Chapter 8 - Forest Soils and Productivity
Contents - Class Homepage
Sections:
Properties Important For Growth - Evaluations of Forest Sites and Soils For Tree/Stand Productivity - Summary - References - Interesting Web Links

 

Back to top PROPERTIES IMPORTANT FOR GROWTH

Site and soil characteristics, combined with disturbances (human or natural) act to control the plant survival, health, density, and growth. If age and management is similar, "Good" sites are capable of supporting more species of trees, higher densities of trees, and larger, faster growing trees as compared to "poor" sites. "Good" sites also tend to support larger wildlife populations and "Good" sites tend to be more flexible in terms of management options. Therefore it is important that we consider just what soil and site characteristics comprise a "good" site.

This "good" site is indicated by the presence of site demanding species such as Cherrybark oak and the diversity of tree and shrub species (> 50 species)

This good site is indicated by the extremely fast growth rates for pine and hardwood species (stand is 60 years old).

The "poor" site is indicated by low species diversity (< 5 tree species), slow growth (stand is 80 years old), and presence of drought tolerant species (scarlet and chestnut oaks).

This "poor" site is indicated by the presence of only very flood tolerant species such as pond cypress, slow growth rates (stand is 57 years old), and low density of trees.

PROPERTIES IMPORTANT FOR GROWTH - Soil Physical Properties
Soil texture refers to the amount of sand, silt, or clay that a particular soil contains. Soil texture is one of the most important soil physical properties because it influences the ability of a soil to provide water, nutrients, and aeration necessary for plant growth. Soils are made of individual mineral particles of sand, silt, or clay, organic materials, water, and air. These individual particles may be aggregated into larger particles known as soil peds. Plant roots, organic compounds, and soil moisture act in concert to provide stability and aggregation to these larger particles and the arrangement and stability of these aggregates are known as soil structure. Some of the more common soils structures are depicted.

PROPERTIES IMPORTANT FOR GROWTH - Soil Physical Properties
Soils are composed of mineral solids (sand, silt, and clay particles), organic solids (organic matter), water, and air. The non-solid portion of the soil, air and water, are contained within the soil pore space (or soil porosity), which are the voids between the solid portions. There are two major types of soil pore spaces, the smaller capillary pores that can retain soil moisture against the force of gravity and the larger non-capillary pores that provide drainage for the soil.

PROPERTIES IMPORTANT FOR GROWTH - Soil Physical Properties
Soil strength is not a constant value, rather it varies as soil texture, soil porosity, soil organic matter, and soil moisture change. Soil strength is an important soil property because it indicates how well plant roots can growth within a particular soil and it indicates how well a particular soil will withstand traffic. Because soil moisture can change so rapidly, it is a major modifier of soil strength. When most soils are dry, they tend to be relatively strong and they are able to support traffic without collapsing the pores. When a soil is moist, the soils become more plastic and have less strength.

Soil cone penetrometer is used to measure soil resistance to penetration, which indicates soil strength.

Measurement of soil strength with a soil cone penetrometer.

PROPERTIES IMPORTANT FOR GROWTH - Soil Physical Properties
Soil Bulk Density is a soil physical characteristic that can be used as an index of soil porosity and soil strength. Soil bulk density is the mass of dry soil in g (no water) divided by the specific gravity of soil (usually 2.6 g/cm3). Mineral soils having bulk densities of <1.2 g/cm3 are fairly easy for roots to penetrate when moist and probably have adequate pore space for air and water movement. Soils having bulk densities > 1.6 g/cm3 are dense soils which may limit porosity, water and air movement, and root growth.

This eroded old field site in the Virginia Piedmont has shallow root growth because of the high bulk density and high soil strength of the B horizon.

PROPERTIES IMPORTANT FOR GROWTH - Soil Physical Properties
Under moist conditions the soil can be compacted by traffic and the soil porosity can be reduced. Under saturated soil conditions, when all of the pore space is filled with water, soil strength is so reduced that the soil may become almost liquid like when a force is applied. Under this condition the soil porosity can also be reduced and the soils particles may actually be rearranged. The decrease of soil pore space with the increase in soil bulk density is referred to as compaction, while the rearrangement of the soil particles is referred to as puddling. Measures of soil strength and bulk density are often used as indicators of excessive, or potentially root limiting, traffic as might occur along heavily traveled hiking paths.

