Nutrient movement off frozen and snow-covered soil

Snow has now fallen throughout much of Illinois, and temperatures have dropped going into the last weeks in 2019. With the recent Illinois Nutrient Loss Reduction Strategy biennial report highlighting P and N levels in Illinois waterways, this is a good time to review the application of nutrients on frozen and/or snow-covered soils.

Last spring, after a short and often-muddy fall fertilizer season, a considerable amount of fertilizer—mostly P in the form of DAP or MAP and K as KCl—was applied during the first week of March when the soil surface was frozen. Between March 3 and March 8, 2019, minimum air temperature averaged less than 15 degrees F, and maximum temperature averaged less than 30 degrees over most of Illinois. This was one of the few times last winter when soils were frozen and there was little or no snow; and many took the opportunity to apply P and K.

Unfortunately, an inch or more of rain fell over most of Illinois south of I-80 followed fertilizer applications on frozen soils in early March 2019. Most of this rain fell during the day on March 9, and as evidenced by the rapid rise in streams and rivers, a great deal of this rainfall ran off the (then-frozen) soil surface.

As part of the ongoing water monitoring that IFCA conducts within their Keep it 4R Crop Program, several samples of water moving out of fields after this rainfall event were collected. In fields where P and K had not been applied recently, average levels of chloride, nitrate-N, ammonium-N, and phosphate-P were 2.3, 0.7, 0.1, and 0.3 ppm, respectively. We expect to see such low values, since these nutrients don’t tend to remain on or near the soil surface during the winter.

Samples from fields where P (MAP) and K were applied on frozen soils in early March had an average of 39.0 ppm chloride, 1.2 ppm nitrate-N, 8.3 ppm ammonium-N, and 19.1 ppm phosphate-P. These fertilizers don’t contain nitrate, so the low nitrate-N number was expected. Both MAP and KCl are highly soluble in water, though, and the elevated chloride (K wasn’t measured), ammonium, and phosphate levels are a clear indication that the rain dissolved fertilizer nutrients and moved them off the field. Samples from a few other fields showed increased sulfate levels, probably coming from ammonium sulfate spread before it rained.

When there is an event like this, higher concentrations come off initially, and concentration declines with time. So we can’t know how much really came off these fields. But as a conservative estimate (because samples were taken some hours after the rainfall began), let’s imagine that the samples had average concentrations, and that an inch of water per acre moved off these fields. An acre-inch of water weighs about 226,600 lb, so if an inch runs off with 19.1 ppm of phosphate-P, about 4.3 pounds of P, or 9.9 lb of P2O5, moved off each acre. KCl is about 47.6% chloride, so 39 ppm chloride would convert to movement of about 9.7 lb K or 11.7 lb K2O per acre.

Another reason for concern about loss of P from events like this is that dissolved P at 20 ppm is 400 times the level (0.05 ppm) in surface waters that many consider to be the point where water starts to become impaired—where P supports the growth of undesirable algae. The major loss from an event like this economic—some of the fertilizer paid for is simply gone from the field. Even more nutrients have likely moved off higher elevations to lower elevations, producing a new “variable-rate” application—but one whose pattern is not likely to line up with existing soil nutrient levels.

Even though events like this are relatively rare, movement of nutrients off fields in this case was both predictable (the rain was forecast) and preventable. It’s understandable that retailers want to apply nutrients when the (frozen) soil will support application, and many producers are in favor of doing this before it gets muddy again. As it turned out, there were not many days in the spring of 2019 when soil conditions were dry enough to allow for surface application of P and K. There are, however, few fields in Illinois with P and K levels low enough to have decreased yields had fertilizer application had been delayed to after planting, or even to the fall, after harvest. Delaying application to the fall would mean adjusting the rate to account for removal by another crop, but would have meant little or none of the loss and redistribution that can result from application to frozen soils.

What about soils that are snow-covered but may not be frozen? Soluble nutrients like MAP/DAP and potash can melt their way down to the soil surface and possibly into the soil, but a “slow melt” with little movement of water off the field is an ideal that we seldom see. With nutrients dissolved in the snow, any water moving off the field as snowmelt will carry a lot of nutrients with it. Deeper snow helps wide (floater) tires stay out of the mud, but it also makes it more likely that rain will fall before the snow finishes melting, and when that happens, a lot of water can leave the field, carrying nutrients with it.

Wet grain, test weight, and late corn harvest

According to NASS, 20 percent—some 2 million acres—of the 2019 Illinois corn crop was still in the field on November 17. Following unprecedented delays in planting, the warm weather in September helped move the crop towards maturity, and frost did not come earlier than normal. So most of the corn in Illinois was at or close to maturity by mid-late October, but temperatures have been below to much-below normal over most of the past month, and this has delayed drydown of the crop. Most of the corn still standing in the field is in the northern third of the state.

One concern with late maturity and harvest of corn is low test weight, which in some cases can mean price dockage at the elevator. Test weight is “bulk density”, or the weight of corn grain per unit of volume, expressed as pounds per bushel (one bushel is 1.24 cubic ft.) The “standard” test weight for corn is 56 pounds per bushel, and this weight (not a volume of 1.24 cubic feet) is the standard unit for marketing corn grain. Test weight is easily measured, but is not easily understood: it’s a complex characteristic, affected by kernel shape, kernel density, and the slipperiness of seedcoats, which affects how well kernels slide past one another. These are all affected both by genetics and by how the crop grows, matures, and is dried for storage.

