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Making Site Improvements to Expand Species Choices
Soil Volume
Example: Reducing Compaction
Soil Alkalinity and Acidity

Example: Correcting Soil Alkalinity
Table: Sulfur Application Rate

Making Site Improvements to Expand Species Choices

Soil conditions are often the most limiting factor for tree growth in downtown areas. In order to use the largest selection of species and provide the greatest potential canopy cover, the ability of the site to support tree growth must be maximized. Common site limitations and techniques for addressing these limitations need to be considered. After improving site conditions, a search in the street tree selector should provide you with many more species choices. Click on one of these buttons for specific information in this process.

Soil Volume

This red maple has reached its maximum size for this planting site. Although the tree survives, it will never reach its genetic size potential. The tree has been growing on this site for 10 years. It is 6” in diameter and has yearly shoot growth increments of less than ½”. These trees look reasonably healthy in early spring, as in this picture, but show premature fall color and dieback by the end of the summer.

In order to support an adequate root system that will see the tree through long hot summers and cold, especially dry winters, soil rooting volume must be maximized. If you have the opportunity to influence the design of tree spaces, much rooting space can be gained. Techniques for maximizing rooting volume include:

  • Increase size of sidewalk cutouts or tree islands and the width of continuous tree lawns
  • Include design elements such as benches or decorative walls that direct pedestrian and vehicular traffic away from tree root zones to prevent compaction of exposed soil.
  • Include pavement-supporting, tree-friendly structural soils, such as CU Structural Soil®, Carolina Stalite, and other similar systems as root break-out zones under pavement.
  • Provide strategically placed structural soils to allow trees to send roots under a paved area, such as a sidewalk or section of a parking lot, to adjacent open soil.

If you must work with the existing design, rooting space can still be maximized within the existing space by increasing the usable soil volume. Compacted soil reduces usable rooting volume by limiting root growth as the soil hardens when dry or becomes suffocating when excessively wet. Even trees in open lawns display the characteristics of the red maple in the photo to the right when soil is severely compacted. An uncompacted soil with good structure and organic matter content will allow more root exploration and have a more desirable mix of plant-available water and air—essentially providing more rooting resources in the same space! Pre-planting techniques include:

  • Remove rubble or other debris that is occupying rooting space
  • Use subsoil tillage to break up soil at lower depths
  • In tree pits, dig out and loosen soil to a 2 ft depth
  • Thoroughly incorporate compost or organic matter to build soil structure
  • Rototill 2” of compost into a tree lawn, median strip, parking lot island, or other accessible site

Reducing soil compaction will have the added benefit of improving water infiltration into the soil profile—providing more water to your trees and less to the supply of urban runoff.

Example: Reducing Compaction
Problem: A severely compacted median in a wide roadway is to be planted with large shade trees as part of the entrance into downtown. The median is 12 ft. wide. Its soil is primarily a clayey subsoil, heavily compacted by road machinery during construction. Even in this wide median, usable rooting volume for a tree is about 300 ft3.

Solution: Subsoil tillage loosens lower layers. Two inches of municipal leaf compost is spread over the surface and tilled in thoroughly with a rototiller to a depth of 6-8 inches. During tilling, debris is removed. Soil with excessive stones or gravel is screened. Soil is now relatively uncompacted, and usable tree rooting volume within the median has been increased to about 1200 ft3.

This is a one-time activity that will pay dividends for many years.

Soil Alkalinity and Acidity
Most trees grow best in neutral to somewhat acid soils (pH 7 and below). Some, such as pin oak and willow oak (Quercus palustris and Q. phellos ) will decline rapidly when soil pHs are above 7. In downtown settings, where concrete is abundant and limestone gravel may be mixed in the soil or used as a base layer for pavement, soil pH can be very high (alkaline). These high pHs limit nutrient availability to plants and often result in chlorotic (pale green or yellow) leaves. Fortunately, there are some species that will tolerate these higher pHs and these are often selected as street trees.

You may want to consider adjusting the pH of the soil at your site in some situations. Typically the need is to lower pH as overly alkaline soil (high pH) is the most common problem with street trees. First, you must have determined the current site soil pH and develop a feel for the soil texture at your site and the sources of alkalinity (see site analysis page for information on gathering this data). The more clay in a soil, and the more widespread the sources of alkalinity, the more difficult it is to lower pH. Even if pH is lowered, it is likely to return to its present level in the long term. For this reason pH lowering is most easily achieved when some or all of the source of alkalinity can be removed, the clay content is relatively low and the desired change in pH level is not too great. In the example below, pH can likely be lowered at least temporarily with a resultant improvement in tree health. Tree species that will tolerate a pH of 7.0 or higher, however, should still be selected, as the pH will likely rise over time.

Example: Correcting Soil Alkalinity
  Current Situation Desired Situation Remedial Action
pH 7.2 6.5 apply sulfur at recommended* rate, till in and retest in 6 weeks**
Soil Texture clayey same none
Sources of alkalinity limestone gravel used as mulch and from nearby sidewalk reduce sources remove limestone gravel mulch and any mixed into soil
*See sulfur table below
**Note that microbial activity is required for sulfur to lower pH. Warm weather and moist soil conditions will speed results.

If the sources of alkalinity cannot be removed—for example if a limestone-based structural soil is in use, or if limestone gravel is mixed into the soil and cannot be removed, then attempts to adjust pH are likely a waste of resources.

Sulfur Application Rate

Sulfur tilled in to 6” depth to adjust pH in lbs/1000 sq ft
Use elemental “agricultural sulfur” in a flaked or pelleted form
Existing pH Desired pH Clay Loam Sandy Loam
8.0 7.0 32 20 8
  6.5 47 29 12
7.5 7.0 16 10 .4
  6.5 32 20 8
7.0 6.5 16 10 4
  6.0 32 20 8
Virginia Cooperative Extension        

Site improvement in urban areas can play an important role in increasing both the species diversity and the canopy cover of our urban forests.

Support provided by the US Forest Service - Urban and Community Forestry