This soil was compacted by the repeated passes of heavy logging equipment near the area where logs were loaded on the truck (deck).

This area was puddled by logging equipment traffic during very wet conditions.

PROPERTIES IMPORTANT FOR GROWTH - Soil Chemical Properties
Soil acidity (or pH) is defined as the logarithm of the reciprocal of the Hydrogen ion (H+) activity or pH = -log Activity H+, but in practice this equation is usually circumvented by the use of a soil pH meter. Soil acidity is the most commonly measured soil chemical property because it is so important to plant nutrition due to its influence on soil organisms such as bacteria and actinomycetes, and plant nutrients such as nitrogen, calcium, magnesium, phosphorus, potassium, sulfur, iron, zinc, manganese, copper, cobalt, molybdenum, and boron.

PROPERTIES IMPORTANT FOR GROWTH - Soil Chemical Properties
The majority of agricultural crops and many hardwood forest species have optimal nutrition when soil pH = 6.0-7.0. Some plants, including rhododendron, azaleas, and blueberries are naturally adapted to very acid soil conditions (pH < 5.0). Others, such as black walnut and Osage orange grow better under slightly acid to slightly alkaline conditions (pH = 6.5 - 8.0). In general, conifers occur on moderately acid soils (pH = 5.0-6.0). Soil pH can be unintentionally modified by activities such as flooding, wildfires, acid deposition, or additions of acid producing fertilizers. Soil pH may be intentionally increased by the addition of lime (Calcium or Magnesium oxides or carbonates) or decreased by the addition of gypsum or other sulfate bearing compounds.

Osage orange trees are generally associated with high pH soils, blueberries tend to grow on acidic soils.

PROPERTIES IMPORTANT FOR GROWTH - Soil Chemical Properties
Soil organic matter refers to the portion of the soil that was formed from the decomposed remains of dead plants and animals. Soil organic matter serves as the primary source of three important plant nutrients: nitrogen, phosphorus, and sulfur. Soil organic matter compounds serve as binding agents, which aggregate smaller soil particle into larger, more stable units. Therefore, soil organic matter is said to improve soil structure. Soil organic matter also improves retention of soil nutrients so that they will eventually be available for plant growth. For example, soils with higher organic matter levels may have better Cation Exchange Capacity (CEC), which means that these soils can retain cation or positively charged nutrients such as ammonium so that the ammonium can be used for plant growth. Soil organic matter also increases the moisture holding capacity of a soil. This is why organic amendments such as peat moss are so widely used for indoor and yard plants. Soil organic matter serves as the food source for many soil organisms. Finally, soil organic matter serves as a tremendous reservoir or pool of terrestrial carbon that would otherwise be released to the atmosphere.

Here sludge from a wastewater treatment facility is being applied to a forest site. This will increase the organic matter on the site.

PROPERTIES IMPORTANT FOR GROWTH - Soil-Plant Relationships
Plants are dependent upon sites and soils to provide water, nutrients, aeration, and stability. The actual location of the site will affect overall climate due to the latitude, elevation, or proximity to climate modifying features such as mountains or large water bodies.

PROPERTIES IMPORTANT FOR GROWTH - Soil-Plant Relationships
The ability of a soil to provide moisture for plant uptake is a function of the soil texture, soil organic matter, size and volume of pore spaces within the soil profile, and landform. Plants are best able to uptake water from soils that are between the somewhat inexact moisture levels known as the Field Capacity and the Permanent Wilting Point of a soil. Field capacity is the moisture that remains in a soil 1 to 2 days following a saturating rainfall event. Permanent Wilting Point refers to the soil moisture at which many plants wilt and will not recover. The quantitative difference between the Field Capacity and Permanent Wilting Point is called the Available Water Capacity because this is the water that is generally considered available for plants.