There are two main reasons for low test weight. The first is the premature end to kernel filling that can result from poor growing conditions, disease of leaves or ears, severe drought, or frost that comes early or that occurs before late-planted corn is mature. Starch deposition in kernels starts at the crown of the kernel and moving towards the base, and when the movement of sugars into the kernel stops before kernels are full-sized, the base of the kernel may be shrunken. The starch deposited under poor conditions can also be less densely packed on the endosperm. The result can be kernels that don’t weigh as much as usual and that don’t fit together very well, both of which can lower test weight. Such kernels have less starch and a lower starch-to-seedcoat ratio, but they might have higher protein and oil, since these are deposited before starch deposition ends.

The market typically rewards sound, dense kernels, so shrunken kernels may be docked in price, with the amount of dockage tied to test weight. Test weight acts as a proxy for harder-to-measure things such as starch content or kernel density. Kernels that don’t fill completely may also contains sugars that didn’t get converted to starch, and these can darken during heated-air drying, causing additional dockage due to kernel damage. The last time we saw a substantial amount of this was on 2009, when cool weather prolonged the season, and test weights were as low as the mid-40s in some fields. We don’t believe there was much of this in 2019, but some fields planted very late with normal-maturity hybrids might not have finished filling before freezing.

The other main reason for low test weights is having high grain moisture when test weight is measured. This is complicated—if all other kernel characteristics stay the same as kernels dry, loss of water weight might lower test weight. But loss of water from kernel starch usually causes starch granules to pack together more tightly, which increases kernel density and test weight. Dry kernels tend to slide past one another more easily, so they pack a little better, which can also raise test weight. As kernel moisture drops into the low teens, it might even be possible for kernels to lose test weight as water is lost but nothing else changes much.

We did a small experiment in 2017 to see how test weight responded as kernels dried down. We selected ears from several different trials, with different hybrids and management factors, including planting date, with the goal of starting with grain at about 30% moisture. Three grain samples were collected from each source, and grain was allowed to dry down in the open air in the lab. At the start and then every few days, samples were stirred, grain moisture was taken using a GAC tester, and test weights were taken using an official (funnel) test weight apparatus. Weights of one-pint volumes of grain were taken using an electronic balance. Testing continued for two to three weeks, and ended once grain moisture dropped to the low teens. Changes in test weight as grain moisture dropped are shown for five grain sources in Figure 1 below.

Figure 1. Changes in grain moisture and test weight measured on grain samples air-dried indoors.

While responses of test weight to grain drying varied some among sources of samples, trends were fairly consistent. From about 30% to about 27-28% moisture, test weight tended to drop by a pound or two, presumably as kernel lost moisture (2 percentage points of moisture weighs about 1 pound per bushel) but little else changed. From 25 down to about 15% moisture, test weights tended to rise more or less as a straight line. The increase in test weight as moisture dropped from 25 to 15% ranged from 5.3 to 7.7 pounds per bushel, and averaged 6.1 pounds per bushel. As moisture dropped below 15%, the rate of increase in test weight slowed, but stopped as moisture dropped to 10 to 11%.

I don’t know if corn grain delivered at, say, 28% moisture with a test weight (measured on wet grain) of 53 pounds per bushel gets docked in price, but if the grain is bright and sound, there is likely to be flexibility in assigning dockage amounts, especially if high-test-weight, dried grain is being delivered as well. Our results show that drying the grain (very gently, without heated air or mechanical disturbance) to 15% moisture should increase test weight of such grain by at least 5 pounds per bushel. Mechanical handling, such as moving grain through augers, rubs kernels together, which might increase test weight a little more.

To put this into perspective, a 56,000-lb truckload (1,000 “wet” bushels) of corn that gets dried from 25% down to 15% moisture loses 1-75/85 = 11.8% of its weight as water, so ends up as 882 bushels of “dry” corn. There is also some mechanical weight loss during drying—for example the “wings” and dust that scatter from driers—but that’s typically only a fraction of a percent, and we’ll ignore it here. If the test weight at 25% moisture is 53 lb/bu, that amount of grain would occupy 1,310 cubic feet, or 1,056 bushels, of bin space. If after drying the test weight is 57 lb/bushel, this grain would occupy only 1,075 cubic feet, or 867 bushels of bin volume. The fact that weight drops by 11.8% while the grain volume drops by 18% shows that about two-thirds of the decrease in volume is due to weight loss and about one-third is due to increase in test weight.

The price of corn and cost of drying may cause some to allow the corn to stay in the field for more days or weeks before they harvest it. If the crop is still standing well after some wind events we had over the past six weeks, and if ears remain firmly attached, then there may be little danger that harvest losses will increase very much or very quickly. Temperatures are expected to be somewhat higher over the next week or so than they have been in recent weeks, and this will help. But daytime high temperatures in the 40s are not going to speed up drydown by very much, and it could easily take a week to lose a point of moisture, although sunshine and some wind might increase this rate. We’ve all heard of corn staying out until February or March and being harvested safely with good quality. While that might mean no direct storage costs, we need to count as “storage” costs any grain that ends up on the ground and not in the bin once we do harvest it. It’s not an easy decision, but we need to stay alert and be ready to harvest as soon as that looks less risky than leaving the crop in the field.

