When considering adjusting your corn nitrogen program for dry weather conditions, consider how N gets to the root system for uptake. Mass flow is the primary mechanism for nitrogen (also sulfur, magnesium, and calcium). Mass flow is where nutrients in soil solution move toward the root as the plant takes up and transpires water through the crop canopy. Also, consider how dry weather affects the plant root system. Root growth will slow in dry soils, the situation we are now experiencing in the upper soil depths. Fortunately, root growth will re-establish, and the mass flow of nitrogen will quickly improve with rainfall. Here are a few considerations for managing N applications.
Nitrogen placement is one area to consider changes within the limitation of equipment available. The nodal root system originating from the lowermost nodes will take up nearly all N. Normally we discourage surface applications of nitrogen due to potential volatilization losses of N in urea form found in UAN or urea. Under dry weather conditions, we have limited opportunities for rainfall to move N into position for uptake. The goal should be to have nitrogen close to the root system and close to roots actively taking up water. So, moving UAN placement closer to the row and a little deeper will improve plant access. Anhydrous application is already deep enough and moving closer to the row increases root injury potential, so no adjustment is needed.
Nitrogen rate is another consideration. Looking at our N yield response data from 1998-2022 does not show that lower yields usually require less fertilizer N than higher yields. We think that’s because the causes of lower yields, which are typically stress from having less available water at critical times, often affect root growth, and so may make it harder for plants to take up the N that is in the soil. If your plan was to apply a rate based on the Corn Nitrogen Rate Calculator (https://www.cornnratecalc.org) which is for corn after soybean and a N:Corn Price between 0.1 and 0.15, the rate is 160 to 181 pounds of N per acre then stay with that amount. If the plan was to apply more than that, then cutting back would be reasonable.
A final consideration is timing. Application systems that make late-season applications possible have become more common. Putting down a reasonable side-dress rate and then basing a later application on rainfall could be a reasonable strategy.
Cover crops would also be something to start looking at now. We know if the yield falls short of normal, we will have excess soil nitrogen left after the growing season. The edge of field studies conducted by USDA-ARS has shown we can recover a substantial amount of this residual N through a cover crop. Exploring available cost-share programs with NRCS and SWCD would be a reasonable way to retain that valuable N on your field for a future crop.