Root injection fertilization, also known as deep root fertilization, is the most effective method for fertilizing trees and shrubs. This process involves sticking a pole into the soil about 8-12 feet, then, under pressure, squirting liquid fertilizer deep into the root zone of the trees.

The liquid fertilizer being injected under pressure helps aerate the soil which opens up air pockets, helping to provide much-needed oxygen to the root system. 

Tree roots only grow and develop when they find oxygen, nutrients, and moisture, all of which are provided through root injection fertilzation. Since this process requires special equipment, it is usually done by an arborist.