#3 Read About Urban Soils
Edited by Len Phillips, updated January 2023
Sections You may go directly to the section by clicking on the titles listed here.
Edited by Len Phillips, updated January 2023
Sections You may go directly to the section by clicking on the titles listed here.
Note: Click on green text in each section for more information.
Urban Soils
The dynamic interaction between vegetation and soil is so strong that it's unclear which is dominant and which came first.
The Forest
The forests help secure and renew the soil. The forest covers and protects the soil from extreme heat and cold while slowing the natural forces of water, wind, and gravity. Crickets, moles, and mites also provide tunneling in the soil, along with thousands of other creatures that use leaf litter as a source of energy and habitat. Previous root and worm channels serve as pathways for root penetration, water, and air movement.
Forest Soil
Soil helps secure and renew the forest. Forest soil provides habitat for a high level of organism population, diversity, and activity. Time, weather, and soil-borne organisms break down decaying leaves, plants, and animals into nutrients that rebuild the soil, and again become available for forest growth. The soil is favorable for water movement and storage, as well as gaseous diffusion. Soil sustains the forest and provides raw materials for its life while the autumn leaves, woody debris, and dead animals accumulate and decay on and in the soil. In addition, all soils store water and nutrients and provide for the most critical functions of the tree.
Urban Soil
An urban soil on the other hand, is a soil material having a non-agricultural man-made surface layer more than 20 inches (50 cm) thick that has been produced by mixing, filling, or by contamination of forest soil. Urban soil is formed naturally or intensely modified by human activity in urban areas. Organic materials are generally removed from farms and forests in the form of topsoil and it is spread in a thin layer to be called a new lawn. In most cases the urban soil is exposed to temperature extremes. Limited organism activity results and some soil components are not able to function. Abrupt changes occur in urban soil density and porosity. Water movement into urban soil is limited, water storage is reduced, and aeration is restricted. Barriers to root penetration are present along with anthropogenic contaminants.
Urban soils present plant growth conditions entirely different from the natural soils that occur on an undisturbed forest or wild landscape. These conditions must be recognized and evaluated to gain an understanding about soil's relationship to tree growth. Interpretative information that is available from agriculture and silviculture cannot be generally extrapolated to urban soils. Therefore, some soil-plant concepts must be modified to meet urban soil needs. Only with a great deal of effort can a healthy soil system be restored to create a healthy root system and then to create a healthy tree. The soil along urban streets or in city parks must be improved by amendment or soil replacement such as CU Soil.
Native trees should only be installed in native soil while cultivars of native trees can be installed in the city.
Role of Soil Volume on Tree Growth
Where soil volume is limited by pavement, tree roots suffer. The highly compacted soils required for constructing pavements do not allow root penetration, resulting in poorly established and declining trees which are all too common in cities. Yet it is precisely these paved areas such as parking lots and streets that most need the benefits of shade trees. Healthy trees need a large volume of non-compacted soil with adequate drainage and aeration and reasonable fertility.
Root Growth in Urban Soils
When roots encounter dense soil, they change direction, stop growing, or adapt by remaining abnormally close to the surface. This surface rooting makes urban trees more vulnerable to drought and can cause pavement heaving. However, if a dense soil is waterlogged, tree roots can also rot from lack of oxygen and constant saturation.
Trees growing in conventional tree pits are surrounded by sidewalks and urban soils that are limited in the amount of soil that is suitable for proper root growth. As a result, trees blow down because the roots became contorted or growth was stunted. Tree roots also have a tendency to follow the weakness in the pavement, such as at the joints. It is at this point where there is sufficient moisture and air to permit root growth and the root expansion causes the sidewalk to heave and buckle.
Planting Sites
Planting sites should be large enough to accommodate the tree's roots at maturity. This means 4 square feet (4.4 sm) of surface area for every inch (2.5 cm) of diameter the tree is expected to attain, or 2 cubic feet (0.06 cm) of soil for every square foot (0.1 sm) of the future crown projection (which is the area under the drip line). The soil should also be about 36 inches (1 m) deep for maximum root penetration. In other words, the more surface area for root growth, the better. Sites with pavement and barriers to healthy root development should be avoided or trees should be selected that will tolerate a small root space. Proper installation techniques are essential for tree root survival.
Urban Soil Replacement
According to recent research, soil replacement using compost or compost-topsoil mix allowed root development in the replacement soils to be greater than in unmodified soils. However, roots did not grow beyond the composted soil and into the existing, unmodified urban soil. Therefore, unless the entire area for the mature tree root space can be improved, it is better to let the tree roots develop in unmodified soils, even though the growth rate will be much slower.
If the soil on an urban site is really bad and is composed only of concrete, metal, building materials, clay, etc. and the soil cannot be improved by amending, then it must be replaced if trees are to grow. It is possible to amend an existing poor soil so that its density is decreased. To accomplish this with inorganic amendments such as sand, approximately 75% by volume would need to be replaced to affect a positive change in drainage. Adding less than 75% actually decreases the porosity of the soil. It is important that any amendment is predominantly uniform and preferably large sized. The best amendment is one of nearly equal size particles so that when they touch each other, large pores are formed.
Potential Solutions
Man-made products such as structural soil makes the space under the sidewalk available for tree roots to find optimum growing conditions on the other side of the sidewalk. There are three well-known solutions to provide suitable soil for mature trees in the city.
The Amsterdam Tree Soil consists of 91% coarse sand, 5% organic matter and 4% clay. This soil is
especially suited for growing trees surrounded by sidewalks and other light bearing loads. The sand
provides the stable base for the walk construction while tree roots can utilize the minerals and organic
matter within the sand.
It is also worth noting that pavement trip hazards are not considered an impediment to pedestrian
movement in parts of Europe as they are in the U.S. Sand mixes are horticulturally better, structurally
almost equal, and far less costly than are other options. However, Amsterdam Tree Soil can be heavily
compacted by construction equipment. When this happens, the soil is no different than regular
compacted soil that is not suitable for tree roots.
Cornell University developed this product and obtained a patent for their mix as ‘CU-Structural Soil™’. The mix must be made by licensed, trained contractors in order to be successful. Success requires
skilled material selection, mixing, and placement. Proportions must be exactly right, and the soil
portion must be sticky enough not to fall out of the mix during placement.
CU-Soil™ can and should be used under pedestrian mall paving, sidewalks, parking lots, and low-use access roads. Any time trees are to be completely surrounded by pavement and adequate soil
volumes are not available, CU-Soil™ should be considered. Planting islands in parking lots can
accommodate more trees, shrubs, and herbaceous plants, without taking up a parking space, by using
CU-Soil™.
The weight of the paving and any surface loading is transferred downward by the cells to the firm soil
at the bottom of the planting pit. The uncompacted soil mix and tree roots fill the entire area around
the cells. Water can enter the system through irrigation, pervious paving, porous drains, and/or catch
basins thereby functioning as a stormwater retention system as well as the ideal growing medium for
city tree roots.
Many other options for growing trees along the city sidewalk are discussed in more detail in Topic 18.
Soil pH
Soil pH is a measure of the acidity or alkalinity of a soil and should be known to insure good tree health. A pH below 7, with 7 being neutral, would indicate an acid soil, and a pH above 7 indicates an alkaline soil. Trees have an optimal range of pH. Most trees thrive on an acidic soil with a pH between 5.5 and 6.5. Acidic soil pH is raised by adding calcium carbonate or lime.
Sulfur or aluminum sulfate should be used to bring down a high pH, but it is not a permanent solution. Instead, the installation of alkaline tolerant trees such as Redpointe® Maple Acer rubrum ‘Frank Jr.’ PP 16769 is a better option. Tree species that will tolerate a high pH should also be considered for areas of buried concrete, near foundations, or surrounded by concrete sidewalks.
Fertilizer and nutrients should be added to correct nutrient deficiencies and care should be taken that the soil does not contain herbicides or other contaminants. Amendments must be thoroughly mixed with the existing soil to a depth of 36 inches (90 cm).
Soil Temperature
Tree roots also need warm soil so they can continue to grow all year round. Even when the soil surface is frozen, the roots will continue growing until the soil temperature falls to about 40° – 45°F (7°C). Mulch in the urban landscape or leaves in the forest soil will protect the roots from soil temperature extremes.
A study at LaSalle University showed that soil protected under 8” (20 cm) of un-compacted snow (acting as a mulch) hovered at 32°F (0°C) until temperatures dropped past -18°F (-26°C) and only ¾” – 2” (2 to 5 cm) deep of soil freezing occurred at -40°F&C.
Soil Preparation
These tips will encourage tree survival by relying on root microorganisms prior to installation:
A specification for the ideal planting soil can be found on the Planting Soil Specification page of this website.
Soil Microorganisms
Soil microorganisms are necessary in all soils because they participate in the food web and supply nutrients in tree available forms. Microbes suppress disease organisms and reduce the potential for temperature and moisture stress. They also create humus, improve the soil structure, produce enzymes and hormones that help plants grow, and decompose pollutants in the soil.
Urban Soils
The dynamic interaction between vegetation and soil is so strong that it's unclear which is dominant and which came first.
The Forest
The forests help secure and renew the soil. The forest covers and protects the soil from extreme heat and cold while slowing the natural forces of water, wind, and gravity. Crickets, moles, and mites also provide tunneling in the soil, along with thousands of other creatures that use leaf litter as a source of energy and habitat. Previous root and worm channels serve as pathways for root penetration, water, and air movement.
Forest Soil
Soil helps secure and renew the forest. Forest soil provides habitat for a high level of organism population, diversity, and activity. Time, weather, and soil-borne organisms break down decaying leaves, plants, and animals into nutrients that rebuild the soil, and again become available for forest growth. The soil is favorable for water movement and storage, as well as gaseous diffusion. Soil sustains the forest and provides raw materials for its life while the autumn leaves, woody debris, and dead animals accumulate and decay on and in the soil. In addition, all soils store water and nutrients and provide for the most critical functions of the tree.
Urban Soil
An urban soil on the other hand, is a soil material having a non-agricultural man-made surface layer more than 20 inches (50 cm) thick that has been produced by mixing, filling, or by contamination of forest soil. Urban soil is formed naturally or intensely modified by human activity in urban areas. Organic materials are generally removed from farms and forests in the form of topsoil and it is spread in a thin layer to be called a new lawn. In most cases the urban soil is exposed to temperature extremes. Limited organism activity results and some soil components are not able to function. Abrupt changes occur in urban soil density and porosity. Water movement into urban soil is limited, water storage is reduced, and aeration is restricted. Barriers to root penetration are present along with anthropogenic contaminants.
Urban soils present plant growth conditions entirely different from the natural soils that occur on an undisturbed forest or wild landscape. These conditions must be recognized and evaluated to gain an understanding about soil's relationship to tree growth. Interpretative information that is available from agriculture and silviculture cannot be generally extrapolated to urban soils. Therefore, some soil-plant concepts must be modified to meet urban soil needs. Only with a great deal of effort can a healthy soil system be restored to create a healthy root system and then to create a healthy tree. The soil along urban streets or in city parks must be improved by amendment or soil replacement such as CU Soil.
Native trees should only be installed in native soil while cultivars of native trees can be installed in the city.
Role of Soil Volume on Tree Growth
Where soil volume is limited by pavement, tree roots suffer. The highly compacted soils required for constructing pavements do not allow root penetration, resulting in poorly established and declining trees which are all too common in cities. Yet it is precisely these paved areas such as parking lots and streets that most need the benefits of shade trees. Healthy trees need a large volume of non-compacted soil with adequate drainage and aeration and reasonable fertility.
Root Growth in Urban Soils
When roots encounter dense soil, they change direction, stop growing, or adapt by remaining abnormally close to the surface. This surface rooting makes urban trees more vulnerable to drought and can cause pavement heaving. However, if a dense soil is waterlogged, tree roots can also rot from lack of oxygen and constant saturation.
Trees growing in conventional tree pits are surrounded by sidewalks and urban soils that are limited in the amount of soil that is suitable for proper root growth. As a result, trees blow down because the roots became contorted or growth was stunted. Tree roots also have a tendency to follow the weakness in the pavement, such as at the joints. It is at this point where there is sufficient moisture and air to permit root growth and the root expansion causes the sidewalk to heave and buckle.
Planting Sites
Planting sites should be large enough to accommodate the tree's roots at maturity. This means 4 square feet (4.4 sm) of surface area for every inch (2.5 cm) of diameter the tree is expected to attain, or 2 cubic feet (0.06 cm) of soil for every square foot (0.1 sm) of the future crown projection (which is the area under the drip line). The soil should also be about 36 inches (1 m) deep for maximum root penetration. In other words, the more surface area for root growth, the better. Sites with pavement and barriers to healthy root development should be avoided or trees should be selected that will tolerate a small root space. Proper installation techniques are essential for tree root survival.
Urban Soil Replacement
According to recent research, soil replacement using compost or compost-topsoil mix allowed root development in the replacement soils to be greater than in unmodified soils. However, roots did not grow beyond the composted soil and into the existing, unmodified urban soil. Therefore, unless the entire area for the mature tree root space can be improved, it is better to let the tree roots develop in unmodified soils, even though the growth rate will be much slower.
If the soil on an urban site is really bad and is composed only of concrete, metal, building materials, clay, etc. and the soil cannot be improved by amending, then it must be replaced if trees are to grow. It is possible to amend an existing poor soil so that its density is decreased. To accomplish this with inorganic amendments such as sand, approximately 75% by volume would need to be replaced to affect a positive change in drainage. Adding less than 75% actually decreases the porosity of the soil. It is important that any amendment is predominantly uniform and preferably large sized. The best amendment is one of nearly equal size particles so that when they touch each other, large pores are formed.
Potential Solutions
Man-made products such as structural soil makes the space under the sidewalk available for tree roots to find optimum growing conditions on the other side of the sidewalk. There are three well-known solutions to provide suitable soil for mature trees in the city.
