Illinois Fertilizer & Chemical Association
Supply · Service · Stewardship

UI Nutrient Bulletins

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.
 

How much nitrogen is gone?

Written by Emerson Nafziger     (View the UofI bulletin)

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 onREC 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.
 
Figure 1. Soil nitrate and ammonium recovered from the top 2 ft. of soil following application of 200 lb. of N as anhydrous ammonia with and without N-Serve (NS) in the fall of 2016 or in March, 2017. Data are averages over 3 on-farm sites in central Illinois.
 
 
Figure 2. Soil nitrate and ammonium recovered from the top 2 ft. of soil following application of 200 lb. of N as anhydrous ammonia with and without N-Serve (NS) in the fall of 2016 or in April, 2017.
 
Data are averages over trials at Monmouth, Urbana, and Perry, Illinois. 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.

Positive signs for nitrogen

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

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.
 
Figure 1. Soil nitrate and ammonium recovered from the top 2 ft. of soil following application of 200 lb. of N as anhydrous ammonia in the fall of 2016 or in April, 2017. Data are averages over trials at Monmouth, Urbana, and Perry, Illinois.
 
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 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 Corn Crop and Sidedress Nitrogen

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

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.

New Grain Phosphorus and Potassium Numbers

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

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.
 
Figure 1. Distribution of grain P levels among 2,335 corn grain samples collected in Illinois over a three-year period, 2014-16.
 
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.
 
Figure 2. Cumulative distribution of corn grain P levels for 2,335 samples collected from 2014-2016 in Illinois. Vertical lines identify the 25th, 50th, and 75th percentile values, and the current “book value” (0.43 lb P2O5 per bushel, at the 97th percentile) is indicated by the vertical line on the right.
 
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.
 
Table 1. Average, 25th, 50th (median), 75th percentile, and “book values” for corn (2,335 samples) and soybean (2,620 samples) grain P and K levels found in the survey.
 
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.

Using the N rate calculator

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

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 theprocess 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.
     
    Figure 1. Economically optimum N rate and yield at that N rate for each of 245 N rate trials with corn following soybean in central Illinois. Each symbol represents one trial. This figure is from the N rate calculator – see text for website address.
     
    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 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 fertilizeralmost 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 comparison 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: No, 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.

Timing Fall Nitrogen

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

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.
 
Figure 1. Soil temperature at 4 inches under bare soil at three Illinois Climate Network sites on October 17, 2017. Source: Illinois State Water Survey.
 

Early-Season Management of Soybean

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

If the old saying that rain on Easter means that it will rain on the next seven Sundays applies to snow, we’re in trouble – it snowed across a wide swath of Illinois on Easter Sunday (April 1) and also on April 8.
 
We had enough dry weather in March to allow some ammonia to go on early, but there has been little opportunity for field work over the last six weeks. Rainfall over the past month has been below normal for the northern third of Illinois and above-normal in the southern half of the state, especially along I-70. Even though it’s not sopping wet in many areas, below-normal temperatures in recent weeks means very slow drying of soils. While we know that conditions can change quickly – even as I write this the forecast has improved for the rest of this week – it’s clear that the spring of 2018 is not going to be one that allows a very early start for field operations.
 

Soybean following soybean

With soybean acreage in Illinois expected to increase some and corn acreage to fall this year, some soybeans in 2018 will follow soybeans. As I’ve written before, there is no particular concern in planting soybeans after soybeans, except perhaps to avoid doing this if soybean cyst nematode egg counts are high. We have no reason to expect that SCN counts are unusually high, but if this will be the third year of soybean in the same field or if there was any hint of SCN damage in the 2017 crop, it might be worth taking a count yet this spring. SCN-resistant varieties are a must in any case.
 
The yield penalty for soybeans that follow soybeans instead of corn varies some by site and year, but most of our research shows this penalty to be modest, usually less than 10%. Averaged over three trial sites and two years (2016 and 2017), soybean following: 1) continuous corn yielded 76.9 bushels per acre; 2) two years of corn yielded 71.4 bushels; 3) one year of corn yielded 69.2 bushels, and; one year of soybean yielded 68.0 bushels per acre. In 2017 we had soybean following two years of soybean, and averaged over three sites, these yielded about 2.5 bushels less than those following one year of soybean.
 
