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 plants above.
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.