- "Amsterdam Tree Soil", a low-cost sand-based soil which can go to 85% to 90% Proctor Density (a standard lab test for compaction). Amsterdam Tree Soil has worked well in Europe. However in the U.S. there is a requirement for compaction to be at 95% for soils under pavement. Amsterdam soil would not be acceptable for agencies like highway departments in the U.S. Unfortunately, a sand-based system compacted to 95% Proctor Density severely limits root growth.
The Amsterdam Tree Soil consists of 91% coarse sand, 5% organic matter and 4% clay. This soil is
especially suited for growing trees surrounded by sidewalks and other light bearing loads. The sand
provides the stable base for the walk construction while tree roots can utilize the minerals and organic
matter within the sand.
It is also worth noting that pavement trip hazards are not considered an impediment to pedestrian
movement in parts of Europe as they are in the U.S. Sand mixes are horticulturally better, structurally
almost equal, and far less costly than are other options. However, Amsterdam Tree Soil can be heavily
compacted by construction equipment. When this happens, the soil is no different than regular
compacted soil that is not suitable for tree roots.
- 'CU-Structural Soil™' or CU-Soil™ is a moderately priced, crushed stone and soil mix that can achieve 95% compaction Proctor Density and still admit roots. CU-Soil™ consists of a mix of crushed stone for structural support, coated with a thin layer of soil for root growth. Voids in the compacted structural soil provide air and water sources for root development that allow roots to grow freely, deeper, and farther away from paved surfaces.
Cornell University developed this product and obtained a patent for their mix as ‘CU-Structural Soil™’. The mix must be made by licensed, trained contractors in order to be successful. Success requires
skilled material selection, mixing, and placement. Proportions must be exactly right, and the soil
portion must be sticky enough not to fall out of the mix during placement.
CU-Soil™ can and should be used under pedestrian mall paving, sidewalks, parking lots, and low-use access roads. Any time trees are to be completely surrounded by pavement and adequate soil
volumes are not available, CU-Soil™ should be considered. Planting islands in parking lots can
accommodate more trees, shrubs, and herbaceous plants, without taking up a parking space, by using
CU-Soil™.
- Soil Cells are the absolute best way to grow trees in the city, according to recent research. But soil cells are also the most expensive of these options presented above, even though they are made with recycled plastic products that suspend the pavement over the ideal soil used to nourish street trees and have the added benefit of providing stormwater retention. Each soil cell is composed of a frame with posts, and a deck or a stack of "pot-like" boxes, depending on the manufacturer selected. The frames or pots can be stacked to various heights and widths before they are topped with a deck that supports the hardscape at the surface. The frames are filled and surrounded with an optimum amount of quality soil volume for tree root growth and stormwater storage. A geotextile fabric and a layer of aggregate (thickness is variable depending on the final paving) are layered on top of the deck. Finally, the surface paving is added.
The weight of the paving and any surface loading is transferred downward by the cells to the firm soil
at the bottom of the planting pit. The uncompacted soil mix and tree roots fill the entire area around
the cells. Water can enter the system through irrigation, pervious paving, porous drains, and/or catch
basins thereby functioning as a stormwater retention system as well as the ideal growing medium for
city tree roots.
Many other options for growing trees along the city sidewalk are discussed in more detail in Topic 18.
Soil pH
Soil pH is a measure of the acidity or alkalinity of a soil and should be known to insure good tree health. A pH below 7, with 7 being neutral, would indicate an acid soil, and a pH above 7 indicates an alkaline soil. Trees have an optimal range of pH. Most trees thrive on an acidic soil with a pH between 5.5 and 6.5. Acidic soil pH is raised by adding calcium carbonate or lime.
Sulfur or aluminum sulfate should be used to bring down a high pH, but it is not a permanent solution. Instead, the installation of alkaline tolerant trees such as Redpointe® Maple Acer rubrum ‘Frank Jr.’ PP 16769 is a better option. Tree species that will tolerate a high pH should also be considered for areas of buried concrete, near foundations, or surrounded by concrete sidewalks.
Fertilizer and nutrients should be added to correct nutrient deficiencies and care should be taken that the soil does not contain herbicides or other contaminants. Amendments must be thoroughly mixed with the existing soil to a depth of 36 inches (90 cm).
Soil Temperature
Tree roots also need warm soil so they can continue to grow all year round. Even when the soil surface is frozen, the roots will continue growing until the soil temperature falls to about 40° – 45°F (7°C). Mulch in the urban landscape or leaves in the forest soil will protect the roots from soil temperature extremes.
A study at LaSalle University showed that soil protected under 8” (20 cm) of un-compacted snow (acting as a mulch) hovered at 32°F (0°C) until temperatures dropped past -18°F (-26°C) and only ¾” – 2” (2 to 5 cm) deep of soil freezing occurred at -40°F&C.
Soil Preparation
These tips will encourage tree survival by relying on root microorganisms prior to installation:
- Maintain a well-aerated soil environment because this stimulates mycorrhizal fungi activity and root growth.
- Permeability of the soil is essential if water, air, and roots are to move through the soil.
- Avoid using vermiculite and perlite as landscape installation amendments because they inhibit the formation of mycorrhizae essential for tree root development.
- Avoid overusing inorganic fertilizers, especially phosphorus, because they inhibit the formation of mycorrhizae.
- Avoid unnecessary use of fungicides unless there is a major outbreak of harmful fungi. Fungicides will kill good fungi as well as the bad fungi.
- The ideal soil for tree root development should be 25% air, 25% water, 5% organic matter, and 45% minerals (soil). By comparison, a typical urban soil is 12% air, 12% water, 1% organic and 75% minerals.
A specification for the ideal planting soil can be found on the Planting Soil Specification page of this website.
Soil Microorganisms
Soil microorganisms are necessary in all soils because they participate in the food web and supply nutrients in tree available forms. Microbes suppress disease organisms and reduce the potential for temperature and moisture stress. They also create humus, improve the soil structure, produce enzymes and hormones that help plants grow, and decompose pollutants in the soil.
Soil Properties
The mineral portion of soil is clay, silt, or sand depending on the particle size. Clay is the smallest and sand is the largest. Soils are named according to texture such as silty clay or loam, which is a soil that has a moderate amount of clay, silt, and sand. The smaller the soil particle size, the greater the water holding capacity of the soil.
Unscreened loam is preferred for best tree growth.
Soil Drainage
How do you know if you have well drained soil that is ideal for growing trees? The first step is to examine the grains of soil. Is it sandy? Is it moist with some organic material? Is it heavy clay and, therefore, wet and perhaps compacted? Construction practices such as cutting and filling, installation of underground utilities, and backfilling against foundations can create great diversity in soil structure. This variability can change drastically with depth and between installation locations.
The term soil structure refers to the manner in which the particles of soil are arranged. Soil texture and drainage are closely related. Sandy soils usually are very well drained but have large pore spaces and poor water-holding capabilities. They are usually associated with dry conditions. Conversely, clayey soils have much smaller pore spaces, are poorly drained, and can suffocate plant roots. The pore spaces in soil are very important for plant growth because the oxygen that occupies them is essential for healthy roots. A tree installed in poorly drained soil and very dry sandy soil will be slow to establish, lack vigor, and often will die very slowly. The arborist must look at a tree's capability to handle the drainage conditions of the soil.
Hydraulic Rate
To measure soil drainage rates, start by digging a narrow hole 12 – 18 inches (30 – 46 cm) deep. Use an auger or post-hole digger if you have one. The idea is to make a fairly small hole in the area where the roots will be. Next, get a yardstick and place it in the hole. Then fill the hole to the top with water. As soon as it has all drained out, fill the hole again and this time, count the inches of water that drains out of the hole in an hour.
Drainage Rates
Inches per Hour of Water Drop Drainage Condition
6 or more Rapid
3 – 6 Good
2 – 3 Fair
Less than 2 Poor
If the drainage is Rapid, you will have to consider irrigation to keep your trees in good growing condition. Also, plan on adding 12 inches (30 cm) of compost to the backfill material surface for mixing into the top 12 inches (30 cm) of topsoil so you end up with a root area soil that is 50% compost. You will also have to keep the tree mulched with compost, wood chips, etc. to replenish the compost in the soil every year. The best way to replenish the compost is to maintain a 2 – 3 inch (5 – 8 cm) layer of mulch on the surface of the previous year's mulch. You might also consider installing trees that do well in sandy soil.
If the drainage is Good, you are all set. However, to keep the soil this way, consider replenishing the soil with a 2 – 3 inch (5 – 8 cm) layer of composted organic material to the top of the previous year's mulch application.
If the drainage is Fair, consider deep aeration to loosen the subsoil. You may also have to use trees that are tolerant of poor draining soils such as red maple, (Acer rubrum) and its many cultivars. Again, compost will help loosen the soil by improving the soil structure. Another option would be to mix coarse sand into the top 36 inches (90 cm) of topsoil to improve the soil structure. Approximately 75% sand by volume would need to be added to affect a positive change in drainage.
If the drainage is Poor, simple soil modifications may not be an option. If a hardpan (a compacted, impermeable layer of soil with an underlying layer of well-drained soil) is present, a hole can be dug down through the hardpan to the permeable layer to provide drainage for the installation hole. If the soil is poorly drained and there is no well-drained layer below, a tile system or permeable pipe will need to be laid at least three feet (one meter) deep. This, however, is expensive and requires the assistance of a professional engineer or landscape architect for creating a proper design. This design will require perforated pipes around the tree pit, connected to a pipe that drains out at a lower elevation. The perforated pipe should be laid in a gravel filled trench. A second design is the construction of a French drain, which is a gravel filled trench. The French drain must also exit at a lower area nearby. Simply adding gravel to the bottom of the installation hole will further decrease oxygen availability to the root system and not solve the problem. Also, consider installing the trees above the existing soil as a raised bed with up to 2 inches (5 cm) of the root ball above grade. Since poor soil is frequent in urban landscapes, consider using something such as soil cells or CU-Structural Soil™ as well as trees that tolerate very poor and constantly wet soil. Recent developments with soil cells are using stormwater runoff directly from catch basins to water the trees. Surplus water flows out from an under-drain that feeds directly into the city's storm sewer.
Since the arborist may not have time to conduct the hydraulic rate test, a quick way to judge the soil drainage is to dig the planting hole. Then fill it half full of water. If the water takes more than an hour to drain, there is a problem, and so corrective action must be taken to provide proper drainage.
Site Assessment
The site conditions should be investigated to obtain information so that the planting design can take advantage of opportunities and avoid problems. The conditions include soil type and drainage, available water and sunlight, exposure to drying winds, and other factors. Attempting to match the requirements of the plant to the site increases the chance of survival, performance, and longevity of the plant selected.
Tree Soil Volume
Ideally, the roots should be able to grow at least to the drip line or crown edge of the tree at maturity. A soil volume of 2 to 3 cubic feet per 1 square foot of crown spread is recommended. Calculate crown projection by taking the mature crown spread of the tree (usually listed in nursery catalogs), squaring it, and multiplying by .7854. For every square foot of "crown projection", the site should have 2 cubic feet of soil available for rooting. For example, a tree that matures at a spread of 30' and has a crown projection of (30) ² x .7854 = 707 and will thus require 707 x 2 = 1,414 cubic feet of usable soil volume for root growth. In an average soil, expect roots to penetrate three feet deep. Use this standard depth as one of the three dimensions of a volume of soil. Back to the example, divide 1,414 by 3; this equals 471 square feet. The square root of 471 is approximately 22. This means a soil volume with dimensions of 22' x 22' x 3' is needed to ensure adequate below ground space for the tree in this example. This simple formula is ideal in the Midwest and Western parts of the US and Canada. However, it is an overestimate for trees growing in the eastern US, the Gulf Coast, and the Pacific Northwest where there is more rainfall. It also applies to all trees except those adapted to growing in very wet or very dry locations.
If time does not permit going through the process above, here is a shortcut:
1. trees larger than 50 feet tall or spread, need 2,700 cu. ft. of soil,
2. trees that grow 30 to 50 feet tall or spread, need 1200 cu. ft. of soil,
3. trees that grow less than 30 feet tall or spread, need 600 cu. ft. of soil,
4. trees that are to grow in structural soils or soil cells, need 1,500 cu. ft. of soil.
Soil Tests
Tests for texture, drainage, and aeration should always be done. Soil pH is one of the most useful soil tests, while iron and manganese contents are needed only under special circumstances. It is not necessary to make soil analysis for all sites. Remember that testing and analysis of contaminated sites requires much more laboratory work and could become a major budget item.
How does the landscape architect/arborist provide suitable soil conditions that will allow the tree's roots to grow and thrive in urban soils? When transplanting trees into urban soils, arborists and landscape architects should select the sites containing as much high quality soil as possible. High quality soils have adequate aeration, moisture, a stable pH, and a good amount of compost or organic matter. If good soils are not available, amendments such as inorganic fertilizer and soil conditioners should be added throughout the entire root zone of the mature tree. Biostimulants and surfactants can be added to the soil. Inoculants of mycorrhizae and beneficial bacteria may also be considered at planting time. Mature trees will also benefit from mycorrhizal injections into the soil before, during, or after a major construction project near their roots. One caution on using mycorrhizae injections – be sure the mycorrhizae are fresh and viable. Too often the products are delayed in shipping so mycorrhizae have died. This author has found positive results by taking a shovel full of soil close to the roots of a nearby tree of the same species being installed. The shovel full of soil is then sprinkled on the roots of the new tree.
The mineral portion of soil is clay, silt, or sand depending on the particle size. Clay is the smallest and sand is the largest. Soils are named according to texture such as silty clay or loam, which is a soil that has a moderate amount of clay, silt, and sand. The smaller the soil particle size, the greater the water holding capacity of the soil.
Unscreened loam is preferred for best tree growth.
Soil Drainage
How do you know if you have well drained soil that is ideal for growing trees? The first step is to examine the grains of soil. Is it sandy? Is it moist with some organic material? Is it heavy clay and, therefore, wet and perhaps compacted? Construction practices such as cutting and filling, installation of underground utilities, and backfilling against foundations can create great diversity in soil structure. This variability can change drastically with depth and between installation locations.
