The extension weed science programs at The Ohio State University, Purdue University, and the University of Illinois recently collaborated to produce suggestions and precautions for use of dicamba in dicamba-resistant soybean. The United States Environmental Protection Agency issued amendments to the Xtendimax, Engenia, and FeXapan labels last October, and this new extension weed science publication offers additional suggestions to help further reduce off-target dicamba movement. Dicamba Precautions
The substantial rain that fell over central and northern Illinois between October 5 and 15 mostly soaked into the soil that was dried out by crop water use, and harvest has moved back to full speed in most areas. With harvest, thoughts turn to application of fall ammonia in central and northern Illinois. Almost everyone is on board with waiting until soil temperatures are at or below 50 degrees before applying ammonia. Cool soil (along with use of nitrification inhibitor) lowers the rate of nitrification, so helps preserve N in the ammonium form. Nitrogen present in the soil as ammonium is safe from loss.
Once air and soil temperatures start to decline in October, it’s natural to anticipate that soil temperatures will reach 50 soon, so some are inclined to start to apply before soil temperatures reach 50 degrees. But if we apply when soil is at 60 degrees and soil temperatures fail to drop quickly, or if they rise again after application, nitrification will continue and will persist as long as soils stay warmer. In fact, nitrification does not stop dead at 50 degrees; as a biological process, its rate drops off as temperature falls, but temperatures need to near freezing for nitrification to stop completely.
So we need to wait to apply fall ammonia not only until soil temperatures are 50 or less, but until we have reasonable confidence that they’ll stay there. In Illinois, we normally consider November 1 to be the date at which we can be reasonably sure that soil temperatures won’t rise again until the next spring. That’s not a sure thing, however – in both of the past two years, soil temperatures have gone above 50 at least once between November and February. But most years it’s a reasonable starting date to balance keeping N safe with getting fall application done.
Minimum air temperatures have fallen into the 40s this past week, which has people wondering if it might be OK to go ahead and start applying now. Minimum soil temperatures 4 inches deep under bare soil (from the Illinois Water Survey http://www.isws.illinois.edu/warm/soil) have dropped to the upper 40s to low 50s over much of the state each day between October 16 and 18 this week. The problem with using only the minimum soil temperature is that it doesn’t represent the actual soil temperature in the ammonia application zone. As Figure 1 shows, minimum soil temperatures (on clear days) are typically five degrees or so less than average soil temperatures for the day. So even though we may need a jacket on cool mornings this week, ammonia applied now is not going to be in soils with temperatures less than 50 degrees for some days or weeks.
Air temperatures are forecast to stay in the 70s the rest of this week, to fall into the 50s (with lows in the mid to upper 30s) next week, then to rise again (with dry weather) for some period after that. We’re already past the average first frost date for central and northern Illinois, and even with more seasonal temperatures coming the last week of October, it doesn’t look like ammonia applied now will be as safe from nitrification and possible loss as will ammonia applied in November.
If the soil is in condition to apply ammonia, soil temperatures are in the upper 40s, and the 10-day forecast doesn’t show above-normal temperatures settling in, the last few days of October might offer an opportunity to start applying ammonia. But what if early November is warmer than normal, and soil temperatures remain above 50? Delaying application, of course, moves us closer to having safer soil temperatures.
Average Illinois fall temperatures have been trending slowly upward for some decades now, and as we have seen the last few years, waiting until November 1 does not assure low soil temperatures as consistently as it did in the past. So if a stretch of warm weather is still in the forecast at the end of October, it might make sense to wait a little longer. Otherwise, patience in waiting another 10 days will likely be rewarded, even if – as is often be the case when doing the right thing – the reward isn’t very visible.
A group of people who work on nitrogen fertilizer met in 2004 to talk about an alternative to the yield-goal-based N recommendation system that had been in widespread use for some three decades. The main concern with the yield-goal-based system was that, as corn yields increased over time, N rate trials were showing that the amount of fertilizer N needed to maximize yields was not going up as fast as yields. On the other hand, the amount of fertilizer N needed on lighter soils, including those in southern Illinois, was consistently higher than the amount suggested using the yield goal.
The result of this meeting was the development of an approach to turn recent N rate trial data into N rate guidelines. For each of several hundred trials in Illinois, we fit a curve to the data then calculated the “(net) return to N” (RTN) – yield produced by N x corn price minus N rate x N price – for each N rate over a range of rates from low to high. For each region – Northern, Central, and Southern Illinois – and for corn following corn and corn following soybean, we averaged the RTN curves from all the trials. The N rate that produced the high point of that average curve was dubbed the “maximum return to N” (MRTN) rate. Because the RTN curves were fairly flat on top, we also decided to include a range of rates over which the RTN was close (the default is within one dollar per acre) to the maximum. So along with the MRTN value is a range, typically about 15 pounds of N on either side of the MRTN, over which the data predicts about the same return to N.
The current version of the calculator that produces the MRTN values based on the latest N response data is located at http://cnrc.agron.iastate.edu/. Seven states – Minnesota, Iowa, Wisconsin, Illinois, Indiana, Michigan, and Ohio – currently use this approach. The calculator uses the trial data from each state or each region within a state, and for trials with corn following corn and corn following soybean to produce MRTN vales and ranges. For Illinois, the calculator runs on data updated through 2016; our plan is to update it every year, adding new data and selectively “retiring” data from older studies. This approach is an economic one, so requires the user to enter prices for corn and for N.
As part of the development of this approach, we looked to see if practices like tillage or if factors like soil type would enable us to provide separate guidelines based on such practices or characteristics. We failed to find anything that would produce different MRTN rates. This doesn’t mean that, for example, different soil types don’t need different amounts of N fertilizer. It simply means that the set of data from, say, trials run in different soil types did not produce differences in N rates that were large enough to justify having separate categories for soil type. The main reason we think this is the case is that N responses vary so much due to things such as weather, soil, and crop conditions.
Over time, with enough additional data, we might be able to make separate MRTN predictions for, say, lighter-textured versus heavier-textured soils. But we’ve also found that year-to-year variations in N response are often very large even within the same field or the same part of a field. This means that it won’t be easy to find factors that show a large enough or consistent enough difference in N response to stand out over time. While it might be tempting to select out and use data that “looks similar” in order to show “differences,” we can’t do that and still claim that the process is unbiased. It would, instead, mean that the results we chose don’t accurately predict future responses.
