Growers seeding into less than ideal conditions or using less than ideal seeding equipment — broadcast, for example — may want to lower their fertilizer rates at seeding then top up if a good stand emerges and if growing conditions improve.
John Heard, fertility specialist with Manitoba Agriculture Food and Rural Initiatives, prepared an article called “Options for applying nitrogen fertilizer in a wet, late spring.” The article says top-dressing nitrogen allows for uncomplicated, quick seeding. Growers don’t have to put as much fertilizer in the drill tank, which extends the time between fills.
Nitrogen options for post planting application are urea, blends of urea and ammonium sulphate (to try to reduce volatile loss potential with straight urea), dribble band UAN, urea treated with Agrotain, and UAN treated with Agrotain.
Ammonium sulphate (15-0-0-21) also provides a sulphur top up, which could be valuable if growers did not apply all the sulphur they needed at seeding.
Urea’s average performance is still acceptable for top dressing, Heard says, especially with good soil contact and favorable moisture. If rain is not in the forecast, Agrotain will limit losses until rains come. Band stripping of dry urea will minimize surface contact and may reduce volatilization potential.
Performance of broadcast UAN solution may not be as high as for other forms of nitrogen, but Heard says spoke-wheel injection or dribble application will improve UAN performance.
ESN is not recommended for post seeding applications.
How much to apply? Growers may need to top dress up to 30 pounds per acre of actual nitrogen to see a noticeable difference with the naked eye. Adding liquid nitrogen to the tank when herbicide spraying provides less than 10 pounds per acre, which will likely not be sufficient if early applications were conservative and the crop is truly deficient. When trying this, consider a test strip to see if it has any effect on final yield.
Nitrogen rate calculators. As growers head to the field, they have a chance to make some last minute adjustments to their nitrogen rates. Here are links to two calculators that can help with the decision: MAFRI Alberta Agriculture