The term soil structure refers to the manner in which the particles of soil are arranged. Soil texture and drainage are closely related. Sandy soils usually are very well drained but have large pore spaces and poor water-holding capabilities. They are usually associated with dry conditions. Conversely, clayey soils have much smaller pore spaces, are poorly drained, and can suffocate plant roots. The pore spaces in soil are very important for plant growth because the oxygen that occupies them is essential for healthy roots. A tree installed in poorly drained soil and very dry sandy soil will be slow to establish, lack vigor, and often will die very slowly. The arborist must look at a tree's capability to handle the drainage conditions of the soil.
Hydraulic Rate
To measure soil drainage rates, start by digging a narrow hole 12 – 18 inches (30 – 46 cm) deep. Use an auger or post-hole digger if you have one. The idea is to make a fairly small hole in the area where the roots will be. Next, get a yardstick and place it in the hole. Then fill the hole to the top with water. As soon as it has all drained out, fill the hole again and this time, count the inches of water that drains out of the hole in an hour.
Drainage Rates
Inches per Hour of Water Drop Drainage Condition
6 or more Rapid
3 – 6 Good
2 – 3 Fair
Less than 2 Poor
If the drainage is Rapid, you will have to consider irrigation to keep your trees in good growing condition. Also, plan on adding 12 inches (30 cm) of compost to the backfill material surface for mixing into the top 12 inches (30 cm) of topsoil so you end up with a root area soil that is 50% compost. You will also have to keep the tree mulched with compost, wood chips, etc. to replenish the compost in the soil every year. The best way to replenish the compost is to maintain a 2 – 3 inch (5 – 8 cm) layer of mulch on the surface of the previous year's mulch. You might also consider installing trees that do well in sandy soil.
If the drainage is Good, you are all set. However, to keep the soil this way, consider replenishing the soil with a 2 – 3 inch (5 – 8 cm) layer of composted organic material to the top of the previous year's mulch application.
If the drainage is Fair, consider deep aeration to loosen the subsoil. You may also have to use trees that are tolerant of poor draining soils such as red maple, (Acer rubrum) and its many cultivars. Again, compost will help loosen the soil by improving the soil structure. Another option would be to mix coarse sand into the top 36 inches (90 cm) of topsoil to improve the soil structure. Approximately 75% sand by volume would need to be added to affect a positive change in drainage.
If the drainage is Poor, simple soil modifications may not be an option. If a hardpan (a compacted, impermeable layer of soil with an underlying layer of well-drained soil) is present, a hole can be dug down through the hardpan to the permeable layer to provide drainage for the installation hole. If the soil is poorly drained and there is no well-drained layer below, a tile system or permeable pipe will need to be laid at least three feet (one meter) deep. This, however, is expensive and requires the assistance of a professional engineer or landscape architect for creating a proper design. This design will require perforated pipes around the tree pit, connected to a pipe that drains out at a lower elevation. The perforated pipe should be laid in a gravel filled trench. A second design is the construction of a French drain, which is a gravel filled trench. The French drain must also exit at a lower area nearby. Simply adding gravel to the bottom of the installation hole will further decrease oxygen availability to the root system and not solve the problem. Also, consider installing the trees above the existing soil as a raised bed with up to 2 inches (5 cm) of the root ball above grade. Since poor soil is frequent in urban landscapes, consider using something such as soil cells or CU-Structural Soil™ as well as trees that tolerate very poor and constantly wet soil. Recent developments with soil cells are using stormwater runoff directly from catch basins to water the trees. Surplus water flows out from an under-drain that feeds directly into the city's storm sewer.
Since the arborist may not have time to conduct the hydraulic rate test, a quick way to judge the soil drainage is to dig the planting hole. Then fill it half full of water. If the water takes more than an hour to drain, there is a problem, and so corrective action must be taken to provide proper drainage.
Site Assessment
The site conditions should be investigated to obtain information so that the planting design can take advantage of opportunities and avoid problems. The conditions include soil type and drainage, available water and sunlight, exposure to drying winds, and other factors. Attempting to match the requirements of the plant to the site increases the chance of survival, performance, and longevity of the plant selected.
Tree Soil Volume
Ideally, the roots should be able to grow at least to the drip line or crown edge of the tree at maturity. A soil volume of 2 to 3 cubic feet per 1 square foot of crown spread is recommended. Calculate crown projection by taking the mature crown spread of the tree (usually listed in nursery catalogs), squaring it, and multiplying by .7854. For every square foot of "crown projection", the site should have 2 cubic feet of soil available for rooting. For example, a tree that matures at a spread of 30' and has a crown projection of (30) ² x .7854 = 707 and will thus require 707 x 2 = 1,414 cubic feet of usable soil volume for root growth. In an average soil, expect roots to penetrate three feet deep. Use this standard depth as one of the three dimensions of a volume of soil. Back to the example, divide 1,414 by 3; this equals 471 square feet. The square root of 471 is approximately 22. This means a soil volume with dimensions of 22' x 22' x 3' is needed to ensure adequate below ground space for the tree in this example. This simple formula is ideal in the Midwest and Western parts of the US and Canada. However, it is an overestimate for trees growing in the eastern US, the Gulf Coast, and the Pacific Northwest where there is more rainfall. It also applies to all trees except those adapted to growing in very wet or very dry locations.
If time does not permit going through the process above, here is a shortcut:
1. trees larger than 50 feet tall or spread, need 2,700 cu. ft. of soil,
2. trees that grow 30 to 50 feet tall or spread, need 1200 cu. ft. of soil,
3. trees that grow less than 30 feet tall or spread, need 600 cu. ft. of soil,
4. trees that are to grow in structural soils or soil cells, need 1,500 cu. ft. of soil.
Soil Tests
Tests for texture, drainage, and aeration should always be done. Soil pH is one of the most useful soil tests, while iron and manganese contents are needed only under special circumstances. It is not necessary to make soil analysis for all sites. Remember that testing and analysis of contaminated sites requires much more laboratory work and could become a major budget item.
How does the landscape architect/arborist provide suitable soil conditions that will allow the tree's roots to grow and thrive in urban soils? When transplanting trees into urban soils, arborists and landscape architects should select the sites containing as much high quality soil as possible. High quality soils have adequate aeration, moisture, a stable pH, and a good amount of compost or organic matter. If good soils are not available, amendments such as inorganic fertilizer and soil conditioners should be added throughout the entire root zone of the mature tree. Biostimulants and surfactants can be added to the soil. Inoculants of mycorrhizae and beneficial bacteria may also be considered at planting time. Mature trees will also benefit from mycorrhizal injections into the soil before, during, or after a major construction project near their roots. One caution on using mycorrhizae injections – be sure the mycorrhizae are fresh and viable. Too often the products are delayed in shipping so mycorrhizae have died. This author has found positive results by taking a shovel full of soil close to the roots of a nearby tree of the same species being installed. The shovel full of soil is then sprinkled on the roots of the new tree.
Soil Compaction
Soil compaction by vehicles or people can reduce pore space and restrict water infiltration, as well as cause physical damage to the roots of existing trees. In compacted soil, oxygen is depleted, carbon dioxide accumulates, and root penetration is reduced. This is detrimental to root growth. Roots need oxygen to grow and survive. Crushed soil pores no longer provide roots with a source of oxygen.
Pore space exists around soil particles, structural units, and the interfaces of infrastructure and different soil types. Large sized soil pores found in sandy soils are usually filled with air pockets, and so provide good aeration but poor water holding capacity. Small soil pores found in clay soils are usually filled with water, and conversely have large water holding capacity but poor aeration. Soils dominated by small soil pores have more total pore space than soils dominated by large pores. Small or capillary pores are divided between tree-available water-filled pores and tree-unavailable water-filled pores. The tree-unavailable water resides in the smallest soil pores where the tree cannot exert enough suction through transpiration to pull the pore water into the root.
There are three attributes of soils:
1. Soil depth – With increasing soil depth there is an increase in carbon dioxide (CO2) concentrations and a decrease in oxygen (O2 ) concentrations. The balance between these two gases will vary depending on the water content and the biological activity.
2. Organic matter – Organic matter provides cationic exchange, water holding capacity, essential elements, detritus (waste) food, and pore space. Organic matter is deposited on the surface as plant litter or near the soil surface as roots breakdown. The decomposing materials then move downward through the soil.
3. Developed structure – The basic soil particles (sand, silt, and clay) are held together in clumps or structural units. Between structural aggregates are soil pore spaces utilized by tree roots as they capture the water and O2 trapped in the pores.
Compaction
Here are the principal reasons to compact soil:
When the soil is randomly fluffed with a pitch fork, the amount of pore space will be 45% to 55%. When the soil particles are pushed closer together, the pore space is removed and the bulk density will increase. Compaction increases the bulk density by reducing the pore space. Compaction is a soil condition that makes it impossible for trees to grow because it prevents roots from penetrating the soil which causes systemic damage and decline of the tree. An ideal soil has 50% pore space, divided among air-filled and water-filled pores. In addition, 45% of an ideal soil is composed of mineral materials and 5% organic material. This ideal does not exist in a compacted soil. The volume of soil space controlled by tree roots is directly related to tree health. In heavily compacted sites, roots cannot grow normally and instead will be concentrated around the edges of infrastructures and filling any moist air space.
Compaction creates many negative impacts:
Components of Compaction
The components of soil compaction do not necessarily occur in order, or on any given soil.
The following components represent the extent and depth of a damaged top surface layer. They can generate soil conditions difficult for tree health.
Measuring Compaction
The primary resources critical to tree growth in the soil are oxygen availability, gas exchange with the atmosphere, and soil strength values. The most commonly used measure for soil compaction is bulk density.
Bulk density is the weight of the soil per unit volume (usually in g/cc). As bulk density increases, total pore space declines and aeration pore space is destroyed. Bulk density as a measure of soil compaction rapidly increases with the first few impacts on the soil surface and then levels-off. In other words, it is not years of traffic, but the first 4 trips that cause the majority of the compaction damage to the soil.
Biological Disruptions
Compaction disrupts the respiration process that powers every function of the tree. As a result, growth regulators are destroyed prematurely or allowed to buildup, causing changes in tissue reactions. Carbon allocation patterns following highly modified growth regulation patterns will change food production, storage, use, and transport processes.
The presence of toxic materials can be highly disruptive to soil health. As oxygen concentrations decline, reduced compounds are generated by the tree roots and associated soil organisms. Reduced compounds are chemicals which are produced in the absence of oxygen. These reduced compounds can build up and damage organisms and move the soil toward anaerobic conditions. In normal soils, these materials, if produced at all, are quickly oxidized or removed from nearby tree roots.
Structural Disruptions
The structure of the tree can also be directly and indirectly impacted by compacted soils. Root decline and death can lead to catastrophic structural failures. Top and root dieback as well as branch drop can also occur. Reduced rooting volume mechanically destabilizes the whole tree.
Compaction Effects
Major soil compaction effects on trees are defined below:
In response to increased compaction, roots thicken in diameter. Thicker roots exert more force and penetrate farther into compacted soil areas. As roots thicken, growth slows and more laterals are generated. Lateral root tip diameters are dependent upon initiation by growth regulators and vascular tissue connections. If laterals are small enough to fit into the pore sizes of the compacted soil, then lateral growth will continue while the main axis of the root is constrained. If the soil pore sizes are too small for even the lateral roots, root growth will cease.
Soil compaction by vehicles or people can reduce pore space and restrict water infiltration, as well as cause physical damage to the roots of existing trees. In compacted soil, oxygen is depleted, carbon dioxide accumulates, and root penetration is reduced. This is detrimental to root growth. Roots need oxygen to grow and survive. Crushed soil pores no longer provide roots with a source of oxygen.
Pore space exists around soil particles, structural units, and the interfaces of infrastructure and different soil types. Large sized soil pores found in sandy soils are usually filled with air pockets, and so provide good aeration but poor water holding capacity. Small soil pores found in clay soils are usually filled with water, and conversely have large water holding capacity but poor aeration. Soils dominated by small soil pores have more total pore space than soils dominated by large pores. Small or capillary pores are divided between tree-available water-filled pores and tree-unavailable water-filled pores. The tree-unavailable water resides in the smallest soil pores where the tree cannot exert enough suction through transpiration to pull the pore water into the root.
There are three attributes of soils:
1. Soil depth – With increasing soil depth there is an increase in carbon dioxide (CO2) concentrations and a decrease in oxygen (O2 ) concentrations. The balance between these two gases will vary depending on the water content and the biological activity.
2. Organic matter – Organic matter provides cationic exchange, water holding capacity, essential elements, detritus (waste) food, and pore space. Organic matter is deposited on the surface as plant litter or near the soil surface as roots breakdown. The decomposing materials then move downward through the soil.
3. Developed structure – The basic soil particles (sand, silt, and clay) are held together in clumps or structural units. Between structural aggregates are soil pore spaces utilized by tree roots as they capture the water and O2 trapped in the pores.
Compaction
Here are the principal reasons to compact soil:
- Increase load-bearing capacity
- Provide stability
- Prevent soil settlement and frost damage
- Reduce water seepage, swelling, and contraction
- Reduce the settling of soil
When the soil is randomly fluffed with a pitch fork, the amount of pore space will be 45% to 55%. When the soil particles are pushed closer together, the pore space is removed and the bulk density will increase. Compaction increases the bulk density by reducing the pore space. Compaction is a soil condition that makes it impossible for trees to grow because it prevents roots from penetrating the soil which causes systemic damage and decline of the tree. An ideal soil has 50% pore space, divided among air-filled and water-filled pores. In addition, 45% of an ideal soil is composed of mineral materials and 5% organic material. This ideal does not exist in a compacted soil. The volume of soil space controlled by tree roots is directly related to tree health. In heavily compacted sites, roots cannot grow normally and instead will be concentrated around the edges of infrastructures and filling any moist air space.
