Illinois Fertilizer & Chemical Association
Supply · Service · Stewardship

Nitrogen Updates

Emerson Nafziger, University of Illinois

Corn planting has moved ahead of the 5-year average, with 66% of the Illinois crop planted by May 1. Early planting usually means an early start to nitrogen uptake. But N uptake is slow for a month or more after planting: in one study we did in 2015, plants at the 4-leaf stage about five weeks after planting had only 4 pounds of N per acre in the above-ground part of the plant. So there’s time both to get N applied to the crop before it needs it and also time for N in the soil to move out of the rooting zone if it’s in the nitrate form and the weather turns wet.

Soil N after fall N application

Dan Schaefer of IFCA and we have continued to sample soils this spring to see how much N applied as anhydrous ammonia last November remains in the top 2 feet of soil, and how much of this N is still in the ammonium form, hence safe from loss even under wet conditions.

Figure 1 shows soil N recovered from samples taken in mid-April. Fall ammonia applications were made in November at the N rates shown, and all included N-Serve. The three sites on the left are farmer fields, and the three on the right are UI research centers. At the Logan county site, soil without N fertilizer had 84 lb. of N recovered, and zero-N plots had 64, 48, and 63 lb. N recovered at the Champaign, Warren, and DeKalb County sites, respectively.

Figure 1. Plant-available nitrogen (ammonium plus nitrate) in the top 2 feet of soil from samples taken at six Illinois sites in mid-April, 2016. Fall N was applied in November 2015 as NH3 at the rate indicated for each site, and included N-Serve®.

Even after subtracting an amount of N in soils without fertilizer, most or all of the N applied as fertilizer last fall was recovered in mid-April sampling. For the three on-farm sites, about half of the recovered N was in ammonium form. The amount of N recovered from three on-farm sites with similar soils in April 2015 was similar to what we found this year, but last spring only about one-third of the recovered N was in the ammonium form. Given the difference between the two winters it’s surprising that more of the N was in the ammonium form this year, but we can take it as good news.

With the exception of DeKalb, amounts of nitrogen recovered at the research center sites were also close to the amount of N applied last fall, and were similar to amounts of N recovered from samples taken in April 2015 at the same sites. The positive from sampling at all six sites is that, after a lot of concern about N loss following warm and sometimes-wet condition this past winter, we aren’t really finding that a lot of the N has been lost. Reports are also that surface water nitrate levels, which were higher than normal in February, have not continued to increase, probably because rainfall and tile flow haven’t been unusually high.

In contrast to what we saw in the on-farm sites this spring, only about one-fourth of the N recovered was in the ammonium form at the research centers, compared to nearly half in the ammonium form in last year’s samples. We don’t have an explanation for the difference between on-farm and research center sites. But unless we get a lot of rainfall, N present as nitrate will remain a ready source of N for the crop.

More unexpected than the high percentage of nitrate we found is the fact that we found so much more N at research center sites in April than we found in February. On April 8 I reported http://bulletin.ipm.illinois.edu/?p=3554 that samples taken from these plots in late February this year showed only about 130 and 160 lb. of N per acre, with about 60 and 43% of the recovered N in the NH4 form at Urbana and Monmouth, respectively. The amount of ammonium-N we recovered changed very little between February and April, dropping from 78 to 60 lb. at Urbana and from 68 to 67 lb. at Monmouth. So the higher amount of soil N we found in April came entirely from the increases in nitrate. I’m at a loss to explain how soil nitrate could increase by 116 and 146 lb. per acre at the two sites over a period of 6 to 8 weeks, with little or no drop in ammonium levels. So I won’t speculate; we can just accept that the N is in the soil and available as the growing season gets underway.

Nitrification inhibitor

In the April 8 Bulletin article cited above I reported that N-Serve® used with fall-applied NH3 had little effect on the amount of soil N or ammonium recovered in February at the Urbana and Monmouth sites. Figure 2 shows the amount of soil N recovered and the percentage of N found as ammonium in the mid-April samples. As we saw in February, adding the inhibitor in the fall gave no consistent effect on either N recovery or the percentage of N as ammonium. That's not surprising, as the levels of ammonium were already fairly low in February and so there wasn't much ammonium present to nitrify. We had these two treatments at several other sites, and will report on those results later.

Figure 2. Soil N recovery in mid-April 2016 following application of 200 lb. N per acre as NH3 with and without N-Serve in November 2015.

Cover crop rye and soil N

Using cereal rye to take up soil N and thereby lower potential for loss is becoming more common. Some are using the cover crop to scavenge residual N following harvest in the fall, and some are planting rye before fall N application, or in some cases even fertilizing the cover crop when it’s planted, to see if this will increase the total amount of N recovered in the spring.

Dennis Bowman, UI Extension educator and Dan Schaefer are managing a cover crop study at the U of I research center near Urbana in which cover crop rye was drilled after harvest of both corn and soybean last fall. These trials received no addition of N before soil and cover crop samples were taken on April 15, 2016 and analyzed for N. The rye was 12 to 18 inches tall at the time of sampling, and its dry weight was 942 lb. per acre following corn and 1,735 lb. per acre following soybean.