In two long-running studies in western Illinois, tillage has had either no effect on yield of soybean following soybean, or has decreased yield. If the soybean stubble was not tilled last fall, it would probably be better to plant soybeans without tillage this spring. We did see a slow start to no-till soybeans under the cool, wet conditions of 2015, and at the Monmouth site that year, no-till soybeans following soybeans yielded 5 bushels less than tilled. This differences was even larger in soybean following corn that year. Soybean following corn tends to yield a little more when tilled than with no-till, in fact, though the difference averaged over years is not enough to pay for tillage operations.
 
Other than normal scouting for disease and weed management, though, there are few other management considerations specific to growing soybeans after soybeans.
 

Cover crop management

Cereal rye planted into corn stalks last fall has made much less growth than normal, especially in comparison to 2017, when February temperature averaged nearly 10 degrees warmer than in 2018 and the cover crop grew for a couple of months before April. The slow growth this year will continue as long as soil temperatures remain in the upper 30s to low 40s as they are now. But rye is a cool-season crop, and will start to grow rapidly once it warms up.
 
Conditions have not been good to kill the cover crop with herbicide, so slow growth may be preferable to rapid growth for now. But a choice will need to be made in the coming weeks about how long to let the rye grow before spraying to kill it. We want enough growth to produce the benefits for which we planted the cover crop, but we also need to manage it so it doesn’t interfere with soybean establishment. If soybean seed can be placed into soil well, this shouldn’t be a big concern. But as long as the weather and soils stay cool, soils will dry slowly, especially once the rye is killed and is no longer taking up water. Lower amounts of residue due to slow growth will help some, but soybean seed placement and crop emergence could still be a challenge, especially if soils continue to dry slowly or the weather turns wetter. Clearing residue off the row will help, if that’s an option.
 

Planting date

In the spring of 2017, soil conditions and temperatures were so favorable early that some people planted a “test” area of soybeans in February to see how they’d do. They did well – soybean are quite tolerant of frost as long as they haven’t emerged yet or have emerged and their “neck” has straightened out to bring leaves and cotyledons to the horizontal. So the period in mid-March with temperatures in the 20s last year didn’t kill the crop, and some of these soybeans yielded as much as those planted in late April.
 
This year, soils in some areas were dry enough to plant by mid-March, and some people again planted soybean then. [Some corn got planted as well, but that no longer attracts the attention that super-early soybean planting does.] Conditions since have been much less favorable than they were a year ago, and this has kept the early-planted crop from emerging, at least in most fields. Only about 30 growing degree days (base 50) have accumulated over the past month at Champaign – that’s maybe a fourth of the number of GDD needed for the crop to emerge. It will be surprising if soybean seed that has been in cold, wet soils for the past month is still viable, but we won’t know for sure until soils warm up. For the curious, digging up seeds and putting them in a damp paper towel in a warm room for a few days will show whether they’re still alive. Even if they’re alive there’s no guarantee that they’ll be able to emerge and become healthy plants.
 
Lost in the attention given to the survival of soybeans planted very early is the question about such early planting – provided the crop survives – affects soybean yield. Given how rare it is that soybeans can be planted by or before mid-March, we have not done trials on this. We mostly have anecdotes, and those may be skewed towards those times when the crop survived. We have seen a few cases, especially in the very dry spring of 2012, when planting in early April was followed by stressful (cool or dry) conditions that limited plant height and yield compared to soybeans planted later. Even if soil conditions allow a March-planted crop to emerge, there is virtually no chance that it will yield more than a crop planted in the same field in late April, and some chance (if it survives) that it will yield less.
 