Compaction creates many negative impacts:
- The volume of ecologically active space is decreased.
- Root space is decreased and made more shallow.
- The food web is disrupted and modified.
- The diversity of living organisms decline.
- Beneficial microorganism associations are eliminated and a few ecological niche generalists succeed.
- Pests (including harmful fungi), favored by the new conditions, consume desirable organisms, and roots are unable to defend themselves.
- Tree roots become more prone to damage and attack at a time when sensor, defense, growth regulation, and carbon allocation processes are functioning at reduced levels.
Components of Compaction
The components of soil compaction do not necessarily occur in order, or on any given soil.
- Compression is most prevalent in wet soils. Compression occurs when large air-filled pore spaces are crushed leading to small water-filled pores.
- Compaction occurs from the movement of sand, silt, and clay particles, destruction of aggregates, and collapse of aeration pores.
- Consolidation is the deformation of the soil destroying any pore space and structure as water is squeezed from the soil matrix. This process leads to increase internal bonding and pore space is eliminated.
The following components represent the extent and depth of a damaged top surface layer. They can generate soil conditions difficult for tree health.
- Crusting is the packing of fine particles and organic matter on the soil surface, preventing water and oxygen infiltration. The primary cause of crusting is the impact of raindrops or sprinkler water.
- Puddling and rutting are a dense crust on the soil surface caused by pressure from foot and vehicle traffic. In saturated soils, under a surface load, there is no place for non-compressible water to go except to the side, squashing soil structure and eliminating pores.
Measuring Compaction
The primary resources critical to tree growth in the soil are oxygen availability, gas exchange with the atmosphere, and soil strength values. The most commonly used measure for soil compaction is bulk density.
Bulk density is the weight of the soil per unit volume (usually in g/cc). As bulk density increases, total pore space declines and aeration pore space is destroyed. Bulk density as a measure of soil compaction rapidly increases with the first few impacts on the soil surface and then levels-off. In other words, it is not years of traffic, but the first 4 trips that cause the majority of the compaction damage to the soil.
Biological Disruptions
Compaction disrupts the respiration process that powers every function of the tree. As a result, growth regulators are destroyed prematurely or allowed to buildup, causing changes in tissue reactions. Carbon allocation patterns following highly modified growth regulation patterns will change food production, storage, use, and transport processes.
The presence of toxic materials can be highly disruptive to soil health. As oxygen concentrations decline, reduced compounds are generated by the tree roots and associated soil organisms. Reduced compounds are chemicals which are produced in the absence of oxygen. These reduced compounds can build up and damage organisms and move the soil toward anaerobic conditions. In normal soils, these materials, if produced at all, are quickly oxidized or removed from nearby tree roots.
Structural Disruptions
The structure of the tree can also be directly and indirectly impacted by compacted soils. Root decline and death can lead to catastrophic structural failures. Top and root dieback as well as branch drop can also occur. Reduced rooting volume mechanically destabilizes the whole tree.
Compaction Effects
Major soil compaction effects on trees are defined below:
- Reduced Elongation Growth – As compaction increases, roots are physically prevented from elongating into the soil by a lack of oxygen because of decreasing pore size and increased soil strength. As roots are put under pressure, elongation slows and stops.
- Essential Element Collection Problems – With less soil volume, there is less physical space to collect
resources. - Shallow Rooting – As roots survive in a steadily diminishing aerobic layer, and as the anaerobic layer expands toward the surface, the physical space available for living roots decline, which means roots are subject to greater stresses.
- Constrained Size of Root Systems – Compaction limits the depth and reach of the tree root systems leading to greater probability of wind throw and problems near the flare.
- Stunted Tree Form – As resources are limited by soil compaction, trees become stunted. Carbohydrate and protein synthesis rates enter decline cycles interfering with nitrogen and phosphorous uptake. The result is a tree with a limited ability to take advantage of resource availability.
- Seedling Survival Problems – Variability in compaction levels constrain newly installed trees. Less bulk density increase and crusting effect are needed for failure of younger trees compared with older, established trees.
- Root Crushing and Shearing-off – The mechanical forces generated in compacting a soil can crush roots, especially small ones. Larger roots can be damaged. Vehicle rutting can shear-off roots as soil is pushed to new locations.
- Soil compaction reduces aerobes and favors low oxygen requiring organisms, like Pythium and Phytophthora root rots or anaerobes.
In response to increased compaction, roots thicken in diameter. Thicker roots exert more force and penetrate farther into compacted soil areas. As roots thicken, growth slows and more laterals are generated. Lateral root tip diameters are dependent upon initiation by growth regulators and vascular tissue connections. If laterals are small enough to fit into the pore sizes of the compacted soil, then lateral growth will continue while the main axis of the root is constrained. If the soil pore sizes are too small for even the lateral roots, root growth will cease.
Dealing with Compaction
Recovery of soil to pre-compaction conditions may not be possible. However, one should consider the following.
Some techniques for renovating compacted soils are:
Prevention Techniques
During construction, the following tips should be implemented to prevent soil compaction:
Compaction Tolerant Trees
Among trees, there is a great variability in reactions to soil compaction. Compaction tolerant trees are tolerant of poor drainage. No tree is tolerant of high soil bulk density. A tree's ability to tolerate compacted soil conditions is associated with four mechanisms:
1) reaction to mechanical damage is effective and fast,
2) continuation of respiration under chronic oxygen shortages,
3) ability to continue to adjust absorbing root systems,
4) ability to deal with chemically reduced toxins.
Recovery of soil to pre-compaction conditions may not be possible. However, one should consider the following.
- Soil compaction should be considered permanent. Studies demonstrate that after one-half century, compaction still afflicts soils under natural forest conditions. Notice during a walk through the woods, how old logging roads are still visible and are today, considered "hiking trails".
- Every urban soil has a compacted layer. Changing management may not change the current compacted zone but could even add a compacted zone.
- Management activities should increase aeration space and reduce soil strength.
- Alleviation of future soil compaction should be part of a soil health plan.
- Seek the assistance of a tree and soil specialist to avoid problems on compacted soils.
- Replace the compacted soil with new soil or build up new soil over the compacted layers.
Some techniques for renovating compacted soils are:
- Aerating the soil with a mechanical aerator, will help correct the problem if it is on the surface and compaction is within the depth of the aerator tines.
- Restarting or improving the detritus energy web in the soil including addition of organic matter and living organisms.
- Vertical mulching, which is a technique that can be used to partially alleviate soil compaction within the CRZ of trees. Vertical mulching is done using a power auger with a 2-inch (5 cm) diameter drill bit. Starting about 8 feet (2.4 m) out from the trunk, drill holes 12 inches (30 cm) deep on an 18-inch (48 cm) grid out to the drip line while trying to avoid damaging roots. The holes should be back-filled with pea gravel, sand, or a mixture with compost.
- Installing drainage systems at the point where compacted soil meets non-compacted soil to drain
water from the root zone. - Cutting radial trenches away from the tree by digging slowly until roots are encountered. From the roots, another 10 feet (3 m) of trench should be dug at least 12 inches (0.3 m) deep sloping downward to a depth of 24 inches (0.6 m). Be sure that there is adequate drainage at the deepest point of the trench. The soil that has been removed can be modified to improve its density before putting it back into the
trench and watered. Turf should also be removed and replaced with mulch. - Tilling farm soil regularly reduces compaction.
- Injecting compressed air in a process known as "fracturing".
- Using a pneumatic soil excavating tool to remove all soil from over the roots and replacing it with a 50:50 mix of soil and compost. The replacement process should be done without causing new compaction.
- Using commercial surfactants to improve water penetration, especially when irrigating or applying fertilizers.
- Sub-soiling techniques, which are useful in treating large areas of compacted soil.
- Remove mulch in the autumn and encourage frost to penetrate soil deeply with unfilled holes punched through the compacted soil. Stay off the bare soil until late spring when the soil has dried and the deep frost has thawed. Avoid re-compacting the soil when replacing the mulch.
- Recent research has found that a backhoe with tines can be used to dig an oversized tree pit and then the soil is replaced with a mix of compost and the excavated soil by dropping the soil into the pit with the backhoe bucket. The soil should be allowed to fall into the tree pit but not compacted.
- The most important thing to encourage rapid recovery from transplanting in compacted soil is to loosen soil around the installation hole in a 10 to 15 foot (3 – 4 m) width. Locate about 25% of the root ball above the surrounding landscape soil by installing the tree on a mound of soil.
Prevention Techniques
During construction, the following tips should be implemented to prevent soil compaction:
- Protect the ecological "foot print" of the tree rooting area or critical root zone (CRZ).
- Restrict site access to the soil surface with fences and fines (for construction code violations).
- Select working conditions (dry, dormant season, surface mulch, etc) that minimize compaction.
- Restrict vibration compaction where possible.
- Try to soften and distribute compaction forces with temporary heavy mulch, plywood driving pads,
and soil moisture content awareness planning.
Compaction Tolerant Trees
Among trees, there is a great variability in reactions to soil compaction. Compaction tolerant trees are tolerant of poor drainage. No tree is tolerant of high soil bulk density. A tree's ability to tolerate compacted soil conditions is associated with four mechanisms:
1) reaction to mechanical damage is effective and fast,
2) continuation of respiration under chronic oxygen shortages,
3) ability to continue to adjust absorbing root systems,
4) ability to deal with chemically reduced toxins.
Soil Bulk Density
The combined influence of soil texture and structure may best be described by the term soil bulk density. Density is the mass of material contained within a given volume. The bulk density takes into account the total soil volume, which is the space occupied by the solid particles plus the pore space occupied by air and/or water.
Soil Bulk Density Determination
Determining soil bulk density requires heating it so that it dries out completely. Weigh the oven dry soil on a balance scale. Pouring the soil into a graduated cylinder and measuring the volume that it occupies can determine the volume of the soil. Bulk density in soils is normally expressed in g/cm3 (weight divided by volume), just like the idea that a box may be heavy or light, depending upon what kind of material is in it. The maximum compaction rate of soil for tree growth would have a bulk density of 1.4 to 1.6 g/cm3. An open friable soil with good organic matter content will have a bulk density of less than 1.0 g/cm3.
Calculate dry bulk density following the equation:
Dry bulk density (g/cm3) = weight of dry soil
volume of sample
Significance of Bulk Density
The bulk density of the soil will play an important role in determining if the soil has the physical characteristics necessary for plant growth, foundations for structures, or other uses. The weight of the soil is also important for lifting or hauling it long distances. Sandy soil has a very high bulk density. Organic soil or compost has a very low bulk density. One of the main reasons that sod farms operate on organic soils is to reduce the cost of transportation because the organic soil is so much lighter and easier to handle than sand or mineral soils.
Sometimes knowing the weight of an acre of soil is desired for erosion comparison purposes. Erosion of 5 tons (4.6 metric T) per acre sounds like a lot. However the weight of an acre furrow slice on average is 1 million tons, so 5 tons per acre by comparison is a very small amount. (Note: an acre furrow slice is one acre in area and 7 inches [18 cm] deep.) Five tons per acre is only about 0.034 inches (0.085cm) thick.
Soil Factors Impacting Weight
Another factor similar to carrying soil is its potential to be moved short distances, such as in B&B tree balls. The soil that is heavier will be more difficult to move. Related to this is the soil's consistency.
Soil Consistency
Soil consistency is the soil's ability to cohere or stick together. The soil's consistency may be evaluated at three moisture conditions: air dry, moist, and wet. Clay soil is noted for its stickiness and large energy requirements in tillage. Farmers refer to it as "heavy," but they really mean it is difficult to plow, not that it has a high bulk density. Clay soils generally will have a lower bulk density than sandy soils (clay=1.3 g/cm3, sand=1.6 g/cm3). Sandy soils have a higher bulk density, but are easier to plow since they have weaker consistency. Thus they are often referred to as "light soils". Tilling the soil also reduces the bulk density by "fluffing" the soil.
Terms commonly used to describe soil consistency are:
Loose – Non-coherent when dry or moist; does not hold together in a mass.
Friable – When moist, crushes easily under gentle pressure between thumb and forefinger and can be pressed together into a lump.
Firm – When moist, crushes under moderate pressure between thumb and forefinger, but resistance is
distinctly noticeable.
Plastic – When wet, readily deforms by moderate pressure but can be pressed into a lump; will form a "wire" when rolled between thumb and forefinger.
Sticky – When wet, adheres to other material and tends to stretch somewhat and pull apart rather than to pull free from other material.
Hard – When dry, moderately resistant to pressure; can be broken with difficulty between thumb and forefinger.
Soft – When dry, breaks into powder or individual grains under very slight pressure.
Cemented – Hard; affected little by moistening.
Porosity
Bulk density is an indirect measure of soil pore space. Under field conditions, pore spaces are occupied at all times by air and/or water. Porosity, when expressed as a percent, is the same thing as percent pore space. Soil particles have irregular shapes, and thus the pores between them vary irregularly in size, shape, and area. Sandy soils have large continuous pores while clays have small pores that transmit water slowly.
Soil Temperature Regimes
The temperature of a given soil at a given time is dependent upon the gains and losses of heat energy. Generally, dark surfaces will absorb more heat than light surfaces. However, the amount of water in the soil is an also important factor in moderating soil temperature. Losses of the absorbed heat are by radiation back into the atmosphere as long-wave radiation, heating the air and cooling the soil.
Soil temperature regimes are used to classify soils. They are defined according to the average annual soil temperature in the root zone. The use of soils for agriculture and forestry is closely related to soil temperature, due to the specific requirements of plants. Over most of the earth, daily soil temperatures below 20 inches (50 cm) deep seldom change. To approximate the mean annual soil temperature, 2Fº (1Cº) is added to the mean annual air temperature.
The combined influence of soil texture and structure may best be described by the term soil bulk density. Density is the mass of material contained within a given volume. The bulk density takes into account the total soil volume, which is the space occupied by the solid particles plus the pore space occupied by air and/or water.
