Edited by Len Phillips

For the purpose of this article, urban soil is compacted rubble or subsoil that has very little or no organic content. Water movement, water availability, and aeration are limited. Barriers to root penetration are present along with contaminants and soil compaction. In urban soil, the topsoil was removed for use somewhere else. What is left is basically dead and devoid of any microorganisms.

Urban soil presents growing conditions entirely different from the forest or native soil that occurs in an undisturbed, wild, native landscape. What I shall call suburban soil in this article, is soil that has topsoil but is lacking the essential micro-organisms necessary to provide good tree growth. This article describes the results of recent research and provides tips on how to grow trees in urban soil.

Essential Needs of Roots in Urban Soil
A healthy soil system results in a healthy root system and a healthy tree. Trees need 17 elements to thrive but soil needs over 90 elements for the soil food web to thrive. Permeability of any soil is essential if water, air, and roots are to move through the soil. Organic matter is also essential for improving soil structure and soil properties.

A recent study suggests the addition of composts could be beneficial in improving the chemical and physical properties of urban soil. According to this research, soil replacement using compost or a compost and topsoil mix allows root development in the replacement soil to be greater than in unmodified soil. However, the roots did not grow from the composted soil into the urban soil. When this happened, the tree only developed to the size that the roots could accommodate. If having a tree less than its full potential is acceptable, this would be a reasonable and cost effective solution for growing trees in urban soil. If it is preferred to have the mature tree at full size, the entire area for the mature root space must be improved. This could mean improving the soil to at least two times the mature tree spread.

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 around the root falls to about 40° to 45°F (7°C). Mulch in the urban landscape and leaves on the forest floor protect the roots from temperature extremes and form a layer of insulation. A study at LaSalle University showed that soil protected under 8" (20 cm) of uncompacted snow (acting as a mulch) hovered at 32°F (0°C) until air temperatures dropped below -18°F (-26°C) and only ¾" - 2" (2 to 5 cm) deep of soil freezing occurred at -40°F&C.

Urban Trees & Worms
In the areas of North America that were covered with ice 10,000 years ago, earthworms were all frozen out and the forests that evolved after the ice receded, developed without earthworms. Recent research suggests that imported European earthworms appear to make the US and Canadian native forest trees behave more drought stressed, but this does not directly kill the trees or cause obvious health declines. When planting native trees in the city, do not encourage the addition any earthworms to the planting pit. If adding clean compost or mulch, be sure it is without earthworms so soil microorganisms will be able to have a positive affect on individual trees.

Soil Tests
A soil nutrient analysis, density, percolation, and texture test should be performed before planting trees. Soil pH is one of the most useful soil tests, while testing for iron (which is pH dependent) and manganese contents are needed only under special circumstances. In a park setting for example, it is not necessary to take soil tests for all planting sites, just test at some typical locations to find any general soil problems that need correction prior to planting. Remember that testing and analysis of contaminated sites requires much more laboratory work and could become a major budget item, but may be necessary.

Soil pH
Most trees prefer an acidic soil. When selecting trees for planting, tree species that will tolerate a high pH (between 7.0 and 8.0) should be considered for urban areas where concrete is buried or the site is surrounded with concrete building foundations, roads, or sidewalks that may increase the soil pH.

Acidic soil pH is raised by adding calcium carbonate or lime. Sulfur or aluminum sulfate should be used to temporarily bring down a high pH, although the planting of alkaline tolerant trees is a better option.

Urban Soil Replacement
There are no universal prescriptions for urban tree planting because each site and each tree is unique. If the soil on an urban site is really bad because it is composed of concrete, metal, building materials, clay, etc. and the soil cannot be improved by amending, then it must be completely replaced if trees are to grow. If this is not possible, use all soil from the planting pit as back fill and keep it aerated during the planting process. Then let the tree grow as it can and plan to replace the tree in a few years. It is important to note that frequent replacements of trees leads to frequent costs without much benefit from the tree as it stays small. Planning ahead is encouraged.

Researchers have reported that it is also possible to amend really bad urban soil so that its structure, composition, and drainage are improved. To accomplish this, use inorganic amendments such as coarse sand, approximately 75% by volume, added to this bad soil to create a positive change in water movement. It is important that the amendment should have relatively large, uniform soil particles so that when they touch each other, large pores are formed.

Suburban Soil
Fortunately, most tree planting sites are in suburban soil and are not really bad. They can be made healthier for trees with amendments of native soil containing tree roots with mycorrhizal fungi on them. Also add compost to provide habitats for the soil micro-organisms that provide better conditions for tree root growth.

Fertilizer and mycorrhizal fungi may be added if the results of a soil test indicate the need to correct deficiencies and care should be taken that the soil does not contain herbicides or other contaminants. The amendments must be thoroughly mixed with the existing soil to a depth of at least 30 inches (76 cm).

Try to increase the surface area of the planting space, so as to allow more oxygen and water infiltration. Roots are the "lungs" of a tree and oxygen is necessary for every process in the tree. Adjust the depth of the hole so the root collar is positioned at or just above grade. Roots can negotiate a multitude of challenges, but these all take time and energy. Improperly planted trees can survive but their vitality and resiliency will suffer.

