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Nitrogen management this fall

Published October 3, 2012

URBANA – Many cleared fields in Illinois are ready for fall operations due to the very early harvest and the chopping, plowing-under, or harvesting for biomass that took place earlier in the summer in fields that did not produce grain. University of Illinois assistant professor of crop sciences Fabián Fernández said that decisions on nitrogen applications this fall rank high in the list of producers’ priorities because the application can affect profitability and the environment.

Fernández offers some guidelines for nitrogen application. “Although recommended management practice may not work very well every year because of environmental conditions beyond our control, I emphasize that these guidelines, if followed, will ensure the greatest chance to protect your nitrogen investment and at the same time enhance environmental protection,” he said.

Nitrogen sources

Anhydrous ammonia (NH3) and ammonium sulfate ([NH4]2SO4) are the only sources recommended for fall application. Ammonia transforms quickly to ammonium (NH4+), and nitrogen in ammonium sulfate is already in the ammonium form. Ammonium is adsorbed into the exchange sites in soil particles and organic matter, protecting it from leaching.

Nitrogen sources containing nitrate (NO3-) should not be used in the fall. Nitrate does not become adsorbed into exchange sites in the soil and can be easily leached or denitrified long before the plants are ready to use it. Common fertilizers that contain nitrate include ammonium nitrate (NH4NO3) and urea ammonium nitrate (UAN).

Another common nitrogen source is urea (CO[NH2]2), which converts to NH3 and then to NH4+ within a few days of application. However, research has demonstrated that this fertilizer should not be used in the fall because, compared to anhydrous ammonia, it is more likely to be lost before rapid nutrient uptake by the crop the following spring.

The same can be said of polymer coated ureas. While the coating initially protects urea, it can diffuse out of the granule too early and the loss potential is higher than for anhydrous ammonia. This is especially true when it is applied too early in the fall or when the coating is damaged during handling, allowing for quicker dissolution of the urea granule.

One of the benefits of anhydrous ammonia is that it kills the nitrifying bacteria that transform ammonium to nitrate at the application point. Moreover, the reaction of ammonia with water to form ammonium creates an alkaline (high pH) environment within the ammonia retention zone, which inhibits activity of nitrifying bacteria.

However, these effects are temporary. Including a nitrification inhibitor with anhydrous ammonia applications can lengthen the period of bacterial inhibition. Research indicates that nitrification inhibitors, such as dicyandiamide (DCD) and N-serve, can protect fall nitrogen against loss and increase the amount of nitrogen present in ammonium form the following spring.

"As with most practices, using a nitrification inhibitor might not pay every year,” said Fernández. “If the following spring is dry and cool, the inhibitor might not be as effective at enhancing ammonium recovery. However, this practice will, overall, offer the best chance to protect your nitrogen investment and the environment.”    

Ammonium sulfate is an excellent source for no-till fields where broadcast applications are preferred. It is always best to apply it before soils freeze so the fertilizer can dissolve and rain can incorporate it into the soil. In fields with minimal slope (less than 5 percent) with low run-off potential, ammonium sulfate can be applied on frozen ground because volatilization losses are unlikely.

“Ammonium sulfate is more acidifying than other nitrogen sources, so make sure to keep an eye on soil pH,” Fernández advised. “As a general rule, 5 pounds of lime is needed to neutralize 1 pound of nitrogen from ammonium sulfate compared to 2 pounds of lime per pound of nitrogen from anhydrous ammonia.”  

Organic fertilizers derived from animals (manure, poultry litter) are good fertilizer sources that can be used in the fall. These products supply nitrogen as well as phosphorus, potassium, and other crop nutrients, and are often less expensive than inorganic fertilizers.

Time of application

When the harvest is early, soil temperature has a significant impact on the efficiency of fall nitrogen applications and the effectiveness of nitrification inhibitors. Nitrifying bacteria are active until soils freeze (32 degrees Fahrenheit), but their activity is greatly reduced once soil temperature goes below 50 degrees Fahrenheit.

“This is why I recommend that the start of fall nitrogen applications be directed by soil temperature and not by calendar date, when harvest is done, or any other consideration,” said Fernández. The same guideline applies for anhydrous ammonia, ammonium sulfate, and manure/organic fertilizers.

“The efficiency of nitrification inhibitors also decreases with warm temperatures. Higher temperatures result in faster breakdown of the molecule responsible for inhibition of nitrifying bacteria,” he continued. “The cooler the temperature, the greater the efficiency of the inhibitor, and the greater chance ammonium does not convert to nitrate.”

