Which is better for fertilizing trees - injecting nutrients from a probe into the soil below the turf root level or applying a time-released fertilizer on the grass for the benefit of the trees. I've read that the probe injection goes to 12 inches deep so it reaches the tree root levels better and the granular fertilizer lets the turf absorb most of the nutrients and never gets below the turf root zone. I've had a tree service that has applied a time-released fertilizer on the grass once in the spring for the trees and said that it won't interfere with the dry fertilizer that my grass service uses. Should I use both services or just one for grass and one for probe injection?

Injecting a liquid product or auguring holes for a granular product has been the traditional way of fertilizing trees. Holes or injectors going down the 12-15 inches is also pretty standard. I am unfamiliar with the technique of granular fertilizer applied o the surface for tree feeding. If you did not observe any adverse affects to the lawn, like excessive growth, then this method may work out ok in your yard. Trees can use more fertilizer than the lawn will tolerate as a surface application usually.