PROPERTIES IMPORTANT FOR GROWTH - Soil-Plant Relationships
Soil texture affects the ratio of soil capillary and non-capillary pores. Coarser textured soils, such as sands, have a higher percentage of non-capillary pores so they drain easily and do not retain as much water as a clay soil. However, because the clay soil has a higher percentage of small capillary pores, clay does not release water so it releases a smaller percentage of its total water as compared to a loam. Most plants are best available to extract water from a soil that is a loam as opposed to a clay or sand.

PROPERTIES IMPORTANT FOR GROWTH - Soil-Plant Relationships
Organic matter also affects a soil ability to retain and release water. In general, increases in organic matter have positive effects on available soil water.

PROPERTIES IMPORTANT FOR GROWTH - Nutrient Cycling
Native forest species have adapted to site conditions over thousands of years and are very capable of surviving without artificial nutrient inputs. Forests accomplish this by cycling nutrients from one component of the soil and vegetation to another in a complex cycle referred to as nutrient cycling. In a simple conceptual model, the forest returns leaves, branches, fruit, and dead trees to the litter layer, which eventually replenishes the nutrients that were extracted from the soil. Some nutrients may be lost to the atmosphere, some may be eroded, and some may be leached, but these small losses are often replenished by inputs such as atmospheric deposition or natural weathering of parent materials. In managed forests, losses from the system would also include removal of nutrients in wood products, but as long as only bole wood is removed, losses are small.

Bole wood removal, as depicted in this truck load of sawtimber, generally removes a relatively insignificant fraction of nutrients from forest soils because a much higher proportion of nutrients is left on the site in the form of leaves, stems, branches, and roots.

Following timber harvest operations, slash should be redistributed across the site to ensure that organic matter and nutrient pools are left relatively intact.

In some situations slash is piled and burned in order to make tree planting operations easier. This practice removes organic matter and accumulates nutrients so that they may not be available uniformly across a site. This practice should be minimized whenever possible.

Nutrients and organic matter may be returned to the soil via natural disturbances. This large Douglas Fir on the Olympic Peninsula is slowly decomposing and is actually serving as a nurse tree for the establishment of younger trees.

PROPERTIES IMPORTANT FOR GROWTH - Nutrient Cycling
The overall nutrient cycle, which involves the atmosphere, vegetation, soil, and geology of a site, is comprised of two smaller cycles: the geochemical and the biological cycle. Sometimes the biological cycle is further subdivide into the biochemical and the biogeochemical cycles. The biochemical cycle considers the internal transfers of nutrients within living trees and the biogeochemical cycle considers the transfer of nutrients between the soil and tree. The geochemical cycle considers the import or export of nutrients into or out of a stand by processes such as atmospheric deposition via dust or precipitation and deposition of sediments.

PROPERTIES IMPORTANT FOR GROWTH - Nutrient Cycling
Trees require 20 essential nutrients (not definite for all species) for survival, growth, maintenance, and reproduction. Carbon, hydrogen, and oxygen are obtained from carbon dioxide and water. Six nutrients are required in relative large amounts (macronutrients): nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. These macronutrients must be obtained from the soil. Micronutrients, or those required in small amounts are boron, chlorine, cobalt, copper, iron, manganese, molybdenum, silicon, sodium, vanadium, and zinc. These also must generally be obtained from the soil. In general, soils are most often limiting in nitrogen, phosphorus, or potassium, thus these 3 elements are the components of the worlds most common inorganic fertilizers.

These 4 year old loblolly pine growing on a coastal plain site were not fertilized.

Four year old fertilized loblolly pine stand.

PROPERTIES IMPORTANT FOR GROWTH - Nutrient Cycling
The ability of a particular site to provide adequate nutrition for a tree depends on numerous factors and the limiting factor for tree growth may actually vary from one season to the next. During one year rainfall may be plentiful, but the soil nutrients cannot be supplied rapidly enough for the tree to fully exploit the moisture. During the next year, nutritional supplies are adequate, but a drought limits growth. This type of limiting situation is referred to as Liebig's Law of the Minimum. Liebig concluded that plants will growth as rapidly as possible until some factor limits their growth. If the limiting factor is supplied, then another factor will eventually become limiting.

