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Should you apply nitrogen in-season?

This article by Dan Kaiser, Extension soil fertility specialist, was originally posted on the MN Crop News Blog.

As we start moving into June, there is an increased risk for nitrogen loss through denitrification. Denitrification is a process by which nitrate is chemically reduced and lost as a gas. Denitrification as a loss pathway is similar to other nitrogen loss pathways as it is driven by water. For denitrification to occur, nitrogen has to be 1) in the nitrate form; and 2) the soil has to be saturated to a point where there is little or no oxygen and there is carbon as an energy source for soil bacteria. The potential for denitrification increases as soils warm due to increased microbial activity.

The greatest risk for denitrification each year tends to begin in June as the summer heat sets in. While measuring exact losses can be difficult, the example from Bremner & Shaw 1958 illustrates how potential loss adds up as soils warm and stay saturated for an extended period of time. What should be noted is that the data shows loss as a percentage of fertilizer applied. Thus, the total loss will depend on the rate of N applied and the amount which has converted to nitrate.

 Soil saturated for 4 days  Soil saturated for 10 days
Soil Temperature   Amount of denitrification (% of NO3-  applied)  Amount of denitrification (% of NO3-  applied)
 50°F  3%  6%
 60°F  6%  12%
 70°F  12%  26%
 77°F  20%  43%
Source: Bremner, J.M., and K. Shaw. 1958. Denitrification in soil. II: Factors affecting denitrification. J. Agric. Sci. 51:40-52.

If soils are saturated and loss potential is large enough, then supplemental N may be required. Before making a decision to apply more N, determine if yield potential is limited due to waterlogged soils or other growing season conditions up to this point. Yield loss may be severe enough in some situations where additional N may not recover lost yield. Low areas of fields where there was water ponding may not be worth an application of N, but other areas in the field may still warrant some N application.

There are two options which could be used to determine if your soils warrant additional N fertilizer. The first is the supplemental N decision worksheet, which helps determine whether conditions have favored N loss using a series of three questions. The second option is taking a pre-sidedress nitrate test. The issue with both methods is that information is lacking in how much fertilizer should be applied if either indicate sufficient loss of N. Generally, 30-40 lbs. of additional N is sufficient as a supplemental application, but in cases of severe denitrification, more N may be needed. Trying to maximize economics should be a priority, so keeping rates reasonable is important to keep costs as low as possible.


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Support for this project was provided in part by the Agricultural Fertilizer Research & Education Council (AFREC).
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