A request for 2019 yields

On October 17, 2019, the UI College ACES put out a news release that described an effort to gather yields from a lot of Illinois corn and soybean fields in 2019. We’re doing this because of the unique opportunity we have to try to get a handle on how planting date affected yields in 2019, so we know better what to expect if and when planting is this late again.

Although late planting is nothing new in Illinois, never before have so many acres been planted as late as in 2019. I describe it in the release as a “giant, unplanned, and involuntary experiment conducted by Illinois farmers.” No one wanted or expected this, but with thousands of fields planted late, we can use planting dates and yields—if we get them from enough fields—to estimate how much effect late planting had on yields. We can also see if changing to an earlier-maturing hybrid for late planting increased yield. It takes a large number of fields for this because variability among fields is so great that having only a few dozen yields from, say, the third week of June won’t give us a very sound estimate of yield.

We already have a fair amount of data from planned experiments in which we planted the same hybrid or variety in the same field over several dates. But we have not planted corn past early June or soybeans past mid-June in most of these trials, and so had little to go on when more than half of the corn and some 80% of the soybeans couldn’t be planted until after June 1 this year.

To have a chance to make this work, we need yields along with planting date, hybrid/variety maturity rating, and yield, from a lot of fields, representing a range of planting dates from early (April) to very late (July). Getting ten or more of these from individual producers from their different fields would be great. Getting a hundred or more from a seed dealer, agronomist, or other retailer who work with numbers of farmers would be even better. We’d like to get these by the end of December, or any time before memory of this year’s harvest begins to fade.

One easy way to submit information for this is online, using the anonymous form found here. With only eight things to fill in (including county and crop), we think it will take only a minute or so to do this for a field, once planting and harvest dates are on hand. You can also fill out a form with information from a number of fields on the form found here: You can return this form as an email attachment, as a file or scan, or you can print and send it by U.S. Mail.

We’ve found a few cases in which information already submitted has “variable” as the field yield, sometimes with a low-to-high range. We don’t have a way to use such data. I suggest you leave out fields like this, or put in yield from a yield map (or a written estimate during harvest) from a uniform part of the field that you think represents the field.

Thanks in advance for helping to make this work. We have no funding for this work, but it’s important enough that we’re willing to put in the effort to make it work. We hope never to have a spring as wet as this last one, but if we do, having this information will give us a much better idea of how to deal with it and of what to expect.


Soil temperatures and fall ammonia application

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

Figure 2 shows the daily maximum, minimum, and average soil temperatures, averaged over six years at Peoria. The daily minimum temperature reached (and stayed below) 50° by October 20, but the maximum soil temperatures at that point were still above 60°, and ammonia applied then would likely have undergo a considerable amount of denitrification over the following weeks. Average soil temperature first reached 50° on October 25, but rose again the first few days of November, and stayed below 50 only after November 6. Almost all of this increase came from 2015 and 2016, when soil temperatures rose to above 60 degrees the first week of November. Although this trend supports the general recommendation that November 1 is the safe date to start to apply ammonia, we can also see that soil temperatures can sometimes rise after that. It may make sense to hold off in a year when soils are still above 50° the last week of October, and warm weather is predicted to continue into November.

Figure 2. Daily minimum, maximum, and average soil temperature averaged over six years (2013-2018) at Peoria. Data are from

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 six years, 2013-2018. Data are from

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

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.

Fall fertilizer considerations in 2019

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.

Fields with a crop in 2019

If neither soil sampling nor P and K application could be done as planned for the 2019 crop, the yield-based estimate of nutrient removal by this year’s crop can be added to the estimate of removal by crops grown since the last application. The urgency of the need to apply “catch-up” P and K depends on soil test levels the last time the field was sampled: if P and K levels are already high, there’s less concern about yield loss even if 2019 ends up being a “skipped” year of replacement. Yields in some fields will also not be as high in 2019 as they were in 2018, meaning less nutrient removal. But any of the immobile nutrients like P and K that were removed with harvest of any crop will need to be replaced at some point if soil test levels are to be maintained.

Other than less nutrient uptake in fields where yields are lower than expected this season, soil sampling and nutrient management can continue as usual in fields where a crop was grown this year. In the drier parts of Illinois, late-planted crops took up water (and matured or will mature) later than normal, although the total amount of water taken up is less where yields are lower. Where it’s dry enough to make it difficult to get a soil probe to the proper depth, we can expect soil samples to show more variability than usual, especially in K test levels. This is due both to variable depth of samples and to the effect of dry soils on K extractability. Samples taken from dry soils often show lower than expect soil test K levels because K cations get trapped in clay lattices. Test levels of pH and P are less affected than the K test by soil moisture before and during sampling. Dry soils are rare in the spring, and so soil test levels, especially of K, are more consistent when measured on samples taken in the spring.

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.

Corn and soybean crops limp towards the finish line

After the worst start to a cropping season in decades, mid-season lack of rain in parts of Illinois, and season-long low crop ratings, it’s time to take a look at what comes next as the 2019 cropping season moves into its final stages.


