One of the hardest management tasks in a wet growing year, like the one much of Kansas experienced this year, is managing nitrogen to minimize loss while keeping the nutrient near the corn root zone for maximum crop uptake.
At an Agco Crop Tour event near Haven in late July, Agco agronomist Jason Lee shared some ideas on nitrogen timing, form and application choices to minimize the three main causes of loss: leaching, denitrification and volatilization.
Severe weather events, such as the wet conditions and flooding of the 2019 growing season, will dramatically increase the potential for major nitrogen loss, especially through leaching and denitrification.
“Leaching is the major pathway for nitrogen loss,” Lee said. “Regardless of nitrogen fertilizer source, after application, eventually microbes in the soil will convert ammonium into nitrate. Once in the nitrate form, the molecule will no longer bind to the soil, and therefore moves freely down through the soil profile with water and past the root zone. That is leaching.”
In-season nitrogen applications and using a stabilizer can help reduce the amount of nitrogen lost in that fashion.
Losses through denitrification tend to occur most where soils are heavily saturated for long periods of time, he said. The water-logged soil limits the oxygen needed by soil bacteria, so they use the nitrate for respiration and convert it into a gas that is lost into the atmosphere.
Temperature also matters. Prolonged saturated soils, coupled with high temperatures, will increase microbial activity and intensify the denitrification process leading to additional loss. Minimizing ponding water through drain tile or avoiding excessive compaction can help reduce loss through denitrification.
For farmers who broadcast urea, there is also a risk of loss through volatilization, especially on soils with high pH and high surface residue. “It is ideal to mechanically incorporate urea or apply right before a small rain event to minimize loss,” Lee said.
No matter what the nitrogen source is, applying it close to periods of peak crop demand is important, he emphasized. For corn, that period begins shortly after the sixth leaf collar stage. Sidedress and split-applications will generally be the most consistent way to minimize nitrogen loss.
Lee also advised producers to do their own nitrogen rate studies to determine optimum rates for their specific soil types and regions. Selecting the optimum nitrogen rate for highest yield and greatest return on investment is difficult, and it can vary substantially from year to year and by field to field. He recommended testing over several years and in multiple fields to get a solid average, because weather conditions during the growing season will ultimately determine how much nitrogen was available to the crop.
“Right before harvest, you can also do a stalk nitrate test,” Lee said. “Clip stalks and test for nitrates. If stalk nitrates are excessively high, you actually applied more than the crop needed. If stalk nitrate levels are very low, you didn’t apply enough.
“If, after a few years, your stalk nitrate tests are consistently too low or too high, then consider adjusting your nitrogen rate accordingly.”