Here are some questions that have been raised about the N rate calculator, along with answers:
Q: Although the N rate calculator has been around for more than a decade and the Illinois Agronomy Handbook describes it as the best current method for making N rate decisions, it still doesn’t make sense that higher yields wouldn’t need more fertilizer N. But there’s no place to enter expected yields, and actual yields from trials aren’t visible. Are the yields in these trial just “average” yields, and doesn’t that mean that if we shoot for really high yields this won’t tell us to use enough N?
A: To better understand this, go to the calculator site and choose “Illinois” then “Central” (region) and “Corn following soybean” as the rotation. Leave the price of N and corn as the default values or put in different values, then hit “Calculate.” A figure with some numbers at the top will appear; the numbers include the MRTN value, which is 172 pounds N per acre at the default prices, and the range of N rates (158 to 186 pounds of N per acre) over which the RTN is close to the maximum. There is also a number called “Percent of Maximum Yield at MRTN Rate” which in this case is 98%. That means that 172 pounds of N produced 98% of the maximum yield, averaged over all of the trials. This is less than the maximum yield because adding enough N to maximize yield costs more money than those last few added bushels of yield are worth. On the left under “Display Charts,” click on “EONR vs. Yield” and a figure (Figure 1) will appear that has actual yields and the N rate required to meet that yield for each trial in the database.
Over the 245 trials in this database, yield at the optimum N rate (the MRTN for an individual trial) ranged from 100 to nearly 300 bushels per acre, and the optimum N rate ranged from less than 50 to more than 250 pounds N per acre. But there is no correlation between N rate and yield; high yields did not require more N and low yields did not require less N. In about a dozen trials, less than 100 pounds of N produced more than 200 bushels per acre, and in about the same number of trials, it took more than 200 pounds of N to produce less than 200 bushels per acre. So knowing beforehand exactly what the yield would turn out to be would have been of no help at all in knowing how much N to use.
The overall best (MRTN) rate of 172 pounds of N is more than the amount needed in nearly two-thirds of the trials and less than the amount needed in about one-third of the trials. We have no way to guess beforehand on which side of this rate the amount needed for an individual field would fall, but we do know that needing less than this amount is more likely than needing more than this amount. Using more N than the MRTN means, on average, spending more on adding N than we get back in added yield. So the MRTN is the best guess at the rate that maximizes return to N while minimizing the amount of N left over after the season.
Q: Since high-yielding corn obviously takes up more N than lower-yielding corn, how can it possibly not need more N fertilizer?
A: It is absolutely true that higher-yielding corn takes up more N: at its maximum N content during the season, which is usually during the kernel dent stage, the crop typically contains 0.8 to 1.0 pound of N per bushel of grain yield. So a 220-bushel crop might have 220 pounds of N in the plants, even if, say, only 160 pounds of fertilizer N was applied. If we estimate that 75 percent of the fertilizer N (120 pounds) got taken up (that’s a higher percentage than is often measured), that means that 220 minus 120, or 100 pounds of the N in the plants had to have come from a source other than fertilizer. Other than the small amount of N (usually less than 10 pounds per acre) fixed by lightning during storms, this only place this N could have come from is the soil.
Q: Can a soil really supply 100 pounds of N to a crop?
A: An acre of soil 6 inches deep weighs about 2 million pounds, so one percentage point of soil organic matter is about 20,000 pounds of SOM in that acre. Soil organic matter is about 5 percent N, so one percent SOM in the top 6 inches of soil would mean about 1,000 pounds of N per acre. This N is in organic form, which is not available to plants. A soil with 12 inches of topsoil and with 3 percent OM would contain about 6,000 pounds of N. On average under Illinois conditions, an estimated 2 percent of soil organic N is mineralized – freed from organic matter and available for crop uptake – each year. Two percent of 6,000 pounds would be 120 pounds of N per acre. So, while such a soil could provide 120 pounds of N to a crop, the actual amount provided depends on soil conditions – mineralization is carried out by soil microbes, so is slower when soils are cool, dry, or wet. Growing conditions can also affect the ability of roots to take up N, and too much when soils are warm or in light-textured soils can result in loss of mineralized N. This variability in soil N supply is what makes it so difficult to guess at how much N the crop will get from mineralized SOM, and so how much it needs from fertilizer.
Q: If more soil organic matter means more N for the crop, shouldn’t we decrease fertilizer N rates in high-organic-matter soils, including in higher-OM parts of a field?
A: If yields were uniform across the field that might make sense. But fields or parts of fields with higher OM tend to yield more, usually because organic matter can store more water, and sometimes nutrients, for the crop. Higher yields mean more plant N uptake, and so in higher-OM parts of a field, increased N uptake is matched with higher amounts of N supplied by the soil through mineralization. This doesn’t always happen – for example lower-lying parts of a field might have higher OM but also be more prone to damage to stands or roots from standing water, which can limit yield, N uptake, and the amount of soil N available to the crop. The soil is usually not capable of providing all of the N the crop needs, however; even when corn (following soybean) yields, say, 180 bushels per acre without N fertilizer, adding N fertilizer almost always adds yield. Some states include (or once included) SOM in their N rate calculations, though, and the amount of soil organic matter might make a reasonable basis for site-specific N management (that is, to apply less N where SOM is higher) within fields. Still, higher yields in such areas can increase the need for N more than higher OM increases the amount of N supplied by the soil.
Q: While I see that selecting rotation (previous crop) is required to run the calculator, why is there no place to enter the adjustment for the soybean N credit?
A: The data used to produce MRTN values for corn following corn is from trials in which corn followed corn, and for corn following soybean, is from trials where corn followed soybean. Most of these trials were in separate fields, and this was done on purpose so we could make separate calculations for the two rotations. In central Illinois, for example, there are 245 corn following soybean and 152 corn following corn trials currently in the database. So the soybean “N credit” – or more accurately, the corn-following-corn “N penalty” – is already built in. The difference between soy-corn and corn-corn MRTN rates is a little more than 40 pounds of N (which was the value of the soybean N credit in Illinois under the yield-based approach) in northern Illinois; a little less than 30 pounds of N in central Illinois; and only 10-12 pounds in southern Illinois. These differences are consistent with the idea that as we move south, more corn residue breaks down by spring and soils are warmer at planting, so residue effects on soil temperature and moisture – and so on mineralization and on N tieup – are less noticeable. It is important not to subtract any “credit” from the MRTN rates for corn following soybeans; doing so will mean not applying enough N.
Q: What if I’m applying manure?