PROPERTIES IMPORTANT FOR GROWTH - Nutrient Cycling
The parent material will influence the quantity and weathering rate of certain essential nutrients. For example coastal plain sediments are often lacking phosphorus. Soils in the valleys of the ridge and valley region are generally deeper and more productive than soils on the ridges because the valleys were formed from weatherable limestone as opposed to the resistant sandstones and granite.

This landowner is adding waste products (boiler ash) to loblolly pine plantations in order to improve site fertility and reduce land filling costs.

Equipment used to apply boiler ash.

PROPERTIES IMPORTANT FOR GROWTH - Nutrient Cycling
The chemical structure of the soil will also affect the degree to which that soil can retain and release nutrients to plants. Organic matter is the most effective compound for retaining positively charged ions (cations) such as ammonium (NH4+) or potassium (K+). Some clays such as montmorillonite and vermiculite are also very effective at retaining nutrients while kaolinite clays are less effective. Some hydrous oxides may also retain very small quantities of cations. Certain anions (negative charges) such as phosphorus may be retained in a soil by iron or aluminum compounds. In general, many anion nutrients, such as nitrate, are readily leached, or washed out of the soil, into stream or ground water.

Montmorillonitic clay, as indicated by the shrink-swell potential of this site, is able to retain cations more effectively than non shrinking clays such as kaolinite.

PROPERTIES IMPORTANT FOR GROWTH - Nutrient Cycling
Organic matter affects the nutrition of forests in several critical ways. Organic matter decomposition generally serves as the most readily available source of nitrogen, phosphorus, and sulfur. Organic matter also serves as an importance source of charge for the retention of cation nutrients. Organic matter increases the ability of a soil to store and release water. Organic matter on the soil surface (litter layer) serves as a pool of recycled nutrients and reduces the potential for nutritional losses through soil erosion. Therefore, maintenance of organic matter is of critical importance to maintaining forest productivity.

Large volumes of organic matter are naturally added in the form of leaves, stems and logs in this older yellow poplar forest in the blue ridge.

Notice the large volumes of organic matter left on site following a harvest operation in the coastal plain of Virginia.

Notice the organic matter left on this site following mechanical site preparation.

An organic soil.

PROPERTIES IMPORTANT FOR GROWTH - Aeration
Soil aeration provides area within the soil that is necessary for retention (capillary pores) and drainage (non-capillary) of water. When the pores are not filled with water they provide space for the movement of soil gases. This is particularly important in flooded situations, where restricted movement of gases could allow toxic organic compounds to accumulate. Poor aeration is often indicated by plant adaptations that allow them to survive under poorly aerated conditions. These specialized plant structures are generically called pneumatophores.

This well aerated soil is evident due to the very bright colors (reds) which indicate oxidation of iron compounds.

This poorly aerated soil is evident because of the dark grey colors which indicates that anaerobic soil organisms are reducing iron compounds.

Aerated and anaerobic conditions can occur in close proximity. These aerobic (red) and anaerobic (grey) soils were taken from a transect across a terrace that was less than 200 yards long.

Aerobic and anaerobic conditions may occur within centimeters of each other. Notice the bright red areas along the root channels, which are well aerated versus the anaerobic areas between the root channels.

Some soil organisms, such as this crayfish burrow can increase soil porosity, thereby influencing soil aeration and drainage.

This cypress knee indicates poorly aerated soils. The specific purposes of cypress knees are not certain, but the knees may improve oxygen or carbon dioxide exchanges or provide anchorage for soft soil. Cypress knees grow to the approximate height of the mean high water table, thus taller knees indicate higher average flood conditions.

These arch roots of red mangroves in the Florida Everglades allow mangroves to survive in poorly aerated soils.

Water tupelo will produce arching roots which have enlarged species between the cells known as aerenchymous tissue. These aerenchymous tissue are specific adaptations to anaerobic conditions.

