Nitrogen Management This Fall

By CnAgri2012-10-11 19:54:22 Print

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 Fabi醤 Fern醤dez 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.

Fern醤dez 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.

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 Fern醤dez. "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," Fern醤dez 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.

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 Fern醤dez. 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."

Fern醤dez 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," Fern醤dez 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 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.