With no fertilizer N added, the amount of soil N recovered in mid-April was low – only 56 lb. per acre following soybean and 42 lb. following corn (Figure 3). When the cover crop was present, the amount of N recovered from the soil plus the cover crop rye following corn and soybean was only 9 and 5 lb. per acre higher, respectively, than the amount recovered from the soil without cover crop. The cover crop contained about half of the recovered N following soybean and less than 40% of the N recovered following corn. In both cases the amount of N left in the soil with cover crops present was only about 30 lb. per acre, which is about as low as soil N levels ever get. So the cover crop took up what N it found, but that was not very much. The cover crop residue contained less than 2% N, or less than half that of a well-fertilized cereal rye crop at the same stage. Its C:N ratio was above 20, reflecting this N deficiency.

Figure 3. Recovery of nitrogen at a Champaign County site on April 15, 2016 with and without cover crop rye drilled into corn and soybean crops after harvest in fall 2015. No fertilizer N was applied in these trials.

Dan Schaefer is also managing an on-farm site in Macon County in which fall ammonia was applied at the rate of 175 lb. N with N-Serve. The site was soybeans in 2015, with one field planted to cover crop rye and an adjoining field without cover crop. Soil and cover crop samples were taken on April 8, 2016 and analyzed for N. With the added N, rye growth was excellent, with more than 2,700 lb. of dry weight per acre.

Nitrogen recovery was very high, with 272 lb. of soil N recovered per acre without a cover crop (Figure 4).  Of this amount, 132 lb. per acre (49%) was in the ammonium form. In the field with cover crop rye, a total of 258 lb. N per acre was recovered, slightly less than the amount recovered without cover crop. Of this total, 140 lb., (54%) was in the cover crop, and 128 lb. was in the soil. Of the amount of N in the soil under cover crop, 116 lb. (84%) was ammonium, indicating that the cover crop took up nitrate as it was formed, leaving relatively little in the soil. The cover crop might have taken up some ammonium, but soil under cover crop had only 16 lb. less ammonium-N than soil without cover crop, so ammonium uptake by the rye was minimal.

Figure 4. Recovery of soil and cover crop nitrogen on April 8, 2016, at a Macon County site with and without cover crop rye and with 175 lb. N as ammonia with N-Serve applied in November, 2015.

While having rye take up most of the fall-applied N would seem like a good way to lock in the N and keep it from being lost, this also means that much of the N ends up in a form that will become only slowly available to the corn crop. In early April, the field with cover crop had only 128 lb. of plant-available N, and while this amount may increase some after the cover crop is killed due to mineralization, this soil has nowhere near as much N as the corn crop will need.

How much of the cover crop’s N will become available to the crop, and when this happens, involves weather, soils, and crop growth, so is highly unpredictable. Some of the N in dead cover crop tissue may still be inorganic (nitrate or ammonium) and this can get into the soil and reach the corn roots relatively quickly. But most of the N is part of proteins and amino acids, and getting it into the soil and to the roots requires microbial activity, with N being released as microbes grow and die off. The cover crop residue in this case had a C:N ratio of only about 10:1, so tieup of N should be minimal as the residue starts to break down. Still, the process by which N cycles through microbes to get to the crop's roots is neither fast nor complete, and the chances that all of the N will all get to this year's corn crop in time are not high.

Whether or not fall N was applied, fields with cover crop rye going into corn this spring are likely to have low amounts of N in the seeding zone due to uptake by the rye. Strip-till done in the fall or in the spring reduces the density of rye roots in the planting strip, so should lessen this problem. If corn seed will go into soil close to residue or roots of rye, it may help to add some N in-furrow or close enough to the seed to allow the seedling access to the N soon after emergence. That should help avoid early N deficiency. But if the rye has made a lot of growth, it may be worth considering sampling to measure soil N at sidedress time to see if the supply is adequate, even if enough N was applied last fall.

Emerson Nafziger, University of Illinois

Corn planting has moved ahead of the 5-year average, with 66% of the Illinois crop planted by May 1. Early planting usually means an early start to nitrogen uptake. But N uptake is slow for a month or more after planting: in one study we did in 2015, plants at the 4-leaf stage about five weeks after planting had only 4 pounds of N per acre in the above-ground part of the plant. So there’s time both to get N applied to the crop before it needs it and also time for N in the soil to move out of the rooting zone if it’s in the nitrate form and the weather turns wet.

Soil N after fall N application

Dan Schaefer of IFCA and we have continued to sample soils this spring to see how much N applied as anhydrous ammonia last November remains in the top 2 feet of soil, and how much of this N is still in the ammonium form, hence safe from loss even under wet conditions.

Figure 1 shows soil N recovered from samples taken in mid-April. Fall ammonia applications were made in November at the N rates shown, and all included N-Serve. The three sites on the left are farmer fields, and the three on the right are UI research centers. At the Logan county site, soil without N fertilizer had 84 lb. of N recovered, and zero-N plots had 64, 48, and 63 lb. N recovered at the Champaign, Warren, and DeKalb County sites, respectively.

