Urban Forest Management Plan
Toolkit

Soil survey data

Soil surveys have been published by the U.S. Department of Agriculture since 1899. Modern soil surveys produced by the USDA Natural Resources Conservation Service (NRCS, formerly the Soil Conservation Service), provide maps of soils types. The properties, capabilities, and limitations of the soil types are reported. Soil surveys were previously produced as hardcopy documents. Surveys that are still in print are available from the NRCS. Copies are also available at many reference libraries, soil conservation district offices, and county agricultural extension offices.

NRCS has been migrating soil survey data to formats compatible with GIS for a number of years. Their most recent product for accessing soil data is the Web Soil Survey (WSS).The WSS provides soil data and information produced by the National Cooperative Soil Survey. The site is updated and maintained online as the single authoritative source of soil survey information in the US. WSS has soil maps and data available online for virtually all of the nation's counties. An interactive online map is used to define your area of interest. Data for an area of interest can be downloaded in tabular database format or as spatial data.

The NRCS also maintains the Soil Data Mart. This site provides access to GIS Soil Survey data by county or area. Data are available for free download for one area at a time.

Unfortunately, soil survey data are lacking for many older urban areas. However, old hard copy soil maps may be available in areas where the WSS and Soil Data Mart show no coverage. For example, much of western Santa Clara County (California) was mapped in a 1958 soil survey before most of this area was converted from agriculture to urban land uses. Data from this survey has not yet been made available in digital format by NCRS.

Structural soil

Structural soil (or engineered soil) is a medium that has been used to grow trees in areas where soil must be compacted to support pavement. The first widely used structural soil was developed by Nina Bassuk and her colleagues at Cornell University. The soil consists of a particular mix of crushed gravel (uniformly about 1 inch diameter) and soil (clay loam). When properly constituted and installed, the gravel in the mix provides a locked weight-bearing matrix that can support pavement. The voids between the gravel pieces are mostly filled with the clay loam, which holds moisture and nutrients needed for tree growth. A hydrogel is added when the components are mixed to prevent separation of soil and gravel components.

Various publications at the Cornell Urban Horticulture Institute website provide information on the development and use of this structural soil. The Cornell University structural soil is patented and trademarked (CU-Structural Soil®). Manufacture and distribution of CU-Structural Soil is licensed to Amereq, Inc. Other engineered soils have been developed that make use of more locally-available materials. Davis soil (75% lava rock and 25% loam) has been tested by researchers from the USDA Forest Service Center for Urban Forest Research (CUFR).

Structural soil's main advantage is its ability to be compacted to load-bearing specifications. In street tree plantings, structural soil is primarily used under pavement. New trees are planted directly into a suitable topsoil rather than the structural soil. The New York City Tree Planting Standards describe the use of CU Structural Soil in street tree plantings.

Due to its limited water holding capacity, trees planted in structural soil need to be irrigated. However, the high permeability of structural soil allows it to function as a reservoir for absorbing storm runoff. Pollutants present in the runoff can then be degraded in the structural soil matrix, rather than flowing into streams or lakes. Researchers at several locations are investigating the use of structural soils to better manage urban runoff. See the Virginia Tech Stormwater Management site and the 2009 CUFR report for more information on these uses of structural soil.

Pervious pavement

Pervious pavement provides another way to manage urban runoff. It allows contaminated water to infiltrate into the soil where soil microorganisms can degrade contaminants. This prevents oils and other toxic materials from flowing directly into surface waters. In a well-designed system, pervious pavement can help reduce stormwater runoff, help meet U.S. Environmental Protection Agency stormwater regulations, and recharge groundwater that can be used by urban trees.

Several types of pervious pavement are now available. The Lake Superior Duluth Streams website is one of various websites that discuss the use of pervious pavement. Pavers of various types and materials allow infiltration to occur through openings between or within pavers. In addition, porous versions of concrete (also see this link) and asphalt pavement are available that allow water to percolate directly through a continuous paved surface.

Tree roots and sidewalk/curb damage

Many researchers and urban foresters have tested a variety of techniques for dealing with conflicts between tree roots and nearby sidewalks and curbs. These include reconfiguration of sidewalks around trees, use of different sidewalk construction techniques and materials, and the use of root barriers. The City of Sunnyvale has used steel plates bolted to roots to inhibit further root expansion in a given direction.

Some references to this subject area: L.R Costello and K.S. Jones 2003. Reducing Infrastructure Damage by Tree Roots, A Compendium of Strategies. Available from ISA or WCISA. Coder, K.D. Tree Roots and Infrastructure Damage.