A: To find the MRTN, run the calculator using the price of commercial fertilizer N, since fertilizer N is usually used to “top up” the N rate (if any more N is needed) in addition to the N from manure. The amount of N from manure is based on rate, type, and first-year availability estimates that can be found in a number of sources, including the Illinois Agronomy Handbook at http://extension.cropsciences.illinois.edu/handbook/pdfs/chapter09.pdf. Failure to credit manure N adequately is a common reason why corn in some fields has access to a total N supply that exceeds, sometimes by a considerable amount, the needs of the crop.
Q: Should I include the N from MAP or DAP in the total fertilizer rate?
A: All forms of N that we apply and that will be available to the crop should be counted in the N rate we apply. MAP and DAP contain N in the ammonium form, so it’s safe from loss at the time of application, though it needs some rainfall to move it into the soil to keep it in place. If we apply these materials in early October, higher temperatures (with some soil moisture) will usually mean that some portion of the N will convert to nitrate before winter. So waiting until soil temperatures are below 50 degrees (but before the soil freezes) or waiting until spring to apply will usually increase the availability to the next crop. We’re doing a study now to try to measure how much of the N from DAP is available following application in the fall or spring. Until we have better answers, I would suggest taking half credit for the N in MAP or DAP if these are applied before mid-October, three-fourths credit if applied in the fall when soils are cooler than 50 degrees and there isn’t heavy rainfall within the six weeks after application, and full credit if application is in the spring, within a few weeks of planting.
Q: Shouldn’t fertilizer N rates differ for different forms of N and different application times – for example, don’t we need less N if we apply in the spring rather than in the fall, and even less if we sidedress most of the N?
A: Most N fertilizers contain most of their N in the form of ammonia or urea, both of which convert to ammonium in a moderately moist soil. Ammonium is held tightly on soil exchange sites, and so is safe from loss. But the process by which bacteria convert ammonium to nitrate begins soon after N fertilizer is applied, and the conversion rate increases as soil temperature increases. Once N is in the nitrate form, it can move in the soil, and can move with water down to tile lines or to beneath the rooting zone. Nitrate can also be denitrified – converted to a gaseous form and lost to the air – if water stands for a few days when soils are warm. The reason we might consider adjusting rate based on N form or application timing is to counter the risk of loss. It is, though, better to manage N to decrease loss than to apply more N in order to counter loss the risk of loss from applying fertilizer N in ways that more often lead to loss.
We can lower the risk of N loss either by slowing the conversion of ammonium to nitrate, or by delaying application to shorten the time between application and crop uptake. We can slow the conversion to nitrate by using inhibitors – for example by adding N-Serve to fall-applied ammonia and by waiting until soils are cold to apply ammonia. Delaying ammonia application to spring is even more effective than applying later in the fall; we have found in recent comparisons that waiting to apply ammonia in the spring (without N-Serve) lowers the rate needed by about 20 pounds per acre, compared to fall ammonia with N-Serve. But compared to applying all of the N at planting, we have not found that we can consistently use less N if we wait to apply some or most of the N as sidedress. That’s because our sampling shows that we typically lose less N than we think between planting and early June when N uptake rates increase, and also because the crop seems to benefit sometimes from having more (or all) of the N available at planting in order to minimize the chances that the crop will develop deficiency. Loss of (nitrate) N only occurs when soils are wet, so little N will be lost if the soil stays relatively dry, even if all of the N is present in the nitrate form.
Q: Why can’t I compare different forms and prices of N on the calculator to see which are most cost-effective?
A: We do not have enough N source comparisons to allow us to calculate different MRTN values with use of different forms or times, though direct comparisons are showing that we might be able to make some adjustments, as discussed above for fall- versus spring-applied N. Most of the calculator database consists of trials with N applied in the spring; some with all of the N applied in early spring, some with all of it at planting, and others with some at planting and the rest as sidedressed N. There are also a number of trials with fall-applied N. But grouping trials by N time and form does not produce responses that are different enough to justify breaking down the database this way. The calculator allows the comparison of up to four different N prices at a time, and those can be from different forms of N. The MRTN rate calculation depends on the ratio of N price to corn price; the higher this ratio (high N price or low corn price), the more yield increase it takes to pay for the last amount of N, and the lower the N rate. So N forms that cost more per pound of N will produce lower MRTN rates. Either price per ton of fertilizer or price per pound of actual N can be put into the calculator, which converts one to the other.
Q: Can I calculate MRTN for fields where I grow cover crops?
A: Not with confidence, at least not yet. Allowing cereal rye or annual ryegrass ahead of corn to grow well into April almost always increases the amount of N needed by the crop, and even with more N the crop may not yield as well. Grass cover crop residue is similar to corn residue in that the breakdown of residue in the spring ties up some N, decreasing availability to the crop. Some of the tied-up N will become available to the corn crop, but the amount and timing of its release is unpredictable, and in most cases some of it will be released too late to be taken up by the crop that season. So it’s not a good idea to take a credit for the N in a grass cover crop to lower the fertilizer N rate. A cover crop legume that grew in the previous season (say, red clover planted into wheat the previous summer), that grew enough in the spring to have fixed some N, and that is killed early enough to break down and to supply some of that N to the corn crop would justify lowering the fertilizer N rate for corn, by perhaps 50 pounds per acre. Growing that amount of dry matter and fixing N would likely require leaving the cover crop grow into late April or, in northern Illinois, into May, so might delay corn planting. If a grass cover crop that overwinters is killed early enough (in March) so that there’s little intact residue at the time corn is planted, N rates and corn yields will probably be little affected by the cover crop. As a general observation, though, cover crop residue is not a particularly reliable source of N for the corn crop that follows it.
Corn and soybean take up relatively large amounts of phosphorus (P) and potassium (K), and much of this P and K ends up in the grain that is taken off the field during harvest. In order to keep soil nutrient levels from dropping over time, the amounts removed need to be replaced by applying fertilizer or manure.
In order to know how much nutrient a crop removes, we need to know how much there is in a bushel of harvested grain. We’ve been using amounts per bushel that are several decades old, and whose origin isn’t clear. There is no indication that these numbers are inaccurate, but newer numbers in other states tend to be lower than the numbers we use in Illinois. It was time to take another look to see if these numbers have changed.
In 2014 Dr. María Villamil and I initiated a survey, funded by the Illinois Nutrient Research and Education Council (which administers fertilizer checkoff funds) to measure P and K levels in corn and soybean grain samples from all regions of Illinois over the three years from 2014 to 2016. With some help from the Illinois Soybean Association and the Illinois Corn Growers Association, and with assistance from a lot of elevators, we collected 2,335 corn and 2,620 soybean grain samples over the three-year period. A commercial lab analyzed nutrient levels in the samples.
