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.