Mature trees generally do not need fertilizer. Fertilizers are not tree food, you do not "feed" a tree when you fertilize. Tree food (sugar) is made by living leaves through the process of photosynthesis. Healthy leaves manufacture sugars which are stored in healthy roots grown in healthy soil.
Mulching the soil within or beyond the dripline helps create a perfect environment for non-woody roots (these absorb essential elements and water from the soil) and woody roots (these store energy reserves and support the tree).
If nutrient deficiencies are detected in a soil test, it may be necessary to fertilize that soil to improve conditions and ultimately, tree vigor. Consult a certified arborist for advice on the best fertilizer for your trees and the best application methods for each. Fertilizer is best applied in late summer or after the trees have completely leafed out in spring.
Warning - Many lawn fertilizers contain weed and feed formulations and broadleaf herbicides that may be picked up by tree roots and harm your trees if applied incorrectly. Remember, mature trees have expansive, actively growing root systems that extend well beyond your tree's branches.