Nitrogen application is one of the many important decisions corn  and soybean  farmers are making now – a decision that impacts both profitability and the environment. Fabian Fernandez, University of Illinois Extension  specialist in plant nutrition and soil fertility, reviews important guidelines that can help protect this nitrogen (N) investment while enhancing environmental protection.
What to apply
The only recommended sources of inorganic N for fall are anhydrous ammonia (NH3) and ammonium sulfate ([NH4]2SO4). This N is adsorbed onto the exchange sites in soil particles and organic matter, and is protected from leaching, Fernandez says. By contrast, N sources containing nitrate (NO3-) should not be used in the fall because nitrate can be easily leached or denitrified. Fertilizers  containing nitrate include ammonium nitrate (NH4NO3) and urea ammonium nitrate (UAN).
Another common N source is urea (CO[NH2]2). This fertilizer should not be used in the fall because it has a greater risk of loss compared with anhydrous ammonia. The same can be said of polymer-coated ureas. While the coating protects urea for a while, urea eventually starts to diffuse out of the granule too early, resulting in high loss potential.
"It is always a good idea to include a nitrification inhibitor with the application of anhydrous ammonia," he says. "Many years of research have indicated that nitrification inhibitors can protect fall N against loss."
As with most practices, the use of a nitrification inhibitor might not pay every year, Fernandez adds. However, they will reduce loss overall. While an inhibitor represents an added cost, a reduction in N efficiency due to losses plus the environmental degradation linked to N loss, also represent added costs. Farmers must carefully consider all these factors when deciding to apply N in the fall.
When to apply
Soil temperature can significantly impact the efficiency of fall nitrogen applications. Nitrifying bacteria are active until soils freeze (32° F), but their activity is greatly reduced once soil temperatures drop below 50° F. For this reason, Fernandez says the start of fall N applications should be directed by soil temperature and not by date of year. This guideline applies equally for anhydrous ammonia, ammonium sulfate and manure/organic fertilizers that can be used in the fall.
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. The cooler the temperature, the greater the efficiency of the inhibitor and the greater the chance that ammonium does not convert to nitrate, he says.
"In most years, the 50° F temperature allows for N applications before soils become too wet or frozen," Fernandez says. "There is no need to increase the risk of N loss by starting applications too early. Also, applying once temperatures are 50° F does not automatically ensure no N loss, though it does provide a better chance to protect your investment."
Air temperatures in Illinois can vary substantially during the early fall. Even if temperatures are approaching 50° F, historically the chance that they will continue to decline without a significant bounceback to warmer levels is very rare before the second week of October in northern Illinois and the third week in central Illinois. On average, soil temperatures reach 50° F and continue to decrease the first week of November in central and northern Illinois.
Up-to-date soil temperatures can be accessed online .
Where to apply
Because temperatures do not stay below 50° F long enough during the winter in all parts of Illinois, fall N application should not be done south of a line roughly parallel to Route 16. In areas near this boundary, evaluate soil characteristics to determine whether fall application is appropriate, he said.
Soils with high potential for nitrate leaching in the fall or early spring (sandy soils or those with excessive drainage) should not receive fall N applications. Also, regardless of location in the state, growers should not apply nitrogen in the fall to soils with high potential for nitrate leaching or soils that are very poorly drained.
Application of manure and other organic N sources should be done as far as possible from environmentally sensitive areas, such as steep slopes and bodies of water. If the application cannot be accomplished in late fall, do not apply on frozen soils in the winter.
How to apply
Soils that are too dry or too wet can result in ammonia loss to the atmosphere from anhydrous ammonia because the application knife tracks may not seal properly. If you use manure or poultry litter, incorporate them into the soil to avoid volatilization.
To determine the economically optimal N rate at various corn and N prices, use the corn nitrogen rate calculator .
"Remember that you don't have to apply the entire amount in the fall," Fernandez says. "If you don't like taking big risks, but a fall application makes sense, it may be better to apply some N in the fall and the rest in spring. Also, research has shown better efficiency of nitrification inhibitors when lower nitrogen rates are used in the fall."
Weigh your options
Although N does not have to be applied in the fall, this timing has both economic and logistic advantages, Fernandez says. Unfortunately, because spring weather conditions greatly influence N efficiency, it is impossible to know in any given fall how risky it is to apply N.
"If you decide fall N application is right for you, the guidelines outlined here will help protect your N investment and enhance environmental protection," he says.
For more information on fall nitrogen applications, read The Bulletin online .