To no one’s surprise, various crop tours in recent weeks have confirmed that corn yields in parts of Illinois are likely to be disappointing. If there is a positive, it’s that the crop may look a little better than we thought it would by now after more than half of it was planted after June 1. While canopy cover and color in early July were a little better than expected, lack of rainfall and a less vigorous root system on late-planted corn meant that water stress began to show up in July. In areas where the dryness continued through August, some fields now show little green leaf area, and ear tips have dropped in drier parts of fields.

The driest parts of the state are the counties around the Quad Cities and in east central Illinois, with rainfall totals in July and August only about half of normal in these areas. This region shows up as being abnormally dry or in moderate drought on the U.S. drought map. Much of northeastern and southern Illinois received at least normal rainfall amounts over the past two months, and a band from St. Louis east along I-70 in south central Illinois shows rainfall totals of 150% or more of normal. Although late planting has gotten most of the attention, rain amounts, including lack of rain in some areas, will be a big part of the 2019 cropping story. That would have been the case even if planting had been early.

Late planting made the lack of adequate water a bigger problem. Many fields showed early water stress symptoms, and ended up with shorter-than-normal plants; both point to soil compaction as a major issue. Soil compaction was certainly an issue this year, but the smaller root systems and drying soils meant that plants weren’t as able to get access to water deeper in the soil as they would have been with earlier planting, even into compacted soils. Soil compaction is always present after tillage and planting using heavy equipment, but roots of early-planted corn can usually make connections to tap water from deeper in the soil even when there is compaction.

Temperatures this season have tracked very close to normal: from May 1 through September 1, the statewide GDD accumulation was about 2,580, 15 GDDs above normal. This total ranges from about 2,300 in northern Illinois, to 2,500 in central Illinois, and to more than 2,700 GDD in the southern part of Illinois. Had the crop been planted at the normal time, some fields in southern Illinois would be starting to dry down by now, and those in central Illinois would be getting close to black layer. But corn planted on June 1 instead of May 1 in northern and central Illinois lost about 350 and 450 GDDs, respectively, and so accumulated only about 1,950 and 2,050 GDDs by September 1. Corn planted on June 15 lost an additional 250 GDDs or so, and so accumulated only about 1,700 and 1,800 GDDs by September 1 in northern and central Illinois, respectively.

If we assume for simplicity that hybrids normally grown in northern and central Illinois require 2,550 and 2,700 GDDs from planting to maturity, corn planted on June 1 would need to accumulate roughly 600 and 650 GDDs, respectively, from early September through maturity. Normal GDD accumulations in September in northern and central Illinois are about 450 and 500, respectively, and accumulating the number of GDDs needed to reach maturity would, with normal temperatures, take until about October 20 in northern Illinois and about October 15 in central Illinois. September temperatures have been above normal in four of the last five years, but we can’t count on that in 2019. Corn planted after June 10 requires more GDD to mature than it is likely to get before the average date of first frost, which is around October 20.

What if the corn doesn’t get enough GDDs to mature fully? According to the Iowa State University publication Corn Growth and Development (PMR 1009), dry matter accumulation slows considerably near the end of the grainfilling period: it takes 380 GDDs to accumulate the last 10% of kernel dry weight, and 205 GDDs to accumulate the last 3% of dry weight. So having the corn stop filling with 200 GDD yet to go should not cost a lot of yield. That depends somewhat on how grainfill ends, though: a hard freeze (28 degrees or less) stops grainfill and starch formation in the kernels quickly, while slow deterioration of the leaf area before grainfill ends allows more sugar to move into the kernels and be converted to starch to add dry weight. Kernels that don’t fill completely tend to have a constricted base where they attach to the cob, and that can mean lower test weight. If frost stops the conversion of sugars to starch, kernels remain unfilled to the tip and also accumulate sugar there, which can slow field drying and can make kernels discolor more easily during heated-air drying.

In the drier areas of eastern Illinois we’re seeing a plants starting to die in patches, with ear tips dropping and leaves drying up. Such patches are typically where soils hold less water, and in some fields also in low areas where there was damage from standing water early or perhaps more compaction at planting. If other areas in the field are still green, we expect patches where the crop died early to show lower kernel weights and yields.

The USDA-NASS will issue the September 1 crop yield estimates on September 12. The August 1 estimate was 181 bushels per acre for Illinois corn, which is down 29 bushels (14%) from the 2018 Illinois corn yield. The corn crop ratings are not very high: 19, 35, and 44 percent of acres were rated as poor or very poor, fair, and good or excellent in the September 1 report. In 2017, only 55% of the crop rated as good or excellent in early September. That year, the Illinois yield estimate went from 188 in August to a final of 201 bushels per acre. This year is not a lot like 2017 (or any other year in the last 40), so we’ll need to wait to see what the new estimate turns out to be.


With 80 percent of the 2019 Illinois soybean crop planted after June 1 and some 10% planted after July 1, we set a new record for late planting of soybean in Illinois this year as well. With such late planting, the flowering and pod setting took place at least two weeks later than normal (average of the last five years); by September 1, nearly 10% of the crop was still not setting pods.