The swollen lower stems of these swamp tupelo also have enlarged intercellular spaces known as aerenchymous tissue that facilitates gas exchange during anaerobic conditions.

PROPERTIES IMPORTANT FOR GROWTH - Anchorage
Soils provide trees with a material in which they obtain anchorage. Some tree species, due to their dense or deep rooting habits are more stable (live oak) and some trees, such as the redwoods are simply so massive that the weight of the tree provides stability. However, the soil features are generally more critical for determining tree stability. Trees having shallow root systems are inherently unstable and wind-throw is common on shallow soils. This shallowness may be due to a very dense horizon, a high water table, or a shallow depth to bed-rock. Soils that have less strength also favor wind-throw of trees. Reduced soil strength may be due to increase soil moisture or due to the natural weakness of sandy mineral soils or organic peats. Wind-throw is a common phenomena in forested wetlands, particularly in areas of high saturation and organic soils.

This very large Douglas fir in coastal Oregon are less subject to wind-throw simply because of their great mass.

This very large redwood was probably the victim of an earthquake.

This tip up mound indicates where a tree succumbed to very wet, unstable soils.

Trees on very wet soils are often victims of wind throw because of shallow root systems combined with weak soil strengths.

Live oaks, due to their deep twisted root systems are less subject to hurricane damage than other oak species on similar sites.

Back to top EVALUATIONS OF FOREST SITES AND SOILS FOR TREE/STAND PRODUCTIVITY

EVALUATION OF PRODUCTIVITY
Ultimately, forest managers are generally interested in site and soil evaluations because they can use this to guide their management decisions. For example soil and site evaluations might be used to choose the most appropriate species to plant in a particular area, or to select the best rout for a hiking path, or to determine how quickly a landowner might be able to receive income from a thinning operation. There are numerous ways in which such evaluations can be made ranging from the simple to the complex.

EVALUATION OF PRODUCTIVITY - Based on Plants
Plant characteristics on a given site may serve as indices of site productivity. Species requirements for water and nutrients vary so that the tree species in a given area may indicate the relative productivity. For example if stand one is composed of northern red oak, yellow poplar, and white pine and stand two is composed of pitch pine, scarlet oak, and chestnut oak, then stand one is probably on a better site. This type of evaluation requires knowledge about species requirements and characteristics and local knowledge.

Longleaf pine is usually indicative of well drained sites. In this instance the longleaf pine have been planted on beds on a somewhat poorly drained site, pointing out a hazard of using species to indicate site quality.

Cherrybark oak is a very site specific tree and often indicates good nutrition and moderately well drained soils.

EVALUATION OF PRODUCTIVITY - Based on Plants
Species richness, or the number of species present in per unit area, can also be used as a simple index of site differences, if the sites to be compared are of similar age and management history. In general, better sites have greater tree species richness values. For example, non-eroded piedmont sites have greater species richness than do eroded piedmont sites of similar age and topography. Missing information regarding previous management often limits the usefulness of this technique.

The number of species present per acre in the bottomland hardwood stand (species richness) indicates that this site is relatively productive.

EVALUATION OF PRODUCTIVITY - Based on Plants
Understory species may also serve as indicators of site differences. For example running cedar is found most often on areas which were formerly tilled (and probably eroded) and dwarf palmetto is commonly restricted to somewhat poorly drained areas. This technique also requires considerable local experience.

Dwarf palmetto is a species that grows with in a relatively narrow range of soil moisture conditions and may sometimes be useful as an indicator species.

Pitcher plants are insect trapping plants that indicate sites having poor nutritional statuses. The plants trap insects and utilize their nitrogen.

The broom sedge in this abandoned field that is being converted to a hardwood plantation indicate an acidic soil conditions.

EVALUATION OF PRODUCTIVITY - Based on Plants
Basal Area is the cross-sectional area of woody vegetation measured at 4.5 feet above the ground. Basal area varies by species, stand density, stand age, and site quality. Basal area is not appropriate for inter-species comparisons, but can be used to compare site quality for similar species at similar ages and densities. For example if one a 50 year old bottomland hardwood stand had a basal area of 200 ft2/acre and another 50 year old had a basal area of 160 ft2/acre, then the second stand was either not fully stocked or was on a poorer site. The only major advantage of this technique is that it can be measured very quickly and easily.