Overall, our data across 26 soybean planting date trials show that soybeans produce full yield if planted anytime between the second week of April and the end of April. The rate of yield loss with planting delay accelerates into May, reaching about 2/10ths of a bushel per day by the end of the first week of May, a quarter of a bushel per day by mid-May, a third of a bushel by the end of May, and 4/10ths of a bushel per day by June 10. These are lower loss rates than we often see presented elsewhere, most of which are based on a limited number of trials. That doesn’t mean we shouldn’t try to plant as early as we can to get full yields, but it does show what most farmers know from experience – that high soybean yields depend more on what happens during the season than on when the crop gets planted, although planting by mid-May increases the chance that the crop will be able to respond to favorable conditions later. A corollary to that observation is that planting soybean into poor soil conditions just to get them planted early can decrease the ability of the crop to respond to favorable conditions later, and thereby end up costing yield.
 

Seeding rate

While 100,000 or even fewer plants per acre will maximize yield in many cases, our research shows that this is not always enough plants. Trying to minimize the seeding rate can end up costing yield and losing money, especially in those cases when emergence and stand establishment are lowered by conditions at or after planting. While responses to plant stand do vary across trials, we have found that 115,000 to 120,000 plants (not seeds) per acre are often needed to produce the highest dollar return on the seed investment. If we plant good seed into good conditions we can expect 85% stand establishment, in which case we should plant about 140,000 seeds per acre, which for most seed companies today is one unit of seed.
 

Nitrogen

Despite that fact that most trials in Corn Belt states in recent years – including our trials in Illinois – have shown little or no yield increase from applications of 45 to 90 pounds of N (100 to 200 pounds of urea) during the growing season, this practice continues to draw a lot of interest. In a set of trials we just finished, applying 45 or 100 pounds of N at planting time produced large increases in yield two years in a row on an irrigated loam soil near the Illinois River at Chillicothe, Illinois. Planting-time N had no effect on yield in most other trials in heavier, higher-organic-matter soils. We did find yield increases in a number of trials when we applied the same amount of N four different times, from planting through early podfilling. While repeated use of N may help explain some “contest” yields, the yield increase from four applications was not enough to pay even half the cost of these applications. Putting that much N on also means a lot of N left in the soil at the end of the season, so more N loss through tile.
 
With so many voices today claiming that N application on soybean “can” increase yield and others saying that it still won’t increase profits, what should producers do? In an ideal world, 500 Illinois farmers would put out a set of N strips (next to strips without N) in a field or two each year, and results would be brought together to produce data-backed expectations of how profitable this practice is on different soil types and across years. One of the reasons that’s difficult today is that so many soybean fields are harvested on an angle to the rows, making yield data collection difficult or impossible. There is also no one to organize such work and little noticeable interest by those who might fund such a project. “Trying” N by applying it to a field or two is sometimes suggested by those who feel that this is a profitable practice. This approach, of course, provides no information on whether or not applying N did anything.
 

For those interested in a “lite” N trial on soybean

Both times that we’ve seen a large yield increase from N on soybeans were on lighter-textured soil with N applied at planting. Applying N at planting typically makes leaves and cotyledons of small plants darker green in color compared to plants without N. In cases where N ends up increasing yield, this darker green color persists into vegetative stages, and plants tend to show increased growth and more green through most of the rest of the season. Where planting-time N doesn’t increase yield, the difference in green color between plants with N and those without N disappears as the plants make vegetative growth, and as their roots get more access to N from the soil and from N fixation in nodules. By the time plants are 6 to 8 inches tall, the effects of planting–time N are often no longer visible.
 
Based on what we’ve seen, I’m suggesting a low-cost alternative to large-scale application of N as a way to see where and how often N might have the potential increase soybean yields. Here’s the outline:
 
