year after that, we have seen some claims recently that soybean “needs its own P and K” and that it shouldn’t have to “settle” for the P and K “left over” from the corn crop. In all but very low-testing soils, where crop roots can have trouble reaching enough P and K as they grow into the soil, research has failed to show a benefit to annual applications of P and K, at least in soils such as those in Illinois. We know for certain that it costs more to apply nutrients every year than only once in two years. There have also been claims that soils tie up P and K over time after they are applied, such that “freshly-applied” nutrients are more available to plants. But applying amounts of P and K that crops remove tends to keep soil test levels fairly constant, suggesting that any tieup of P and K is not a permanent “loss” of these nutrients; as long as soil test levels are adequate, both crops get enough even if their roots don’t encounter fertilizer granules as they grow.
A sound approach to determining rates for P and K is to add up the amount removed over the last two years (assuming a biennial application) and to apply that amount in preparation for the next two years. A year ago in a Bulletin article I reported the results from a recent NREC-funded grain nutrient sampling project in Illinois. We set grain removal levels as the values below which 75% of sample values fell, so a little higher than the average amounts of nutrients we found in the grain samples. In some 2,100 grain samples of both corn and soybeans, we found removal levels of 0.37 lb. P2O5 and 0.24 lb. K2O per bushel of corn grain, and 0.75 lb. P2O5 and 1.17 lb. K2O per bushel of soybean grain. These are 10 to 15% lower than previous “book values” used in Illinois and many other states, and are in line with levels reported within recent years by Iowa State University scientists.
Even with slightly lower P and K removal levels than we have used in the past, high yields mean removal of a lot of nutrients from fields. In a field that produced 240 bushels of corn in 2017 and 75 bushels of soybean in 2018, we calculate that harvested grain over the last two years removed 0.37 x 240 + 0.75 x 75 = 145 lb. P2O5 and 0.28 x 240 + 1.17 x 75 = 155 lb. K2O per acre. At current estimated retail prices of $520 per ton for DAP and $370 per ton for potash, the fertilizer to replace these amounts would cost about $123 per acre, not including the application cost.
The still-used “200-200” application (200 lb. DAP, or 92 lb. P2O5 and 200 lb. potash, or 120 lb. K2O) every other year was enough to keep soil test levels moving up when using such rates was common. That’s because yield levels were much lower than in recent years; Illinois corn and soybean yields from 1961 through 1979 averaged 96 and 31 bushels per acre, respectively. Having applied rates in many fields exceed removal for decades is why soil test levels are as high as they are in such fields today. But using that amount of fertilizer at today’s yield levels will mean a steady drop in soil test values as more nutrients are removed than are replaced.
Low crop prices often have some people wondering if they might cut back some on P and K in order to save money, presumably until crop prices are higher (or fertilizer prices are lower) in a year or two. Despite imaginative claims of “hidden hunger” and some overwrought interpretations of tissue testing levels, P and K deficiency symptoms are very rare in Illinois; we tend to see such symptoms when soils dry out after planting and roots have trouble growing into soils enough to take up adequate P and K, even when soil test levels are high. Such symptoms are more common in compacted soils and in no-till fields, but we hardly ever see such symptoms with spring rainfall is normal.
With adequate soil test levels of P and K in most fields and with crops that are good at extracting these nutrients, delaying the application of some or even all of the P or K for a year or even two years is likely to have little or no effect on the yield of the next crop(s). Still, nutrients removed by the most recent crops do need to be replaced, if not before the next crop or two then after that; higher soil test levels now provide more leeway. The real risk comes from allowing removal to exceed replacement over years, to the point where even good root systems can’t take up enough nutrients, and yields suffer. Reaching that point in most Illinois fields would take more than a year or two, but Illinois soils cannot generate enough P and K to meet the needs of high-yielding crops, so getting to that point is inevitable if the neglect continues. We can “kick the can” of nutrient replacement “down the road” for now, but that will mean having to replace ever-growing amounts of nutrients later, as grain, along with its nutrients, continues to come off the field every year.

Dicamba Buffers, Training and Licensing: What to Know for 2019

Written by Aaron Hager    (View the U of I bulletin)

The United States Environmental Protection Agency (EPA) renewed the labels of three dicamba-containing products used in dicamba-resistant soybean varieties on October 31, 2018. These renewed labels also contain new restrictions and requirements that did not appear on the original labels. Each application must completely satisfy all label requirements and restrictions, but the following three new requirements might necessitate additional forethought and planning.

Additional in-field buffers

Fields that exist in counties that might harbor endangered terrestrial dicot plant species must have an in-field, 57-foot omnidirectional buffer. The new 57-foot buffer will occur on three sides of the field and be in addition to the required 110-foot downwind buffer. Non-sensitive areas, as defined in the renewed labels, can be included in the omnidirectional buffer calculation. This new buffer requirement includes fields in at least 29 Illinois counties (Figure 1).

Training requirement

Approximately 11,000 individuals in Illinois completed the label-mandated auxin training prior to applying these dicamba products in 2018. Some have mistakenly assumed this particular training had to be completed only once, but the dicamba training must be completed every year. Prior to the 2018 application season, dozens of face-to-face training sessions were held around the state, but it appears there will be fewer of these offered prior to the 2019 application season. An option to complete the training via on-line modules likely will become available.

Licensing requirements

EPA mandates that only certified applicators (not operators) are allowed to purchase and apply these dicamba-containing products. This requirement applies to both private and commercial applicators. Historically, licensed operators working under the direct supervision of a licensed applicator have done the majority of pesticide applications in Illinois but this no longer permissible with the dicamba formulations applied to dicamba-resistant soybean. According to the Illinois Pesticide Act for commercial applicators: “A person may make application to the Director to become licensed as a licensed commercial applicator…only after successfully demonstrating comprehension of the general competency standards and one or more of the technical category areas of pesticide use.” The Illinois Department of Agriculture requires Field Crops as the technical category for application of dicamba to dicamba-resistant soybean. Additionally for private applicators: “A person may become certified or recertified as a private applicator by: 1) attending a training session conducted by the University of Illinois Cooperative Extension Service which has been approved by or is in cooperation with the Department and by successful completion of a written, closed book, competency examination; or 2) in lieu of attendance at a training session, successfully complete a written closed book examination. The closed book examination will consist of questions pertinent to general competency standards for which a correct answer is to be selected for each question from multiple choice answers.”
Figure 1.

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.