Once established, most canopy trees do not need fertilization. Unless there is a verified nutritional deficiency, applying fertilizers to canopy trees the way we would our tomatoes or rose bushes, often leads to problems such as nitrogen loading and attracting unwanted pests.
In general, nutritional deficiencies are best diagnosed via a leaf tissue analysis, which is like a blood test for a plant. These tests are performed mostly by university laboratories but some government and private labs do them as well. This type of test identifies exactly what plant nutrient is in deficit, abundance, or over abundance. Then, fertilization of only those deficiencies takes place. But that’s only half of the solution. The important question to answer is “why did a nutritional deficiency occur?” If a nutritional deficiency does occur, it is often an indicator that something in the tree’s environment is not functioning properly. Is the planting area more wet because drainage has been changed? Has there been poor quality backfill soil or new soils placed around the tree after construction? Have there been new additions to the soil environment such as coffee grounds (highly acidic), egg shells (highly alkaline) or did someone wash out painting materials under a tree? Did someone get too heavy-handed with the bleach mixture when they were power washing the driveway? These are all common culprits in urban environments.
Fertilizing alone only treats the symptom. It is important to find and treat the cause in order to permanently correct the symptoms.