In order to see if yield level was related to grain nutrient level in a way that would allow adjustment of the per-bushel nutrient level to the field yield, we gathered estimated yields from the fields from which samples came in 2014, and for some of the 2015 samples. We chose yield level instead of soil type or soil nutrient test level because yield level is far more likely to be known for a field. While we found slight correlations between yield and nutrient level in a few cases, adjusting grain levels based on yield would have made so little difference in the results that using yield level to adjust grain nutrient per bushel was not justified.
Grain P and K numbers in some cases where slightly different from one year to another, and from one region of Illinois to another. But overall, we couldn’t find any consistent effect of location or crop year on nutrient levels. This means that nutrient levels were not clearly tied to anything else about the samples, including where in Illinois the samples came from, which of the three years the samples were collected, or to the yield levels of the fields from which samples were taken.
Figure 1 shows the distribution of corn grain P levels among the 2,335 samples that we collected. Values ranged from less than 0.2 lb. P2O5 per bushel to more than three times that amount. While we expected some variability among samples, this variability means uncertainty about nutrient levels in a given load of grain; they could be below, at, or above the average values we found in the survey.
To be on the safe side, we chose the 75th percentile – the point at which three-quarters of the values are below and one-quarter are above – as the new value to use for removal. Figure 2 shows the cumulative distribution of corn grain P values, with the 25th, 50th, and 75th percentile values identified by vertical lines. The number on the right is the “book value” for corn grain P that is currently found in the Illinois Agronomy Handbook. This value (0.43 lb. P2O5 per bushel) exceeds 97 percent of the values we found in the survey. So the older number might not be “wrong” – one could choose it in order to be very sure to cover any possibility for unknown samples. But that would mean overestimating P removal by 6 percent on average, and over years that would add up.
We took this same approach for corn K, soybean P, and soybean K. Table 1 has averages and quartile numbers, as well as “book values” for the other nutrients. Average and 50th percentile (also called the “median”) are not all exactly the same because the distribution is not perfectly uniform, as illustrated in Figure 1. The median is a little better value to use for such things, because extremely low and high values, though rare, affect the average but not the median.
For corn, the new grain removal numbers of 0.37 lb. P2O5 and 0.24 lb. K2O per bushel are both about 15 percent lower than the book values currently in the Illinois Agronomy Handbook. For soybean, the new numbers of 0.75 lb. P2O5 and 1.17 lb. K2O per bushel are 12 and 10 percent lower than the book values, respectively. Because we used the 75th percentile values as the removal numbers, these values are 4 to 8 percent higher than the average or median values; in other words, they’re a little higher than actual removal for a field with average grain nutrient content.
The new numbers we found are very close to those that Iowa State University reported several years ago, after going through a similar exercise and using the 75th percentile values. It’s possible that new numbers are lower than the older values because nutrient levels have dropped as yields have increased. It’s also possible that older numbers were not based on very many samples, or that, in order to make sure that these numbers would never underestimate actual removal, they were chosen as the highest values found.
What seems clear is that improved varieties and management have not led to increases in per-bushel nutrient removal. We did collect some data on hybrids and varieties as part of this study, and will use those to see if there are consistent differences based on genetics. We know from rate studies that adding P or K fertilizer tends to bump up removal rates even when there is no yield response. This could be because roots encounter these nutrients in a concentrated form after fertilization and so take more up without really needing the additional amounts.
How much difference will using the new numbers make? Over two seasons, one with 200-bushel corn and the next with 60-bushel soybeans, P removal using the old book values comes to 137 lb. P2O5 per acre, and using the new values to 119 lb. P2O5 per acre, or 13 percent less P removed. For potassium, the old values calculate to 134 lb. K2O per acre while the new ones calculate to 118 lb. K2O per acre, or 12 percent less.
These are not large changes, but using these replacement numbers instead of the old numbers might mean that soil test values increase slightly less over years. Adding fertilizer in excess of removal is not the only way soil tests can rise; for example, nutrients can be brought up from deeper in the soil to the surface. Soil test levels often rise slowly if at all when nutrients are replaced, though, and lowering the amount used to replace nutrients by 10 percent may have little noticeable effect on soil test levels over time.
This project included wheat grain sampling as well, but we were unable to take many samples in 2014, and so we took some additional samples in 2017 for which we don’t yet have data. The removal numbers for wheat based on 625 samples through 2016 were 0.47 lb. P2O5 per bushel and 0.28 lb. K2O per bushel. The book value for wheat P in the Illinois Agronomy Handbook is 50% higher than actual removal, and after adjusting that number (from 0.90 to 0.60 lb. P2O5 per bushel) the new removal numbers are 22 and 8 percent less than the book values for P and K, respectively. We’ll calculate these again once we have the data from the 2017 wheat samples.
The program is set for the 36th annual University of Illinois’ Northwestern Agricultural Research Center Field Day. The program will begin at 8 am on Wednesday, July 26th.
Topics and speakers will include:
- Lessons from 35+ Years of Research at the Northwestern Illinois Ag Research Center – Emerson Nafziger – Extension Specialist, Crop Production, University of Illinois
- Reducing Tile Drainage Nitrate Loss: Chippin’ Away with Woodchip Bioreactors – Laura Christianson – Extension Specialist, Water Quality, University of Illinois
- 2017: Insect Headlines in Western Illinois – Kelly Estes – Coordinator, Illinois Cooperative Agricultural Pest Survey Program, Illinois Natural History Survey
- Important Soybean Diseases in Illinois – Roger Bowen – Visiting Research Scientist, Dept. of Crop Sciences, University of Illinois, USDA-ARS
The Northwestern Illinois Agricultural Research and Demonstration Center is a 320 acre facility, established in 1980, 1 mile North and 4 miles West of Monmouth at 321 210th Avenue. Each year, more than 50 different projects are conducted by up to 12 campus-based project leaders and the center superintendent.
For more information about continuing education units offered at the Field Day visit the Northwestern Illinois Agricultural Research and Demonstration Center website.
If you need a reasonable accommodation to participate in this program, please contact Greg Steckel (309) 734-7459, email@example.com by July 5.
The weather has turned from cool and wet to warm and dry, with thoughts now turning to when it might rain next. The US Drought Monitor at http://droughtmonitor.unl.edu/ shows no drought in the Corn Belt, and water use is still low, but some plants whose roots are not growing well or are in compacted soil are starting to show afternoon leaf curling, and water demand is increasing as plant growth rates increase. We hope rainfall returns soon.