Soil Bulk Density Determination
Determining soil bulk density requires heating it so that it dries out completely. Weigh the oven dry soil on a balance scale. Pouring the soil into a graduated cylinder and measuring the volume that it occupies can determine the volume of the soil. Bulk density in soils is normally expressed in g/cm3 (weight divided by volume), just like the idea that a box may be heavy or light, depending upon what kind of material is in it. The maximum compaction rate of soil for tree growth would have a bulk density of 1.4 to 1.6 g/cm3. An open friable soil with good organic matter content will have a bulk density of less than 1.0 g/cm3.
Calculate dry bulk density following the equation:
Dry bulk density (g/cm3) = weight of dry soil
volume of sample
Significance of Bulk Density
The bulk density of the soil will play an important role in determining if the soil has the physical characteristics necessary for plant growth, foundations for structures, or other uses. The weight of the soil is also important for lifting or hauling it long distances. Sandy soil has a very high bulk density. Organic soil or compost has a very low bulk density. One of the main reasons that sod farms operate on organic soils is to reduce the cost of transportation because the organic soil is so much lighter and easier to handle than sand or mineral soils.
Sometimes knowing the weight of an acre of soil is desired for erosion comparison purposes. Erosion of 5 tons (4.6 metric T) per acre sounds like a lot. However the weight of an acre furrow slice on average is 1 million tons, so 5 tons per acre by comparison is a very small amount. (Note: an acre furrow slice is one acre in area and 7 inches [18 cm] deep.) Five tons per acre is only about 0.034 inches (0.085cm) thick.
Soil Factors Impacting Weight
Another factor similar to carrying soil is its potential to be moved short distances, such as in B&B tree balls. The soil that is heavier will be more difficult to move. Related to this is the soil's consistency.
Soil Consistency
Soil consistency is the soil's ability to cohere or stick together. The soil's consistency may be evaluated at three moisture conditions: air dry, moist, and wet. Clay soil is noted for its stickiness and large energy requirements in tillage. Farmers refer to it as "heavy," but they really mean it is difficult to plow, not that it has a high bulk density. Clay soils generally will have a lower bulk density than sandy soils (clay=1.3 g/cm3, sand=1.6 g/cm3). Sandy soils have a higher bulk density, but are easier to plow since they have weaker consistency. Thus they are often referred to as "light soils". Tilling the soil also reduces the bulk density by "fluffing" the soil.
Terms commonly used to describe soil consistency are:
Loose – Non-coherent when dry or moist; does not hold together in a mass.
Friable – When moist, crushes easily under gentle pressure between thumb and forefinger and can be pressed together into a lump.
Firm – When moist, crushes under moderate pressure between thumb and forefinger, but resistance is
distinctly noticeable.
Plastic – When wet, readily deforms by moderate pressure but can be pressed into a lump; will form a "wire" when rolled between thumb and forefinger.
Sticky – When wet, adheres to other material and tends to stretch somewhat and pull apart rather than to pull free from other material.
Hard – When dry, moderately resistant to pressure; can be broken with difficulty between thumb and forefinger.
Soft – When dry, breaks into powder or individual grains under very slight pressure.
Cemented – Hard; affected little by moistening.
Porosity
Bulk density is an indirect measure of soil pore space. Under field conditions, pore spaces are occupied at all times by air and/or water. Porosity, when expressed as a percent, is the same thing as percent pore space. Soil particles have irregular shapes, and thus the pores between them vary irregularly in size, shape, and area. Sandy soils have large continuous pores while clays have small pores that transmit water slowly.
Soil Temperature Regimes
The temperature of a given soil at a given time is dependent upon the gains and losses of heat energy. Generally, dark surfaces will absorb more heat than light surfaces. However, the amount of water in the soil is an also important factor in moderating soil temperature. Losses of the absorbed heat are by radiation back into the atmosphere as long-wave radiation, heating the air and cooling the soil.
Soil temperature regimes are used to classify soils. They are defined according to the average annual soil temperature in the root zone. The use of soils for agriculture and forestry is closely related to soil temperature, due to the specific requirements of plants. Over most of the earth, daily soil temperatures below 20 inches (50 cm) deep seldom change. To approximate the mean annual soil temperature, 2Fº (1Cº) is added to the mean annual air temperature.
Compost Improves Soil Structure
Prehistoric farmers discovered that if they mixed manure with other organic waste, the mixture would change into a fertile soil-like material excellent for growing crops. Arborists can benefit from using this age-old technique, today.
Dealing with Compost
In today's cities, autumn leaves are a major concern and their collection is a big budget item. Many cities that have suitable space and means, have adopted a program to compost autumn leaves to create leaf mold compost that is an excellent amendment to soil as well as being a product that can be sold to local nurseries, landscape companies, and home gardeners.
A successful municipal composting operation requires careful attention and planning to the site selection, the collection system, the management of the materials, utilization of the product, and community involvement and support.
Healthy compost provides all the organisms and nutrients required and are in the proper forms for a tree to take up. Healthy compost holds nutrients in non-leachable forms so they remain in soil until the plant requires the nutrients.
Composts that are stable and possess significant amounts of humic acids have the ability to bind nutrients and heavy metals as moisture passes through the product layer. The organic nature of compost also improves the cation exchange capacity of the soil, increasing sites in which nutrients can be bound as well as providing a home for microorganisms to proliferate. Organisms found within compost have the ability to degrade organic contaminates such as hydrocarbons. Furthermore, as humic materials accumulate in the soil, the productivity of plants increase.
Large Scale Composting
The municipal composting site is frequently a political decision and one that often generates neighborhood opposition. This opposition comes from a lack of understanding about how the composting process works. One often used argument deals with leachate from the compost. However, leaves, grass clippings, and wood chips or wood debris do not create leachate problems. Animal manure wastes, while providing nutrients to the compost, could cause leachate problems. Another uninformed complaint deals with odor. However, since a properly managed leaf composting operation generates only minor odor concerns, there is actually little reason for complaint.
The composting site can be any former landfill, unused park, or vacant land. The site must have a total acreage that will equal approximately one acre of compost site per square mile of municipality, or 1 acre per 30 miles of streets, whichever is less. It is more efficient to operate the entire operation at one site, but in larger cities this is not always feasible. If residents are expected to deliver leaves to the site themselves, the site needs to be accessible to most everyone in the municipality. If the municipality operates a citywide clean up, then access is less important. The site should be gently sloping, well drained, and hard. Surface drainage should not go directly into brooks or catch basins. The site should be on a constant slope and open so there is room to maneuver equipment as well as store the leaves.
Newly arriving leaves should be piled in windrows that are as long or as wide as the available site and as high as the municipal equipment (loaders) can reach for maintaining the piles. The pile should be as wide as it is high and should run up and down the slope so that the pile does not trap rainwater. The windrows are designed to let air and moisture enter the pile. One tip many communities take advantage of is to build twin windrows when the leaves are being delivered to the site. At the first turning of the piles, they are combined into one. Since the most rapid amount of leaf size reduction occurs in the first month, the twin piles can easily be managed as one large pile after that first month. As a general rule of thumb, 1000 cubic yards of leaves on the street will eventually become about 200 cubic yards of composted leaf mold.
Size reduction can also be accomplished by mechanically shredding the leaves prior to composting. Shredding will reduce the amount of time necessary to complete the composting process by almost 50%. Uncomposted shredded leaves can also be used as mulch spread in gardens or around a tree. However, since the leaves decompose very quickly, they have to be replaced annually. Large pieces of shredded leaves can also be blown away if the landscaped area is windy.
Ingredients and Procedures:
The following provide all the necessary steps required to insure successful composting.
Turning the pile on a rainy day allows moisture to be mixed throughout the entire pile as the compost is being turned. It also provides an opportunity to utilize equipment that would not normally be working on outdoor projects in the rain. If the piles are near residential areas, rainy day turning takes advantage of the fact that residents' windows are likely to be closed, minimizing the chance for odor release and reduce the chance of complaints from neighbors.
As the composting action proceeds, the pH value fluctuates from acidic in the beginning to neutral at completion regardless of the product being composted. When the compost cycle is completed, the row may be screened to remove any uncomposted leaves, sticks, etc. The material is then piled to cure. Curing allows the compost to stabilize so nutrients are released when added to the soil instead of consumed by bacteria and continued decomposition.
Small Scale Composting
There are many small scale programs and methods of composting available for homeowners and small companies. For example, a 14-day compost is good for small amounts of residential yard waste material. This is accomplished with a rotating barrel that contains fresh green grasses or leaf clippings and is rotated every two or three days. By contrast, static composting, which is simply making a pile of leaves and letting them decompose without any management, requires 2 to 3 years and anaerobic decomposition that produces odor and alcohol problems. There are many other methods in between these two extremes.
Residential Composting
Many communities actively encourage residents to develop backyard composting areas to eliminate leaves and grass from ever getting into the municipal waste stream. The making of composted leaf mold is actually a simple process, but a thorough knowledge of the process is required in order to insure success.
Many residents have compost piles in their yards. Compost piles can be filled with any biodegradable, non-animal waste product. Municipalities encourage this effort because it saves the cost of removal while enhancing the local environment, home garden, and soil quality.
Compost bins are best made with wood slats or wire sides and in three sections so the compost can be moved from one bin to the next as the compost decomposes. Wood posts and beam designs are also quite popular as are plastic containers that are designed to be compositors. This concept is very popular with small landscape companies who have a small tractor and a front end loader to move the compost piles from one wood section to the next, right down a line to a pile of cured compost at the end.
Another popular residential approach and this author's favorite, is the trench method. Begin by digging a long open trench in the garden. The compost products are placed in the trench and then covered with soil from a second trench right beside the first. Dig the trenches a shovel blade deep. If trimmings from trees and shrubs are included, go deeper so the branches and twigs are completely buried. In the second year, consider going at right angles from the last trenching in the garden.
In an average suburban garden, during the course of the summer enough compost material will be generated to cover an area that is 8 feet wide and 25 feet long. In the autumn, repeat the process again with leaves. This process takes a lot of effort but it works. In a 15-year period, up to 1 foot of rich, black, well-drained and unscreened loam can be produced and it will grow terrific vegetables and flowers. In the 30 years this author has been growing his vegetables in the trenches, the elevation of the garden is now two feet higher than the land was when he started and you should see the size and number of his vegetables.
Compost develops good soil structure by binding pieces of soil together and by building airways and passageways through the soil. Good movement of air and water are vital to the health of plants and the soil/food web itself. While it seems contradictory, good soil structure allows water to drain from excessively wet soil and helps soil to hold water when soils start to dry out.
Compost Mulch
Another method which many experts feel is the absolute best way to deal with waste and compostable products is to use it as a mulch. Lush meadows and healthy forests require soil started from mulch, also known as that fresh layer of organic matter (dead grass and leaves) on the surface. Under that fresh layer of mulch is just-started compost (decaying mulch), followed by finished compost a few inches below the surface. Below all the compost comes the healthy parent soil with compost mixed throughout. This soil is ideal for healthy trees. In one gram of healthy soil there are as many as 3 billion organisms plus a whole range of interactions between all these organisms.
A host of organisms, including bacteria, worms, and fungi, break down the organic matter in mulch and compost and begin to change it into a form that will enhance tree growth by the humidification process and produce rich soil. Research shows that this process utilizes nitrogen from air above, not the soil below. In addition, this process also sequesters carbon in the soil which provides a modest assist toward slowing the global warming trend.
Organic Matter Content
High-quality organic soil amendments usually contain at least 20% – 40% organic matter. Concentrations as low as 25% are often adequate for mulches. Soil typically makes up the remainder of the compost dry matter. A moderate amount of inorganic content is desirable as foundation material for compost blankets, filter berms and similar installations.
Particle Size Distribution
Particle size is determined by passing the compost through a set of sieves and determining respective weight fractions retained on each sieve size. Different distributions serve different purposes. For example, at least 90% of a compost to be used as a turf or landscape soil amendment should pass through a 5/8-inch screen. Conversely, composts with larger particles serve as excellent mulches.
Compost Types
Composts can be dominated by either bacteria or by fungi. Bacteria dominated compost is best applied to herbaceous plants. For the bacteria to dominate, it should be made from green materials such as 25% high nitrogen ingredients, 45% green plants, and 30% woody material. High nitrogen materials include manure, grass clippings, and legumes such as alfalfa, peas, clover, and bean plant residues. Green material includes any green plant debris, kitchen scraps, and coffee grounds, which all contain sugars and proteins that bacteria love. Woody material includes wood chips, sawdust, and paper products. The more frequently you turn the pile, the more the compost tends to become bacterial.
Fungi dominated compost is good for mulching and for woody plant growth. Fungal compost consists of approximately 25% animal manure, 50% green plant material, and 25% shredded wood plant material. Any kind of disturbance to the compost pile destroys fungi by breaking up the mycelia. Fungal compost is especially useful for suppressing disease and introducing fungi for root development immediately after tree installation. Fungal composts can not be turned so it is best they be applied to the surface as a mulch after one turning.
Benefits of Compost
Compost improves low-quality soil by adding:
Compost serves as:
Compost improves poor soil by:
Uses of composted leaf mold are many:
Compost may be added to existing soil for optimizing tree growth. In certain situations, such as growing trees in a large container, there is a need to amend the planting soil. When compost is applied as a soil amendment, it should be spread evenly and deeply mixed into the soil. The final amended soil should contain at least 20% to as much as 40% leaf mold compost. Compost amendment is not recommended for sites where trees will be installed in the sidewalk, boulevard, or tree lawn because tree roots often fail to venture out into poorer quality urban soil resulting in root-bound conditions and a shortened tree life span.
Prehistoric farmers discovered that if they mixed manure with other organic waste, the mixture would change into a fertile soil-like material excellent for growing crops. Arborists can benefit from using this age-old technique, today.