  1. After planting and before emergence, choose a uniform spot at least 20 feet away from endrows or edge of the field, and put flags in the corners of an area 15 feet x 15 feet square. We expect to see N effects more often on soils that are lighter in texture and lower in organic matter, so place this accordingly, in two or three fields or parts of fields with contrasting soil types if that’s an option. If possible record GPS coordinates for each site.
  2. Weigh out enough N fertilizer – urea or lawn fertilizer (without herbicide) – to provide 50 pounds of N per acre on the 225 square feet you flagged out. Calculate this by dividing the number 25.83 by the percentage of N in the product (urea may be 46-0-0, or 46% N; lawn fertilizer might be something like 27-0-4, or 27% N) to give the amount of product needed. As an example, if using urea (46% N), you would need 25.83/46 = 0.56 lb. of product. Multiply this by 16 to get number of ounces, or by 453.6 to give number of grams, if you have a gram scale. If you have a measuring cup but not a scale, urea weighs roughly 3/4ths as much as the same volume of water, so a cup (8 fluid ounces) of urea weighs about 6 ounces.
  3. Apply the fertilizer carefully, by hand or using a hand spin-spreader, uniformly over the soil inside the square.
  4. The “data” to be taken can, for most people, just be a photograph with the image split between the area with N and an area (outside the square) without N. I suggest taking a photo at about the V2 stage (two full trifoliolate leaves present), and another one about a month later, at perhaps V6-V7, when plants may be a foot tall or so. Find the side of the square that gives the best contrast under existing light conditions. Feel free to supplement the photo by noting what you can see (or not see) by eye.
  5. If the second photo shows no difference in greenness of plant size between those that received N and those that didn’t, the experiment could end there, with the conclusion that N probably made no lasting difference on growth, and so is unlikely to increase yield. If the plants with N are still greener and/or larger than those without N, that would be a signal to come back once or twice more, to see if the differences persists to the podfilling stages in mid- to late August, and again before leaves drop.
  6. If plants inside the treated square are visibly different than those outside, and there’s enough ambition and curiosity, you could harvest 15 or 20 randomly-selected plants inside the treated area and outside the treated area, and take a photo with the two sets of plants next to one another to show any visual effects on height or pod number. Those interested could count the number of pods, or even thresh the plants (in burlap bags works best) and weigh seed to estimate yield. Calculating yield would require an estimate of number of plants per foot of row. Yield estimated this way are highly variable, so they may not line up with what we thought we’d see based on plant size and appearance.
 
I’d be happy to look at photos from such comparisons; if there’s enough interest I could also develop a small reporting form to make a record for each trial. I’ll also be glad to send a layout for anyone interested in doing a strip trial with and without N. I can be contacted by email (link below, on my name) or my cellphone number is (217) 369-1997.
 

N Rate Calculator Updated

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

Last month (March 2018) we used data from 2017 N rate response trials to update the N rate calculator that provides best-estimate N rate guidelines for different regions and previous crops (corn or soybean) in Illinois. The updating process, which is currently being done by spring each year in Illinois, involves adding the new data and taking out some of the older data.
 
Many people understand the idea of using data from previous research to try to predict how a management factor will work the next time (in this case, in fields in 2018) – after all, that’s what applied research is all about. But responses to N are highly variable across even nearby fields within a year, and that can make it difficult to place a lot of confidence in the results as a predictor of what will happen the next year. Which N response curve from last year would we expect to do the best job of telling us how much N to use this year?
 
Let’s use the data from Illinois trials in which corn followed soybean in 2017 to illustrate this. These data (shown in Figure 1) represent a large amount of work; many of these trials were done on farmer fields, organized and conducted by Dan Schaefer of the Illinois Fertilizer & Chemical Association, and some are done within our N research projects at University of Illinois sites. Both IFCA’s and my research projects are supported by the fertilizer checkoff, administered by the Illinois Nutrient Research & Education Council.
 
Figure 1. Nitrogen rate responses of corn following soybean in 51 on-farm trials in Illinois in 2017. Yellow triangles show the calculated optimum N rate (EONR) and yield at that rate for that trial, and the green circles show the MRTN rate and the yield at that N rate.
 
In order to show actual corn yields at different N rates, Figure 1 has straight lines connecting the yields, each of which is an average over three or four reps at each N rate. To work with the data, we have the computer calculate a “best-fit” line for each set of data, and the equation for this line produces a smooth curve. That equation is also used to calculate what N rate is required to maximize yield, and what that yield is. We can’t possibly hope to set N rates so that one of them is the “best” rate – we have to produce the shape of the response defined by the actual data points, then work with that.
 