Engineered Soil
Research teams have been investigating innovative ways to maximize the potential of trees to control stormwater in a series of studies supported by the USDA Forest Service's Urban and Community Forestry Grants Program. The studies have focused on stormwater storage below the pavement surface in engineered soil. Engineered soil, also called structural soil, consist of uniformly sized stone, coated with a thin layer of soil. They can effectively filter and trap pollutants despite the relatively small amounts of soil. The engineered soil also provides the compaction necessary to support sidewalks and roadways while providing ample pore space for roots, air, and water. Since engineered soils can have a porosity of 35% they can be used as a stormwater management reservoir below pavement. Depending on the amount of precipitation the reservoir collects and the infiltration rate of the subsoil, the challenge may be to design the system so that it drains in a reasonable time to provide trees with enough water but without submerging their root system for too long.

Soil Microorganisms
When trees are brought from the forest and planted in urban or suburban soil, they will not do well. The major cause of this is the lack of microorganisms in the soil potentially due to a very low soil organic matter content. Soil microorganisms consist of animals such as algae, viruses, insects, protozoa, arthropods, bacteria, fungi, and nematodes. They carry out numerous biological functions such as organic decomposition, creation of humus, suppression of pathogens, and improvement of soil properties.

Soil micro-organisms are necessary in all soil because they participate in the food web and convert soil nutrients from organic matter, such as leaves, into 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 Mycorrhizae
Highly disturbed urban and suburban soils are oftentimes missing mycorrhizal fungi. Since 1994, numerous companies have manufactured mycorrhizal fungal spores in a variety of mulch, liquid root dip, and soil injection products. All of these products include an array of mycorrhizal fungi designed to stimulate root growth. Early research indicated that comparing trees without any treatment, to trees receiving root inoculations, the latter showed greater growth, and those inoculated via the injection method were double the average growth of the control trees.

On the other hand, other research and experiments introducing mycorrhizal fungi to trees have generally failed to demonstrate any significant improvement in tree growth. The best results have been in highly disturbed urban soil, where native mycorrhizae populations are unnaturally low. Other research reported that mycorrhizae will not perform efficiently in highly fertilized soil. Therefore the tree has to rely on less efficient root hairs, which reduces the effectiveness of the fertilizer.

Most plants form mycorrhizal associations without any human interference. Even in urban soil, mycorrhizal fungi will be introduced naturally over time as wind and animals carry fungal spores into the area. Introducing mycorrhizal fungi into urban soil may help but the species introduced will likely be different than what the tree will eventually develop on its own.

Soil Preparation
These tips will encourage tree survival:

• Maintain an environment supportive of soil microorganisms including mycorrhizae.
  • Maintain a well-aerated soil environment,
  • Avoid using vermiculite and perlite as soil amendments because they inhibit the formation of mycorrhizae,
  • Avoid overusing inorganic fertilizers, especially phosphorus, because they inhibit the formation of mycorrhizae,
  • Incorporate organic matter as it is important for soil structure as well as for food for microorganisms,
• Avoid unnecessary use of fungicides unless there is a major outbreak of harmful fungi.
• Avoid using herbicides prior to soil preparation and in any amendment soil.

• The ideal soil for tree root development should be 25% air, 25% water, 5% organic matter, and 45% minerals. By comparison, a typical urban soil is 12% air, 12% water, 1% organic and 75% minerals and a typical suburban soil is 18% air, 18% water, 4% organic and 60% minerals.

Selecting the Right Tree
First of all, the right tree is a tree that will do well with the urban atmosphere; it will tolerate urban soil; and it will grow despite all the abuse and lack of care that one finds in our cities. The mature tree size is an important factor in selecting the right tree. One obvious reason is not planting a large shade tree under the utility wires or in a small space above ground as well as limited root space below ground. The right tree should also be resistant to serious insect and disease problems. Make use of plant diversity in urban tree selections.

Sources

• Bartens, J, S. Day, J.R. Harris, J. Dove, and T. Wynn. "Can urban tree roots improve infiltration through compacted subsoil for stormwater management", Journal of Environmental Quality, 37:2048-2057, November-December 2008.
• Burton, Dennis, "The Trouble with Worms", The Schuylkill Center for Environmental Education, 2008.
• Chau, K. C., W. Y. Chan, and L. M. Marafa, "Planter Soils", Arboricultural Journal, 24:189-208, 2000.
• Craul, Dr. Phillip J., "Soils", City Trees, Vol. 36, Number 6, November/December 2000.
• Hale, Cindy, "Earthworms of the Great Lakes", University of Minnesota, Natural Resources Research Institute, 2006.
• Melendrez, Michael Martin and Dr. Michael Karr, Ph.D., "Soil Ecology and the Soil Food Web", Nursery News, July 2004.
• "Mycorrhizae", Archive #31, Online Seminars for Municipal Arborists, March/April 2010.
• Nixon, Will, "Forest Soil and Earthworms", American Forests, Autumn, 1995.
• Personal communications with Julia Bartens, PhD Candidate in Urban Forestry at Virginia Tech.
• Phillips, Jack, "Nature of Tree Care III", Tree Care Industry, January 2008.

• Xiao, Dr. Qingfu, "Engineered Soil", Archive #25, Online Seminars for Municipal Arborists, March/April 2009.

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