By Michael Snyder
Can I Fertilize a Forest
Like I Fertilize a Garden?
Forest soils certainly benefit from the addition
of plant nutrients. Elements like nitrogen, phosphorous, potassium, calcium, and magnesium
are the building blocks of leaves, twigs, trunks,
and roots, and they regulate or activate countless
physiological processes in the microscopic life of
plants – functions like water movement, enzyme
activation, and stress signaling and response.
No mineral nutrients in the soil below, no living
Some forest stands are naturally flush with
nutrients. Plant-available minerals in the soil
come from the weathering of rocks, deposition of
airborne particles relocated from somewhere else,
and from the recycling of decomposed organic matter from dead
plants and animals on the site. Their continual cycling between
soils and trees is vital to the maintenance of soil minerals.
But not all soils contain sufficient nutrients for healthy tree
growth. Some soils are just naturally depauperate, some have
been exhausted by erosion or poor management practices, and
some have been depleted by repeated harvesting and removal
in the form of grass, wool, milk, or logs over many decades.
Minerals can also be leached from soil in drainage water.
Recently, we’ve learned that some minerals, like calcium, can
be leached at accelerated rates by inputs of acid precipitation.
Such losses of essential nutrients lead to deficiencies that reduce
growth and jeopardize forest health.
So can you fertilize a forest? Yes. Fertilization of forest trees
– particularly with nitrogen – has been a common practice in
intensive plantation silviculture in the Southeast and Northwest
since the 1960s. Most is applied by aircraft, unless there is
adequate spacing between rows of trees where it can be done by
tractor or skidder-mounted equipment. The vast majority of such
applications use dry, pelletized forms of synthetic fertilizers.
There have been experimental applications of fertilizer to
northeastern forests. For example, in 1999, a 30-acre hardwood stand at the Hubbard Brook Experimental Forest in New
Hampshire was amended with over 50 tons of calcium dropped
from a helicopter in an attempt to restore that which had been
leached away by acid precipitation. By following the forest
ecosystem’s response over the past 16 years, researchers documented that increases of calcium in such conditions stimulated
a significant increase in growth of forest vegetation.
While these findings are significant, they do not necessarily
indicate that amending forest soil with a helicopter is the best
solution to a forest health problem. For starters, it is highly
impractical and, unless you’ve got your own aircraft, prohibitively expensive. Moreover, there are many possible reasons
beyond fertility why a forest stand might exhibit slow growth,
discolored or misshapen foliage, or dieback.
Fertilization simply will not fix the limitations of a site that
is too wet or too dry, and it cannot overcome destructive logging practices that erode soils or damage tree stems and roots.
Similarly, fertilization cannot prevent defoliation by insects (in
fact, it might just nourish them). And an overcrowded stand
where trees have no room for expansion will likely benefit far
more from a good thinning.
Fertilization won’t improve the growth of trees already
growing on a nutrient-rich site, and if overdone, it can actually
have a deleterious effects on trees and the greater environment.
Indeed, high soil concentrations of even the most essential
nutrients can be toxic to plants and excessive nutrients can run
off and pollute nearby waters. Effects on wildlife have not been
adequately studied and remain largely unknown.
Fertilization may be a workable idea if your forest is a young
plantation of southern pines and your sole objective is growing
timber as fast as possible, or if your forest is an abandoned
surface mine and you are heaven-bent on restoring its vegetation.
Otherwise, it’s probably not worth the associated expense,
practical difficulties, or environmental risks. If you really want
to enhance your forest soil’s productivity, advocate for clean air,
retain leaves, twigs, and branches from harvested trees, practice
good silviculture and careful logging, and return your raked
yard leaves to the woods from whence they came.
Michael Snyder, a forester, is commissioner of the Vermont Department of Forests,
Parks, and Recreation.
Foul-looking forest fertilization using fowl feces.