Each set of N response data shown in Figure 1 has two symbols associated with it. One is a yellow triangle that marks the N rate – and yield at that rate – where the last pound of N produced just enough additional yield to pay for that pound of N. We call this rate the “economic optimum N rate” or EONR. As a default, we often set the cost of 10 pounds of N as equal to the price of one bushel of corn – for example, N might be $0.40 per pound and corn $4.00 per bushel. In that case the EONR is the N rate at which the last pound of N produces a tenth of a bushel of yield. Above that N rate, the cost of added N is not covered by the additional yield; below that rate, each pound of N adds more yield than it takes to cover its cost. In both cases, the return to N is less than it is at the EONR.
 
Each set of response data also has a green circle (like the EONR triangles, many of these are not exactly on the line because the lines aren’t the fitted curves) that gives the “maximum return to N” (MRTN) N rate calculated by the N rate calculator (the 2017 version, which ran using data through 2016), and the yield at that N rate for that response curve. There are three vertical arrays of the green circles, each corresponding to the Illinois region in which the trials were conducted; MRTN values produced by the N rate calculator decrease from the north (those on the left) to central (middle set of circles) to southern (circles on the right) regions of Illinois for corn following soybean. The green circle shows how well using the MRTN rate would have done in that field in 2017 compared to what we now know (because we had an N rate trial there) was the actual best rate (the EONR, shown by the yellow triangle) in that field in 2017.
 
At first glance, the fact that the EONR values (yellow triangles) on Figure 1 are so spread out, and that most of them are not very close to the green circle on the same curve, looks like the MRTN rate failed to predict the best N rate for that field in 2017. Of the 51 sites, the actual EONR is to the right of the green circle – that is, the MRTN rate wasn’t high enough – at 21 sites. At the other 30 sites, the MRTN rate was higher than the actual EONR – the yellow triangles are to the left of the green circles. Across all 51 trials, the average MRTN was 172 pounds N per acre, and the average EONR was about 168 pounds N per acre – the difference was only 4 pounds of N. The average yield at the MRTN rate was 226 bushels per acre, and at the EONR was 229 bushels per acre, or 3 bushels more. Using N and corn prices of $0.35 per pound and $3.50 per bushel, using the EONR (which, of course, we couldn’t have known before the season) in each field would have returned about $12.50 per acre more than using the MRTN rate in each field. The two sites with the highest EONR values (those farthest right in Figure 1) by themselves added almost 4 pounds to the average EONR value and almost $3 to the higher return from using the EONR. Though we seldom see such high EONR values, we leave them in the database because they represent possible (future) outcomes, and we can’t justify leaving them out.
 
Focusing on the yellow triangles in Figure 1 makes clear what we discovered some years ago – that there is no correlation between EONR and the yield at the EONR across a set of N response trials. In fact, a trendline drawn through the EONR values slopes down slightly, meaning that across this set of trials, higher yields needed a little less N than lower yields. The only explanation for this that makes sense is that moisture and temperature conditions that resulted in high yields also increased the amount of N provided by mineralization of sol organic matter, and may also have increased the ability of the roots to get access to that N. At the extremes, in one trial the yield was 278 bushels at 101 pounds of N, and in another the yield was 203 bushels at 226 pounds of fertilizer N. We estimate that corn needs to take up about one pound of N for each bushel it yields. If so, fertilizer provided only 101/278 = 36 percent of the N needed in the first trial, but 226/203 = 111% of the N needed in the second trial.
 
Even though on average the MRTN performed well in 2017, it clearly did not do so in every trial field, or even in most of these fields. This points out a fundamental difficulty that dogs every attempt to predict “best” N rates for a field or for different parts of a field: nothing we have been able to measure predicts with any accuracy how much N from fertilizer a given field will need at the start of a given season. With yield goal as a basis for N need pretty much sidelined by the finding that higher yields don’t need more N than lower yields, and with both yields and N responses highly influenced by (unpredictable) weather, what can we use to predict how much fertilizer N a field will need?
 
