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Top-dressing/Side-dressing N (06/18/20)

There are always farmers who top-dress or side-dress as part of their normal growing season plan.

There are always farmers who top-dress or side-dress as part of their normal growing season plan. These are usually farmers who have learned that high-clay soils and sandier soils, particularly in the wetter eastern regions of ND, are prone to early-season N loss through leaching and/or denitrification. This year, there are many more farmers who wisely planted rather than wait for N fertilizer delivery and application.               

For small grains and other solid-seeded crops, sooner is better than later, with broadcast NBPT treated urea or streamed UAN. For row crops, particularly corn, but also sunflower, the side-dress could go on at any time this year. Usually, it would be wise to wait until V5 to begin application, but with the late planting the calendar is already on the tail end of the traditional wet season, so the risk of leaching and denitrification with early side-dress is less than normal.

If rainfall has been normal to drier than normal, using the N recommendations in place, less soil nitrate-N test, previous crop credits, and less any N applied to date would be a reasonable rate to use. If rainfall has been above normal, then some loss of N has already happened, so the question is what rate to use? There are two methods that might be considered beyond a guess. The first would be a pre-sidedress soil test. This test was developed in Iowa and probably performs the best in Iowa. The test involves taking soil samples 1-foot deep in a transect across the corn rows and analyzing for nitrate. The Iowa testing protocols do not account for spatial variability, which I spent the first 10 years of my NDSU career studying. There will be spatial variability within the field, so taking one sample in a small area to characterize an entire field is unreasonable (this is also a major problem with the y-drop nitrate test box protocol). To characterize the field, the field should be zone sampled similarly to what should be done for a preplant N application. The reason I haven’t spent more time than necessary on the pre-sidedress soil test calibration is that I make it a habit not to research anything I don’t think a farmer/consultant would do. But if one took the time to sample correctly, the pre-sidedress N test might have some value, although the 4 years of work I conducted were not very supportive of the results.

The second method, and one I prefer to recommend, is the use of an N-sufficient strip in the field that will show differences between an area with sufficient N and any areas that may have N shortages. The preferred time to make the strips (~ 100-200 feet long, width of an application, an extra 150-200 lb/acre N) would be at preplant base-N application time. However, that time is now past, so what to do? The small grains are up, but the row crops are mostly not up. In the small grains, stream the normal N rate or 50 lb/acre more with an ATV sprayer or any sprayer as soon as possible in a strip about 100 feet long in an area generally typical of the field soil and in an area that can easily be revisited. Then either watch the greenness of the strip and compare it to the field, or use an active-optical sensor to provide a number of reflectance that would be better than the eye to provide an indication whether topdress would be of value. For corn, again with an ATV dry spreader or any spreader, apply 150-200 lb/acre N, or whatever the original difference in preplant and side-dress might be plus about 100 lb N/acre to a strip about 100 feet long, then watch the strip compared to the rest of the field. Ideally, an active-optical sensor, such as the GreenSeeker or the Holland Scientific Crop Circle sensor would be used, and NDSU has published corn algorithms available to direct rates in different parts of the field either by active site-specific N application, or by mapping zones, then applying the N in separate operations.

Another benefit of the N sufficient strip is that if the strip is more yellow or less green than the rest of the field, that indicates S deficiency, and S should be applied immediately; then the N needs should be assessed about a week later after a rain.


Dave Franzen

Extension Soil Specialist


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