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Thinking of getting an early start on fertilizer applications? Here are a few things to consider.

nitrogen fertilizer minnesota urea

By: Dan Kaiser, Extension nutrient management specialist; Fabian Fernandez, Extension nitrogen management specialist; & Jeff Vetsch, U of M SROC researcher

The record warm winter has been raising questions about early spring fertilizer applications. With soils not frozen, yet fit enough for fieldwork, what are the risks of early fertilizer application?

Most fertilizers are water-soluble and will dissolve readily at this point. If a soil is not frozen, any fertilizer that dissolves will react with the soil and stay where it was applied. Therefore, applications of phosphate and potash fertilizers present little risk for loss if they are not incorporated in the soil, unless excessive quantities of water are running off the soil surface. In addition, because the nitrogen in MAP, DAP, and AMS is in the ammonium form, it is not subject to volatility.

In cold soils (less than 50°F), the accumulation of nitrate nitrogen from fertilizers is low. Nitrification is still possible if soils are not frozen, but the amount of nitrate accumulation should be minimal as this is a microbial-driven process. Similarly, denitrification, which is one of the processes by which nitrate can be lost, is low in cold soils because it is done by microbes. On the other hand, while N loss from nitrate might be low at this point, nitrogen loss through ammonia volatilization is a bigger concern because it occurs readily even at cooler soil temperatures.

Is early urea application a bad idea?

Volatilization of ammonia is greatest with urea that is not incorporated. Urea molecules undergo a process called hydrolysis that splits the urea into ammonia gas and carbon dioxide. If this process occurs near the soil surface, the soil might not be able to retain ammonia and it would be lost via volatilization. Incorporation of urea to a depth of at least three inches within four days of application is suggested to limit volatilization. There is research showing that the risk of ammonia volatilization is greater when urea is surface-applied in early or late winter compared to late April or early May. While soil seems dry, there is enough moisture to dissolve (hydrolyze) the urea granules and start ammonia volatilization.

Liquid forms of N such as 28% or 32% UAN should also not be applied too early. Urea ammonium nitrate solutions contain half of the N as urea so volatility can be an issue. A larger problem is that a quarter of the N in UAN is already in the nitrate form at application, and nitrate is subject to loss if there is excess water.

While urease inhibitors are effective to reduce ammonia volatilization for up to 14 days, they will not be effective long enough if urea is applied in March and left on the soil surface. It takes about a quarter-inch of rainfall to incorporate urea left on the soil surface deep enough into the soil to protect it from volatilization. So, a urease inhibitor is effective only if it lasts long enough for urea to get incorporated. It should also be noted that nitrification inhibitors such as DCD and Instinct will have no effect on ammonia loss because ammonia is produced before nitrification can even occur.

What about anhydrous ammonia?

Since anhydrous is knifed into the soil, the risk for volatilization loss is substantially less. However, application in March may warrant a nitrification inhibitor. Since soils are still cool, the overall effect of the nitrification inhibitor will be great compared to application closer to planting. This is because of two reasons: first, the inhibitor last longer because degradation is slower at cooler temperatures now compared to closer to planting, and second, the time between application and nitrogen uptake by the crop is longer now, so there is a longer window of time for nitrogen loss to occur.

Other considerations

With our current field conditions, it would be a good idea to consider soil testing in fields that haven’t been tested or where you think you might have carryover nitrate. In continuous corn or fields with a manure history, a two-foot soil sample analyzed for nitrate might provide some insight into fields where some nitrate can be credited for this year’s crop. It would also be a good time to consider other things like lime application if you have been putting it off.

The main thing to avoid right now is application of urea or UAN as there is greater risk for loss with those fertilizers when we still have two months before there is a crop in the field growing enough to utilize these nutrients. One thing about urea and UAN is there is a lot of flexibility with when these fertilizers can be applied after planting. There is still plenty of time for N fertilizer application this spring, so there is no need to jump the gun and apply too early. Also, this is a year where, because of the low nitrogen loss potential so far, there might be more nitrogen already in the soil than is typical. So, crops will have all the nitrogen they need for a while after planting, which should give you more time into the growing season to make your nitrogen application.

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