Dealing with Compost
In today's cities, autumn leaves are a major concern and their collection is a big budget item. Many cities that have suitable space and means, have adopted a program to compost autumn leaves to create leaf mold compost that is an excellent amendment to soil as well as being a product that can be sold to local nurseries, landscape companies, and home gardeners.
A successful municipal composting operation requires careful attention and planning to the site selection, the collection system, the management of the materials, utilization of the product, and community involvement and support.
Healthy compost provides all the organisms and nutrients required and are in the proper forms for a tree to take up. Healthy compost holds nutrients in non-leachable forms so they remain in soil until the plant requires the nutrients.
Composts that are stable and possess significant amounts of humic acids have the ability to bind nutrients and heavy metals as moisture passes through the product layer. The organic nature of compost also improves the cation exchange capacity of the soil, increasing sites in which nutrients can be bound as well as providing a home for microorganisms to proliferate. Organisms found within compost have the ability to degrade organic contaminates such as hydrocarbons. Furthermore, as humic materials accumulate in the soil, the productivity of plants increase.
Large Scale Composting
The municipal composting site is frequently a political decision and one that often generates neighborhood opposition. This opposition comes from a lack of understanding about how the composting process works. One often used argument deals with leachate from the compost. However, leaves, grass clippings, and wood chips or wood debris do not create leachate problems. Animal manure wastes, while providing nutrients to the compost, could cause leachate problems. Another uninformed complaint deals with odor. However, since a properly managed leaf composting operation generates only minor odor concerns, there is actually little reason for complaint.
The composting site can be any former landfill, unused park, or vacant land. The site must have a total acreage that will equal approximately one acre of compost site per square mile of municipality, or 1 acre per 30 miles of streets, whichever is less. It is more efficient to operate the entire operation at one site, but in larger cities this is not always feasible. If residents are expected to deliver leaves to the site themselves, the site needs to be accessible to most everyone in the municipality. If the municipality operates a citywide clean up, then access is less important. The site should be gently sloping, well drained, and hard. Surface drainage should not go directly into brooks or catch basins. The site should be on a constant slope and open so there is room to maneuver equipment as well as store the leaves.
Newly arriving leaves should be piled in windrows that are as long or as wide as the available site and as high as the municipal equipment (loaders) can reach for maintaining the piles. The pile should be as wide as it is high and should run up and down the slope so that the pile does not trap rainwater. The windrows are designed to let air and moisture enter the pile. One tip many communities take advantage of is to build twin windrows when the leaves are being delivered to the site. At the first turning of the piles, they are combined into one. Since the most rapid amount of leaf size reduction occurs in the first month, the twin piles can easily be managed as one large pile after that first month. As a general rule of thumb, 1000 cubic yards of leaves on the street will eventually become about 200 cubic yards of composted leaf mold.
Size reduction can also be accomplished by mechanically shredding the leaves prior to composting. Shredding will reduce the amount of time necessary to complete the composting process by almost 50%. Uncomposted shredded leaves can also be used as mulch spread in gardens or around a tree. However, since the leaves decompose very quickly, they have to be replaced annually. Large pieces of shredded leaves can also be blown away if the landscaped area is windy.
Ingredients and Procedures:
The following provide all the necessary steps required to insure successful composting.
- Air – Ideally, perforated pipes should be run through the compost pile. A more practical solution is to loosely stack the leaves in long, narrow rows using a front-end loader. Oxygen is essential for preventing anaerobic decomposition while promoting aerobic composting and a minor, but not unpleasant odor.
- Turning – Turning the pile mixes the materials, re-aerates the compost, and provides a check on the progress of composting. The frequency of turning speeds the composting. The minimum interval is four days; the more practical is one month. The more you turn the pile, the more the compost tends to become bacterial because any kind of disturbance destroys fungi by breaking up their mycelia.
- Nutrients – Nitrogen is very important for feeding the composting bacteria. The best source would be to add manure to the compost during the turning procedure. If manure is unavailable or not allowed, the next best sources of nitrogen are weeds, grass clippings, aquatic weeds and commercial nitrogen fertilizer. Except for the fertilizer, all these products will also supply the heat required for composting.
- Bacteria – Commercial bacterial compost starters are available; however, the occasional mixing of compost with previously composted soil should provide sufficient quantities of bacteria seed. If the same site is used year after year, the bacteria can be obtained by scrapping up the top inch or two of soil when turning the pile over for the first and second time. After the composting action has begun, additional bacteria do not need to be added.
- Heat – The optimum temperature is 140°F (60°C). This is no trouble to obtain in summer but
composting over winter requires special insulation with hay or uncomposted leaves, which will also protect the pile from winter rains. When temperature has cooled to 100°F (38°C), the compost action is finished. Weeds, green vegetation, and manure speed up the heating and composting action. The top 2 feet of surface area are unlikely to decompose as readily as the interior of the pile. Therefore, when turning the pile, exteriors should be moved to the center of the new pile and the centers moved
out to cure at the edge.
- Moisture – Rainfall is generally sufficient, but a sprinkler may be necessary to supplement natural rainfall and to insure the moisture content remains at a wet but not dripping condition.
Turning the pile on a rainy day allows moisture to be mixed throughout the entire pile as the compost is being turned. It also provides an opportunity to utilize equipment that would not normally be working on outdoor projects in the rain. If the piles are near residential areas, rainy day turning takes advantage of the fact that residents' windows are likely to be closed, minimizing the chance for odor release and reduce the chance of complaints from neighbors.
As the composting action proceeds, the pH value fluctuates from acidic in the beginning to neutral at completion regardless of the product being composted. When the compost cycle is completed, the row may be screened to remove any uncomposted leaves, sticks, etc. The material is then piled to cure. Curing allows the compost to stabilize so nutrients are released when added to the soil instead of consumed by bacteria and continued decomposition.
Small Scale Composting
There are many small scale programs and methods of composting available for homeowners and small companies. For example, a 14-day compost is good for small amounts of residential yard waste material. This is accomplished with a rotating barrel that contains fresh green grasses or leaf clippings and is rotated every two or three days. By contrast, static composting, which is simply making a pile of leaves and letting them decompose without any management, requires 2 to 3 years and anaerobic decomposition that produces odor and alcohol problems. There are many other methods in between these two extremes.
Residential Composting
Many communities actively encourage residents to develop backyard composting areas to eliminate leaves and grass from ever getting into the municipal waste stream. The making of composted leaf mold is actually a simple process, but a thorough knowledge of the process is required in order to insure success.
Many residents have compost piles in their yards. Compost piles can be filled with any biodegradable, non-animal waste product. Municipalities encourage this effort because it saves the cost of removal while enhancing the local environment, home garden, and soil quality.
Compost bins are best made with wood slats or wire sides and in three sections so the compost can be moved from one bin to the next as the compost decomposes. Wood posts and beam designs are also quite popular as are plastic containers that are designed to be compositors. This concept is very popular with small landscape companies who have a small tractor and a front end loader to move the compost piles from one wood section to the next, right down a line to a pile of cured compost at the end.
Another popular residential approach and this author's favorite, is the trench method. Begin by digging a long open trench in the garden. The compost products are placed in the trench and then covered with soil from a second trench right beside the first. Dig the trenches a shovel blade deep. If trimmings from trees and shrubs are included, go deeper so the branches and twigs are completely buried. In the second year, consider going at right angles from the last trenching in the garden.
In an average suburban garden, during the course of the summer enough compost material will be generated to cover an area that is 8 feet wide and 25 feet long. In the autumn, repeat the process again with leaves. This process takes a lot of effort but it works. In a 15-year period, up to 1 foot of rich, black, well-drained and unscreened loam can be produced and it will grow terrific vegetables and flowers. In the 30 years this author has been growing his vegetables in the trenches, the elevation of the garden is now two feet higher than the land was when he started and you should see the size and number of his vegetables.
Compost develops good soil structure by binding pieces of soil together and by building airways and passageways through the soil. Good movement of air and water are vital to the health of plants and the soil/food web itself. While it seems contradictory, good soil structure allows water to drain from excessively wet soil and helps soil to hold water when soils start to dry out.
Compost Mulch
Another method which many experts feel is the absolute best way to deal with waste and compostable products is to use it as a mulch. Lush meadows and healthy forests require soil started from mulch, also known as that fresh layer of organic matter (dead grass and leaves) on the surface. Under that fresh layer of mulch is just-started compost (decaying mulch), followed by finished compost a few inches below the surface. Below all the compost comes the healthy parent soil with compost mixed throughout. This soil is ideal for healthy trees. In one gram of healthy soil there are as many as 3 billion organisms plus a whole range of interactions between all these organisms.
A host of organisms, including bacteria, worms, and fungi, break down the organic matter in mulch and compost and begin to change it into a form that will enhance tree growth by the humidification process and produce rich soil. Research shows that this process utilizes nitrogen from air above, not the soil below. In addition, this process also sequesters carbon in the soil which provides a modest assist toward slowing the global warming trend.
Organic Matter Content
High-quality organic soil amendments usually contain at least 20% – 40% organic matter. Concentrations as low as 25% are often adequate for mulches. Soil typically makes up the remainder of the compost dry matter. A moderate amount of inorganic content is desirable as foundation material for compost blankets, filter berms and similar installations.
Particle Size Distribution
Particle size is determined by passing the compost through a set of sieves and determining respective weight fractions retained on each sieve size. Different distributions serve different purposes. For example, at least 90% of a compost to be used as a turf or landscape soil amendment should pass through a 5/8-inch screen. Conversely, composts with larger particles serve as excellent mulches.
Compost Types
Composts can be dominated by either bacteria or by fungi. Bacteria dominated compost is best applied to herbaceous plants. For the bacteria to dominate, it should be made from green materials such as 25% high nitrogen ingredients, 45% green plants, and 30% woody material. High nitrogen materials include manure, grass clippings, and legumes such as alfalfa, peas, clover, and bean plant residues. Green material includes any green plant debris, kitchen scraps, and coffee grounds, which all contain sugars and proteins that bacteria love. Woody material includes wood chips, sawdust, and paper products. The more frequently you turn the pile, the more the compost tends to become bacterial.
Fungi dominated compost is good for mulching and for woody plant growth. Fungal compost consists of approximately 25% animal manure, 50% green plant material, and 25% shredded wood plant material. Any kind of disturbance to the compost pile destroys fungi by breaking up the mycelia. Fungal compost is especially useful for suppressing disease and introducing fungi for root development immediately after tree installation. Fungal composts can not be turned so it is best they be applied to the surface as a mulch after one turning.
Benefits of Compost
Compost improves low-quality soil by adding:
- stable organic matter,
- beneficial soil microbes that out-compete soil diseases for nutrients,
- water-holding capacity,
- nutrient-holding capacity
Compost serves as:
- a soil amendment,
- a turf top-dressing,
- a mulch,
- an erosion-control agent,
- a water-quality enhancer
Compost improves poor soil by:
- promoting plant establishment,
- improving moisture retention,
- costing less than adding topsoil,
- improving structure, infiltration, and drainage capacity.
Uses of composted leaf mold are many:
- It can be mixed as 1/3 compost, 1/3 sand, and 1/3 soil to create topsoil.
- It can be substituted for peat anytime peat is required in construction or for greenhouse potting soil.
- It makes an excellent top dressing for turf areas and mulch in a garden, eliminating the need for fertilizer.
- It is an excellent cover for construction restoration.
- Many communities with successful composting operations no longer purchase topsoil. Other communities sell the material to topsoil contractors, greenhouses, nurseries, garden shops, and residents; or they use the material as incentives for sales of other surplus products such as woodchips
and lumber from forestry operations. - Some communities have a Give-Away Day while other communities sell it for $25 – $45 per yard, unscreened and delivered.
Compost may be added to existing soil for optimizing tree growth. In certain situations, such as growing trees in a large container, there is a need to amend the planting soil. When compost is applied as a soil amendment, it should be spread evenly and deeply mixed into the soil. The final amended soil should contain at least 20% to as much as 40% leaf mold compost. Compost amendment is not recommended for sites where trees will be installed in the sidewalk, boulevard, or tree lawn because tree roots often fail to venture out into poorer quality urban soil resulting in root-bound conditions and a shortened tree life span.
Soil Food Web
The Soil Food Web refers to the entire complex system of soil particles, pores, microorganisms, nutrients, moisture, and organic matter that all work together to make a healthy soil. Organic sustainable systems are all about soil health, which translates into a healthy tree with a deep root system, and the rhizosphere having symbiotic relationships with the microbes.
Healthy Soil
Healthy trees only need nineteen elements in contrast to healthy soil that needs over ninety elements. A healthy tree needs all the nutrients in forms that are readily available for root absorption in order to grow and to defend itself from insects, disease, and environmental conditions.
The best way to create a healthy soil is to treat the soil with compost to feed the existing soil microbes that in turn breaks down minerals into tree available forms. Conventional systems using synthetic fertilizers, pesticides, and herbicides, will weaken and kill the soil microbes over time to create depleted monocultures in the soil. As the seasons go by, the need to use more and stronger synthetics with more water and resources to maintain the same yields is not sustainable.
Other Microorganisms
In addition to over ninety elements, healthy soil contains six main microorganisms: bacteria, fungi, protozoa, nematodes, arthropods, and earthworms. Also found in soils are other microorganisms such as cysts, amoeba, flagellates, bacterial colonies, nematodes, ciliates, decomposing plant cells, fungal hyphae and spores, and actinomycetes hyphae and spores.
These microorganisms assimilate pollutants, neutralize chemicals, cleanse water, breakdown elements into tree-available forms, create humus, and provide a myriad of other functions. The live soil biology is about microorganisms living between and within the soil particles. Between micro-spaces are spaces filled with water, air, and available food that create a broad range of habitats.
Interactions of Organisms
There are five trophic levels of organism interactions within the soil food web. Trophic levels are the feeding position in the food chain. In soils,:
1. On the First trophic level are the smallest microorganisms that utilize organic matter and residue from plants, animals, and microbes.
