Figure 1. Leaf marginal burn from post-emerge applications of urea treated with NBPT.
- Nitrogen loss may have occurred in fields where UAN solutions were applied or nitrification inhibitors were not used.
- Delaying N applications too late into the growing season may result in disappointing yields, and a greater risk of having residual N in the soil profile after harvest.
- All sources of post-applied N are equally effective when managed properly.
- The burn from broadcast post-applied urea is not necessarily a direct fertilizer burn.
Spring often brings cool, wet weather to many areas across the country. Under these circumstances, there is minimal risk of nitrogen loss from fields that receive fall or spring nitrogen with a nitrification inhibitor. However, fields receiving UAN as 28% or 32% could lose up to a quarter of the total nitrogen applied. The mechanisms by which N can be lost include leaching below the young root system, or through denitrification, which results in gaseous loss to the atmosphere. Soil testing that includes analysis for ammonium (NH4+) and nitrate (NO3-) will reveal the extent of these losses.
Post N applications as either urea or UAN solution provide a great opportunity to include sulfur in AMS or ATS to meet early season crop needs. Much of the sulfate (SO4-) form of sulfur that was applied either last fall or early spring may have also moved below the young crop’s root system under wet conditions. Persisting light green coloration to the crop canopy may indicate sulfur deficiency. The ideal ratio of nitrogen to sulfur in the growing crop is 15:1, while ratios of 20:1 or greater indicate a deficiency. Undetected sulfur deficiency in the crop treated with additional nitrogen application can lead to greater sulfur deficiency. This is due to the codependency of these two nutrients for chlorophyll production and protein formation.
When is a good time to plan for post-emergence N applications? There is no reason to delay post-emergence N applications on medium-to-heavy textured soils. As soon as field conditions allow, post-emergence N should be applied. Nitrogen must move into the plant with soil water. The longer a post-application of N is delayed, the greater the risk of not receiving adequate rain to get the N into the plant. Delaying N applications too late into the growing season may result in disappointing yields due to N deficiency and a greater risk of having residual N in the soil profile after harvest. This is dependent on soil moisture conditions after any delayed applications. Unless water can be controlled, typical mid-season rainfall can be impossible to predict and delaying N applications may be risky.
Is one source of post-applied N better than the other? A pound of N is a pound of N whether applied as urea, UAN or anhydrous ammonia. All sources of post-applied N are equally as effective when proper management of the source is considered. Marketing and infrastructure (equipment capabilities and fertilizer storage) are usually the reasons one source is favored over the other, not because of performance.
What type of burn should be expected from post-applied urea? The burn from broadcast post-applied urea is not necessarily a direct fertilizer burn. Symptoms will usually develop a day or two after application and will be more obvious on the newer leaf tissue (leaf after application). The burn will be associated with leaf margins (Figure 1). The exact mode of burn has yet to be defined but is thought to be related to N accumulation at the leaf margins. This accumulation is due to the evaporation of water carrying urea at the leaf’s edge, reaching levels high enough to induce the characteristic marginal scorching. The degree of leaf margin burn appears to be related to the temperature and humidity the day of and after the application. The leaf burn is nearly always cosmetic (if apparent) and no yield loss should be anticipated.
What forms of post-applied nitrogen match BMP’s for N use? All forms are in-line with nitrogen best management practices. Urea or UAN treated with a NBPT based urease inhibitor (e.g. Agrotain1), and surface-applied, are a safe and reliable option for post-N applications to corn. Consistent yield response of broadcast post-applied urea with NBPT has been well-proven in scenarios where supplemental N was needed. Broadcast urea is evenly distributed and readily available to corn roots once carried into the soil surface by rainfall. If the corn plants will respond to supplemental N applications, urea treated with NBPT will not be disappointing. Likewise, UAN is still a reliable source of post applied N and remains a popular choice. UAN and NBPT dribbled between the rows does take a little longer for corn roots to reach it as roots need to grow and the UAN needs rain to disperse it from the point of application. Anhydrous ammonia remains an old standby. Anhydrous ammonia applied as a liquid quickly dissipates in the soil to a gas as it quickly seeks out soil moisture to associate with. Once dissipated and in soil solution it is readily available for plant uptake.
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Keywords: Corn, Fertility, Nitrogen