An understanding of forest soils will help owners manage their woods for improved tree health and more efficient growth of trees. Forest soils should be thought of as a living layer or mantle that allows for trees to be anchored to the earth and obtain necessary minerals and water. The mantle includes layers, called horizons, with the uppermost being the organic horizon often called the “duff.” Below the organic horizons are the mineral horizons, and usually far below the surface is the parent material or bedrock. The science of soils is as complex as the science of forestry or the science of biology; this article will only scratch the surface of the influence of soils on trees and woodland management.
The characteristics of soils important for woodlands include their texture, organic matter and pH. Texture is the size of the soil particles that might include some combination of fine clay, medium silt and coarse sand. Organic matter particles in the mineral horizons help hold or bind the nutrients such as calcium and magnesium that are necessary for tree growth. Micro-organisms and invertebrates (e.g., fungi, bacteria, centipedes) are dispersed throughout the organic layers and play important roles in decomposition. Texture and organic matter, plus other factors, influence the moisture-holding capacity of the soil. The soil’s pH describes soil acidity and influences the availability of nutrients. These three characteristics of soils will impact which trees occur in an area, how well they grow, the limitations imposed on certain woodland operations and the opportunities for other types of woodland operations.
Most forest stewardship plans include a discussion of forest soils. Unfortunately, most of these discussions are of limited use to the owner, or to anyone lacking a strong background in soil science. These discussions often mention the name of the soil series, the depth of the soil horizons or layers and perhaps some chemical attributes of the soil. A partial example of such a soil description from a stewardship plan might read:
“These loams belong to the Mardin series of soil and are most commonly found in previously glaciated areas, specifically on broad hilltops and slopes that range between 0 percent and 50 percent. They are well-drained soils with a dense fragipan that begins 14 to 26 inches below the soil surface.”
This information can easily be found in the county’s soil survey (paper copy) or the more widely available Web Soil Survey (WSS) produced by the Natural Resources Conservation Service.
Specific information is also available in the WSS that would be more useful to most landowners. Information is available, for example, to address how different tree species respond to the soil, if soil conditions might limit the building of roads or landings and if the soils are prone to drought or poor drainage of moisture.
If a tree species is suited to a particular soil it will have better health, better growth and recover more quickly after a stressful event than a species that is growing “off-site.”
One indicator of whether a tree is suited to a particular soil is the tree’s site index on that soil. Site index is the expected height of a tree species at a specific age, usually 50 years old.
As an illustration, if a tree species growing on a given soil type has a site index of 65, we would expect an upper canopy tree of that species to be approximately 65 feet tall when the tree is 50 years old. If sugar maple has a site index of 55 on one soil and 70 on another soil, the latter soil is better matched to the needs of sugar maple resulting in a tree that will grow better and have fewer problems with health. WSS provides the full range of information about the physical and chemical properties of soils.
Part two of this series will focus on potential resources and examples from woodland owners.
ForestConnect is a joint research and extension program funded by the Cornell University Agricultural Experiment Station (Hatch funds) and Cornell Cooperative Extension (Smith Lever funds) received by Cornell University College of Agriculture and Life Science from the National Institutes for Food and Agriculture, U.S. Department of Agriculture. Additional educational resources are available.