One such possibility is a way to predict how much N will come from mineralization of soil organic N, which by subtraction could help estimate how much fertilizer might be needed in a field. Another is a way to estimate how much N is lost due to unfavorably wet weather before the plant has a chance to take it up. These attempts have so far not been very successful; we still don’t have a way to know, in May or June, whether the crop in a field needs 120 pounds of N or twice that much to produce the yield it will produce this season. Our best guess today is the MRTN, perhaps tweaked up or down depending on circumstances. Each MRTN value has a range of about 15 pounds of N on either side over which we expect return to N to be very close (within $1 per acre) of the return at the MRTN.
 
Updating the N rate calculator with recent data and dropping out some older data doesn’t change its output by very much in Illinois, because we have so much data already there: there are 267 N responses for corn following soybean in the database for central Illinois alone. The update this spring added about 3 pounds of N to most MRTN values using current N and corn prices. At $500 per ton of ammonia and with corn at $3.85, the MRTN for corn following soybean in central Illinois is 183 pounds per acre. That rate is expected to produce 99% of maximum yield; by definition, the optimum N rate never produces maximum yield because the last N has to pay for itself, and only if N were free would that be at the rate needed to produce maximum yield. The range over which the return to N would be expected to be within one dollar per acre of the maximum is 168 to 200 pounds of N per acre.
 
Other numbers produced by the calculator include the cost of N at the MRTN ($54.90/acre), the amount of material needed (223 pounds of ammonia), and the net return to N at the MRTN ($314.62 per acre.) Note that the calculator subtracts the yield without N from each N response – the return to N is only the yield added by fertilizer N times the corn price minus the MRTN N rate times the price of N. With the N cost of $54.90 per acre and the gross return of $314.62 + $54.90 = $369.52, we can calculate that this amount of N increased yield by $369.50/acre ÷ $3.80/bushel = 97 bushels per acre. The “true” nitrogen use efficiency (NUE) is the MRTN divided by the amount of yield added by N, or in this case 183 lb. N/acre ÷ 97 bushels/acre = 1.9 pounds of N for each bushel added from using N fertilizer. We often just divide N rate by yield for a field because we can’t know what the yield was without N, so when yields are high and N rates reasonable, we typically have NUE values of only 0.7 or 0.8 lb. N per bushel. That’s helpful in that it helps us see that we don’t need such high N rates to get high yields. But it includes the N supplied by the soil, not just that added as fertilizer.
 

Crop conditions and potential in mid-June

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

Warm temperatures continue in Illinois, with growing degree day (GDD) accumulations since May 1 running from 150 above average in northern Illinois to about 250 GDD above average in the rest of the state. With GDD accumulations of 900 to 1,000 since May 1, the corn crop planted in early May is at V10 to V14, about 30 to 60 inches tall, and needing only about 350 to 450 more GDD to tassel and silking. With daily accumulations at about 25 GDD, much of the crop will be showing tassels and silks by the end of June and first days of July.
 
The corn crop condition ratings as of June 10 were 83% good + excellent, one of the highest early-season ratings we’ve ever had. In most fields, the crop looks outstanding, with probably the best stands we’ve ever had, and in most cases very good crop canopy development and color. On the other hand, plants are showing leaf curling in the afternoon in some areas, indicating that the water supply in the soil is not high enough to sustain maximum rates of photosynthesis now.
 
A lot of the rain over the past month has been from thunderstorms rather than broad movement of fronts; as a result, its distribution has been very uneven. During the first half of June, rainfall ranged from less than a half inch in parts of western and southwestern Illinois to more than six inches in southeastern Illinois. Even in those areas showing average or above-average rainfall, there are places that the storms missed, and where soil water is starting to run short. The US drought monitor shows “abnormally dry” conditions in several western Illinois counties, and in a small pocket in northeastern Illinois.
 
While the well-watered areas with deep soils have enough soil water now to get the crop through pollination, normal to below-normal temperatures will assure that the crop has high potential to set the kernel numbers needed for high yields. In areas where plants are showing stress in the afternoon now, we expect that this will set in a little earlier, and last a little longer, each day that temperatures remain high and there’s no rainfall. Canopy development is good so far, but a good canopy also means faster water usage, and a crop that’s head-high has a “crop coefficient” (the proportion of evaporation that the crop takes up) of about 0.7. This value reaches a maximum of about 0.8 at full crop canopy.
 