Figure 1. Plant-available nitrogen (ammonium plus nitrate) in the top 2 feet of soil from samples taken at six Illinois sites in mid-April, 2016. Fall N was applied in November 2015 as NH3 at the rate indicated for each site, and included N-Serve®.

Even after subtracting an amount of N in soils without fertilizer, most or all of the N applied as fertilizer last fall was recovered in mid-April sampling. For the three on-farm sites, about half of the recovered N was in ammonium form. The amount of N recovered from three on-farm sites with similar soils in April 2015 was similar to what we found this year, but last spring only about one-third of the recovered N was in the ammonium form. Given the difference between the two winters it’s surprising that more of the N was in the ammonium form this year, but we can take it as good news.

With the exception of DeKalb, amounts of nitrogen recovered at the research center sites were also close to the amount of N applied last fall, and were similar to amounts of N recovered from samples taken in April 2015 at the same sites. The positive from sampling at all six sites is that, after a lot of concern about N loss following warm and sometimes-wet condition this past winter, we aren’t really finding that a lot of the N has been lost. Reports are also that surface water nitrate levels, which were higher than normal in February, have not continued to increase, probably because rainfall and tile flow haven’t been unusually high.

In contrast to what we saw in the on-farm sites this spring, only about one-fourth of the N recovered was in the ammonium form at the research centers, compared to nearly half in the ammonium form in last year’s samples. We don’t have an explanation for the difference between on-farm and research center sites. But unless we get a lot of rainfall, N present as nitrate will remain a ready source of N for the crop.

More unexpected than the high percentage of nitrate we found is the fact that we found so much more N at research center sites in April than we found in February. On April 8 I reported http://bulletin.ipm.illinois.edu/?p=3554 that samples taken from these plots in late February this year showed only about 130 and 160 lb. of N per acre, with about 60 and 43% of the recovered N in the NH4 form at Urbana and Monmouth, respectively. The amount of ammonium-N we recovered changed very little between February and April, dropping from 78 to 60 lb. at Urbana and from 68 to 67 lb. at Monmouth. So the higher amount of soil N we found in April came entirely from the increases in nitrate. I’m at a loss to explain how soil nitrate could increase by 116 and 146 lb. per acre at the two sites over a period of 6 to 8 weeks, with little or no drop in ammonium levels. So I won’t speculate; we can just accept that the N is in the soil and available as the growing season gets underway.

Nitrification inhibitor

In the April 8 Bulletin article cited above I reported that N-Serve® used with fall-applied NH3 had little effect on the amount of soil N or ammonium recovered in February at the Urbana and Monmouth sites. Figure 2 shows the amount of soil N recovered and the percentage of N found as ammonium in the mid-April samples. As we saw in February, adding the inhibitor in the fall gave no consistent effect on either N recovery or the percentage of N as ammonium. That's not surprising, as the levels of ammonium were already fairly low in February and so there wasn't much ammonium present to nitrify. We had these two treatments at several other sites, and will report on those results later.

Figure 2. Soil N recovery in mid-April 2016 following application of 200 lb. N per acre as NH3 with and without N-Serve in November 2015.

Cover crop rye and soil N

Using cereal rye to take up soil N and thereby lower potential for loss is becoming more common. Some are using the cover crop to scavenge residual N following harvest in the fall, and some are planting rye before fall N application, or in some cases even fertilizing the cover crop when it’s planted, to see if this will increase the total amount of N recovered in the spring.

Dennis Bowman, UI Extension educator and Dan Schaefer are managing a cover crop study at the U of I research center near Urbana in which cover crop rye was drilled after harvest of both corn and soybean last fall. These trials received no addition of N before soil and cover crop samples were taken on April 15, 2016 and analyzed for N. The rye was 12 to 18 inches tall at the time of sampling, and its dry weight was 942 lb. per acre following corn and 1,735 lb. per acre following soybean.

With no fertilizer N added, the amount of soil N recovered in mid-April was low – only 56 lb. per acre following soybean and 42 lb. following corn (Figure 3). When the cover crop was present, the amount of N recovered from the soil plus the cover crop rye following corn and soybean was only 9 and 5 lb. per acre higher, respectively, than the amount recovered from the soil without cover crop. The cover crop contained about half of the recovered N following soybean and less than 40% of the N recovered following corn. In both cases the amount of N left in the soil with cover crops present was only about 30 lb. per acre, which is about as low as soil N levels ever get. So the cover crop took up what N it found, but that was not very much. The cover crop residue contained less than 2% N, or less than half that of a well-fertilized cereal rye crop at the same stage. Its C:N ratio was above 20, reflecting this N deficiency.

Figure 3. Recovery of nitrogen at a Champaign County site on April 15, 2016 with and without cover crop rye drilled into corn and soybean crops after harvest in fall 2015. No fertilizer N was applied in these trials.

Dan Schaefer is also managing an on-farm site in Macon County in which fall ammonia was applied at the rate of 175 lb. N with N-Serve. The site was soybeans in 2015, with one field planted to cover crop rye and an adjoining field without cover crop. Soil and cover crop samples were taken on April 8, 2016 and analyzed for N. With the added N, rye growth was excellent, with more than 2,700 lb. of dry weight per acre.

