Shallow soils are common on hilltops, hillsides, and throughout mountainous regions, where the thin topsoil may be the only thing that lies between the ground surface and subsurface rock layers. They usually lack subsoil and may be poor in quality as well as thin.
When shallow soil lies over bedrock, the soil was usually washed away by erosion. This is often the case in hilly country. Shallow soils in level areas may be over hardpan, a rocklike layer caused by eons of calcium or iron deposits at the same level. Some surprised homeowners find that their shallow soils are the result of an underlying concrete sidewalk, former tennis court, or other buried construction.
Shallow soils have less than 50 cm depth of solum. Soils are considered very shallow if they extend less than 10 inches deep before hitting an impervious layer that retards root growth. They are considered merely shallow if they are between 10 and 20 inches deep.
Trees and shrubs that grow in shallow soils have less physical support from their root systems. Large plants, especially trees, growing in shallow soils are consequently vulnerable to wind and may blow down. Large trees on shallow soil should be evaluated by an arborist to see if they are safe.
Gardeners have several alternatives when dealing with shallow soils in a garden.
If shallow soil is due to hardpan, the hardpan may be broken up to allow drainage and root penetration.
In some cases, the only problem caused by the shallow soil is one of drainage. This may be solved by laying drain lines on top of the bedrock to carry the water away. The topography of the site will determine whether or not this is feasible.
Raised beds are an attractive and comparatively easy way to deal with both hardpan and other shallow soils. They can be used to garden even on top of old driveways or patios. Simple raised beds can be made of railroad ties, or of any other material that will contain the soil.
Another way to deal with thin soils is to seek those plants known to have shallow root systems. Plant flowers, groundcovers, or a lawn, or select trees from the list below.
These trees have shallow root systems and will thrive in shallower soil than other trees, but may still pose a hazard of blowing over if they grow too large.
|Common Name||Botanical Name|
|Evergreen ash||Fraxinus uhdei|
|Pacific dogwood||Cornus nuttallii|
|Silver maple||Acer saccharinum|
|Tree of heaven||Ailanthus altissima|