Nitrogen application is one of the many important decisions growers 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 investment while enhancing environmental protection.
What to apply
The only recommended sources of inorganic nitrogen for fall are anhydrous ammonia and ammonium sulfate. This nitrogen is adsorbed onto the exchange sites in soil particles and organic matter, and is protected from leaching, Fernandez says. By contrast, nitrogen sources containing nitrate should not be used in the fall because nitrate can be easily leached or denitrified. Fertilizers containing nitrate include ammonium nitrate and urea ammonium nitrate.
Another common nitrogen source is urea. 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 adds. "Many years of research have indicated that nitrification inhibitors can protect fall nitrogen against loss."
As with most practices, the use of a nitrification inhibitor might not pay every year. However, they will reduce loss overall. While an inhibitor represents an added cost, a reduction in nitrogen efficiency due to losses plus the environmental degradation linked to nitrogen loss, also represent added costs. Farmers must carefully consider all these factors when deciding to apply nitrogen 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 nitrogen 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 said.
Up-to-date soil temperatures can be accessed at www.isws.illinois.edu/warm/soiltemp/displaymap2.asp?day=0&data=bstmax.