Nitrogen recovery was very high, with 272 lb. of soil N recovered per acre without a cover crop (Figure 4).  Of this amount, 132 lb. per acre (49%) was in the ammonium form. In the field with cover crop rye, a total of 258 lb. N per acre was recovered, slightly less than the amount recovered without cover crop. Of this total, 140 lb., (54%) was in the cover crop, and 128 lb. was in the soil. Of the amount of N in the soil under cover crop, 116 lb. (84%) was ammonium, indicating that the cover crop took up nitrate as it was formed, leaving relatively little in the soil. The cover crop might have taken up some ammonium, but soil under cover crop had only 16 lb. less ammonium-N than soil without cover crop, so ammonium uptake by the rye was minimal.

Figure 4. Recovery of soil and cover crop nitrogen on April 8, 2016, at a Macon County site with and without cover crop rye and with 175 lb. N as ammonia with N-Serve applied in November, 2015.

While having rye take up most of the fall-applied N would seem like a good way to lock in the N and keep it from being lost, this also means that much of the N ends up in a form that will become only slowly available to the corn crop. In early April, the field with cover crop had only 128 lb. of plant-available N, and while this amount may increase some after the cover crop is killed due to mineralization, this soil has nowhere near as much N as the corn crop will need.

How much of the cover crop’s N will become available to the crop, and when this happens, involves weather, soils, and crop growth, so is highly unpredictable. Some of the N in dead cover crop tissue may still be inorganic (nitrate or ammonium) and this can get into the soil and reach the corn roots relatively quickly. But most of the N is part of proteins and amino acids, and getting it into the soil and to the roots requires microbial activity, with N being released as microbes grow and die off. The cover crop residue in this case had a C:N ratio of only about 10:1, so tieup of N should be minimal as the residue starts to break down. Still, the process by which N cycles through microbes to get to the crop's roots is neither fast nor complete, and the chances that all of the N will all get to this year's corn crop in time are not high.

Whether or not fall N was applied, fields with cover crop rye going into corn this spring are likely to have low amounts of N in the seeding zone due to uptake by the rye. Strip-till done in the fall or in the spring reduces the density of rye roots in the planting strip, so should lessen this problem. If corn seed will go into soil close to residue or roots of rye, it may help to add some N in-furrow or close enough to the seed to allow the seedling access to the N soon after emergence. That should help avoid early N deficiency. But if the rye has made a lot of growth, it may be worth considering sampling to measure soil N at sidedress time to see if the supply is adequate, even if enough N was applied last fall.

Dan Schaefer, IFCA Director of Nutrient Stewardship (CCA, CPAg, 4RNMS)

The late July 2016 N-WATCH test results reflect what most of the N-WATCH sites in the overall program show, which is a minimal amount of plant-available nitrogen remaining in the upper 0-1 feet of the soil profile. This is expected based upon N-WATCH results at this time of the year, and is consistent with what we have seen in the past three years of testing.

Residual N that is detected in the 1-2 foot soil profile has the potential to be lost prior to the next cropping year from leaching or denitrification. Growers should discuss their N-WATCH results with their agronomic advisers and determine the potential economic and environmental benefits of a cover crop this fall for fields going into soybeans in 2017.

Please contact Dan Schaefer at (217) 202-5173 or dan@ifca.com for questions about the N-WATCH program.

N-WATCH is funded by a grant from the Illinois Nutrient Research & Education Council.

 
IFCA has several participating sites that provide live information complimenting the research that Dr Nafziger discusses.

Click on the site markers for more info.
 
 
 

Nitrogen on Corn in 2016: A First Look

Written by Emerson Nafziger

The 2016 cropping season was a good one in Illinois, with planting a little ahead of normal and good May moisture and temperatures to get the crop off to a good start. June was warm and, in most parts of Illinois, drier than normal; parts of western Illinois received less than an inch of rainfall for the month. Temperatures and rainfall returned to normal in July and August, though there was the usual variability from region to region, including much-above-normal rainfall in the southern end of the State.
 
With good May soil conditions, mineralization got off to a fast start, and the crop in most fields was dark green by the end of May and starting to grow rapidly. Without N loss conditions in June, N from both fertilizer and mineralization stayed in the rooting zone, and N availability to the crop was outstanding. Even no- or low-N strips stayed dark green in trials into the middle of June, much later than we normally see N deficiency developing.
 
The retention of N in the soil and its availability to the crop carried through the season to diminish the need for fertilizer N. Figure 1 shows a response to N in an on-farm trial in DeWitt County, Illinois. Not only did about 150 lb. of N maximize yield at 230 bushels per acre, but it made almost no difference whether the N was applied in the fall or in the spring. We know from our N tracking that most of the N was in the nitrate form by the time crop uptake started in late May; we can see here that in the absence of N loss (wet) conditions, nitrate stays in the soil and is available for plant uptake just like ammonium.
 
