The primary call we received this week was regarding corn that was going to be replanted or not planted yet and preplant fertilizer was applied many weeks ago and whether or not there was a need for additional preplant N. There are a few things that should be considered when dealing with preplant N on corn and this goes for both first plantings as well as replants.
At the V6 growth stage corn only requires 25-30 lb N/acre to maximize grain yield and the majority of this N is obtained from the soil- not fertilizer-N. Previous research has shown that for preplant N rates ranging from 30-90 lb N/acre, only ~6 lb N/acre from the fertilizer actually makes it in the corn plant by the time producers are applying sidedress N.
Although corn is a large consumer of N and requires large quantities to maximize grain potential, the majority of this N is needed much later in the season past the V8 growth stage. Almost 40-50% of the N uptake by corn occurs after the R1 growth stage (silking) which means the window of opportunity to apply N to corn and maximize yield is very wide, much wider than previously thought.
Lastly and probably the most important thing to consider is fertilizer incorporation. Incorporation of preplant fertilizers into the bed is critical to ensure that the small corn plant can access these nutrients and use them in an efficient way and this goes for all preplant fertilizers, not just N!
When preplant fertilizer is applied to the surface of freshly pulled beds the fertilizer tends to stay evenly distributed across the soil surface but is still a significant distance from the corn seedling roots. However, when fertilizer is broadcast across stale beds, (such as in a replant situation) most of the fertilizer falls to the furrow and is about as far from the corn seedling roots as you can get.
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This restricts the young corn plants access to the nutrients and typically means that they will not take up those nutrients until the roots reach the furrow which occurs around the V5-V8 growth stage. Preplant nutrient applications in corn are not recommended if they cannot be incorporated into the bed as the nutrient is often too far from the plant for efficient use prior to the V6 growth stage.
Therefore, if you are broadcasting preplant fertilizer or want to put out additional nutrients in a replant situation do not do so unless you are willing to repull the beds allowing incorporation of the fertilizer.
1. Should I apply more N to my replanted corn?
The easiest answer and this applies to almost every situation is NO. Current preplant N recommendations for silt loam soils is 30 lb N/acre and for clay soils is 60 lb N/acre and we want these to be incorporated into the bed.
If you have already applied at least these rates for your first planting there is no need to apply additional N for your replant.
2. How should I adjust my season total N rate for replanted corn?
Current season-total N rate recommendations for corn on silt loam soils is 220 lb N/acre and 290 lb N/acre for clays. For replanted corn there is no need to automatically make adjustments to these rates just because you are having to replant.
The best approach is to continue with your current N management plan (the one you had before the replant) and monitor the corn throughout the season. One way to ensure you have enough N is to take an earleaf sample at the R1 growth stage.
If the N content is >3.0% there is no need for additional N. If the N content is <3.0% then 45-60 lb N should be applied to maximize grain yield.
Remember that corn has only taken up 50-60% of its season total N at R1 so an application at this point still has plenty of time to be taken up and utilized by the plant.
The attached flowchart provides a good step by step approach to how N fertilizer should be managed in Arkansas furrow-irrigated corn. Please let us know if you have additional questions or need any help.