As I have been reporting in recent posts, our soil N sampling is continuing to show that most of the N we applied to the crop earlier is still present. The amount of fall-applied N we recovered here at Urbana on May 31 was down only slightly from the amount recovered on May 17, and, at about 170 lb. N per acre (after applying 200 lb. last fall), is only about 15 lb. less than we found at this time in 2016. We recovered more than 240 lb. of N from NH3 applied in April, and more than 100 lb. of N from soil that hadn’t received any N fertilizer. These are also in line with what we’ve seen in early June in the past.
While the soil N supply seems to be holding up fairly well as soils dry, the crop in many fields is showing symptoms of the stress it’s been through. One of the most common is uneven growth. Our corn planted here on April 20 emerged fairly well, but in places where water stood temporarily, we see some lower stands and considerable variability down the row in plant size and growth stage. It’s hard to guess what caused this, but it’s likely that it will affect overall yield potential as plants that are behind now struggle to compete.
In areas in many fields where water stood, crop color continues to be paler than normal. This is related to both the effect of water on root health and growth, and perhaps to loss of some nitrate from the soil around the roots. We expect that color of these plants will improve some, but we can’t say with certainty that the these plants still have their full yield potential, especially if it takes another week or more for the color to improve.
April-planted corn in central Illinois has now reached the V6 stage (6 leaves with collar visible) or beyond, and above-normal temperatures are helping growth accelerate as the stem begins to elongate. The need for water increases as plants get larger. Roots take up water near them and dry out the soil there, so root growth need to increase in order to maintain the water supply to the plant.
We normally consider some dry weather in June as a positive, since it encourages roots to growth deeper. But with the difficult start to the season this year, including low soil oxygen, cool temperatures, and water that likely moved nitrate down more than normal, roots may not be able to grow down fast enough to keep up with the demand for water and N. Plants in many fields are showing leaf curling by the afternoons during the current stretch of warm, sunny weather. More leaf area means more demand for water, and we can expect the crop to continue to struggle.
Managing sidedressed nitrogen
The best measure of the N supply to the crop right now is crop color. With the sunshine and warm temperatures, many early-planted fields or parts of fields where water didn’t stand are showing considerable improvement in crop color, with leaves now taking on the dark green color we hope to see. If a field or part of a field is paler in color than plants of similar size in the same field or other fields, then it’s probably not getting enough N with the water it takes up, and as discussed above, it may not be taking up enough water.
A lot of questions have come up about how to manage N now that conditions are good for application and the crop is starting to take up N more rapidly. Here are some questions and answers on the topic:
If corn was replanted or planted late, should the amount of N applied be lowered to reflect lower yield potential?
Our research does not show that lower yields usually require less fertilizer N than higher yields. We think that’s because the causes of lower yields, which are typically stress from having less available water at critical times, often affect root growth, and so may make it harder for plants to take up the N that’s in the soil. If the plan was to apply the 160 to 180 lb. of N needed to produce the best return for corn following soybeans (200 to 210 lb. N for corn following corn) then stay with that amount. If the plan was to apply more than that, then cutting back would be reasonable.
Should I plan to apply sidedressed N more than once over the next month?
While the idea of “spoon-feeding” N has some appeal, we have found very little benefit to delaying some of the N until later during vegetative growth. As soils dry out, concern will increase about whether applied N is getting to the roots, and applying N more than once will bring that same concern each time. Chances for multiple applications to pay for themselves are low by now, and they’ll get even lower, especially if it remains relatively dry.
Should we use inhibitors with N applied now?
With the crop starting to take up N, there is simply no need to try to keep applied N in the ammonium form as long as possible, which is what nitrification inhibitors do. Plants take up mostly nitrate, but always have access to some ammonium. There’s no problem associated with this mixture, and there is no benefit to trying to increase the amount of ammonium. Using a urease inhibitor to slow the loss of urea (as ammonia gas) might be useful if applying urea or UAN, but only if urea is applied to the soil surface. Even then, a half inch or more of rain will carry surface-applied urea into the soil, which will capture any volatilized ammonia. There is no value in using a urea inhibitor with injected UAN. Finally, using extended-release forms of N is inappropriate when the crop has reached the stage of rapid uptake. The N needs to get into the soil and available to the crop as soon as possible. There is a time for inhibitors, but it is not during sidedress unless there’s no alternative to surface placement of urea and the weather is in a dry pattern.
What about N placement?
It is important to get sidedressed N into the soil near the roots as soon as possible so uptake can get underway. Nodal roots take up nearly all of the N, and these roots originate at the lowermost nodes of the plant – they are shallow near the plant and deeper farther away. When the surface soil dries out, roots in the top several inches of the soil may not be actively taking up water, so aren’t taking up N. In that case, applying UAN solution near the row may improve access of the plant to the N, compared to shallow placement in the row middles. But soils near the plant typically dry out first, and roots may be more active farther away from the rows but several inches deep. It may be worth increasing application depth if UAN is injected between the rows. Anhydrous ammonia should also work well, but roots of V6 plants are well out into the row middles, and may be damaged slightly by injection of NH3.
Is nitrogen management the key to bringing this crop back to full yield?
History tells us that the water supply will be the key to how the 2017 corn crop does. The difficulty, of course, is that we can’t do much about the water supply; but we can do something about nitrogen, including adding more N to compensate for what we think might have been lost. We do need to use sound management in applying any N that still needs to go on, and it needs to go on quickly in early-planted fields. But it’s unlikely that making extra trips or applying a lot more N than we had planned to apply is going to be profitable.
Just a quick overview of some insect issues presenting themselves recently.
Lots of reports of armyworm being found in wheat and corn. With reports of wheat harvest starting/getting close, reports of armyworm in corn seem to be taking over. I’ve seen a range of larvae stages from 2nd-4th instars. Injury to the whorl and ragged leaf margins is usually noticed around field margins first. Armyworm larvae are night feeders and will usually spend the day in soil cracks, under dirt clods, or in the whorl. Control may be justified when 25% of the plants are being damaged. Things to consider: hot spots in the field and also size of the larvae. Larvae greater than 1 ¼” will have completed most of their feeding.
Reports of lightning bugs in the central part of the state along with degree day accumulations suggest rootworm hatch is underway.
We picked up our first corn earworm moth last week. To date, we’ve trapped a total of 10 moths in Champaign. We expect those numbers to slowly pick up over the next couple of weeks.
A few fall armyworm moths have been picked up in our traps here in Champaign. We expect those numbers to increase in the next few weeks as well.
European Corn Borer
We have yet to pick up any moths in our trap in Champaign. We also have been sampling action sites (dense stands of tall grasses along roadsides ditches or waterways) by making 100 sweeps with a standard insect sweep net. To date, no moths have been recovered in any of the samples statewide.
