I have an Emperor Francis cherry tree from Raintree thats in its 3rd year (second producing) in a 20 gallon pot. It looks great; flowers and fruits every year, but I'm wondering if it wouldnt benefit even more from a fertilizer spike

Off from the main stem are three branches that almost form the letter "Y", and blossoms appear on all branches, but I'd like it to do more.

Would fertilizer be ok in the third year, or have I missed the boat?