Two main factors will combine to limit soybean yields in much of Illinois in 2019. One is that late planting has, at least in many areas, resulted in lower numbers of pods that are filling. Reasons for this are complex, but include: 1) late canopy formation, which likely limited the supply of sugars needed to set pods; 2) less favorable (lighter green) canopy color, at least in some fields; 3) lower than normal numbers of nodes with pods, especially in dry areas where plants are short; and 4) low pod numbers per node. We see very little of the three-to-six pods per node (in the central part of the stem) that we saw in the 2018 soybean crop, even in fields that appear to have made fairly good vegetative growth. Many plants have only two or three pods per node, and only 10 to 12 nodes with pods. There appear to be more productive (pod-bearing) branches than normal in some fields, possibly because main stem growth was limited so branches had more resources. In some plants I’ve seen, a third to half of the pods are on branches. We don’t know if this affects yields compared to having most or all of the pods on the main stem.

Another factor that is likely to lower soybean yields in 2019 is the late start of podsetting followed by the late start of seedfilling. This is because, compared to August, days in September are shorter and average temperatures are lower, meaning that the amount of daily photosynthesis is lower. This isn’t a problem at the beginning of September, but it is by the end of the month. Based on temperature and daylength changes, we would expect the amount of daily photosynthesis (on a day with full sunlight) in central Illinois to drop by about 55% from September 1 to September 30. Most of this is due to lower temperatures.

One of the difficult-to-predict differences between early- and late-planted soybeans is the timing of crop maturity. We gauge the end of seedfilling by when the crop canopy loses its color, which signals that the leaves have exported their nitrogen to the seed with the last of the sugars. That happens quickly—often over only two days or so—and we don’t have a very good way to guess when it’s going to happen. We think that the signal to end seedfilling originates in the pods—plants without pods stay green, and even the leaf that feeds a single node without pods may stay green. It’s possible that the timing of this signal is related to how many pods are on the plant and to what extent the seeds in these pods have filled.

We’re now starting to see the loss of leaf color in early-maturity varieties, even those that were planted late. We know, of course, that early varieties mature earlier than later ones, both because the period of pod formation is shorter in early varieties, and because seedfilling starts earlier. Based on the low pod numbers we’re seeing this year, it appears unlikely that early-maturing soybeans are going to produce high yields. That can indicate that fields that mature later may not have great yields, either.

We’ll need to wait to find out how well seeds fill before leaves lose their color, but we should keep in mind that soybean yields are more closely tied to seed numbers (per acre) than to seed weight, and in the parts of Illinois most affected by late planting and dry weather, we aren’t seeing the high seed numbers we’d need for high yields. At 120,000 plants per acre and 3,000 seeds per pound, yield in bushels is the number of seeds per plant divided by 1.5. At 2.5 seeds per pod, each pod per plant would mean 1.67 bushels per acre, and 30 pods per plant would mean 50 bushels per acre.

Will the August 1 NASS estimate of 55 bushels per acre for Illinois soybeans hold up? I don’t have a basis to judge, but there are both some very good and some very poor soybeans in Illinois fields. Having a stretch of warm, sunny weather in September would help to fill the green pods on green plants in many of the late-planted fields. But pod numbers are not going to increase, and that means that many fields will not produce yields this year as high as those we saw in many areas in 2018.

Let us know if you see these diseases in Illinois!

There are two, fairly new and / or important diseases  to keep an eye out for in 2019.  We are actively seeking samples of symptomatic plants for research to help us understand the biology, ecology, and management of these pathogens.  If you have a suspect sample, please send to the UIUC plant diagnostic clinic for confirmation (cost will be covered), and or contact me via email, telephone, or twitter.


The first is a disease that we started working on in late 2017- Tar spot on corn.  I mentioned in earlier articles that I expect this disease will start to show up more in after canopies close, provided we get sufficient rain.  It has been dry, so unless you have seed fields under irrigation, it is likely that tarspot thusfar is extremely low in incidence or nearly absent in your fields in Northern Illinois.  Look for raised black spots that typically have a small yellow halo.  These spots can expand along the vein, giving them a diamond shape, but also can take various amorphous forms. Make sure you pay attention to the lower canopy.

Example of black tar spot on a corn leaf. N Kleczewski


Below is the current map showing counties where we have confirmed the presence of tar spot in at least one field.  If your county is in this region of the state, ensure that you are keeping an eye on your crops and their growth stage.  There is still a fair amount of corn that has yet to tassel and plenty of crop development that needs to take place before the season is finished.

tar spot incidence map at the county level in Illinois as of 8/7/19.


The second disease is Red crown rot on soybeans.  This disease is something we want to keep an eye on because it is new, it shouldn’t be here, and it can look very similar to Sudden Death Syndrome.  Red crown rot likes it warm and wet.  The fungus that causes this disease colonizes the roots of the plant, and produces a toxin that accumulates in the foliage, typically after R3.  This results in interveinal chlorosis that resembles SDS, or other diseases such as brown stem rot, or stem canker.  As the plant nears senesce, the fungus will produce red fungal structures at the base of the stem that give the disease it’s name.  When you split the stems roots will often be black and rotted and the center pith of the lower stem will have a grayish appearance.   This disease caused significant losses in a field near Pittsfield last season, so those in that area should pay particular attention for this one (but so should everyone!)

Signs of Red crown rot on soybean.


A split stem showing gray inner pith characteristic of Red crown rot.