A few reports of stalk borer in corn. These younger larvae did not have the distinct purple “saddle” that we often use to ID these insects. Young larvae will be light brown with a narrow white stripe running down its back from head to tail. There will be a similar white stripe on each side of the body that is interrupted by a purplish-brown band that circles the front third of the body. As larvae get bigger they move from the grass hosts they are quickly outgrowing to larger hosts, such as corn.
Stalk borers will attack corn in 2 different ways:
- Burrow into stalks at ground level and chew upwards through the center of the plant. Wilted leaves will be the first obvious symptom with this injury along with the potential for some plants to buckle at ground level.
- Some larvae may crawl to the top and feed down through the rolled leaves into the stalks. Ragged leaves, along with frass on the leaves may allude to this type of feeding which may also cause wilting of the top half of the plant.
I’ve also gotten reports of stink bug injury in southeast Illinois. Stink bug can cause three different types of injury – tillering, stunted plants, and may even kill small seedlings. Signs of stink bug injury will include oval holes where they have inserted their needlelike mouthparts. The stink bug sticks the base of the plant or their leaves with their small needle-like mouth. When they do this, they are causing these plants to die as a small seedling, produce stunted plants- such as misshapen ears instead of tassels- or suckering- the production of tillers form the base of damaged plants. Most often, you see this pest in fields that are no-tillage, near wooded areas, or conventional fields as well as along field edges.
Slug injury has been reported across much of southeast Illinois as well and as far north at Livingston county in both corn and soybeans. Slugs are generally a problem in no-till fields; especially if they are late-planted. Slugs take advantage of the smaller plants and as the plants grow, they quickly outpace the slugs. Feeding generally occurs on the lower parts of the plants. Symptoms may resemble that of the corn flea beetle (narrow, irregular tracks or scarring), but the presence of a slime trail will be indicative of the presence of slugs.
Once again, we received sustained flights across much of Illinois. Several counties saw repeated significant flights. This map gives you a good indication of projected cutting dates, but several counties like Champaign and Lee saw 4-5 significant flights, so the bottom line is that some of this late planted corn will still be susceptible for BCW injury. Speaking of BCW injury, I have had a few reports of feeding around the state. No indication of anyone area being more severe than another. There is still a lot of small corn across the state that could be susceptible.
I’ve also fielded a few call on centipedes and millipedes last week, though with the warm, dry weather, I don’t suspect to hear much more about these.
Information compiled with the assistance of Kaela Miller, University of Illinois Agriculture Communications.
The welcome return to Illinois of drier and warmer weather has allowed most of the remaining crops to be planted, and has brought a lot of improvement to the corn crop that struggled through cool, wet weather during the first and third weeks of May. The plants in many fields have gotten back their green color (or have gotten it for the first time) and the early-planted crop is about to enter the period of rapid growth.
At Willard Airport near Champaign, 316 growing degree days accumulated in April, and 422 GDD accumulated in May. Corn planted in mid-April has by now accumulated about 600 GDD, enough to bring it to growth stage V6 or so. Our corn planted at South Farms on April 20 is a stage or two behind that, probably due to its having experienced cool weather with limited sunshine and wet soils.
I reported here two weeks ago (May 18) on what we’re finding in as we track soil N by sampling this spring. We’re sampling trials at four research center sites where we applied 200 lb. of N as NH3 in mid-November last fall or in early to mid-April this spring.
Planting was delayed by wet weather at the DeKalb site. Samples taken at planting (May 17) showed 266 and 283 lb. N per acre in the top 2 feet for fall-applied and spring-applied N, respectively. These values are only 10 to 20 lb. less than we found on April 24, even though more than 5 inches of rain fell between these two dates. The plots with no N fertilizer had 133 and 135 lb. N at the earlier and later sampling dates; these are on the high side of normal for the soil there. Soil N levels as high as those at DeKalb give us no cause for concern about N loss at this point in time.
At the Monmouth, Urbana, and Perry (Pike County) sites, samples were taken after spring N application in mid-April, in early May after planting, and again in mid-May. On average, 4.5 inches of rain fell between the mid-April and early May samplings, and 2.1 inches fell between the early May and mid-May sample dates. Soils without N fertilizer averaged 90, 77, and 93 lb. N per acre on these three dates.
As I pointed out in my last article, we expect soil N values to rise in May due to increased mineralization as soils warm. That they dropped by early May probably reflected some movement of N out of the soil, or at least movement to more than 2 feet deep. That they increased between early and mid-May is a positive sign that mineralization is now exceeding N movement down. This means that the soil is now in good shape to help supply N for the crop.
Changes in soil N averaged across the three sites for each sampling time are shown in Figure 1. Here again the news is positive; there was a small (10-lb.) drop in soil N with fall-applied N from early to mid-May, while soils receiving spring-applied N showed a small (20-lb.) increase, reflecting the addition of mineralized N. The soil N amounts, while not quite as high as those found at these sites over the same period in 2016, certainly appear to be adequate to meet the needs of the corn crop this year.
Finding more NH4 in early May compared to mid-April for spring-applied N is mostly a consequence of sampling variability, but most of the spring-applied ammonia has converted to nitrate by now. The drop in soil N following fall N application is likely due to the fact that such a high percentage of this N was nitrate already in April, and so more of it moved down as water moved through the soil. We saw similar differences in nitrate percentages with fall- and spring-applied ammonia in 2016, but never had the loss conditions we saw this year, so there was no penalty to having the N present as nitrate. It may well be that 200 lb. of soil N remaining after fall application will still be enough to supply the crop’s N need this year, but this illustrates the risk of having a lot of nitrate present long before crop uptake starts.
There is no doubt that N has moved out of fields this spring and into streams and rivers. The Illinois Fertilizer & Chemical Association reported this week that nitrate levels in Lake Vermilion and Lake Decatur, while not off the charts, were high enough to require nitrate removal by municipal water providers for a few weeks. Levels in Lake Springfield and in Lake Bloomington are also elevated. Although we saw only a small net change in soil N, we believe that the amount of N produced by mineralization moved out of the soil in our trials, and so it’s not surprising to see that some of it reached surface water after exiting the field in tile water.
A little arithmetic helps put N losses from fields into perspective. Let’s say that a field of nearly level soil receives 5 inches of rainfall, and that 4 inches of water enters the soil. Of this, 1 inch remains in the soil (bringing the soil to field capacity) and 3 inches moves down and eventually exits the field through the tile system. One acre-inch of water is 27,154 gallons, which weighs about 226,600 pounds. So 1 part per million (ppm) of nitrate-N is 0.2266 lb. of N in one acre-inch.
