Fall N Availability: Do These Scenarios Apply To You?

 

This week we’ve been exploring how the nitrogen cycle and rainfall timing affect fall N availability. Keeping this in mind, consider these real-world scenarios on the likelihood of nitrogen loss from fall-applied fertilizer.

 

 

Anhydrous ammonia is fall-applied in early October when soils are at 60℉. Soil temperatures don’t drop below 50℉ for another 3 weeks, and rainfall is 50% above average until the soil freezes. Nitrogen loss is likely to be high in this scenario. Much of the applied nitrogen has converted to nitrate form by November and can move down through the soil with the leaching rainfall.

 

Anhydrous ammonia is fall-applied in early November when soils are below 50℉. Rainfall is 50% above average until the soil freezes, but the soil never gets above 50℉again until the following spring. Nitrogen loss is likely to be low in this scenario. While leaching has likely occurred with the above-average rainfall, very little ammonia is likely to have converted to the vulnerable nitrate form before soil freeze-up. Research conducted in south-central Minnesota on the timing of swine manure application over two years found that November applications out-yielded September applications in corn by an average of 12 bushels/acre.

 

Anhydrous ammonia is applied with a nitrification inhibitor like N-Serve that delays the conversion of ammonium to nitrate in early November when soils are below 50℉. Soil temperatures don’t get above 50℉ until the following April. Nitrogen loss is likely to be low in this scenario. Very little ammonia is likely to have converted to the vulnerable nitrate form before soil freeze-up. This represents the lowest-risk scenario you can have with fall application of nitrogen. A 15-year study conducted in south-central Minnesota on the timing of nitrogen application found that late October application of anhydrous ammonia with N-Serve out-yielded late October application of anhydrous ammonia without an inhibitor by 9 bushels/acre. However, over this same 15-year period, spring application of anhydrous ammonia without N-Serve out-yielded the late fall application with an inhibitor by 3 bushels/acre.

 

Anhydrous ammonia is applied with a nitrification inhibitor in mid October. Soils remain above 50℉ for the next two weeks, and then stay below 50℉ until the following April. Nitrogen loss is likely to be moderate in this scenario. This is a situation where the use of a nitrification inhibitor is very likely to produce higher yields than if an inhibitor was not used.