Dr. Dan Talks Agronomy


By Daniel Davidson
DTN Contributing Agronomist

Sidedressing nitrogen became a lost art for a few years in our history. Many farmers abandoned the practice as farms grew larger and application windows grew tighter.

This isn't the first practice to make a comeback, but it may be one of the most important. The current input cost crunch, the necessity to manage spring nitrogen loss and new application technology are driving the resurgence. The good news is there are other benefits to spoon-feeding nitrogen and providing a fresh supply of nitrogen that includes ammonia.

When you apply N either as ammonia, urea ammonia nitrate (UAN) or urea most of the N is in the ammonia form. When soils warm above 50 degrees, much of that ammonia naturally converts to nitrate. It's the nitrate that is vulnerable to loss from leaching and denitrification. That is why most N management strategies include tactics to keep it in the ammonium form as long as possible.

Charles Shapiro, University of Nebraska agronomist, said there is evidence that today's corn hybrids respond to having N available as ammonia. "Applications of nitrate containing fertilizers provide a source of ammonium for 2 to 3 weeks after application," Shapiro said. "Some agronomists consider ammonium important along with nitrate and some racehorse varieties respond to having ammonium present in addition to nitrate."

Nitrogen is still applied alone in the fall as ammonia or in spring as ammonia, UAN or urea. In the spring, UAN is often added with herbicides as a weed and feed. Growers can be putting down 150 to 200 pounds of N up front before the crop is planted.

Shapiro pointed out that applying too much N that early can drive organic matter decomposition too quickly in the spring instead of letting organic matter decompose more naturally and mineralize N more slowly, like a slow-release fertilizer.

We are often told that adding N helps break down residue that has a high carbon to nitrogen ratio. Growers sometimes apply 30 to 50 pounds per acre of N on cornstalks or wheat stubble to drive decomposition or compensate for the corn-on-corn N penalty.

However, over-applying too much N early can mineralize N more quickly and put that additional N at risk of loss. "Decomposing organic matter too early can dump too much N in the system and lead to more environmental loss," Shapiro said.

Spoon-feeding N in doses -- in the fall, spring, at planting and sidedressing in-season -- may be an important element in soil health. Soil microbes need four things: 1) Good soil structure; 2) Good aeration; 3) Labile (active) soil carbon; and 4) A pool of nitrogen. If the soil is moist and well-aerated, the microbes will feed and break down residue and mineralize a supply of nutrients as long as there is some nitrogen and labile carbon (the preferred source of carbon for energy) present.

The benefit of healthy soil with a high level of microbial activity is its ability to recycle and mineralize nutrients. So having a fresh supply of nitrogen available longer through the summer may maintain higher levels of microbial activity. Unfortunately, as we move into July and then August these supplies naturally diminish and so does microbial activity.

In addition to the economic benefits of higher yields with spoon-feeding, this practice has positive environmental implications. The less nitrogen fertilizer applied and the closer to time of usage by plants, the less chance there is of unused nutrients finding their way into water supplies. Along the way microbes are being fed, enhancing microbial activity and nutrient recycling.

Yes, sidedressing puts additional trips across the field on our already crowded list of summer chores. However, spoon feeding goes down easier when you consider that more N is being kept for the crop, it protects the environment and feeds the microbes critical to soil health.

Dan Davidson can be reached at AskDrDan@dtn.com

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