That means that if evaporation (also called “potential evapotranspiration” or PET) on a warm, sunny day is 0.25 inches – PET has been high as 0.3 inches on the warmest days in Illinois in recent weeks – the crop takes up 0.25 x 0.7 = 0.18 inches of water. Our best soils can store as much as 10 to 12 inches of plant-available water in the top three feet, so at field capacity the water supply can last six weeks or more without rainfall. That’s under ideal conditions, though – the crop will often show stress effects before the soil water is completely depleted. It’s been dry enough in parts of Illinois that the soil water supply is not enough to keep the crop well-supplied now.
 
As pollination approaches, the effect of water stress on the crop will increase. If it rained everywhere today, the crop could probably recover its full yield potential in most fields. But if leaf rolling starts by noon, the crop is producing less than half the normal amount of sugars through photosynthesis on that day, and the closer the crop gets to pollination the larger the effect of lost sugars will be. Today’s hybrids are bred to produce silks, pollen, and some fertilized kernels under stress conditions, but if it stays dry over the next weeks where the crop is already showing stress, kernel numbers will be lowered. Lower kernel numbers mean lower yield potential.
 
Corn plants that develop under high temperatures and with plenty of water tend to be taller than usual. In areas where the crop has been showing stress symptoms in the past week or two, though, we can expect plants to end up shorter than normal. Any water stress during rapid stem elongation – between V8 and tasseling – results in less elongation of cells in those internodes that are expanding during that time, and this results in shortened internodes and plants. As we saw in 2017, shorter plants can still yield very well, but that requires that they get adequate water by a week or so before tasseling to assure that the pollination process can proceed normally.
 
On a more positive note, the benefits of relatively dry weather include the development of good root systems, a good supply of soil nitrogen, and little development of diseases. The dark green leaf color shows that the N supply has been adequate. Where it has rained the soils have not stayed wet except in low-lying areas, and so there has been little potential for N loss. Some rains have been so intense that much of the water runs off, and dry period before the rain meant that the soils could take in several inches of water before they became saturated. Where water has stood or is standing now in fields, though, we can expect both some root damage and some denitrification, with the potential of considerable yield loss in those areas. Fortunately, this area is not as large in size as it’s been in some recent years.
 
By now the question of whether the corn crop needs more N to follow normal rates applied earlier should be answered: the deep green leaf color of the crop means that it’s not likely to run out of N so well-supplied with N as shown by its canopy color, there’s almost no chance that it will need more N than is on the soil now. Our N-tracking results from this spring confirm what the crop is telling us – that it has plenty of N. Those who put on a normal amount early with the idea that they’d come back to apply more if the yield potential looks good can skip the additional application this year.
 
The Illinois soybean crop in mid-June has the same high crop condition rating as the corn crop. Stands are good in most fields, and plants have begun to develop rapidly, after the usual lag that we often see, which in some cases might have been lengthened by application of certain herbicides. In a planting date study we have here at Urbana, early varieties planted on April 25 are at V6-V7 and 15-18 inches tall, with a lot of flowers now. As is the case in corn, canopy health is very good. Growth so far has been good even in dry areas, because soybean plants don’t use water very fast when they’re small so more water remains in the soil.
 
We see flowers appear before the longest day of the year when soybeans are planted relatively early, and when temperatures are warm in June. The appearance of flowers requires a certain night length, and if it takes 10 days after the longest day (June 21) to reach that night length, then that night length also occurs 10 days before June 21. But if plants aren’t past stage V3 or if nights are relatively cool, soybean plants won’t flower before the summer solstice. Even when they do flower in mid-June, limited numbers of flowers might turn into pods. We will need a lot of flowers appearing after June 21 along with good growing conditions in order to set the number of pods needed for high yields.
 
Our best hope now is for a return to normal temperatures and rainfall by July to get both corn and soybeans on the way to reaching their current potential for high yields. Little potential has been lost so far, but the next weeks will spell the difference between average and very good crops.