Scientists will tell you any applied nitrogen regardless of form and time applied is vulnerable to loss. Soil can't naturally hold any in reserve unless it's in the organic form. Or can it?
I recently spoke to a farmer during an event in Illinois about how he was adjusting his nitrogen (N) program on corn in response to the nutrient loss reduction strategies we are facing. He is splitting applications between fall ammonia, spring and sidedress. Not a bad idea, but I was curious why he doesn't just drop the fall ammonia and then sidedress more. His response was that soil has a natural balance point and can hold a certain amount of N without risk of loss so that is what he applied in the fall.
His rationale for fall N applications went against all my training. I've always thought that any N applied as a commercial fertilizer was at a risk of loss due to leaching or denitrification since ammonium converts to nitrate and nitrate can leach or denitrify. I understand why applying N in the fall works -- it is convenient, soil is in better condition, growers have more time and N is usually cheaper. Yet in my mind, N should be applied in the spring and early summer.
I reached out to a few university experts who study N about this balance point notion. Richard Ferguson, soil scientist at the University of Nebraska put it most bluntly. "No, not really. Any N applied in the fall will have some risk of being lost, as will N applied in the spring."
John Sawyer at Iowa State University agreed there isn't a balance point where an N rate is stable and over that value N can be lost via drainage. "The drainage loss accelerates once you get past the economical optimum rate, but there are increases in drainage nitrate concentration at rates below that EONR (Economic Optimum N Rates --that's another reason why economic optimal rates are important.
"As for denitrification, if nitrate is in the soil then loss will occur if conditions are right for denitrification, regardless of nitrate concentration. Of course more nitrate in the upper [soil] profile will be greater loss potential, but no tipping point [exists]," Sawyer said.
I have also heard that CEC (cation exchange rate) can dictate N Rate and the holding capacity of soil. For example, there is a rule that states you should never apply more nitrogen than the CEC (cation exchange capacity) multiplied by 10 at any one time (CEC x 10 = lbs. N). This rule has always puzzled me and gone against scientific rationale.
University of Illinois agronomist Emerson Nafziger said the concept of an N threshold in the soil has been around for years. The "formula" seems to be that the CEC times 10 is the "most" N that should be applied at once. "I suppose the implication is that the timing would be in the fall, but I think people see it as any one time, no matter when applied," Nafziger said.
Nafziger added that no one has described why soils are "limited" in their ability to hold on to ammonium ions. He added that the first stage for ammonium is as dissolved N in the soil solutions -- where it's vulnerable to leaching, denitrification and nitrification or being picked up by exchange sites. "Of course, ammonium is converted to nitrate fairly quickly, so I guess that would 'clear space' on exchange sites for more ammonium. As a monovalent [can form one bond] cation, ammonium is held pretty tightly on exchange sites though."
If your soil has a CEC of 10 that equates to 100 lbs. of N, which isn't enough to produce a corn crop so it would require multiple applications -- which is a good practice. If anything, the CEC x 10 rule confirms the value of multiple applications on low CEC soils. "It certainly doesn't make much sense for a Southern Illinois field with a CEC of 12 to be restricted to putting 120 lbs. of N on -- they would need to make more than one application, which seems to be the goal anyhow," Nafziger added. "The puzzling part to me is what value this whole thing has and it doesn't address whether the soil has a tipping point for holding N."
Read a study from Purdue on economic nitrogen rates here: http://bit.ly/…
Dan Davidson can be reached at AskDrDan@dtn.com
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