What do you use on pecan trees?

A nitrogen fertilizer is what is used to fertilize pecan trees (Carya illinoiensis).

Specifically, ammonium nitrate is an effective, reasonably priced nitrogen fertilizer for pecan trees. It is sold in packages marked 33-0-0 for 33% weight of nitrogen and 0% each of phosphorus and potassium. Another effective, reasonably priced nitrogen fertilizer is ammonium sulfate (21-0-0), at 21% nitrogen.