At this point, all I can say is do not fertilize. Until you identify the problem, fertilizing can make it worse. The symptom you describe has many causes. Any info you can give will help pinpoint the problem.

I need more info. Are the leaves brownin from the edges? Are they turning from green to brown, or green to yellow to brown? Are the leaves otherwise pale green or gregular green. Are they limp? Do all leaves have the same symptom? Have you been watering - how often. Where is the tree planted. Sun, shade, where water drains, or where water drains too? Check the base of the trunk. Any moweritis? (Damaged bark due to mowing or weed whipping.) Have you used any weed killer this year? What kind? How was it last year? Did you plant it any deeper than what it was in the original pot? Did you replace soil when you planted, or use the orignal stuff you dug out? ......

Rick