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.