Issue No. 6, Article 8/May 2, 2008
Sidedressing Nitrogen for Corn
Due to this spring's wet soil conditions, we are off to a late start with corn planting in Illinois and much of the Midwest. In addition, with the high price of corn many acres originally planned to grow soybean might be planted with corn this year. Whether farmers decided over the winter to plant more corn or whether they were preparing to apply nitrogen (N) in a preplant application, most have not been able to get in the field yet. However important N is to optimizing yield, at this point, when soils become dry enough to enter the field, the priority should be planting, not fertilizing. The reduction of yield potential due to delayed planting is far more important than the benefit of applying N before planting. Thus, application of N at this time will have to be done as sidedress; or, if an RTK auto-steer guiding system is available, the application could be done between rows prior to planting or before seed emergence. However, even with auto-steer, caution should be exercised not to apply N when the soil is still too wet. Trying to apply fertilizer under suboptimal soil conditions can be more detrimental to yield than waiting until soils are sufficiently dry. Soil compaction, which is likely under wet conditions, can limit both root proliferation and the availability of water and nutrients later in the growing season. Also, if anhydrous ammonia is being applied under wet conditions, some N can escape from the soil because of poor closure of the knife track. If the soil dries quickly and cracks when corn is starting to grow, it can damage the emerging plants as ammonia gas escapes the soil. So in most situations this year, it will be better to delay N application until after planting, when soil conditions are more favorable.
Planting without applying N will likely not negatively impact corn yield potential since most soils supply ample amounts of N relative to the requirements of young corn plants. Typically there is no loss of yield if N is applied before the 5th-leaf stage. Although sidedress applications this year represent a change of plans for many, this practice provides several advantages over preplant applications. Increased efficiency of N use is possible because N is being applied closer to the time plants will need it, and the window for potential loss is reduced. At the time of sidedress, typically there has been enough time for weed control, so N is being applied to fertilize corn, not weeds. Another advantage, especially at this time of high N fertilizer costs, is that N rates with sidedress applications can be reduced if loss of yield potential has occurred. It is recommended to reduce rates by 20 lb N acre-1 down to 80 lb N acre-1 as the minimum (for very late planting dates) for each week that planting has been delayed from the optimum planting time for your area. Optimum planting dates are April 10 to 15 in southern Illinois, April 20 to May 1 in central Illinois, and May 1 to 10 in northern Illinois.
All common N fertilizers can be used for sidedress applications. However, in rank of preference from most to least desirable, it is best to apply 1) injected anhydrous ammonia or UAN solution between rows, 2) broadcast of solid ammonium-containing fertilizers such as ammonium nitrate or ammonium sulfate, 3) broadcast urea, 4) dribble UAN solution between rows, and 5) broadcast UAN solution. In the case of surface applications, it is necessary to incorporate the fertilizer by rain or irrigation to move N to the root zone. With injection applications it is recommended to apply N between rows to reduce the potential for plant injury. There is no advantage to trying to apply N close to the row since roots will grow into the in-between-row position by the 4th-leaf stage. It is also possible to apply N in every other row instead of every row without negatively impacting yield, because every row will have N applied on one side or the other. Of course, the outside injectors should deliver half the rate since the injector will pass between those rows twice. (See chapter 11 of the Agronomy Handbook for more information on how to apply N in every-other-row schemes.)
Although it is not desirable, if you are concerned that corn might be too big by the time you can make N applications, it is possible to apply N with high-clearance equipment or by aerial application. When doing aerial application it is important to keep rates below 125 lb N acre-1 and to avoid doing the application when the canopy is wet to reduce plant injury. If liquid applications are to be used, the rate should not exceed 10 lb N acre-1. For the time being, though, let's hope we can get all corn planted soon and prepare to sidedress N before the 5th-leaf stage.--Fabián Fernández