Well for starters, EVERYTHING NEEDS TO EAT!
Even unhealthy trees will grow. They have to in order to survive, but for a tree to be healthy it needs essential nutrients. A tree in good health and vigor can defend itself against environmental stresses, plant parasites, disease and insect infestation. The goal is to provide these nutrients using the most effective products, which are the slow release fertilizers (3-1-1) with the most efficient method of delivery, which is by soil needle injection.
Urban soils are deficient of the basic nutrients needed by trees to survive. This is due to the soil damage and sometimes total removal of soil that occurs during the home development process. In most cases the organic humus layer is replaced by a turf grass or landscaping that actually competes with the trees for nutrients. Remaining or planted trees are left to fend for themselves with the added stresses thrown at them through the season. Because the soil cannot be replenished, it should be supplemented.
Trees and shrubs are living organisms. They require food and water just as humans do.
Rely on an Arborist who has the proper certifications (Board Certified Master Arborist / Registered Consulting Arborist), and whose expertise lies in Tree Health Care to diagnose the tree issues you may have on your property.