The concept of basal area basically uses the cross-sectional area of standing trees to indicate stocking, volume, site quality, and other factors. In order to understand basal area, just consider the circular area of the ends of the logs. Basal area is merely this area for standing trees.

This individual is using a prism to quickly measure basal area.

This tupelo cypress stand on a good site had a very high average basal area: > 300 ft2/acre.

This cypress stand on a poor site had a very low average basal area: < 30 ft2/acre.

This white pine plantation had a high basal area of >250 ft2/acre.

EVALUATION OF PRODUCTIVITY - Based on Plants
Net Primary Productivity (NPP) is the sum of the above-ground and below-ground plant biomass produced per unit area. Measures of NPP are labor intensive, slow, and expensive, but NPP can be used for comparison of site quality for similar species. However, the stands should be of similar ages and general histories. This technique is often used by researchers for detailed investigations of site quality, but is seldom applied by land managers. Forest managers may sometimes use total merchantable volume per unit area (e.g., cords/acre, tons/acre) for general comparisons of site quality.

This person is sorting plant material by species before weighing them in order to estimate aboveground plant biomass per unit area..

EVALUATION OF PRODUCTIVITY - Based on Plants
Site index, the most commonly used estimate of forest site index, is defined as the total height of a dominant or co-dominant tree at a specified base age. On the west coast 100 years is a commonly used base age, in Virginia 25 years is the base age often used on intensively managed lands while 50 years serves as the base age on extensively managed lands. Site index is a good general technique because it is simple, inexpensive, is not strongly influenced by stand density, and provides good comparisons of site differences. Site index is not appropriate for stands where the largest trees have been harvested or situations such as abandoned agricultural lands where no trees are present.

These students are using an increment borer to estimate tree age.

A clinometer can be used to quickly and easily measure tree heights.

EVALUATION OF PRODUCTIVITY - Based on Plants
Foliage samples from trees can be analyzed for nutrient content, in certain specialized situations, such as Christmas tree plantation management,. The nutrient contents can then be used as indices of site adequacy. Unfortunately, the optimal nutrient requirements for many forest species are not known, thereby limiting the usefulness of this technique.

A typical Christmas tree plantation.

EVALUATION OF PRODUCTIVITY - Based on Soil
Topographic factors, such as slope percent, aspect, and micro-relief are sometimes used as an index of forest site quality in the Appalachian region. The three topographic factors are determined at a given location the appropriate value for each is selected from them. The summation of the three values provides a Forest Site Quality Index (FSQI) which can be used to estimate site index. This technique was developed for the Appalachian Mountain region and works best when applied to broad site types as opposed to small, specific areas. The method is simple, fast, and works fairly well, but its major weakness is that it is totally topography driven and does not take soil depth into account.

A clinometer is a useful tool for measuring slope percentages.

A hand compass is necessary for determining the aspect of a slope.

EVALUATION OF PRODUCTIVITY - Based on Soil
Baker and Broadfoot (1979) developed a technique for estimating site index for a variety of bottomland hardwood based on soil, topography, and land use history. Basically, the technique consists of answering a series of questions about the geology, nutritional status, aeration status, and moisture availability status based on factors such as depth of the A-horizon, micro-topographic position, and depth to reduced soil conditions. Each answer selected has a numeric value, the summation of all answer values provides an estimate of the site index for a particular species. Several modifications of this technique have been developed for estimation of site index when vegetation is either absent or is not of desirable species or quality.

EVALUATION OF PRODUCTIVITY - Based on Soil
The United States Department of Agriculture Natural Resource Conservation Service (NRCS), formerly the Soil Conservation Service (SCS), has evaluated, mapped, and interpreted soil information by county for many States. Approximately 3/4 of Virginia has soil maps and interpretations, known as soil surveys, which provide a wealth of information to forestland managers. For mapped counties, soils are mapped to the series level and estimates of slope class, soil depth, soil texture, site index, common tree species, appropriate trees for planting, site limitations, and many other site factors are provided. The soil survey, in the counties having a modern survey, is one of the best management and planning tools available.