Corn and soybean crops at mid-season, 2019

The 2019 Illinois corn crop reached 50% planted during the first week of June, more than a month later than the average of the past five years. The soybean crop reached 50% planted a few days later than corn, and more than three weeks later than the average of the past five years. May rainfall was above normal over most of Illinois, and June brought near-normal rainfall over much of the state. Still, the late planting coupled with too much or too little rainfall after planting produced July crop condition ratings of only about 40% good + excellent for both crops, compared to an average of some 70% over the past three years.


While corn crop condition ratings haven’t improved over the past month, the warm temperatures and good sunlight have resulted in good growth of the crop in most fields, except in low, wet spots. Leaf color of the crop has also improved, as soils have dried enough to allow the roots to reach N from both fertilizer and from mineralization. This has helped the crop plants and crop canopy to recover some from the rough start.

The 20% of the corn crop planted between mid-April and mid-May has reached or completed pollination in most fields, in line with growing degree day accumulations: at Champaign, about 1,400 GDDs accumulated between May 15 and July 15, enough to get a 110-day RM hybrid to, or close to, pollination. That number is only about 1,140 at DeKalb, where little corn was planted by mid-May anyway. With little disease or insect pressure and with dark green leaves, most of the fields that have reached or finished pollination should end up with high kernel counts, which means high yield potential.

Most of the Illinois corn crop was planted at the end of May or during the first half of June, and little of this late-planted crop has reached pollination: only 19% of the Illinois crop pollinated by July 14, which is about the same percentage of the crop that was planted by mid-May. June 1 through July 15 GDD accumulations totaled about 930 at DeKalb, and about 1,060 at both Champaign and Mt. Vernon, in southern Illinois. At current GDD accumulation rates of 25 to 30 GDD per day (this will decrease some with the cooler weather next week), corn planted in early June will reach begin to pollination towards the end of next week. Corn planted after June 10 will probably not pollinate until early August, although early hybrids—we can estimate about 10-11 fewer GDD for each day earlier in RM, or about 2 days earlier for each 5-day drop in RM—may pollinate by the end of July. Objective yield estimates, which use kernel counts, will be difficult to make early this year, with so little of the crop through pollination by the end of July.

There is ongoing concern about how the season will end for the late-planted crop, including whether or not it will mature before frost. If fall frost comes at or after its 50% date, which is October 15 to 20 depending on location in Illinois, the late-planted crop should be at or close to the GDD needed to mature by that time, as long as the maturity of the hybrids planted was adjusted for very late planting. From June 1 through September 30, GDD accumulations for northern, central, and southern Illinois average about 2,400, 2,600, and 2,800, respectively. Including October 1 to 15 adds about 150 GDD to these totals, but delaying planting from June 1 to June 15 subtracts about 300 GDD, or about 20 GDD per day of delay. So in northern Illinois, corn planted on June 10 is expected to accumulate only about 2,350 GDD by the average date of first frost. That’s enough to get a 95-day RM hybrid close to maturity, but not a 110-day hybrid. While a decrease is the number of GDDs required for a hybrid when it’s planted late has been noted, we have not seen a decrease in the number of GDDs needed to get the crop to its current stage so far this year, and any decrease in the number of GDDs required from now to maturity is likely going to come at the cost of losing yield due to late-season stresses that limit grainfilling.

The main challenge facing the corn crop at this point, especially the late-planted crop, is having enough water to maintain growth and to set and fill enough kernels to reach its full yield potential after the difficult start to the season. While there is enough soil moisture to maintain the crop in most Illinois fields now, the late-planted crop especially is showing symptoms of water stress under high temperatures this week. We think this is mostly because the root system of the late-planted crop is somewhat limited, both due to compaction from planting into wet soils and also because late-planted corn that develops under high temperatures tends to favor top growth over root growth. So even though the late-planted crop likely has more water in the soil now than the early-planted crop (which has used several more inches of water up to now), access to this water is somewhat limited, and the result is leaf-curling by early or mid-afternoon. In drier areas, even the early-planted crop is showing some afternoon stress, especially on slopes where the soil holds less water.

The cylinder formed by a curled corn leaf under water stress cuts the windspeed across the leaf surface and lengthens the distance the water vapor has to move once it exits the leaf, so greatly decreases the rate of water loss. That might seem like a good thing, but when leaves aren’t losing water very fast they also aren’t taking in carbon dioxide very fast—that is, their photosynthetic rates are low. Low photosynthetic rates mean slow growth, and a prolonged period of low photosynthesis means lower yield potential. A few days of water stress before pollination won’t lower yield potential by very much, but a week or more of decreased photosynthesis will decrease kernel number, and so will lower yield potential. If such stress continues after pollination, more kernel abortion will lower yield potential even more.

Although the yield potential in early-planted fields without standing water damage appears to be high now, getting the crop to reach the potential with high kernels numbers—16 to 20 million kernels per acre—will require about 10 more inches of water this season. Depending on soil and current soil water supplies, half to three-fourths of that amount of water will need to come from rainfall over the next 50 days or so. The late-planted crop is likely to set somewhat fewer kernels, but it will likely need 11 or 12 more inches of water in order to pollinate and fill kernels successfully. Some rain soon might help to increase the size and competency of the root system and so help the crop better extract water that’s in the soil. But as pollination approaches, the plant shifts its allocation of sugars (from photosynthesis) away from roots and stalks to development of the ear and to completion of the pollination process. So in fields where the crop is under water stress now, the chances of getting good yields will continue to diminish the longer it goes before rain arrives. Cooler temperatures will slow the rate of this decrease by slowing the use of water, but in areas that are dry now, there needs to be water falling from the sky before pollination if this crop is to regain some of the yield potential lost with late planting.