If the 3 inches of water that exits our field has 12 ppm of nitrate-N, 0.2266 x 12 x 3 = 8.16 lb. of N per acre leaves the field. Tile line monitoring shows that N movement out of tiled fields is often in the range of 20 to 30 lb. of N per acre per year, typically carried by 8 to 10 inches of water leaving the field through the tiles. We tile fields to improve their productivity, but how much water moves out of a tiled field during the season depends on how much enters the soil in excess of crop uptake, so mostly on rainfall. While we can’t control the amount of water that moves out, we need to do what we can to minimize how much N this water carries with it.
One good way to minimize N loss through tiles is to avoid applying more N than the crop needs to reach its yield potential. Even with the unusually wet weeks this spring that often came after much or all of the fertilizer N had been applied to fields, the fact that we are finding good amounts of N in the soil now should give us confidence that we don’t need to increase N rates this year.
If the crop continues to green up nicely over the next week, that’s because its root system is enlarging out into the soil, and that the roots are finding N as they go. It helps that uptake remains slow – the crop has so far taken up no more than 10 lb. N per acre or so, and its uptake rate hasn’t yet hit 1 lb. per acre per day in most fields. Once the crop reaches stage V8-V9 in mid-June, uptake rates will reach 3 to 4 lb. per acre per day, if not higher. By then, mineralization will be in full swing, and that, along with the N from fertilizer, should be able to meet the crop’s need.
In an N timing study in 2016, we tracked leaf color with a SPAD meter as the color declined before we applied N and as it recovered after we applied N. We found that if we applied 100 lb. of N at planting, both leaf color and full yield potential recovered after we applied the rest of the N, even if that was as late as tasseling or even later. That means we have time to watch the crop for signs of deficiency and to apply more N only if and when such deficiency develops. There’s reason to believe that such deficiency won’t develop as the soil and crop conditions return to normal.
The heavy rains of late April and early May have paused and the weather has warmed enough to allow corn and soybean planting (or replanting) to resume in Illinois, except in the low spots in some places.
With a lot of nitrogen fertilizer applied early, and with rainfall totaling 5 inches or more over most of the state in the two weeks before May 10, many people are worried about N loss and the possible need to apply more nitrogen than planned.
Although we wish the weather had stayed warm, the return of cooler weather along with the rainfall did slow nitrification – the conversion of ammonium to nitrate – slightly, and also slowed the denitrification process. Both nitrification and denitrification are biological processes, so are faster at higher temperatures. We know from finding nitrate in the soil that there has been a lot of nitrification. Denitrification requires both saturated soils and warm soils, so there has been much less of it, mostly in soils where water stood.
Soils with standing water warm slowly, which has limited denitrification. According to the Illinois Climate Network of the Illinois State Water Survey, soil temperature 4 at inches under bare soil at Bondville, west of Champaign, was less than 60 degrees between April 27 and May 8, which was during the wettest stretch of weather. Soil temperature is now above 70 at that location, and where water is still standing, denitrification is underway. In many such areas, it will be some time before a crop can be planted, and adjustments to fertilizer N may be in order if and when the crop can be established.
Ammonium moves little in the soil, so nitrification is required to mobilize nitrogen. We know from our N-tracking research, which is funded by NREC using fertilizer checkoff dollars, that N applied last fall was about 70% nitrate by early May, and that ammonia applied in March or April was more than half nitrate when the weather turned wet.
Soil samples taken from the same fields before and after the heaviest rainfall period do not show large decreases in N in the top 2 feet of soil. In one set of three on-farm N-tracking trials in central Illinois, samples taken in mid-April and again on May 9 following either fall or March anhydrous ammonia application with and without N-Serve show no change in soil N over that period, regardless of when N was applied (Figure 1). What looks like effects of N-Serve comes from sampling variability.
We found very similar results at three Crop Sciences Research & Education centers (Figure 2). In the on-REC trials, we applied spring N in April instead of March, which explains why spring-applied N showed less conversion to nitrate. But in both sets of trials, the amount of N recovered after the high rainfall in late April and early May was within a few pounds of that recovered in mid-April. Finding no change in soil N doesn’t mean there was no movement out of the top 2 feet of soil, it only means that the amount of N that moved out was about the same amount as was produced by mineralization of soil organic N during this period. We saw some large increases in soil N as the soil warmed in May in 2016.
We have seen some other numbers that show more disappearance of N from the top 2 feet of soil than I’m reporting here. This could be part field location – we do the on-farm and on-REC trials in parts of fields that don’t flood, and that could mean less N movement or loss. On the other hand, N movement down through the soil may be greater in higher positions in the landscape if soils there are somewhat better-drained.
Soil drainage characteristics are an important factor in movement of water and nitrate. Soil texture is a critical component of drainage, but field tiles change the relationship between texture and water movement. As an example, a typical Drummer silty clay loam in eastern Illinois allows hardly any water to move through it unless it’s tile-drained. Tile becomes the exit route for soil N into surface waters, replacing denitrification as the main way N is lost in such soils. So tile drainage changes the assumption that heavy-textured soils will lose N to denitrification while lighter-textured soils lose more to leaching.
Do we adjust nitrogen for this crop?
While it seems likely that some N has moved out of the upper soil as a result of rainfall before crop N uptake began, it is premature to conclude that we need to apply more N than we had planned to apply. If soils dry out and rainfall returns to normal, root extraction will resume once plants are larger, and this can help draw water towards the surface, bringing N with it, including some N that moved below 2 feet but not out in tile drainage. As soils dry and warm, mineralization will kick into high gear. Last year, under good temperatures and without unusually heavy rainfall, we saw mineralization provide as much as 150 lb. of N per acre or more to the crop.
One indication that the topsoil has not been stripped clean of nitrogen is the good recovery of green leaf color that we’re seeing as the soil dries out. Most fields are not as dark green as we saw at this point in 2016, but as the root system starts to expand and as soils continue to warm, this will change. The corn crop at this point looks like it does not because of lack of N, but due to temperature and rainfall and their effects on soil conditions that affect crop growth and early development.
While it’s premature to revise N management based on what has happened so far, we can’t rule out the possibility that the crop may need more N than it might have needed with drier spring weather. The good news is that we still have time to make such decisions: the crop takes up barely one pound of N per acre for every inch of growth it makes up to knee-high or so. As long as soils conditions remain favorable, a crop provided with normal amounts of fertilizer N almost never runs out of N during vegetative development, at least to the extent that we can see it. This year will be no exception.