Figure 1. N responses from fall- and spring-applied anhydrous ammonia in an on-farm trial in DeWitt County, Illinois in 2016. Optimum points are the N rate and yield at the point where the last addition of N provides just enough yield increase to pay for that N. Figure 1. N responses from fall- and spring-applied anhydrous ammonia in an on-farm trial in DeWitt County, Illinois in 2016. Optimum points are the N rate and yield at the point where the last addition of N provides just enough yield increase to pay for that N.
 
Dan Schaefer of IFCA coordinated dozens of on-farm trials similar to the one shown in Figure 1. Some had fall versus spring N timing comparisons, some had all early versus some early plus sidedress, and others just compared yields at different N rates. Figure 2 shows results from 26 trials conducted across central Illinois in 2016. 
 
 Figure 2. N responses from 26 N rate trials in corn following soybean in central Illinois, 2016. Each line connects the data points from one trial, and the optimum points (triangles) are calculated from curves (not shown) fitted to the data. The MRTN points are calculated as the yield at 175 lb N/acre, which is the MRTN (optimum N rate) calculated for central Illinois corn following soybeans at a N to corn price ratio of 0.1 ($0.375/lb. of N and $3.75/bushel of corn.)
 
In 2015, high N loss conditions and damage from standing water resulted in high optimum N rates. In 2016 we found just the opposite: Figure 2 shows that relatively low rates of N were needed to maximize yield in nearly every case. Of the 26 trials, only five had an optimum N rate higher than the MRTN rate, and on average across trials, only 150 lb. of N was needed to produce an average yield at the optimum N rate of 225 bushels per acre. Some like to calculate “efficiency” of (fertilizer) N by dividing yield by N rate; here, we calculate a very high efficiency of 2/3rds of a lb. of N per bushel of yield, or 1.5 bushels per lb. of N used.
 
We ran a new study at a number of sites this year to compare the application of N rates at planting to keeping 50 lb. of N back and applying it dribbled next to the row at tasseling. Figure 3 shows the results of the corn following soybean trial at Urbana.
 
Figure 3. Response to N applied as UAN at planting (early) compared to applying all but 50 lb. of N at planting them dribbling the remaining 50 lb. next to the row at tasseling.
 
Responses to late-split timing of N at other sites were all similar to that in the trial shown in Figure 3. We had three corn following corn trials and four corn following soybean trials, and in none of them did keeping back 50 lb. of N to apply late provide a benefit to either yield or return to N; that is, late-split application did not pay the added application cost. This makes sense given the low N loss conditions in 2016. We would expect to see some loss and possible response to late supplemental N following a wet June, though we did not see much response to a single treatment (150 lb. N early versus 100 early and 50 at tasseling) in 2015.
 
We’re seeing N “at its best” in 2016; it was there in abundance when the crop needed it, and adding the supply of N from soil organic matter meant that the crop needed less fertilizer N than it has typically needed, even at high yield levels. We can’t depend on this to happen in 2017, but we see clearly that the common idea that “high yields require high N rates” often does not hold true. There is certainly no need to raise rates for next year, and fields that received more N than was needed in 2016 (according to N response curves that is probably most fields) might have added to the pool of soil N that can be tapped by the 2017 crop, whether that’s corn or soybean. Keep in mind, though, that what we saw in 2016 was mostly a response to the (June) weather and crop off to a good start; we will need to watch how things develop in the spring of 2017 to know if we’ll have a repeat.

Spring Nitrogen Management

Written by Emerson Nafziger     (View the UofI bulletin)

Most corn producers have made plans on how to supply the 2017 Illinois corn crop with nitrogen. But with the stakes high, unusually early N application this past winter and early spring, the delay in fieldwork due to rainfall over the past week, and ongoing pressure to “get nitrogen right,” some might be rethinking plans as the season gets underway.
 
I presented a webinar on the topic of spring N management on March 30, 2017; the link to the recording can be found at https://ifca.com/. In this article we’ll look at some of the data presented during the webinar and will discuss what these findings mean for spring-applied N. This work is funded by the Illinois Nutrient Research and Education Council, using fertilizer checkoff dollars.
 

Is fall-applied N still present?

A first question for those who applied N last fall is whether the N is still present and how much of it has been converted to nitrate. Dan Schaefer of IFCA and his group sampled soils at three on-farm sites in mid-November, mid-December, late January, and early March, following application of 200 lb. N as NH3 with and without N-Serve in late October last fall. The amount of N recovered from the top 2 feet of soil hasn’t changed; 240 lb. N was recovered on December 16 and 238 lb. on March 3.
 
Nitrate as a percentage of the N recovered increased some over the winter, from 55% nitrate in December to 67% nitrate in early March. In 2016, about 60% of recovered N was nitrate when soils were sampled in March, and about 70% was nitrate in April samples. So from what data we have, it appears that, at least in years with relatively mild winters, we can expect more than half of the N to be converted to nitrate by April. Using N-Serve in the fall hasn’t consistently lowered the percentage of nitrate in spring samples, though variability in the samples makes this an imprecise measurement.
 
Is having most of the fall-applied N in the nitrate form by planting time a problem? Not unless the conditions are conducive to N loss before crop uptake begins. At Urbana, nitrate as a percentage of recovered N reached 80% by early May, and was above 85% by early June in both 2015 and 2016. The amount of soil N recovered stayed constant during May; any N that might have been lost from the soil plus N taken up by the crop didn’t exceed the amount of N provided by mineralization. Most importantly, the N was still there when crop uptake began.
 
