What does that mean?
Do I need to fertilize my trees?
- 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
- 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
What conclusions are supported by research?
When should I fertilize the trees?
- Young deciduous trees benefit from some additional nitrogen
- 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
- 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
Where can I get more information?
- 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.