We are sampling soils about every two weeks up to tasseling, and I’ll keep you updated with what we find. But because we have not seen this sort of weather event at this time of the season in recent years, we don’t have a good way to relate soil N at a specific crop growth stage to (future) plant need. Nitrogen deficiency develops over time, and is almost always more related to current soil moisture than to the amount of soil N. So if soils do not get extra wet or extra dry over the next month, this season could turn out to be much more typical than we think expect.
Some of you may recall my reporting that we’ve had some hints that short-term deficiency during early vegetative development might lower yield potential slightly, even if overall the N amount used is adequate and plants never show deficiency. If soils stay warm this is likely to be a non-issue, but if you have the ability to apply some N with the planter or otherwise close to the seed that might be a useful strategy. As soils continue to warm, the likely advantage of this is decreasing as planting is delayed in places.
April has been a little warmer and drier than average so far this year, which has allowed a good start to corn planting and some progress in soybean planting. This is expected to change, with above-normal rainfall and below-normal temperatures over the next 10 days or so, through the first week of May.
It rained on Easter Sunday most places in Illinois, which according to the old saying means that it should rain on each of the seven Sundays after Easter. It did not rain in most places the first Sunday after Easter (April 23), so that prophecy won’t be fulfilled this year. That hardly means it can’t turn wet.
Above-normal growing degree day accumulations have meant fast emergence for corn. In central and southern Illinois, corn planted by April 19 accumulated, by April 25 or 26, the 115 or so GDD required to emerge. With lower temperatures expected over the next ten days, corn planted on April 25 or 26 may take almost twice as many days to emerge as corn planted in mid-April.
The drop in temperature along with rain on April 26 (and more to come) has some people concerned about the “imbibitional chilling injury” that can accompany such conditions. This can happen when the water available to the corn seed has a temperature in the lower 40s or less. Uptake of cold water damages membranes, and this in turn may cause abnormal seedling development and failure to emerge.
If the corn seed can take up some warmer water before soil (and water) temperatures drop, we often see less injury or none at all. So corn planted early this week should be out of danger. Corn planted on April 25 or 26 may be at risk, but rain that fell on April 26 was not very cold, and with air temperatures expected to rebound into the 70s the last two days of April, along with the (warmer) rain that’s predicted, we hope not to see much of this problem from this round of weather.
A larger concern is how seeds and seedlings might be affected by the rainfall expected over the next few days, followed by the slow rise in temperature that is predicted. Seeds that are starting to germinate need oxygen, and will usually not survive the low oxygen levels in saturated soils for more than a couple of days. They will survive longer if soil temperatures are cool, both because that slows growth and lowers oxygen demand, and also because cool water carries more oxygen into the soil. If soils start to dry off early next week, survival will a concern mostly where water stands.
Young seedlings have the advantage of having roots that might find pockets with more oxygen, but they still depend on seed reserves to grow, especially if it’s cool and cloudy, and before leaves have much green area. These reserves are mostly used up by the time the plant has two leaves, and diseases can invade the endosperm, especially in cool, wet soils. So we can expect seedlings to live for maybe three or four days if they are submerged, and a few days longer than that if only the roots are in saturated soil. If plants remain alive, chances for seedlings to revive and thrive increase considerably once oxygen gets to the roots again.
Soybean issues are not unlike those with corn, although soybeans die in saturated soils a little more quickly than corn, and fewer soybean fields have emerged. Cooler soils will help seeds survive longer, but diseases like Pythium often thrive on cool, wet soils. The need to replant soybean fields can be assessed after emergence of the first seedlings in a field, by checking to see if seeds that haven’t emerged are still alive. Presence or absence of a healthy radicle (emerging root) is the easy test to see if a seed is alive.
In plots where we applied 200 lb. N as anhydrous ammonia last fall, samples taken in mid-April this spring had about 230 lb. N per acre in the top 2 feet of soil. That’s 25-30 lb. more N than we recovered in mid-November last fall. Where we applied no fertilizer N, we recovered 56 lb. N per acre last fall and 90 lb. N this spring. So the amount of N from fertilizer changed hardly at all over the past five months, and (net) mineralization added some N. We recovered about 30 lb. more N last fall and 26 lb. more this spring where we had used N-Serve®. Because the amounts were different last fall before N loss could have occurred, we can’t be sure if this difference is due to use of the inhibitor.
With the mostly dry conditions we have had over the winter and early spring, finding little or no loss of N, while a relief, was not unexpected. In the November samples, 70% of the N was in the ammonium form, safe from movement out of the soil and from denitrification. In April, however, only 25% of the recovered N was in the ammonium form. These percentages were the same whether or not we had used N-Serve®. The 75% of the soil N that is now nitrate can move deeper into the soil – including into tile lines – as water moves. It can also denitrify, releasing the N back into the air, under saturated soil conditions.
It would be premature to predict the loss of fall-applied N at this point. If rains come too fast for soils to take in the water, the resulting runoff will be a real problem for erosion and for forming ponds in lower-lying parts of fields. But runoff water normally carries little N off the fields if the N is not on the soil surface. In most tile-drained fields, which typically have heavier soil textures, water movement down is not very fast, and if conditions turn drier next week, water carrying nitrate will move back up as the water at the soil surface evaporates. Denitrification will start after a few days in standing water; it takes time for the oxygen to be depleted. The rate of denitrification will be fairly slow, however, until soil temperatures, which now range from the mid-50s to the lower 60s, get somewhat warmer. Having soils dry in the meantime will allow oxygen back in, which will stop denitrification.
While we know that corn plants benefit from having N in the soil when and where their roots emerge and start to grow, a return to soil conditions that encourage plant growth will also mean a resumption of mineralization, which will help provide N to the plants. Any ammonia or urea-based fertilizer N that was applied this spring should still be mostly in the the ammonium form, which should remain in the soil after any heavy rains that may come in the next week.
While we will keep looking to see how well N is remaining in the soil, there is no need to try to replace N before we can tell it’s missing. The priority instead is on emergence and health of the crop, and that mostly depends on the weather over the coming weeks. Having cool temperatures linger is probably a bigger concern than heavy rain at this point, except where ponds might form long enough to that kill the plants.
The other concern, of course, is getting the rest of the crops planted. If the weather remains cool, emergence and growth will be quite slow even if it does eventually dry up enough to resume planting. So warmer temperatures will help both to dry things out and to get the planted crop growing. If it helps, you might remember that we had almost no corn planted in Illinois by this time in 2014, and we harvested our highest yield ever.