In comparison to fall-applied N, N applied as NH3 before planting in 2016 had low nitrate initially, then nitrate percentage increased steadily through most of May, reaching 80% of recovered N by early June. While this longer retention of ammonium in the soil is a positive in that ammonium doesn’t move and nitrate does, whether or not this affects the amount of N available to the crop in June depends on whether or not soil conditions are favorable for N loss (that is, wet) during May and into early June. If that happens in 2017, our N tracking project should be able to measure changes in soil N, and we’ll make those results available.
 

Choosing nitrogen rates

While it’s easy to get caught up in questions of N timing and form, we first need to decide how much N to use. The 2016 season brought normal to below-normal June rainfall, little N loss, and high rates of mineralization; as a result, relatively low N rates produced relatively high yields. Adding the 2016 data to the database that powers the N rate calculator (at http://cnrc.agron.iastate.edu/) actually brought the Illinois rates down by a few pounds of N. At current corn and N prices, guideline rates for corn following soybean are 154, 172, and 179 lb N per acre in northern, central, and southern Illinois, respectively, and 200, 200, and 189 lb. N per acre for corn following corn.
 
The calculator guideline rates and the “profitable” N rate ranges found there represent a good starting point for determining N rate for corn in 2017. The calculator uses actual N response data from hundreds of trials to come up with guideline rates. The calculated rate may not be exactly what is required for a given field, though it takes an N rate trial in the field to know that. Some 60 to 65% of the trials in the database have “best” (most profitable) N rates that are lower than the overall best rate. So an N rate trial in a given field is more likely to show a best rate that’s lower than the guideline calculator rate than it is to show one that’s higher than the guideline rate. Choosing high rates in order to be “safe” carries both economic and environmental costs.
 

Will the crop run out of N?

One concern that seems to have increased in recent years is the fear that the corn crop will run out of N at some point during the season, even if enough N is applied early. In fact, it’s rare to have the crop run out of N during pollination and later (grainfilling) stages when enough N was applied early in the season and leaves have good color at tasseling time. In 2016, nearly every field had good color at tasseling time.
 
Any N deficiency symptoms that appear during second half of the season are almost always due to having soils too dry, or, less commonly, too wet; such symptoms almost never come form having too little N in the soil. Water uptake is needed to bring N to the roots and into the plant; under dry conditions, water uptake slows or stops, and so N uptake slows or stops. The “firing” that starts with lower leaves during dry periods is completely due to lack of water, and adding extra N to the soil before the crop fires will do nothing to alleviate it. Only water can fix this problem, and leaf area that fires usually doesn’t come back to healthy green. Under very wet conditions, roots function poorly and may be unable to take up adequate nutrients, including N. Roots standing in water are also unable to sustain the plant in ways unrelated to nutrient supply.
 
So even if we apply enough N, might the crop still run out of N if yield potential turns out to be higher than expected? Again, we see no evidence of this. The crop typically contains a maximum (a few weeks before maturity) of 0.9 to 1.0 lb. N per bushel of yield, so we know that high yields require that the crop take up more N. But we also know from N rate trials that yields of 225 to 250 bushels are often produced at N rates as low as 150 lb. N per acre or less. The extra N in such fields comes from mineralization of the N contained in soil organic matter. Fields and parts of fields with higher organic matter typically produce higher yields as well as more mineralized N, making it easier for the N needs of the crop to be met. In 2016, we saw yields as high as 180 bushels per acre where no fertilizer N had been applied. It is not at all unusual to have the soil provide 150 lb. or more of N to the crop. In lighter soils with lower organic matter, we would expect this amount to be lower, though yields without fertilizer N can be surprisingly high.
 
One idea being marketed today is to test or model soil N during vegetative development and to apply more N if the test shows low soil N levels. This seems to make sense, but we don’t have good guidelines to tell us how much N needs to be in the soil at a certain stage of crop development to assure that there’s enough for the rest of the season. Soil N levels drop fairly rapidly as N is taken up by the crop. In 2016, we found that during the 18 days before tasseling, soil N levels dropped by about 3 lb. N per acre per day, to less than 10 ppm nitrate in the top 2 feet of soil, without having the crop ever show deficiency symptoms on the way to high yields. Over this same period, the crop took up almost 6 lb. of N per acre per day, about twice the rate at which soil N disappeared. Mineralization presumably made up the difference. Much of the N in the soil is in the ammonium form, especially when soil N levels are low, so nitrate levels, which are often used to measure soil N, can be as low as 3 or 4 ppm as the corn approaches pollination without any cause for concern.
 
We know from N uptake studies that some 70% of the crop’s N requirement is taken up by pollination, with uptake rates as high as 6 to 8 lb. N per acre per day right before tasseling, and averaging perhaps 5 lb. N per acre per day for the 30 days before tasseling under good conditions. N uptake rates slow after that, to maybe 2 lb. N per acre per day after pollination to 1 lb. per acre per day or less by mid-grainfill. Mineralization rates may be high enough to supply most of the N the crop needs to take up after pollination, with little need for N supplied (earlier) as fertilizer.
 
