- Category: Programs
Consider Soil in Fall-applied Ammonia Rates
by Lauren Quinn, University of Illinois at Urbana-Champaign
"There have been a number of studies to compare yields with fall- versus spring-applied ammonia or other treatments. But our study is different because we are tracing the nitrogen from the fertilizer ammonia into either the grain or the whole corn plant above ground. That's what makes this unique," says Richard Mulvaney, professor in the Department of Natural Resources and Environmental Sciences at Illinois.
Mulvaney and his graduate student, Kelsey Grieshiem, used a stable isotope of nitrogen, 15N, in formulating the tagged ammonia. They applied it at a typical rate of 200 pounds per acre in mid- to late November in six Illinois fields in 2016 and 2017.
The fields differed in soil type and crop rotation. Four were Mollisols, which Mulvaney describes as prairie soils: rich, black, and productive. The two others were Alfisols, or timber soils, which are typically poorer in comparison to Mollisols. Two of the Mollisol fields were cropped to continuous corn, while the rest were a under a corn-soy rotation.
After the fall ammonia application, the researchers looked for the 15N isotope in corn plant and grain material at harvest in the following growing season. Any nitrogen not tagged with the isotope was assumed to have come from natural nitrogen stores in the soil, rather than from the fertilizer.
"Just as we expected, the poorest soil showed the highest uptake efficiency while the richer soils were much lower," Griesheim says. "On average, only 21% of the nitrogen applied was recovered in the grain, ranging from 34% at the poorest Alfisol to 12% for the richest Mollisol.
"Farmers apply ammonia in the fall thinking they've supplied nitrogen to their corn crop for the coming year. But based on our results, most of the fertilizer nitrogen will not be taken up by the crop."
If it were up to Mulvaney, he would recommend nitrogen applied as a side dressing, which delivers the fertilizer when the plant is actively growing. "You're fighting time with fall applications," he says. "You're counting on keeping the nitrogen in the soil for six months before the next crop needs to take it up."
The study also evaluated the effectiveness of N-serve (nitrapyrin), a nitrification inhibitor commonly applied in the fall with anhydrous ammonia. This product is meant to slow down the microbial conversion from ammonium, which is immobile in the soil, to nitrate, which, during a wet spring, can leach away or be lost as a gas.
"Nitrapyrin has long been used in conjunction with fall-applied ammonia with the motive of increasing uptake efficiency. We didn't find that to be the case at all. And in fact, in our study, the only significant effect of the product was a yield decrease on continuous corn," Mulvaney says.
Should farmers apply anhydrous ammonia in the fall?
Griesheim says, "Considering the low uptake efficiencies observed in our study, farmers should think twice before putting their nitrogen on in the fall. Low fertilizer recoveries mean less return from the farmer's fertilizer investment and a higher risk of environmental pollution."
Adds Mulvaney, "To make matters worse, farmers have been taught for decades that they should follow yield-based recommendations, such that the soils that generate the highest yields need the most fertilizer. But that inverts reality. The highest yielding soils need the least fertilizer. Our research supports that."
New Thoughts on Pre -Emergence Options in Cotton
Clearly, after all the struggles in recent years controlling grass weeds, many are looking to change their weed management program. The most frequent question recently has been on the best pre emergent (PRE) options to more account for barnyardgrass, jungle rice and goosegrass.
In an attempt to do a better job on grass most have decided to move from the traditional Cotoran and/or Caparol. Rather many are trying to decide if herbicides such as Brake, Dual Magnum, Prowl H20 or Warrant would be a better fit for their cotton.
Prowl H2O is by far the herbicide many have decided to go with to help improve their grass control. I believe this is a good option as well. Unfortunately, shortages have been reported and it will not be an option for some. If you are able to use Prowl H20, please consider using at least a low rate of Cotoran to help with the Palmer control. The Treflan/Prowl resistant Palmer amaranth identified by Dr. Rhodes in the late 1990s is still present in some fields and Cotoran will help here.
Another good option is Brake. In our research a tankmix of Brake and Cotoran can provide good grass control. Remember Brake needs a good ½” of rainfall to become activated. As such it would often work more consistently applied with the burndown early PRE-plant where it would be more likely to catch a rain than PRE-applied. Another caveat with Brake is that for soils with higher clay content that 16 oz/A rate is too low. Even bumping up to 24 oz/A maybe not enough depending upon the clay content of the soil and the weed pressure.
Dual Magnum and Warrant are also labeled options. Dual Magnum will provide the most consistent grass control in most situations. The exception to this would be in a cover crop where often in our research Warrant provides better residual control. Of all the herbicides mentioned these two would have the highest probability of harming stand when used as a PRE. This can be mitigated to some extent by using a split-shot approach where ½ to 2/3 of the full rate is applied PRE and the rest early Post emergence around the typical thrips application window (1 to 2 lf).
Raddish Cover Crop Traps Nitrogen:
Matt Ruark of the University of Wisconsin-Madison and colleagues wanted to know more about the effect of this nitrate uptake in the following growing season. They established test sites in three Wisconsin locations and studied them for three years. At each site, some plots received the radish cover crop and some did not. The radish cover crop was planted in August after a wheat harvest. Corn was planted the following spring.
The research showed that radish significantly reduced the nitrate content in the soil as compared to the test plots with no cover crop. This finding confirmed the results of several earlier studies. It showed that radish did take up nitrogen, in the form of nitrates, from the soil.This research supports the use of radish as a cover crop as a trap crop for fall nitrogen. However, what happens to that nitrogen afterward remains unknown.
There was no consistent evidence that nitrogen was returned to the soil as the radish crop decomposed. Radish did not supply nitrogen to the corn crop. The researchers concluded that in the Upper Midwest the nitrogen in radish could not replace fertilizer.
Ruark commented, "Radish grows well when planted in late summer and traps a lot of nitrogen. But the way it decomposes doesn't result in a nitrogen fertilizer benefit to the next crop. We don't know exactly why. We were hoping it would provide a nitrogen benefit, but alas, it did not."
What happens to the nitrogen? The decomposition pattern of radish needs to be explored more fully to learn more. And perhaps, Ruark said, radish could be more beneficial if mixed with a winter-hardy cover crop.