2. On the Second trophic level are decomposers and root feeders such as mycorrhizal and saprophytic fungi
and bacteria.
3. On the Third trophic level are shredders, predators, and grazers also known protozoa, amoebae,
flagellates, and ciliates.
4. On the Fourth trophic level are higher level predators that are arthropods and nematodes.
5. On the Fifth trophic level are highest-level predators like birds, animals, amphibians, and reptiles.
These animals spend their life in the soil converting food to energy and nutrients. One's waste becomes another one's food while releasing available nutrients for trees. These animals are part of and work in perpetuity creating different life cycles, such as the carbon cycle, nitrogen cycle, water cycle, etc. Soil animals follow seasonal patterns and change with temperature and moisture conditions to provide optimal growth. Some thrive in hot or cold, and others in wet or dry conditions. Soil animals are active at different times of the year and interact with the soil and trees to provide beneficial functions that result in nutrient cycling, water flow, and pest control. The food web differs in every location and has a unique proportion of soil animals within these locations. This is a result of different soils, vegetation, and climatic factors.
The ratio of fungi to bacteria is characteristic of the type of soil use.
Microorganism Functions
In the soil, many of these organisms and soil animals are living in the rhizosphere surrounding tree roots. Bacteria feed on plant cells taking proteins and sugars that are released by the roots. Then protozoa and nematodes feed on the bacteria thereby releasing nutrients that are absorbed by the tree roots.
In plant litter, fungi are dominant and are able to decompose complex carbon with fungal hyphae. In addition, macro-arthropods, worms, centipedes, millipedes, sow bugs, and other shredders break up this litter into smaller chunks that are easier for fungi to consume. Fungi then create enzymes that allow the complex litter compounds to decompose further so that bacteria, earthworms, and arthropods can digest the organic matter repeatedly.
Soil surface aggregates contain aerobic bacteria and fungi as well as many fecal pellets of earthworms and other invertebrates. Arthropods and nematodes that cannot burrow through soil move in between the pore spaces while other arthropods and nematodes sensitive to desiccation live in the water-filled pores between soil particles.
Organisms reflect their food source. For example, protozoa are abundant where bacteria are plentiful and bacteria dominate over fungi. In these situations, nematodes that eat bacteria have higher numbers than nematodes that eat fungi. Management practices can change food webs. For example, when agricultural tillage is reduced, the ratio of fungi to bacteria increases, while earthworms and arthropods become plentiful. Bacteria dominated soils have a pH of 7 - 7.5 that is desirable for grasses, vegetables, and annuals. Fungal dominated soils are more acidic with a pH 5.6 - 6 that is desirable for perennials, shrubs, trees, and evergreens.
Management and Soil Health
A healthy soil effectively supports tree growth, protects air and water quality, and ensures human and animal health. The physical structure, chemical make-up, and biological components of the soil together determine how well a soil performs these services. In every healthy system, the soil food web is critical to major soil functions including:
The interactions among organisms enhance many of these functions. Successful land management requires approaches that protect all resources, including soil, water, air, plants, animals, and humans. Many management strategies change soil habitats and the food web, and alter soil quality, or the capacity of soil to perform its functions.
Although the effect of pesticides on soil organisms varies, high levels of pesticide use will generally reduce the food web complexity. An extreme example is the repeated use of methyl bromide, which is observed to eliminate most soil organisms except a few bacteria species.
Carbon Sequestration
Land management practices may increase the amount of carbon sequestered as soil organic matter thereby reducing the amount of carbon dioxide (CO2), released into the atmosphere. As the soil food web decomposes organic material, the microorganisms might release carbon into the atmosphere as CO2, but more often, convert it to a variety of soil organic matter that stays in the soil for decades. This stabilized organic matter is humus.
The Soil Food Web refers to the entire complex system of soil particles, pores, microorganisms, nutrients, moisture, and organic matter that all work together to make a healthy soil. Organic sustainable systems are all about soil health, which translates into a healthy tree with a deep root system, and the rhizosphere having symbiotic relationships with the microbes.
Healthy Soil
Healthy trees only need nineteen elements in contrast to healthy soil that needs over ninety elements. A healthy tree needs all the nutrients in forms that are readily available for root absorption in order to grow and to defend itself from insects, disease, and environmental conditions.
The best way to create a healthy soil is to treat the soil with compost to feed the existing soil microbes that in turn breaks down minerals into tree available forms. Conventional systems using synthetic fertilizers, pesticides, and herbicides, will weaken and kill the soil microbes over time to create depleted monocultures in the soil. As the seasons go by, the need to use more and stronger synthetics with more water and resources to maintain the same yields is not sustainable.
Other Microorganisms
In addition to over ninety elements, healthy soil contains six main microorganisms: bacteria, fungi, protozoa, nematodes, arthropods, and earthworms. Also found in soils are other microorganisms such as cysts, amoeba, flagellates, bacterial colonies, nematodes, ciliates, decomposing plant cells, fungal hyphae and spores, and actinomycetes hyphae and spores.
These microorganisms assimilate pollutants, neutralize chemicals, cleanse water, breakdown elements into tree-available forms, create humus, and provide a myriad of other functions. The live soil biology is about microorganisms living between and within the soil particles. Between micro-spaces are spaces filled with water, air, and available food that create a broad range of habitats.
Interactions of Organisms
There are five trophic levels of organism interactions within the soil food web. Trophic levels are the feeding position in the food chain. In soils,:
1. On the First trophic level are the smallest microorganisms that utilize organic matter and residue from plants, animals, and microbes.
2. On the Second trophic level are decomposers and root feeders such as mycorrhizal and saprophytic fungi
and bacteria.
3. On the Third trophic level are shredders, predators, and grazers also known protozoa, amoebae,
flagellates, and ciliates.
4. On the Fourth trophic level are higher level predators that are arthropods and nematodes.
5. On the Fifth trophic level are highest-level predators like birds, animals, amphibians, and reptiles.
These animals spend their life in the soil converting food to energy and nutrients. One's waste becomes another one's food while releasing available nutrients for trees. These animals are part of and work in perpetuity creating different life cycles, such as the carbon cycle, nitrogen cycle, water cycle, etc. Soil animals follow seasonal patterns and change with temperature and moisture conditions to provide optimal growth. Some thrive in hot or cold, and others in wet or dry conditions. Soil animals are active at different times of the year and interact with the soil and trees to provide beneficial functions that result in nutrient cycling, water flow, and pest control. The food web differs in every location and has a unique proportion of soil animals within these locations. This is a result of different soils, vegetation, and climatic factors.
The ratio of fungi to bacteria is characteristic of the type of soil use.
- Grasslands and agricultural soils usually have bacteria-dominated food webs.
- A highly productive agricultural soil will have ratios of fungal to bacterial biomass near 1:1.
- A deciduous forest is fungal dominated and the ratio of fungal to bacterial biomass is 5:1 to 10:1.
- A coniferous forest is fungal dominated and the ratio of fungal to bacterial biomass is 100:1 to 1000:1.
Microorganism Functions
In the soil, many of these organisms and soil animals are living in the rhizosphere surrounding tree roots. Bacteria feed on plant cells taking proteins and sugars that are released by the roots. Then protozoa and nematodes feed on the bacteria thereby releasing nutrients that are absorbed by the tree roots.
In plant litter, fungi are dominant and are able to decompose complex carbon with fungal hyphae. In addition, macro-arthropods, worms, centipedes, millipedes, sow bugs, and other shredders break up this litter into smaller chunks that are easier for fungi to consume. Fungi then create enzymes that allow the complex litter compounds to decompose further so that bacteria, earthworms, and arthropods can digest the organic matter repeatedly.
Soil surface aggregates contain aerobic bacteria and fungi as well as many fecal pellets of earthworms and other invertebrates. Arthropods and nematodes that cannot burrow through soil move in between the pore spaces while other arthropods and nematodes sensitive to desiccation live in the water-filled pores between soil particles.
Organisms reflect their food source. For example, protozoa are abundant where bacteria are plentiful and bacteria dominate over fungi. In these situations, nematodes that eat bacteria have higher numbers than nematodes that eat fungi. Management practices can change food webs. For example, when agricultural tillage is reduced, the ratio of fungi to bacteria increases, while earthworms and arthropods become plentiful. Bacteria dominated soils have a pH of 7 - 7.5 that is desirable for grasses, vegetables, and annuals. Fungal dominated soils are more acidic with a pH 5.6 - 6 that is desirable for perennials, shrubs, trees, and evergreens.
Management and Soil Health
A healthy soil effectively supports tree growth, protects air and water quality, and ensures human and animal health. The physical structure, chemical make-up, and biological components of the soil together determine how well a soil performs these services. In every healthy system, the soil food web is critical to major soil functions including:
- sustaining biological activity, diversity, and productivity,
- regulating the flow of water and dissolved nutrients,
- storing and cycling nutrients and other elements,
- filtering, buffering, degrading, immobilizing, and detoxifying organic and inorganic materials that are potential pollutants.
The interactions among organisms enhance many of these functions. Successful land management requires approaches that protect all resources, including soil, water, air, plants, animals, and humans. Many management strategies change soil habitats and the food web, and alter soil quality, or the capacity of soil to perform its functions.
Although the effect of pesticides on soil organisms varies, high levels of pesticide use will generally reduce the food web complexity. An extreme example is the repeated use of methyl bromide, which is observed to eliminate most soil organisms except a few bacteria species.
Carbon Sequestration
Land management practices may increase the amount of carbon sequestered as soil organic matter thereby reducing the amount of carbon dioxide (CO2), released into the atmosphere. As the soil food web decomposes organic material, the microorganisms might release carbon into the atmosphere as CO2, but more often, convert it to a variety of soil organic matter that stays in the soil for decades. This stabilized organic matter is humus.
Humus
Dead and dying plant and animal residue make up the bulk of organic matter that feeds the soil food web and are in the form of carbon and nitrogen. Carbon material is brown, and nitrogen material is mostly green with some exceptions like blood, coffee, soybean, cottonseed, and fish-meal. Feeding the soil with organic carbon, nitrogen, air, and water is feeding the soil biology, in a manner that will build up over time and create humus that is a stable organic material in soil. Humus is stable because bacteria and fungi have helped form molecules that are too complex and large for soil organisms to decompose. Because humus is very stable it lasts for a long time. It was here on earth before mankind in layers of fourteen feet deep on average. Now mankind's activities have depleted this thick layer of organic matter to an average of four to six inches and this level is still decreasing.
Humus, sometimes called soil organic matter, is the most stable form of compost. Humus is the final result of the natural composting process for leaves, plants, and other organic matter. Humus is seen as the rich, dark layer at the top of natural sub-soils which is also the bottom of the forest topsoil.
Humus itself has no nutritional value, but it greatly contributes to soil fertility from its physical and chemical properties. Humus particles can store positively charged forms of essential potassium, calcium, and magnesium. Humus also stores large quantities of carbon for years. Humus binds to heavy metals such as mercury and lead, making them less readily absorbed by plants or leached into groundwater.
Humus not only provides the soil with good aeration, it also provides water holding capacity essential tor tree growth. In other words, humus greatly contributes to the storage and uptake of some fertilizer elements, but is not a fertilizer itself.
Organic Soil Conditioners
Organic matter serves as a reservoir for nutrients and improves soil structure, drainage, aeration, cation exchange capacity, buffering capacity, and water-holding capacity. It also provides a source of food for beneficial microorganisms. Generally speaking, soils high in organic matter have better physical conditions compared to soils low in organic matter.
Organic matter is usually less than 10% of the total weight of mineral soils. To actually change the organic matter content of a soil, very large amounts of organic materials must be applied. Research has shown that it takes 5 – 15 pounds (2 – 7 kg) of fresh plant residue to produce 1 pound (½ kg) of stable humus.
Soil Microorganisms
Here is how humus relates to soil microorganisms. First of all, soil microorganisms consist of animals such as algae, viruses, insects, protozoa, arthropods, bacteria, fungi, and nematodes. They carry out numerous biological functions such as creation of humus. They also decompose organic matter, suppress pathogens, improve soil properties, and convert soil nutrients from organic matter into tree-available forms. They improve the soil structure, produce enzymes and hormones that help plants grow, and decompose pollutants in the soil. Microorganisms also suppress disease organisms and reduce the potential for temperature and moisture stress.
As we all know, plants remove carbon dioxide (CO2) from the air and convert it to sugar and energy via photosynthesis. The oxygen in the CO2 molecule is released back to the air and the carbon becomes part of the plant tissue. The carbon is later released from the plant tissue when the plant dies and falls to the ground. There it is consumed by soil microorganisms. The soil microorganisms produce humus and glomalin which are loaded with carbon. Glomalin, an organic glue, acts to bind organic matter to mineral particles in the soil as tiny clumps that actually improve soil structure, and the carbon is deposited on the surface of these particles. This process locks carbon into the soil.
Bacteria
Humus relates to bacteria as one of the main microorganisms in the soil that decompose organic matter and create humus. Nitrogen gas from the air is chemically bound by nitrogen-fixing bacteria into soluble or insoluble organic compounds that degrade in time, releasing soluble nitrogen compounds like ammonia. Most bacteria convert nitrogen into a form that is available to plants. Bacteria will also convert insoluble mineral phosphorus and iron into soluble products that plants can use. Bacterial waste products become humus.
Fungi
Fungi, another main organism in the creation of humus, decomposes dead plants, pine needles, bark, wood, and animal matter. These saprophytes act as recyclers of dead organic matter, obtaining food from this material. Hyphal tips release enzymes that eventually decompose and release organic materials into the surrounding environment. Fungal waste products that cannot be broken down further become soil humus.
Compost
Compost is the partially degraded organic matter that becomes humus when soil organisms have completed their activities. Leaf compost is the top natural fertilizer on the planet, but there are many other good types of compost like cow manure, kelp and fish-meal, and mixes of organic and inorganic minerals. Compost used as a top-dressing instead of mulch is easy and a fast way to improve the growth of trees and shrubs.
