Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
bacteria and actinomycetes apiece (Bollen 1974). Topsoil depth is highly correlated with the
nutritional status of the soil and, in forests of the western United States, has been found to be
highly correlated to site productivity (Steinbrenner 1981). Topsoils possess humus, which is what
gives topsoils their dark color (Figure 5-26). Humus is a stable end-product of decomposition,
important for nutrient storage, soil structure, and water-holding capacity.
Sites lacking topsoil have significantly reduced productivity, and obtaining even
minimal revegetation can be very difficult. Planted seedlings often fail or growth
is significantly reduced, resulting in inadequate plant cover to protect the soils
from erosion. Growth of planted trees can be reduced by one-third to one-half
when planted in subsoil instead of topsoil (Youngberg 1981). Restoring these
sites to functioning plant communities is unlikely without mitigating measures.
Topsoil – How to Assess
On undisturbed sites, topsoil is visually differentiated from the underlying subsoil
by having darker colors, less clay, better soil structure, and higher abundance of
fine roots. In forest soils, topsoil depths can be more difficult to differentiate from
subsoils because the color changes are not always distinct. Other attributes, such
as the abundance of roots, lack of clays, increased soil structure, and lower bulk
density, can be used instead. Section 5.11.1, Topsoil, provides a general approach
to surveying a site for topsoil. On construction sites, topsoil has either been
removed and stockpiled or mixed in with the subsoil.
Mitigating for Lack of Topsoil
Minimize Soil Disturbance on “Fragile Soils”
Where soils are especially fragile and reconstructing topsoil conditions is difficult, it is particularly
important to keep the “disturbance footprint” to a minimum (Claassen and others 1995). Sites
with fragile soils include decomposed granitic soils, serpentine soils, high elevation soils, and
very acidic or basic soils.
Salvage and Reapply Topsoil
An effective practice in revegetating highly disturbed sites is salvaging and reapplying topsoils
(see Section 10.1.4, Topsoil). This practice has been found to greatly increase plant growth
and ground cover (Claassen and Zasoski 1994). Topsoil salvage and application require good
planning, implementation oversight, and topsoil surveys. In the planning phase, a survey of the
planned road corridor identifies the location of topsoil, the depth, and nutrient status through
laboratory testing (see Section 5.11.1, Topsoil). After topsoil is removed and appropriately stored,
it is reapplied to the disturbed site, ideally at depths similar to pre-disturbance reference sites.
Create Manufactured Topsoil
When topsoil is not available, “manufactured topsoil” can be created in situ or produced offsite
and imported (see section 10.1.4.5, Manufactured Topsoil). Manufactured topsoil will lack the
native seed bank and some of the biological components of topsoil, but it can re-create a
rooting zone high in nutrients and organic matter, with good water-holding capacity, porosity,
and infiltration.
Create Planting Islands
If sources of manufactured or natural topsoil are scarce or too costly for broad scale applications,
placing available topsoil in strategic locations, such as planting islands, can create a mosaic
of productive growing sites.
Figure 5-26 | Topsoil
Topsoil is the upper soil horizon and is
generally darker, more friable, and has
more roots than subsoil.
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
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