Fernández noted that every year, anxiety levels rise when soil temperatures are falling steadily but not reaching 50 degrees Fahrenheit. However, in most years, the 50 degrees Fahrenheit temperature allows for nitrogen applications before soils become too wet or frozen. There is no need to increase the risk of nitrogen loss by starting applications too early.

“Also, while applying once temperatures are 50 degrees Fahrenheit does not automatically prevent nitrogen loss, it does provide a better chance to protect your investment,” Fernández said.

Air temperatures in Illinois can vary substantially in the early fall. Even if air temperatures are reaching 50 degrees Fahrenheit, a significant bounce back to warmer temperatures before the second week of October in northern Illinois, and the third week in central Illinois, is very likely. On average, soil temperatures reach 50 degrees Fahrenheit and continue to go down in the first week of November in central and northern Illinois. Daily maximum 4-inch bare-soil temperatures for Illinois this week have been in the mid to upper 60s.

Up-to-date soil temperatures can be found at http://www.isws.illinois.edu/warm/soiltemp.asp. However, because soil temperatures can be influenced by a number of factors (such as residue cover, soil color, and drainage), it is always best to monitor soil temperatures in individual fields before applying nitrogen.

Where can fall nitrogen be applied?

Fall nitrogen application should not be done south of a line roughly parallel to Illinois Route 16 because temperatures do not stay cold enough for long enough during the winter. In areas near this boundary, evaluate soil characteristics to determine whether fall application is appropriate.

“Soils with high potential for nitrate leaching in the fall or early spring, such as sandy soils or those with excessive drainage, should not receive fall nitrogen applications,” Fernández said. “Also, regardless of location, soils with high potential for nitrate leaching or soils that are very poorly drained should not receive fall nitrogen applications.”

Due to the length of time between application and utilization by the crop, manure and other organic nitrogen sources should be applied as far as possible from environmentally sensitive areas, such as on steep slopes and near bodies of water. “If the application cannot be done in late fall, do not apply on frozen soils in the winter; it is better to wait until spring,” Fernández said.

How to apply nitrogen

Fernández stressed the importance of making sure that soil conditions are fit for the application when applying anhydrous ammonia.

“Ammonia losses to the atmosphere can be the result of soils that are too dry or too wet, as the application knife tracks may not seal properly,” he explained. “When soils are dry, increasing application depth or reducing application rates can help minimize volatilization losses. In wet soil conditions there is little that can be done. If you use manure, poultry litter, or  other animal-derived fertilizers, make sure to incorporate them in the soil to avoid nitrogen volatilization.”

How much N

To determine the economically optimal nitrogen rate at various corn and nitrogen prices, use the Corn Nitrogen Rate Calculator at http://extension.agron.iastate.edu/soilfertility/nrate.aspx.

“While the calculator is designed to help you make the most profitable decision for nitrogen management, it does not account for carryover nitrogen that the crops might not have used because of dry conditions,” said Fernández. “Also, if you applied manure or the soil has high potential for nitrogen mineralization, such as a field coming off of alfalfa, you will need to adjust the values from the calculator to reflect what will be available next year.”

After determining the amount of nitrogen needed, it is important to remember that it is not necessary to apply the entire amount in the fall. Many fields still have high nitrate levels because of the drought, and an undetermined amount will be present for the next crop. In such cases, the remaining nitrogen should be enough to allow the plant to get started until sidedress time. If nitrogen is not present because of excessively wet conditions last spring, chances are that a fall nitrogen application could suffer similar losses. A cautious approach would be to apply some of it in the fall and wait until spring to apply the rest.

Applying nitrogen in the spring or splitting the application could be beneficial for several reasons. Supplying nitrogen closer to when the plant will need it can increase nitrogen-use efficiency because there is less chance for leaching or denitrification. Also, nitrification inhibitors are more efficient when smaller nitrogen rates are used in the fall. An ongoing study showed that, over three years, fall applications reduced yield 17 percent relative to pre-plant applications done within 3 weeks of planting. The difference in yield, averaged across nitrogen rates, was 23 bushels per acre less with fall than pre-plant applications.

“We are conducting the study this year, but I currently do not have yield information,” Fernández said. “I suspect differences this year might not be as large because there was very little nitrogen loss potential in the spring and because the drought was more limiting than any other factor.”

Use caution

Anhydrous ammonia is under a lot of pressure inside the nurse tank, and when released it reacts quickly with water. If it comes in contact with skin, eyes, or mucous membranes, ammonia will cause dehydration and burns. Use extreme caution when handling this material.