Apply nitrogen or plant corn?
If N is going to be sidedress applied, then rates can be adjusted from results of the late spring soil nitrate test (LSNT). Soil samples, 0-12 inch depth, are collected when corn is 6-12 inches tall with rate adjustment based on the measured nitrate-N concentration. Using the LSNT could be especially helpful this spring when there is question about N supply in manured fields. The large rainfall this spring has moved carryover nitrate deeper in the soil profile. A concern with the LSNT this spring is that it will miss that nitrate and therefore over-estimate needed application.
Late sidedress N considerations
If corn becomes too tall for normal sidedressing equipment, it is possible to use high clearance equipment to apply N. The N source typically will be UAN solution, with equipment available to either dribble the solution onto the soil surface with drop tubes or shallow inject with coulter-shank bars (coulter-disk injected) or dry urea, which can be broadcast spread across the top of corn.
Research in Iowa has shown corn can respond to mid- to late-vegetative growth stage N application when there is deficient N supply, but there can be loss in yield potential. Reduced yield occurs more frequently when soils are dry at and after application (applied N not getting into the root zone) and with severe N stress. Best responses occur with sufficient rainfall shortly after application to move N into the active root zone.
If attempts to get N applied preplant or early sidedress have failed, or there are concerns about N supply from prior fertilizer or manure applications, then mid- to late-vegetative-stage application can be a helpful rescue. If possible, have some non-N limiting (approximately 50 percent more than normal rate) reference strips or areas in the field to use for comparison. These areas can be used to visually determine if corn would respond to additional N, or as a check to see if earlier N applications or carryover N is not sufficient. These reference areas are also needed for N stress sensing tools (such as chlorophyll meters or canopy sensors) to help guide application rates. These reference areas should be planned and N applied early in the season, or be field areas that are known to be non-N deficient. Plant and canopy sensing can begin when corn is at approximately the V9-V10 growth stage. If late N application is needed, it should be applied as quickly as possible and not later than the tassel stage.
- Plant corn when conditions are fit, don’t rush.
- Fertilize first if it does not delay corn planting.
- In other situations, sidedress N.
- Make certain needed N fertilizer products will be available.
- Make certain sidedress equipment will be available.