Roadside Revegetation

IMPLEMENTATION GUIDES
most topsoil piles. Climates with lower moisture and temperatures can be more favorable to
long- term storage. A study in Alberta, Canada, for instance, revealed that topsoil had very little
respiration or organic decomposition after three years in a stockpile due to the influence of
the cold, dry climate (Visser, Fujikawa, and others 1984). Dry topsoils store longer and maintain
greater populations of viable mycorrhizal fungi (Miller and Jastrow 1992). If a topsoil pile is
to be held over winter in areas of moderate to high rainfall, it should be kept dry by covering
with plastic (which will also keep the piles protected from erosion and weed establishment).
The size of the pile can also affect the viability of the topsoil. The interior of large piles
maintains higher temperatures and are usually anaerobic, which can be detrimental to soil
microorganisms. Microbial biomass levels and mycorrhizal fungi have been found to be very
low in the bottom of large stockpiles (Ross and Cairns 1981; Miller and Jastrow 1992). Most
projects limit topsoil piles to 3 to 6 ft in height. This is not always possible, especially when
topsoil storage space is limited. Under these circumstances, the size of the topsoil pile can be
quite large. To reduce the negative effects associated with very large piles, topsoils should be
salvaged dry and kept dry during storage. Large piles should be stored for as short a time as
possible. In addition to maintaining the viability of the topsoil, minimizing storage time will
reduce the risk of weed infestation.
Standard specifications often call for temporary seeding of topsoil piles. The benefits of this
practice are erosion control and maintenance of mycorrhizae inoculum through the presence
of live roots. This practice should be evaluated for each project to avoid the introduction of
undesirable plant species in the seeding mix if non-native species are used. Alternatives to
this practice include hydromulching without seeds or covering with plastic, however, both
practices loose their effectiveness with time.
Reapplying Topsoil
The depth of topsoil application is generally
based on the amount of topsoil
Total soil nitrogen (or
A other nutrient of interest)
available and the desired productivity
in salvaged topsoil
of the site after application. As a rule,
0.14 %
the deeper topsoil is applied, the higher
the site productivity will be. If the objective
is to restore a site to its original
productivity, the placement of topsoil
should be at a depth equal to or greater
B
C
Soil bulk density
Fine soil fraction
1.1
70
gr/cc
%
than the topsoil horizon of undisturbed
Nitrogen for soil layer
reference sites. Sufficient topsoil quantities
however, are rarely available in the
(A * B * C * 270) =
Minimum or threshold
quantities needed to restore disturbed
N levels
sites to their original topsoil depths.
This often leads to applying topsoil too
Minimum topsoil application:
E/D
thinly across a project site. There may be
* 12 =
a minimum topsoil depth below which
the application of topsoil is not effective.
Research on a northern California road construction site (Claassen and Zasoski 1994) suggests
that a depth of 4 to 8 inches was required for an effective use of topsoil. On sites where the
subsoil is unfavorable for plant establishment (e.g., very high or low pH, high sodium, high
salinity), minimum depths of greater than 12 inches of topsoil should be considered (Bradshaw
and others 1982).
Determining minimum topsoil application depths can be based on the minimum amount
of nitrogen required to establish a self-maintaining plant community. A threshold of
approximately 700 kg/ha (625 lb/ac) of total nitrogen in the topsoil has been suggested for
sustaining a self- maintaining plant community in a temperate climate (Bradshaw and others
From soil test of post construction
soils - reported in gr/l, ppm, mg/kg, ug/g
divide by 10,000 for %
Unless known, use 1.5 for compacted
subsoils, 1.3 for undisturbed soils,
0.9 for light soils such as pumice
100% minus the rock fragment content -
from estimates made from sieved soil
prior to sending to lab
D 2,911 lbs/ac ft Calculated amount of total nitrogen in
1 acre feet of soil
E 1,100 lbs/ac Determined from reference sites or
minimum thresholds from literature
F 4.5 inches The minimum thickness of topsoil to apply
to meet minimum thresholds of nitrogen
Figure 10-24 | Salvaged soil can
be tested for nitrogen content
Soil testing of salvaged topsoil can be
used to calculate the thickness to apply
in order to meet minimum nitrogen
levels.
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
253