Fall Applications Before a Wet Winter

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November and December this last year was warm and wet for winter.

For farmers, one of the biggest challenges is planning for the weather, which can be difficult when trying to stop nutrients from leaving a field.

The Department of Agronomy at Iowa State University says fall application of nutrients should be done after the ground has dropped below 50 degrees. At that temperature, the biological process of converting ammonium nitrogen to nitrates, called nitrification, is really slowed down. But this year, after soils got to that temperature they warmed again.

If nitrogen is in the soil as ammonium then the wet conditions won’t move it as easily, but if it gets converted water can take it away.

But that doesn’t mean applied nitrogen was lost this winter ISU Extension Dr. John Sawyer says, “Even though they were thawed an not frozen, the soils were cold. Denitrification would not be going on because it’s a biological process so compared to a summer or springtime event when soils were warm, we would not have expected denitrification losses, but we could have had leeching of nitrogen.”

Leeching is losing nitrogen by water going down through the soil. It’s different from runoff, which is when water runs on top of the soil. According to Sawyer, runoff does not lose much nitrogen; however, it does create erosion and causes phosphorous concerns.

While there isn’t much landowners can do to change the weather, the Department of Agronomy does have guidelines for safer times to apply nutrients: the late fall. Sawyer says only earlier fall applications could have lost out on nutrients.

Even then, he says producers shouldn’t assume their fertilizer is gone, “Producers should wait and see how things go in the spring use some testing in the spring. Perhaps decide on how those applications are performing and not just make a quick decision to apply more nitrogen. I think we’d rather see them waiting and see how the springtime goes.”

Fall application is an important tool for farmers who have limited time and resources while planting in the spring or have too steep of hills to apply fertilizer during the growing season.



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