Especially in the late-planted crop, there is some talk about needing to apply foliar fungicide and, in some cases, perhaps more N fertilizer. If enough N was applied earlier, there should be no need to apply more, especially if the upper leaves have a dark green color in the morning before stress begins to show. Some have suggested that late-planted corn will more likely need foliar fungicide, with the idea that protecting the canopy is more important when (with late-planted corn) there is less leaf area, or perhaps with the expectation that fungal diseases will more likely attack corn that pollinates late. The planting date studies that we’ve conducted in Illinois have, since 2010, included foliar fungicide applied at pollination as a treatment. Results do not support the idea that fungicide increases yield of late-planted corn more than it does of early-planted corn. Averaged across 21 trials in the northern half of Illinois, foliar fungicide increased the yield of corn planted in early April by 7 bushels per acre, and increased yield of corn planted in late May by 6 bushels per acre. Across 12 trials in the southern half of the state, fungicide increased the yield of corn planted in mid-April by 7 bushels, and increased the yield of corn planted in early June by only 2 bushels per acre. With corn crop prospects so closely tied to the water supply for the rest of this season, spending money on additional inputs (besides irrigation) may not be very helpful to the bottom line.


Most of what I mentioned about corn crop condition and planting above applies to soybean as well, with even fewer soybean acres planted early (in April). May was not a great month for early-planted soybeans in any case this year, even if they had good enough stands and didn’t need to be replanted. Greg Steckel and Marty Johnson were able to plant soybeans in a planting date study on April 9 at Monmouth. Those plants are now about 18 inches tall, and, in 15-inch rows, they have formed a near-complete canopy. Most of the soybeans in Illinois, though, are “canopy-challenged” now, and it’s likely that some of the 30-inch rows will not form a complete canopy at all. Late planting also resulted in a slow start to flowering—only 12 percent of the crop flowered by July 14 this year, compared to 77% on the same date in 2018, and a 5-year average of 54%.

Like corn, late-planted soybeans in the drier parts of Illinois are showing symptoms of water stress. These aren’t as noticeable as they are in corn, but are mostly seen as a slight drooping of leaves in the afternoon as the stress intensifies. The result is the same, though—plants under stress for much of the day do not do photosynthesize very well, and so they do not make very fast growth. This is compounded by the fact that late-planted soybeans have small root systems that can’t get access to very much soil water, and as a result, leaf area and plant height are increasing slowly. Root damage from wet soil conditions after planting also contributed to this in some places, and growth has been slow in these fields or parts of fields as well. As we know from past experience, soybeans are likely to “come around” and make good growth once they get past this period of slow growth, but it’s certainly better when that happens in late June than in late July.

While we’ve historically said that soybeans are less sensitive to late planting than corn, that was more true when soybeans yielded 40 to 50 bushels per acre than when they yield 70 or 80 bushels per acre, as they have done in recent years. Rapid early growth and canopy formation is key to high soybean yields, and this has not happened—and will not happen—in Illinois in 2019. With only about 20% of the Illinois soybean crop emerged by two weeks before the longest day of the year (June 21), the hours of sunshine on that day did little for the crop. The only path to good yield potential this year will be through a turn to unusually favorable weather, with good rainfall and temperatures in August and good rainfall and above-normal temperatures in September. We know from doublecrop soybeans that yields of 50 bushels per acre or more are possible when planting is late, but that does not tend to happen when growth through mid-July is as limited as it is in many field this year. We don’t have much information on how soybeans planted in late June or early July will do in central Illinois, but the slow growth of the crop since planting is a cause for concern.

Along with symptoms of water stress in some soybean fields is a lightening of canopy color that may indicate lack of adequate nitrogen. In fact, the N fixation process is sensitive to plant stress, and this could be a factor. It’s also the case that soybean leaves tend to give up their N (that is, break down proteins and export the N to the rest of the plant) as stress continues, in some cases even dropping leaves like they normally do at the end of the season. There are two reason why loss of leaf color is a concern. One is that soybean plants store significant amounts of N in their leaves as podsetting starts, and how much N is stored this way is a major factor how many pods set and how well seeds fill. The other concern is that paler leaves means lower current photosynthesis, and so slower growth. The “dark green blanket” of plants and leaves that we saw in nearly every field in mid-July in 2018 is not present in many fields this year, and this makes it hard to be optimistic about this year’s crop.

As with late-planted corn, some have proposed adding N or using foliar fungicides as a way to lower the stress on soybean plants and to increase yields this year. But when growth is limited by the amount of water available, either because soils are dry or because roots don’t have access to the water in the soil, then adding other inputs is unlikely to add much yield. In the soybean planting date studies we conducted since 2012, we included a treatment comparing foliar fungicide (usually with an insecticide as well) with no fungicide at each planting date. Over six trials in southern Illinois, foliar fungicide produced an average yield increase of 4.1 bushels per acre when planting was in mid-May, and of 3.7 bushels per acre when planting was in mid-June. Across 17 trials in central and northern Illinois, foliar fungicide increased yield by 2.5 bushels per acre when planting was in late April, and by 1.7 bushels per acre when planting was in early June. We have not done similar studies using fertilizer N, but have found such limited response to using N on soybeans that it’s unlikely that it will ever pay on soybeans, especially on soybeans that undergo stress that limits early growth.

