While much of the field work around Minnesota has slowed, the rising cost of fertilizer and uncertainty surrounding fertilizer availability for spring 2022 may have some still thinking about late fall application options. Urea can be a tempting option for fall application because it is easy to apply with a floater, but is this a good practice?
Urea is a dry source of nitrogen that can be lost to the atmosphere via ammonia volatilization and to water via nitrate leaching. Risk of volatilization is more immediate and can occur regardless of soil temperature. If urea is applied on the soil surface, or even with shallow incorporation, it is at risk of loss via volatilization as it dissolves.
Many people think that as long as it is 50 degrees or colder, urea cannot be lost to water. While products containing NBPT, such as Agrotain, can help delay this process, there still can be risk of loss if urea is not incorporated into the soil mechanically or with at least a quarter inch of rain.
If soils are frozen or nearly frozen, no commercial fertilizer should be applied. Fertilizer needs to be incorporated or have enough time to react with the soil in order to reduce the risk of N loss.
While most soils were not fully frozen through all of November, dry soils with little rainfall can prevent urea from breaking down, even when soils are permeable enough to allow fertilizer to move into the soil. And when the soil freezes, urea will stay on the surface until the spring when runoff risk is highest.
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While the conversion of urea to nitrate can be rapid, there are other risks of loss before that point. When you combine potential concerns with ammonium volatility and potential leaching or runoff losses of nitrogen, urea is much riskier to apply the later in the fall you go.
While there are products out that can reduce the risk for loss, they do not last forever and may not be bulletproof. Urea is best applied and incorporated to a depth of three to four inches to prevent loss and should be applied in a way that there is no risk of the product dissolving and leaving the field where it is needed.
Can you apply fertilizer over snow when the forecast is that it will melt within the next few days?
While we do not wish to promote that practice, the answer is that it depends on where the water will go. If the soil is not frozen underneath the snow, and there is no risk that the snowmelt will run off the field, the overall risk for application over snow will be very low.
I would not suggest trying this with more than one or two inches of snow on the field and without knowing that the soil is not frozen underneath, and I would not suggest this practice with urea due to concerns with volatility.
The simple answer is that this is not a suggested practice and there is some risk with this type of application.