As another way to look at the question of running out of N and the need to apply N late, we conducted N rate studies at several locations in 2016, in which we either applied all of the N at planting or all but 50 lb., which we then applied by dribbling the N solution at the base of the row at tasseling. Figure 1 below shows results from this study at Urbana with corn following soybeans, and Figure 2 for corn following corn. Results were remarkably consistent at the different sites where we had these trials in 2016; optimum N rates and yields at those rates were the same whether we applied all of the N early or kept 50 lb. back to apply late. We may see different results in 2017, but in 2016, keeping back some N to apply into tall corn in mid-season did not cover any of the cost that such an application would incur.
Figure 1. Response to N rate, with N applied either all at planting or all but 50 lb. at planting and 50 lb. dribbled into the row at tasseling. Data are for corn following soybean at Urbana in 2016.
 
Figure 2. Response to N rate, with N applied either all at planting or all but 50 lb. at planting and 50 lb. dribbled into the row at tasseling. Data are for corn following corn at Urbana in 2016.
 

N form, timing, and additives

A major part of our NREC-funded nitrogen work in the past three years has been an evaluation of different ways to apply N to the corn crop. One part of this was a comparison of fall- and spring-applied N, using anhydrous ammonia over a range of rates. Dan Schaefer of IFCA conducted these studies as replicated, field-scale strips in farmer fields. Over ten site-years, it took 18 more lb. of N (169 versus 151) to produce about one less bushel of yield (219 versus 220) using fall-applied N compared to spring-applied N. At current prices, spring-applied N netted $11 per acre more than fall-applied N at the optimum N rate for each. Those are small differences; using guideline N rates (which are higher than optimum rates we found) would have produce virtually identical yields whether the N was applied in the fall or in the spring. Given that we often see a little more loss of N through drainage tile with fall application, those able to apply in the spring may see small gains in terms of better efficiency and less loss of N.
 
We also evaluated the effect of applying all of the N as UAN at planting versus a split application, with 50 lb. of N at planting and the rest applied using UAN at sidedress. Averaged over ten site-years, optimum N rates and yields at those rates were very similar for these two methods (Figure 3). Splitting the application required 9 lb. more N and yielded 1.6 bushels more, so netted about $2.50 per acre more than applying all of the N at planting. Unlike the fall- versus spring-applied N study, though, optimum N rates in the sidedress study were a little higher than the guideline (N calculator) rates; using guideline (lower rates) would have given a slight edge to planting-time N.
 
Figure 3. Response averaged over 10 site-years to N rate, with N applied as injected UAN all at planting or applied at 50 lb. at planting and the remaining N sidedressed as injected UAN at stage V5.
 
As part of N rates studies completed so far at 10 site over 3 years, we applied the same N rate (150 lb. per acre) using a variety of N forms, timing, and additives. Among the 15 treatments in these trials from 2014 through 2016, only 10 bushels per acre separate the highest from the lowest yields (Table 1). The two highest yields came from applying dry urea with Agrotain® (urease inhibitor) or as SuperU® which incorporates both urease and nitrification inhibitors. We did not include urea without an inhibitor, so do not know how much the inhibitors contributed. Other treatments that yielded more than the average included UAN injected at planting (our designated “check” treatment), 100 lb. N at planting followed by 50 lb. UAN, either injected at V5 or dribbled mid-row at V9, and UAN all injected at V5.
 
Table 1. Yields and yield ranks across 10 site-years, 2014 through 2016, for 15 different times and forms of N used to apply 150 lb. of N per acre. Sites included DeKalb, Monmouth, and Urbana in all three years, and Perry in 2016.
 
Yield averages not followed by the same letter are significantly different; seven of the 15 treatments did not yield significantly less than the highest-yielding treatment, and five treatments did not yield statistically more than the lowest-yielding treatment. The lowest-yielding treatments included UAN with Agrotain broadcast at planting; UAN dribbled between rows at planting or at V9; and NH3 injected at or before planting, with or without N-Serve®. As an observation, treatments with lower yields were those that included surface application of UAN or application of N in a way that likely meant some delay before plant roots could get access to the N. There may have been some loss of surface-applied N to volatilization, but N broadcast as UAN on the surface may also not have moved down to the roots quickly.
 
We added several treatments after 2014, and because the 2015 and 2016 seasons differed considerably in June rainfall, we’ll look at the data for 2015 and 2016 separately, across three sites in 2015 and four sites in 2016. With the inclusion of seven of the ten site-years averaged in Table 1, of course, yield levels and trends were similar to those that included the 2014 date. Only 12 bushels per acre separated the highest- and lowest-yielding treatments, and the designated check (150 lb. N as UAN injected at planting) produced 221 bushels per acre, higher than six of the 19 treatments and not statistically less than the highest-yielding treatment (Table 2).
 
