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Fall Nitrogen: When, Where, How Much, What Kind?

Fall Nitrogen: When, Where, How Much, What Kind?
Be sure soil temps are 50 degrees at 4 in. before applying N sources Nitrate containing fertilizers have potential for leaking Split N application between fall and spring


Are you “leaking nitrogen (N)?” Mark David, a biogeochemist at the University of Illinois (U of I) says the Corn Belt is “leaking N.” For many years he has been critical of the way farmers have been applying N to corn and contends the excess winds up in the Gulf of Mexico. In his latest comments, he says “farmers are not to blame,” and says it is the efficient tile system of the Corn Belt that moves nitrates quickly into the ditches and streams. If you are finished with harvest, and will soon be applying anhydrous ammonia to 2011 corn ground, will some of your investment be lost through “leakage?”


If your combine is in the machine shed and you are antsy to begin fall tillage and N application that is certainly understandable, given the 2009 nightmare. Conditions have been much better this year, but just because you are ready to apply N, U of I Soil Fertility Specialist Fabian Fernandez says the soil may not be ready to receive it. In his Sept. 24 newsletter, he says, “It is no time at all yet for anyone to be in the field applying N.”


Fernandez knows what you suffered through last fall and counseled many farmers who needed advice on when and how to apply N in 2010, because it was not done last year. He is aware the time window is short, but he says, “Managing N well is important, because this nutrient is both one of the most expensive inputs in today's farming operations and one that poses environmental concerns.”


What type of nitrogen to use


Ammonium is a positively charged ion with four hydrogen atoms and is tightly held by the negative charge of soil particles. But with microbial action in the soil, hydrogen atoms are replaced by oxygen atoms and it becomes a negatively charged nitrate, which is repelled by the other negatively charged soil particles. That allows it to leach into your tiles, and “leak” as Mark David contends. The solution is to apply the ammonium after the microbes in the soil begin to slow their activity. Fernandez says the threshold is generally below 50º F, but they remain active until the soil reaches 32º F. Those are considered temperatures taken at the 4-in. level, so Fernandez says delay your ammonium application until your soil thermometer drops below 50º at the 4-in. level. So timing of the application is one way to keep the N in the soil, and not lose it through leakage.


Another way is to vary the N source. The preferred method is anhydrous ammonia because it seeks water in the soil for bonding to become ammonium and is slow to convert to a nitrate that will leak. It can be applied with a nitrification inhibitor, such as N-serve, which blocks the action of the microbes. Successful application of anhydrous ammonia depends on a good moisture level in the soil, which is sufficient to bond with the ammonia, but which is not too wet to prevent the knife furrow from closing and allowing it to escape into the air. Fernandez says he strongly encourages use of inhibitors when ammonia is being applied in the fall.


An alternative is ammonium sulfate, which is a good source of N for no-till fields, but must be applied at temperatures below 50º F so microbial action will not convert it to a nitrate and it will leak away. The ammonium will attach to soil particles and become a stable source of N. However, forms of N, such as ammonium nitrate or UAN, are in forms that are less stable and will not attach to soil particles. Those will leak into the tile lines when applied in the fall.


Urea converts to the stable form of ammonium within a few days of application, but it is less effective as a fall applied source of N than anhydrous ammonia. Its lower efficiency is due to the potential for it to become a nitrate in the spring before the crop can use it, and leak away.


How much nitrogen to use


How much N should you apply? When it was cheap, a lot was applied. When it became expensive, less was applied, and some fields did not get any N the past several years. However the question of application rate should be a function of soil type, moisture, the price of corn and the price of the N. Those factors are all included in a decision aid calculator. Fernandez says split your application between fall and spring, “it is important to remember that it is not necessary to make the entire application in the fall. Some producers might find it beneficial for their production system to apply a portion of the total rate in the fall and reserve the rest for a later application in spring.”


Any final Fernandez tips to make the most of your nitrogen dollar?

1) Application in the spring, close to the time of rapid uptake, maximizes yield because there is less chance for leaching or denitrification.

2) Most often, though, under normal spring conditions there is little or no difference between fall and spring times of application.

3) Fall applications have both economic and logistic advantages. Soil conditions are typically more conducive to application, there is more time available than during the busy planting season, equipment and labor are better distributed, and often there are price incentives to buy anhydrous ammonia.

4) The spring typically is wet, and soil compaction, especially for manure application, is of greater concern. Also, waiting until spring to apply fertilizer can delay planting, damage crops, and delay application of fertilizer to meet early nutrient uptake needs of the crop.

5) If you don't like taking big risks but a fall application makes sense, it may be better to apply part of the N in the fall and wait until spring to apply the rest.



Fall application of N should wait for soil temperatures at the 4-in. market to drop below 50º F. Do not apply nitrate containing fertilizers because of the potential for leakage, since nitrates are a form of ammonia which does not bind with soil particles. Make a split application of N between fall and spring, to take advantage of time in the fall, but also ensure some of it will be available at the time of crop uptake.

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