EVALUATION OF PRODUCTIVITY - Based on Soil
In some situations, soil surveys are not suitable because of their age or they have not been completed. In these situations, a forest soil consultant can be hired to create a map based on the criteria specified by the landowner. Many large timber corporations routinely create their own soil database that they combine with a Geographical Information System to facilitate management decisions.

This GIS generated map is useful for rapidly evaluating site and soil conditions for large areas to facilitate planning.

EVALUATION OF PRODUCTIVITY - Based on Soil
For smaller tracts or smaller landowners it may not be practical to contract soil mapping. In these circumstances, two types of information about soils are often collected for management decisions. Surface soil samples may be collected and sent to state soil laboratory so that analyses of soil acidity and major required plant nutrients can be completed. Also, soil profile descriptions can be examined and interpreted in order to estimate site productivity. With experience, this technique can provide excellent interpretations and features that are commonly found to be important to tree growth are factors such as total soil depth, depth of the A horizon, presence of root restricting layers, and presence of anaerobic (waterlogged) horizons.

Soil augers are useful tools for evaluating soils without having to actually dig a soil pit.

SOIL/SITE MANAGEMENT
Soil management techniques to improve or maintain site productivity have been used for thousands of years and are most often associated with agricultural practices. However, soil management is also a common consideration in forests, although the intensity is generally far less than for agriculture.

One of the most basic ways in which forest managers address site and soil considerations is by selection of species that are most suitable for the site. For example, a forester might decide to replant the more drought tolerant longleaf pine on a dry sand hill and she might plant the more flood tolerant baldcypress in order to reforest a streamside management zone in an agricultural field.

SOIL/SITE MANAGEMENT - Organic Matter
Soil moisture management for forest land generally consists of protecting the litter layer and organic matter so that forest lands maintain high rainfall infiltration and minimize soil erosion. This effort is usually passive, or may consist of simply redistributing harvest residue across a site. In some relatively unique circumstances, forests have received organic matter additions, usually for nutritional or waste management reasons.

This machine is designed to grind slash and incorporate it into the soil profile.

SOIL/SITE MANAGEMENT - Irrigation
In certain situations, soil moisture management becomes more intense. Forest nurseries, where millions of seedlings may be grown annually for regeneration activities, irrigation is a common activity. Irrigation of forest stands in the United States is rare, but during the past decade, several large landowners have begun somewhat experimental irrigation efforts in order to produce fast growing, short rotation (<10 years) hardwoods on high quality sites.

A forest nursery with aboveground irrigation system.

SOIL/SITE MANAGEMENT - Drainage
Soil aeration/drainage is a much more commonly activity than irrigation. During the 1950’s – 1970’s large areas in the coastal plain were drained by installation of ditches and water control structures in order to increase the site index for pine plantations. Large scale drainage efforts have been reduced by the provisions of the Clean Water Act, but ditch maintenance is a commonplace activity. Creating elevated beds or mounds so that seedlings can become established on wetter sites is also a common soil management technique that addresses soil moisture and aeration.

SOIL/SITE MANAGEMENT - Fertilizer
Forest are very efficient at cycling nutrients from the soil to the plant, to the leaves, and back to the soil, therefore forest fertilization occurs on a relatively small percentage of the total forest. However, two fertilizers, nitrogen and phosphorus, are commonly applied to intensively managed, short rotation pine and hardwood plantations in the coastal plain region and these fertilizers can sometimes boost forest productivity by as much as 25%.

Forest nurseries, such as this cottonwood nursery, require regular fertilization.

This bulldozer has a fertilizer hopper mounted on the top of the blade that allows distribution of fertilizer, which is incorporated into the soil by the bedding plow pulled behind the bulldozer. This type of operation takes place just prior to planting.

Older stands may be fertilized by aerial applications.

