What does that mean?

Do I need to fertilize my trees?
injecting fertilizer
                                                    Injection of fertilizer below roots of turfgrass

What conclusions are supported by research? When should I fertilize the trees? Where can I get more information?

ANSI.  1998.  A300 (Part 2).  American National Standard for Tree Care operations.  Tree, Shrub, and other woody plant maintenance -- standard practices (fertilization).  New York: American National Standards Institute.  Smiley, E. Tom, et al.  2002.   Best Management Practices Series - Tree Fertilization.  Champaign, IL:ISA.  Both can be ordered online from the ISA.  For other information, advice and help on this topic, call offices of your State Urban Forestry Coordinator or University Extension service, or visit urban forestry web sites.