As I’ve often stated, the crop canopy will “tell the tale” of this year’s corn and soybean crops better than any other thing we can look at. If in mid-August we see fields with dark green leaves, we can have hope for good—if not great—yields.

Southern rust in Illinois- it’s complicated

This week we started picking up Southern rust in the southern Illinois.  Thusfar, reports indicate that disease severity is low.  However, the recent hurricane remnant and warm forecasts may mean that we may see the disease progress somewhat in the coming days and weeks.

When people in Illinois hear the words southern rust, it brings back memories of a few years ago when the disease moved in and environmental conditions favored disease development for a prolonged period of time.  Many fields suffered losses as a result of the disease.  This year the situation is complicated and different from a few years ago.  First, we are dealing with extreme differences in planting dates throughout the state.  One field may have tasseled a week or two ago and the field across the road might just now be reaching V10.  Second, yield potential in late planted fields is likely to be substantially lower than typical, meaning that there is less yield to protect and less money to cover potential application costs.  Third, commodity prices, although they may have increased slightly in recent days, are low, making it hard to justify the cost of applying a fungicide unless necessary and the potential to recover costs is high.


Southern rust.  Image A. Sisson.


Common rust vs Southern rust on corn. Image C. Bradley.

Let’s take a minute and first go over southern rust, then move back into what factors you should consider before making a fungicide application to manage this disease in 2019.

Southern rust is caused by the obligate fungal pathogen Puccinia polysoraPuccinia polysora produces fuzzy, raised structures called pustules on leaves and stalks of corn.  Pustules contain thousands of small orange spores.  When you rub these pustules between your fingers, the spores may leave a dusty orange coat on your fingers, hence the reason it is called a “rust.”  Pustules of Southern rust are orange to light tan, and often small and circular.  Pustules are mostly found on the upper leaf surface, which can help distinguish it from the less damaging common rust.

Spores from pustules can be dispersed miles on air currents, allowing the disease to spread rapidly.  Under hot humid conditions, spores of the fungus can infect suceptible corn, and symptoms can be observed within 3-4 days.  Within 7-10 days, spores are produced and can be dispersed.  The cycle of spore-infect-spore can continue as long as conditions are conducive and corn plants are green.  Conditions that favor disease development include hot temperatures (morning low of 75°F and daytime high of 93°F) and at least 4 hr of consecutive leaf wetness.  Outside of these conditions disease progress can occur, but at a slower rate.  Our colleagues to the South state that Southern rust can continue chugging along at 110 degrees.  That’s pretty impressive.


Southern rust does not overwinter in Illinois and blows into the region from warmer regions.  In years where it develops to a significant degree early in southern regions, it can move into Illinois during critical stages in crop growth.  In general, we see the disease move in most years in late July or early August.  This means that in years when plantings are delayed, the disease can arrive on time but plants may be at greater risk for yield loss because the earlier  infections occur the more yield can be impacted.  Experience from our Southern colleagues indicates that stalk integrity isn’t likely to be affected unless you see significant infections during the vegetative stages of crop development.

Example lifecycle of Southern rust. Note that it blows in from warmer, southern regions.

Now that we are on the same page about this disease, what about management?  As I mentioned previously things this season are complicated.  Let’s start by considering management decisions.  Below is a table modeled after one produced by fellow plant pathologist and Jason Statham look-alike Travis Faske at the University of Arkansas, depicting the likelihood that a fungicide application for Southern rust will provide a benefit:

Growth Stage Southern rust present Forecast favors S rust (75-93F) min 4 hrs leaf wetness Benefit of fungicide?
Vegetative yes yes yes
VT/R1 yes yes yes
R3 yes yes yes
R4 yes yes Unlikely
R5 yes yes No
Maturity yes yes No


Experience from the South indicates that trying to hold off an application until VT/R1 if possible is going to give you the highest likelihood of coming out even or ahead of this disease.  If you apply during the vegetative stages, realize that that means that you might need to come back again and make a second application.  You now have likely doubled your application costs.

Now let’s consider some other aspects of controlling this disease in our late planted crops.  If you were unable to switch to a shorter day hybrid, and decide apply a fungicide for managing S. rust at say, R3, your plant will be protected from disease, and retain greenness later into the season.  Depending on your location in the state, this means you might need to consider frost, and what that means for your yield potential and crop harvestability.

Before making an application, consider these points and also run the numbers.  Remember that fungicides DO NOT INCREASE YIELD POTENTIAL.  They do not increase yield.  They protect potential yield by mitigating losses due to fungal disease.

That being said, you can calculate the amount of protected yield required to pay for a fungicide application by using the formula Yield protected (bu/A) = application cost ($/A) crop price ($/bu).  After calculating your required protected yield, you can then determine if potential yield, frost, and other factors will make it worthwhile to spray.


Continue to scout.  For updates on Southern rust and it’s presence in Illinois and surrounding states click here