SOIL/SITE MANAGEMENT - Compacted Soil
Some forest sites have compacted soil layers that benefit from tillage operations such as bedding, disking, or sub-soiling. These operations can reduce soil compaction by breaking up artificial hardpans and by incorporating organic matter into the soil, thereby reducing compaction and increasing moisture and nutrient holding capacity. Examples of areas that could be improved by tillage operations include an old agricultural field that has a plow or traffic pan or a compacted trail that was trafficked under wet conditions.

Back to top SUMMARY

The geologic parent materials in Virginia's physiographic provinces have experienced long term exposure to climatic forces and organisms as modified by topography that have produced specific soil conditions. The physical and chemical properties of these soils control the ability of the soils to provide water, nutrients, aeration, and stability for the growth of forest trees. Forest soils, due to the nature of the vegetation, presence of a litter layer, and reliance on nutrient cycling, are managed differently from agricultural soils. Quantification of site differences for tree productivity may entail use of plant, topography, or soil based indices. Such site differences are commonly used to evaluate forest site for subsequent management options. It is important to note that all of the major forest environment initiatives, such as the Sustainable Forestry Initiative, Forest Health, Forestry Best Management Practices, and Ecosystem Management, all use soil based values to judge the efficacy and sustainability of management regimes.

As a final qualifying test, all graduating foresters at Virginia Tech must be field tested with a shovel.

If you do not agree with the sentiments expressed on this T-shirt, then reread this section.

Back to top REFERENCES

Brady, N.C. and R.R. Weil. 1996. The Nature and Properties of Soils. 11th edition. Prentice Hall, New Jersey. 740 p.

Baker, J.B. and W.M. Broadfoot. 1979. Site Evaluation For Commercially Important Southern Hardwoods. USDA Forest Service, Southern Station General Technical Report SO-26, New Orleans, Louisiana.. 51 p.

Brooks, K.N., P.F Ffolliott, H.M. Gregersen, L. F. DeBano. !997. Hydrology and the Management of Watersheds, 2nd edition. Iowa State University Press, Ames, Iowa. 502 p.

Buol, S.W. 1973. Soils of the Southern States and Puerto Rico. Southern Cooperative Series Bulletin No. 174. USDA SCS in Cooperation with the Southern Agricultural Experiment Stations. 105 p.

Furman, R. W., D. A. Haines, and D. R. Miller. 1984. Forest Meteorology and Climatology, Chapter 3, p. 97-141. In Wenger, K.F., ed., 1984. Forestry Handbook. Wiley and Sons, New York. 1335 p.

Jenny, H. 1941. Factors of Soil Formation: A System of Quantitative Pedology. First edition. McGraw-Hill Book Company, New York. 281 p.

Klock, G.O., R. G. Cline, and D. N. Swanston. 1984. Geology and Soils, Chapter 2, p. 65- 96. In Wenger, K.F., ed., 1984. Forestry Handbook. Wiley and Sons, New York. 1335 p.

Pritchett, W.L. and R. F. Fisher. 1987. Properties and Management of Forest Soils. 2nd edition. Wiley and Sons, New York. 494 p.

Soil Science Society of America. 1997. Glossary of Soil Science Terms. Soil Science Society of America. Madison, Wisconsin. 134 p.

Sumner, Malcom E. Handbook of Soil Science. CRC Press, New York. 2081 p.

Back to top INTERESTING WEB LINKS

Virginia Department of Environmental Quality, provides soil and water related information

Virginia Department of Forestry, provides information regarding riparian buffers, water quality, forestry best management practices

US Environmental Protection Agency Office of Wetlands, Oceans, and Watersheds, provides lots of water related information.

Topographic maps for entire U.S.

USDA Forest Service handbook - Silvics of North America

Natural Resource Conservation Service, provides information about soil series, technical notes, soil survey manual, water and climate, data bases, maps, and more.

Soil Science Society of America, provides a detailed glossary of soil science terms.

Geology Department of William and Mary University, provides some nice figures and descriptions of the Physiography Provinces of Virginia.