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Earlier warm conditions and dry soil complicate the math on anhydrous ammonia

By: RealAgriculture Agronomy Team

October 12, 2021

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With granular fertilizer prices where they’re at and with uncertainty about supply in ’22, anhydrous ammonia is an attractive option to get nitrogen into the field this fall. But what’s the best way to minimize losses?

Jeff Schoenau, professor of soil fertility, and strategic research chair at the University of Saskatchewan, says that once ammonia is in the soil, it has to react with water to convert into ammonium, the form that can bind to soil particles.

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“What we’re really dealing with this fall is some pretty dry soils, and really what that means is that we’re looking at going deeper in order to ensure that the ammonia gas has the opportunity to react with some water, and be converted to ammonium ions before it reaches the soil surface, and gases off,” says Schoenau.

Schoenau says that nitrification inhibitors can stop the conversion of ammonium into the more easily lost nitrate form, even if for a short period of time. “Certainly the losses of nitrate are accentuated under high moisture conditions, when soils are very wet, through denitrification, and also potentially leaching,” he adds.

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Despite the dry topsoil conditions, that conversion from ammonia, to ammonium, to nitrate can still take place deeper in soil, especially if the fall is warm, says Schoenau. The consequence of that , simply put, is loss — of fertilizer and the time and money it took to apply it.

If soils become wetter later in the fall or in early spring, those are peak periods where loss can still happen via denitrification.

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So, is it better to apply in fall, or in spring?

“I think you need to be thinking in fall applications, making sure that the soil has cooled down, if you’re not using a nitrification inhibitor, cooler soils will tend to slow that rate of conversion of ammonium to nitrate,” says Schoenau. “So we tend to like to make fall applications of anhydrous ammonia when the soil has cooled off, for example, below 10 degrees centigrade.”

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Previous research has also shown that fall applications of anhydrous ammonia showed to be just as efficient as spring applications, provided that the nitrogen stays in the ammonium form.

Structure of the soil also influences ammonia retention, says Schoenau. If the soil is very dry, has poor structure, low organic matter, or is high in clay content, and the soil comes up in clods behind the knife, that’s going to increase gassing off of ammonia, because large pore spaces just allow direct loss.

The same can be said about frozen soils that might have a bit of moisture — if big lumps are coming up at the time of application, it’s also a red flag for potential loss.

“Of course, if the soil is frozen as well, there’s not a lot of good opportunity for that ammonia gas to react with the liquid water it needs to,” says Schoenau.