What does that mean?

• Fertilizing trees refers to the practice of adding supplemental nutrients (chemical elements) required for normal growth and development.
• You can't "feed" a tree, since trees are autotrophs: they use nutrients to feed themselves by making sugar in the leaves through photosynthesis.

• Often, you may not. A reasonably fertile soil will have enough nutrients to support healthy growth on most established trees.
• Trees adjust their growth and development rates to the level of nutrients, and will usually make out all right as long as the roots can continue to grow.
• In nature, trees get nutrients from air, recycled organic matter, beneficial microbes, and soil minerals. 
• In urban settings, the recycling of organic matter is often reduced, beneficial microbes may be minimal, and some minerals can be unavailable because of the soil pH.
• Clayey soils (common on urban sites) present special problems in the availability of micronutrients like iron and manganese for some trees. 
• Lack of water, organic matter, and soil air often limit growth of urban trees much more than nutrient levels.

                                                    Injection of fertilizer below roots of turfgrass

• Young deciduous trees benefit from some additional nitrogen (e.g., 10-6-4). 
• Conifers rarely need fertilization at all, since most are genetically adapted to low-nutrient soils.
• A layer of organic matter maintained under the tree crown will increase fertility, microbial activity, soil air, and water retention--all factors that increase tree growth.
  
• Serious pest and structural problems can result on trees that are overfertilized, especially when a predominantly water-soluble fertilizer is used.  
• Surface application is the easiest and cheapest method of fertilizing ornamental trees.
• Trees surrounded by turf benefit from the application of additional nitrogen every few years, because grass competes well against trees.  Soil-injected fertilization can put the nutrients just below the grass roots so that the trees benefit the most.
• If contracting out fertilization,  make sure that you specify "work to be carried out according to ANSI A300 standards."  These standards specify
• a determination of deficiency
  
• a specific objective 
• manner most beneficial the plant
• preference for at least 50% water-insoluble nitrogen and a salt index below 50
  

• Fertilizers with at least 50% water-insoluble nitrogen should be applied in early fall or early spring. 
• Predominantly water-soluble fertilizers should be applied in late May or early June in the Northeast.

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.