Soil conditions are among the most critical site considerations for tree growth and survival. The Urban Soils Module describes how to identify and test soil and ways to prevent and solve problems at a planting site. Specific problems can be identified by having soil tested at a laboratory. Some of the characteristics to consider when selecting a site are:
Soil texture influences soil fertility and the way air and water move through the soil.
Soil interfaces are abrupt changes in the texture that interrupt the normal movement of water in the soil. These changes can be caused by a variety of activities, such as erosion; mudslides; and the addition of fill soil, bricks, and concrete.
Compacted soils are a major cause of tree-related decline in urban areas. Are there signs of compacted soil, such as hardness, standing water, or poor plant growth? If the soil is compacted, the site may have poor soil structure, aeration, and drainage. A bulk-density test of the soil or use of a soil penetrometer reveals the amount of compaction.
Soil Moisture and Drainage
|Photo Credit: Raina Sheridan|
Soil moisture is influenced by various factors, such as soil texture and structure, precipitation patterns, hardpan, and soil interfaces. Poorly timed irrigation systems can also cause soil moisture problems.
Soil Nutrients, Fertility, and pH
Since leaves and other tree litter are usually raked and not left on the ground to decompose in urban areas, the soil may lack an organic component. Construction activities, such as removal of topsoil and cement wash-out areas (where cement trucks wash their chutes with water), may also alter soil nutrient availability. Sometimes soil color is a clue to nutrient levels in soil. Before designing and executing a planting project, test soil pH and nutrients, where appropriate.
The temperature of the soil influences root growth by controlling the rate of chemical and biological processes. Temperature extremes can freeze or dry out roots. If the site is in a paved area, such as a parking lot, or is a container, soil temperatures may be higher than normal. Adding mulch to the planting area is one way to help moderate soil temperature.
Soil contamination may kill the roots. Does it look as if the soil has been contaminated by construction activities, chemical spills, excessive use of herbicides, or other disturbance? An unusual soil color or odd smell may indicate chemical contamination at the site.
Salt greatly influences tree physiology because it binds with important nutrients the tree needs, thus prohibiting the tree from absorbing those nutrients. Salt also absorbs water in the soil that the tree needs. Is the site located near the coast or near roads that are salted to melt snow and ice? Selecting a species tolerant to salt is usually recommended for these sites.
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By: Ed Macie, Regional Urban Forester, USFS Southern Region