Humus and Humic Acid
Humus contains the remains of plants and organic matter as well as the digestive remains of microorganisms and invertebrates. The soluble pieces of humus are called humic and fulvic acids. Humic and fulvic acids buffer pH swings in the soil. Humic acid slows decomposition reactions in soils. These acids are very complex and easily immobilized by soil mineral matter. They can improve germination of fungi, but the germinated fungus has to rapidly find a root to colonize or it will die.
Commercial Humates
Humates are mineral salts of humic and fulvic acids. Commercial humates are products derived from oxidized lignite, an earthy, coal-like substance associated with lignite outcrops. Humates marketed for agricultural purposes may be soluble or insoluble and may be fortified with commercial fertilizer. Commercial humates contain between 30% -- 60% humic acid. Commercial humates do not resemble soil organic matter and therefore cannot be expected to perform the same function. In addition, the low rate of application normally recommended is insignificant in comparison with organic matter already present in most soils. Mixed humates contain 1.2% -- 1.5% nitrogen, of which only a small portion of nitrogen is needed by the plant in a given year. At the recommended application rate, commercial humates would add less than 1 pound (0.5 kg) of nitrogen per acre.
Sources
The test that follows contains 80 questions. Before taking the test be sure you have read the article carefully. The passing grade is 80% on the entire test.
ISA will award 4.0 CEUs* for a passing grade. SAF members will earn 1.0 Cat. 1-CT credit for a passing grade. The cost for taking this test is $20 per credit. If you purchase an annual subscription for 15 credits, the cost per credit is reduced by 50% (see Annual Subscription link below). We will report all passing test scores to ISA and/or SAF. If you are a member of ISA and SAF we will report your passing test scores to both for no additional cost. Tests with passing scores may be submitted only once to each organization.
*Members of ISA may apply the 4.0 CEUs toward Certified Arborist, Tree Worker Specialist, Municipal Specialist, or BCMA science credits.
California UFC members will receive credit for passing the test. Please add your CaUFC number after your ISA and/or SAF certification number.
ASCA and MTOA members may submit your ISA certification record to these organizations and receive credits one for one.
To take the test by the pay per test option, click on the 'Pay Now' button below where you can send payment online securely with your credit card or Pay Pal account. After your payment is submitted, click on ‘Return to Merchant' / gibneyCE.com. That will take you to the test sign in page followed by the test. Members with certifications from both ISA and SAF, please be sure to add both of your certification numbers. These numbers are important for reporting purposes.
To take the test as an annual subscriber with reduced rates, click on Password and enter your test password which will take you to the test sign in page. If you would like to become a subscriber see our Annual Subscription page for details.
When you have finished answering all questions you will be prompted to click ‘next’ to send your answers to gibneyCE.com. You can then click ‘next’ to view your test summary. A test review of your answers is available upon request.
All passing test scores are sent from gibneyCE.com to your organization(s) at the end of every month and they will appear on your certification record 4 to 6 weeks* after that. ISA maintains a record of CEU credits on their website *SAF requires 5 passing test scores before reporting.
Test re-takes are allowed, however you will have to pay for the retake if you are using the pay per test option. You can spend as much time as you would like to take the test but it is important not to leave the test site until you have answered all the questions and see the 'sending your answers' response.
Dead and dying plant and animal residue make up the bulk of organic matter that feeds the soil food web and are in the form of carbon and nitrogen. Carbon material is brown, and nitrogen material is mostly green with some exceptions like blood, coffee, soybean, cottonseed, and fish-meal. Feeding the soil with organic carbon, nitrogen, air, and water is feeding the soil biology, in a manner that will build up over time and create humus that is a stable organic material in soil. Humus is stable because bacteria and fungi have helped form molecules that are too complex and large for soil organisms to decompose. Because humus is very stable it lasts for a long time. It was here on earth before mankind in layers of fourteen feet deep on average. Now mankind's activities have depleted this thick layer of organic matter to an average of four to six inches and this level is still decreasing.
Humus, sometimes called soil organic matter, is the most stable form of compost. Humus is the final result of the natural composting process for leaves, plants, and other organic matter. Humus is seen as the rich, dark layer at the top of natural sub-soils which is also the bottom of the forest topsoil.
Humus itself has no nutritional value, but it greatly contributes to soil fertility from its physical and chemical properties. Humus particles can store positively charged forms of essential potassium, calcium, and magnesium. Humus also stores large quantities of carbon for years. Humus binds to heavy metals such as mercury and lead, making them less readily absorbed by plants or leached into groundwater.
Humus not only provides the soil with good aeration, it also provides water holding capacity essential tor tree growth. In other words, humus greatly contributes to the storage and uptake of some fertilizer elements, but is not a fertilizer itself.
Organic Soil Conditioners
Organic matter serves as a reservoir for nutrients and improves soil structure, drainage, aeration, cation exchange capacity, buffering capacity, and water-holding capacity. It also provides a source of food for beneficial microorganisms. Generally speaking, soils high in organic matter have better physical conditions compared to soils low in organic matter.
Organic matter is usually less than 10% of the total weight of mineral soils. To actually change the organic matter content of a soil, very large amounts of organic materials must be applied. Research has shown that it takes 5 – 15 pounds (2 – 7 kg) of fresh plant residue to produce 1 pound (½ kg) of stable humus.
Soil Microorganisms
Here is how humus relates to soil microorganisms. First of all, soil microorganisms consist of animals such as algae, viruses, insects, protozoa, arthropods, bacteria, fungi, and nematodes. They carry out numerous biological functions such as creation of humus. They also decompose organic matter, suppress pathogens, improve soil properties, and convert soil nutrients from organic matter into tree-available forms. They improve the soil structure, produce enzymes and hormones that help plants grow, and decompose pollutants in the soil. Microorganisms also suppress disease organisms and reduce the potential for temperature and moisture stress.
As we all know, plants remove carbon dioxide (CO2) from the air and convert it to sugar and energy via photosynthesis. The oxygen in the CO2 molecule is released back to the air and the carbon becomes part of the plant tissue. The carbon is later released from the plant tissue when the plant dies and falls to the ground. There it is consumed by soil microorganisms. The soil microorganisms produce humus and glomalin which are loaded with carbon. Glomalin, an organic glue, acts to bind organic matter to mineral particles in the soil as tiny clumps that actually improve soil structure, and the carbon is deposited on the surface of these particles. This process locks carbon into the soil.
Bacteria
Humus relates to bacteria as one of the main microorganisms in the soil that decompose organic matter and create humus. Nitrogen gas from the air is chemically bound by nitrogen-fixing bacteria into soluble or insoluble organic compounds that degrade in time, releasing soluble nitrogen compounds like ammonia. Most bacteria convert nitrogen into a form that is available to plants. Bacteria will also convert insoluble mineral phosphorus and iron into soluble products that plants can use. Bacterial waste products become humus.
Fungi
Fungi, another main organism in the creation of humus, decomposes dead plants, pine needles, bark, wood, and animal matter. These saprophytes act as recyclers of dead organic matter, obtaining food from this material. Hyphal tips release enzymes that eventually decompose and release organic materials into the surrounding environment. Fungal waste products that cannot be broken down further become soil humus.
Compost
Compost is the partially degraded organic matter that becomes humus when soil organisms have completed their activities. Leaf compost is the top natural fertilizer on the planet, but there are many other good types of compost like cow manure, kelp and fish-meal, and mixes of organic and inorganic minerals. Compost used as a top-dressing instead of mulch is easy and a fast way to improve the growth of trees and shrubs.
Humus and Humic Acid
Humus contains the remains of plants and organic matter as well as the digestive remains of microorganisms and invertebrates. The soluble pieces of humus are called humic and fulvic acids. Humic and fulvic acids buffer pH swings in the soil. Humic acid slows decomposition reactions in soils. These acids are very complex and easily immobilized by soil mineral matter. They can improve germination of fungi, but the germinated fungus has to rapidly find a root to colonize or it will die.
Commercial Humates
Humates are mineral salts of humic and fulvic acids. Commercial humates are products derived from oxidized lignite, an earthy, coal-like substance associated with lignite outcrops. Humates marketed for agricultural purposes may be soluble or insoluble and may be fortified with commercial fertilizer. Commercial humates contain between 30% -- 60% humic acid. Commercial humates do not resemble soil organic matter and therefore cannot be expected to perform the same function. In addition, the low rate of application normally recommended is insignificant in comparison with organic matter already present in most soils. Mixed humates contain 1.2% -- 1.5% nitrogen, of which only a small portion of nitrogen is needed by the plant in a given year. At the recommended application rate, commercial humates would add less than 1 pound (0.5 kg) of nitrogen per acre.
Sources
- Personal conversations with Dr. Nina Bassuk, Urban Horticulture Institute.
- Carpani, Shane, "Soil Requirements for Healthy Urban Trees", GreenBlue Blog - Sustainability in the Urban Landscape, July 2015.
- Chau, K. C., W. Y. Chan, and L. M. Marafa, "Planter Soils", Arboricultural Journal, 2000.
- Cooper, Terry, "Soil Bulk Density", The University of Minnesota, 2005.
- Cornell Plantations, "Urban Tree Collection", Cornell University, 2003.
- Craul, Dr. Phillip J., "Soils", City Trees, Vol 36, Number 6 November/December 2000.
- Dragon, Curtis, "Organic Earth Care", Archive #33 from Online Seminars, July/August 2010.
- Edwards, Clive A., "Soil Arthropods and Earthworms", The Ohio State University.
- Gilman, Edward F., "Planting trees in landscapes", Environmental Horticulture Department, IFAS, University of Florida, 2004.
- Goldstein, Jerome, "Leaf Composting Installations", Compost Science/Land Utilization, p. 32-34, Sept/Oct 1980.
- "Guide to Arbor Care", Plant Health Care, Inc. 2003.
- Hartin, Janet, "Compost Improves Soil Structure and Plant Growth", LCN, July 2010.
- Haynes, John F. and David Harlan Cade, "Alternative EC", Landscape Architect and Specifier News, November 1999.
- Ingham, Dr. Elaine, "The Relationship between Plants and Soil ", Archive #30 from Online Seminars, January/February 2010.
- Martens, Mary-Howell R., “Just What the Doctor Ordered”, Acres USA, February 2001.
- Melendrez, Michael Martin and Dr. Michael Karr, Ph.D., "Soil Ecology and the Soil Food Web", Nursery News, July 2004.
- Moldenke, Andrew R., "The Web & Soil Health, Soil Bacteria, Soil Fungi, Soil Protozoa, and Soil Nematodes", Oregon State University.
- Neely, Dan, and Gary Watson, "The Landscape Below Ground II", International Society of Arboriculture, 1998.
- Petersen, Lars Bo, "Road Salt Effects on Trees", Journal of Arboriculture 26(5).
- Phillips, Len "Custom Soils Lock CO2 Away", Archive #28 from Online Seminars, September/October 2009.
- Phillips, Len, "Soil Microorganisms", Archive #24 from Online Seminars, January/February 2009.
- Phillips, Len, "Urban Soil Updates", Archive #34 from Online Seminars, September/October 2010.
- Rivenshield, Angie, "Soil Amendments to Reduce Compaction". Graduate Thesis, Cornell University
- Smith, Kevin, "Humus and Soil Fertility" Tree Care Industry, September 2010.
- "Soil Biology Primer", Soil and Water Conservation Society in cooperation with the USDA NRCS
- Trowbridge, Peter J. and Nina L. Bassuk, "Trees in the Urban Landscape", John Wiley & Sons, Inc. 2004
- University of Missouri, "Soil Bulk Density", The Cooperative Soil Survey, 2009.
- "Vertical Mulching", Tree Conservation Notes, Athens - Clarke County Community Tree Program
- Watson, Gary W. and E. B. Himelick, "Principals and Practice of Planting Trees and Shrubs"', International Society of Arboriculture, Savoy, IL 1997.
- Watson, G. and Dan Neely, "The Landscape Below Ground", International Society of Arboriculture, 1993.
The test that follows contains 80 questions. Before taking the test be sure you have read the article carefully. The passing grade is 80% on the entire test.
ISA will award 4.0 CEUs* for a passing grade. SAF members will earn 1.0 Cat. 1-CT credit for a passing grade. The cost for taking this test is $20 per credit. If you purchase an annual subscription for 15 credits, the cost per credit is reduced by 50% (see Annual Subscription link below). We will report all passing test scores to ISA and/or SAF. If you are a member of ISA and SAF we will report your passing test scores to both for no additional cost. Tests with passing scores may be submitted only once to each organization.
*Members of ISA may apply the 4.0 CEUs toward Certified Arborist, Tree Worker Specialist, Municipal Specialist, or BCMA science credits.
California UFC members will receive credit for passing the test. Please add your CaUFC number after your ISA and/or SAF certification number.
ASCA and MTOA members may submit your ISA certification record to these organizations and receive credits one for one.
To take the test by the pay per test option, click on the 'Pay Now' button below where you can send payment online securely with your credit card or Pay Pal account. After your payment is submitted, click on ‘Return to Merchant' / gibneyCE.com. That will take you to the test sign in page followed by the test. Members with certifications from both ISA and SAF, please be sure to add both of your certification numbers. These numbers are important for reporting purposes.
To take the test as an annual subscriber with reduced rates, click on Password and enter your test password which will take you to the test sign in page. If you would like to become a subscriber see our Annual Subscription page for details.
When you have finished answering all questions you will be prompted to click ‘next’ to send your answers to gibneyCE.com. You can then click ‘next’ to view your test summary. A test review of your answers is available upon request.
All passing test scores are sent from gibneyCE.com to your organization(s) at the end of every month and they will appear on your certification record 4 to 6 weeks* after that. ISA maintains a record of CEU credits on their website *SAF requires 5 passing test scores before reporting.
Test re-takes are allowed, however you will have to pay for the retake if you are using the pay per test option. You can spend as much time as you would like to take the test but it is important not to leave the test site until you have answered all the questions and see the 'sending your answers' response.