 
Of the four treatments added in 2015, UAN with Instinct II® (nitrapyrin) injected at planting produced below-average yields, though not statistically less than that of the check (UAN injected at planting.) The other three added treatments included 100 lb. N as UAN injected at planting followed by split-applying 50 lb. as UAN. Dribbling UAN into the row at V5 was a very good treatment, yielding only 2 bushels less than the highest yield. The last two treatments including dribbling the split N between rows or at the base of the plants at tasseling time; these also yielded well, at 221 and 222 bushels per acre, respectively, about the same as the check (Table 2).
 
Treatments that ranked considerably higher in 2015 (wet June) than in 2016 (normal to dry June) included 100 lb. N at planting followed by either 50 lb. N injected at V5, or by 50 lb. dribbled into the row at VT; and the treatment with all of the N sidedressed between the rows at V5. It’s possible that rainfall in late May and early June moved the sidedressed N to the plant roots a little sooner in 2015, and it’s also possible that enough planting-time N had moved out of the root zone that year to make adding the last 50 lb. in the row at tasseling a little higher-yielding.
 
Treatments that ranked considerably higher in 2016 than in 2015 included urea + Agrotain broadcast at planting, ESN broadcast at planting, and 100 lb. N at planting with 50 lb. dribbled between the rows at VT. There was enough rainfall in May of both years to move urea into the soil without too much problem, so it’s not clear why these performed better in 2016. But both were good treatments across all sites. It’s also not very clear why dribbling 50 lb. N down the row middle at tasseling was better in 2016 than dribbling it into the row, the reverse of what we found in 2015. Again, these were both reasonably good treatments, but not better than the check (UAN injected at planting.)
 

Summing up

Yields levels were relatively consistent among sites and years, ranging from 185 to 248 bushels per acre; we didn’t really see the tough conditions that we know can happen. We also found somewhat lower N responses than we expected; the 150-lb. N rate we chose in order to spread the yields from different N treatments was either more than the optimum N rate or within 20 lb. of the optimum at six of the ten site-years. So the high-loss conditions under which some treatments might be expected to do much better than others were not very noticeable in this study, at least during the first three years.
 
Given all that can happen when we apply N fertilizer in a way that we think will produce high corn yields, it’s no big surprise that this research has not so far identified clear “winners” or “losers” among the different ways we managed N. With top-to-bottom yield ranges as high as 36 and as low as 12 bushels among sites, expecting treatments to “hold rank” across such different environments may not be very realistic.
 
The ability to separate yield averages statistically is directly related to how well treatments held rank across sites-years. When a treatment ranks high at some sites and low at others, its overall average is in the middle, and the statistical comparison, which measures how well the results predict future performance, becomes less certain. That’s why so many of the treatment yields averaged over sites (as in Table 1) are followed by the same letter – we can’t be sure that a treatment that yielded 4 or 5 bushels more than another treatment will do that again next time, because it didn’t do that consistently across trials so far.
 
These results show, though, that just about any way we are managing N now is probably working reasonably well. We did not expect that treatments involving dry urea, protected against loss and broadcast at planting, to perform as well as they did. We don’t think that these results suggest a push towards broadcast urea application, but it is a common practice in many parts of the world, and if costs and availability move us in this direction, it appears to be workable. Treatments that did not do as well as we might have expected included applying UAN solution on top of the soil, whether that was all at planting or at other times. Anhydrous ammonia applied at or before planting also produced lower yields than expected.
 
These results seem to point to the benefit of having much of the N in the soil into which the roots grow, and to have it there relatively early in the season. Though we didn’t measure soil N in this study, most of the treatments that produced below-average yields were ones that supplied most of the N only at or after the plants had grown for a month or more. Treatments such as UAN dribbled or NH3 injected between rows at planting might have placed the N out of reach of early root growth. In contrast, broadcasting urea or injecting UAN between rows at planting might have resulted in more N in the soil where the roots grew early.
 
Even if the hypothesis that having more N in the vicinity of the roots holds up in further research, yield differences we found over sites were probably not large enough to justify many changes in how we manage N. As an example, incorporating broadcast UAN, which is normal practice, might be adequate to provide the roots with early access to N. And, if it stays dry for several weeks after planting (which did not happen in these trials), broadcasting urea might not work as well as we saw it work so far.
 
We might, though, want to consider the need for N near the roots during early growth as we plan N programs. This could be as simple as applying more of the N early and less at sidedress, or of applying sidedress N closer to the row for better access by the roots. As is always the case, weather conditions will have a large influence on how necessary, useful, or successful our best-chosen strategies turn out to be; no responsible N management program is completely safe.
 
One approach that has appeal, but that adds considerable economic and environmental risk, is to “just apply more” in order to make certain the crop won’t “run out” of N. We have seen how rarely the crop runs out of N when normal N rates are applied. Our work is also showing that loss of N (movement out of the top 2 feet of soil) is less than we expected, especially when we account for the amount taken up by the crop. With the equipment and knowledge we have today, everyone can manage N responsibly and with confidence that the crop will get the N that it needs. As is always the case, good weather helps a great deal to make N work, and we wish good weather for everyone as the season gets underway.
 

Dealing with Cool and Wet Conditions

Written by Emerson Nafziger     (View the UofI bulletin)

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.
 

Nitrogen

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.