Forest soil quality measures the condition or health of soil relative to the requirements of one or more of the organisms in a forest. This includes physical properties (e.g., soil type and depth), chemical properties (e.g. nutrients and toxins), and biological properties (e.g. organic matter and symbiotic microbes such as mycorrhizae). Some soil features are related to bedrock geology while other features have evolved out of past land use or from atmospheric inputs. Vermont soils are heterogeneous, which makes determining soil quality difficult without on-site measurement. However, productive and healthy soils are critical to maintaining forest health and sustaining tree growth.
Soil productivity is the capacity of soil to support plant growth and in forests is often reflected in the growth of forest vegetation or the volume of organic matter produced on a site. Not all trees have the same requirements for optimal growth and can differ in response to soil conditions. Some general descriptions of site and soil productivity are listed below.
- Healthy soils are productive soils.
- Healthy soils have an abundance of organic matter and are thus able to hold more water and reduce runoff and evaporation.
- The Natural Resource Conservation Service found that 1% organic matter in the top 6 inches of soil can hold about 27,000 gallons of water per acre.
- Forest soils with higher levels of macronutrients (nitrogen, phosphorus, potassium, calcium, magnesium) are typically associated with more productive forest stands (Mroz et al. 1985).
- Soil nutrients occur in two major forms: inorganic compounds dissolved in water or attached to minerals, and organic compounds part of living organisms and dead organic matter. Soil organisms are constantly transforming nutrients between these two forms.
- When inorganic compounds are consumed by organisms to support growth, they are immobilized. When organisms excrete inorganic waste compounds, they are mineralized and available for use in soils.
- Soil organic matter is a vital component of productive soils and plays a major role in soil water retention, cation exchange capacity, aeration, drainage, and nutrient supply (Jurgensen et al. 1986, Jurgensen et al. 1997, Burger 2002).
- Healthy forest soils are home to 100 million-1 billion bacteria, hundreds of yards of fungal hyphae, several hundred thousand amoebae and nematodes, and 20,000-25,000 arthropods. These numbers are per square foot of soil or less. Earthworms are not native in Vermont, and can reduce soil productivity.
- Fine woody material protects soils from erosion, provides important energy and nutrients to stream ecosystems, and live fine woody material (i.e., saplings and shrubs) is an important understory habitat component (Elliot 2008).
- Coarse or large woody material (CWM) includes dead and down woody material such as logs greater than 3-4 inches in diameter at the small end and, large branches and stumps. CWM provides habitat for insects, fungi, microorganisms, and amphibians, and provides cover and runways for small mammals and winter den sites for bears and other wildlife (Elliot 2008).
- As logs and other CWM material decay, the suite of organisms finding habitat will shift. CWM is also an important component of stream ecosystems because it provides cover for fish, helps create deeper pools, provides a substrate for stream insects and microorganisms that are important food sources in streams, and is important in the maintenance of nutrient and hydrologic cycling.