Topsoil is that uppermost layer of soil capable
of growing and supporting vegetation. On a typical
highway project, earthwork normally starts by scraping
the ground surface, then transporting the material
and placing it where fill or embankments are needed.
The first layer to be excavated, usually the topsoil,
is often placed at the bottom of the fill. The last
layer excavated is left exposed on the final grade
surface. This process results in the placement of the
nutrient- and organic matter-rich topsoil at depths
that are inaccessible to plant roots. The exposed geological
parent material is usually a suboptimal planting medium
because it is low in organic matter and often has plant
nutrient imbalances.[N][N][N]
Just 12 state DOTs (24 percent) say they stockpile
desirable/uncontaminated topsoil to facilitate revegetation.
Topsoil has become a commodity that contractors often
strip and sell. Soils management on construction sites
is an important factor in invasive species control.
In addition to removing native cover that may effectively
compete with invasive species and opening new pathways
for infestation, the change in soil characteristics
that may occur with loss of topsoil can also bolster
invasives. One Florida study on roadside soils and
invasive species suggested that disturbance alone may
not increase the presence invasive species, but that
a change in soil characteristics through the addition
of clay and limerock from road construction may enhance
invasion.[N]
Claassen et al. have performed a large percentage
of the studies on topsoil usage and compost, with funding
from FHWA and Caltrans. Among their many helpful recommendations
are for DOTs to test nitrogen content in soils and
ensure slow-release of amendments over three years;
to use compost over use of commercial fertilizer; and
to ensure that compost amendments have adequately decomposed.
Where topsoil is not available other amendments can
be used, the quantity and quality of the Nitrogen (N)
materials applied is critical. The N release should
be slow enough to keep plant-available N at modest
levels, but the total amount of N amended should be
high enough so that the site does not run out of N
before the plant community is well established. The
N amendment should be able to support three to five
years' plant growth, for example. Controlled release
of N is important because excessive N availability
promotes weedy annual grass growth, drying out the
site and crowding slow growing perennials. While the
maintenance of moderate, sustained nitrogen levels
may be achieved from commercial, slow release fertilizer
sources, the inclusion of organic matter in the amendment
is also important to improve the hard setting and poor
water holding capacity of low organic content materials.[N]
Biomass associated with compost has been more effective
than N amendments that were evenly disturbed throughout
the profile (0-30 cm) or applied deeply within the
profile (20-30 cm).[N]
Studies of plant communities established on "problem
soils" amended with commercial fertilizers have
shown vigorous initial growth, but that vegetative
cover often becomes sparse or nonexistent within several
years.[N]
In addition to transportation related studies, those
of fertilized mine reclamation spoil observed that
revegetated areas tended to be highly productive for
two to five years followed by a sharp decline in plant
growth and nutrient availability.[N]
Reapplication of topsoil to subsurface materials enhanced
reestablishment of vegetation by increasing nutrient
availability, water holding capacity, and microbial
activity.[N]
Compost can be used to replace the organic matter and
nutrients and can act as a surface mulch to protect
against erosion, extreme temperatures, and droughtiness.[N]
Long-term nitrogen release rates from most yard waste
compost materials approached the N release rates of
moderately fertile soils. Composts were shown to be
able to regenerate the N availability characteristics
of low-nutrient substrates that have been stripped
of topsoil organic matter. Well-cured composts and
co-composts (biosolids blends) approached the N release
rates of highly fertile soils. Compost application
provides longer N release duration compared to chemical
fertilizer and also provides organic materials for
improved infiltration and microbial activity. Potential
compost sources and soils at the site should be analyzed
before amendment, as compost products and the soils
that are to be revegetated vary in fertility and water
availability.
Topsoil is recommended on all disturbed sites and
slopes 2H:1V or flatter that should be permanently
seeded or used as a planting medium for plantings or
nursery stock. Topsoil may be added to a rock mulch
to enhance slope protection and provide soil for seed
germination and plant growth. Topsoil can be mixed
with organic material such as compost or manufactured
soil amendments to improve the growing capability of
seeded and planted vegetation.
Topsoil normally should not be used on slopes steeper
than 2H:1V or on sandy or silty slopes steeper than
3H:1V. Additionally, topsoil should not be placed on
frozen, extremely wet, or smooth slopes.
To the extent practicable, aboveground vegetation,
including litter, should be mixed or otherwise incorporated
into the topsoil prior to excavation. Topsoil should
be excavated from the existing roadway shoulder to
a depth of 150 mm (6 inches). For new alignments, topsoil
should be excavated to the depth it exists and stockpiled.
The topsoil should be placed into stockpiles at locations
designated on the plans. Stockpiles should be treated
with temporary soil stabilization and erosion control
measures as per Stormwater Pollution Prevention Plan
(SWPPP). Topsoil stockpile height should not exceed
three meters (10 feet).
Stockpiling topsoil will result in the disruption
and loss of beneficial soil microorganisms, and if
stockpiled over a length of time (six months +/-) may
result in total or partial loss of soil microorganisms.
If topsoil is stockpiled prior to placement, the
top one foot of the stockpile material should be mixed
with the remainder of the stockpile to ensure that
living organisms are distributed throughout the topsoil
material at the time of final placement.
The use of micro-organism inoculates may be necessary
to re-establish micro-organisms in topsoil material
that has been stockpiled for over a nine (9) month
period.
Apply a temporary soil stabilization and erosion
control treatment to the exposed topsoiled areas to
protect the topsoil prior to permanent seeding.
Soil binders may be applied to disturbed soil areas
or soil stockpiles requiring short-term protection.
Soil binders consist of applying and maintaining polymeric
or lignin sulfonate soil stabilizers or emulsions,
may be used to stabilize stockpiles. A variety of soil
binders are available for use. Prior to use, the manufacturers'
specifications should be reviewed and compared to the
site-specific conditions. In selecting a soil binder,
the following criteria should be considered: availability
of product; ease of cleanup; degradability (how the
product degrades and what its by-products are); length
of drying time; erosion control effectiveness; longevity;
mode of application and availability of application
equipment; and water quality impact.[N]
Apply soil binders per manufacturer's specifications.
Soil binders should be nontoxic to plant and animal
life.
Soil binders shall not be applied to frozen soil
or areas with standing water.
Soil binders should not be applied during or immediately
before rainfall.
Avoid over-spray onto hardscaped areas.
Check protected areas to ensure proper coverage and
re-apply soil binder as needed, or implement additional
BMPs.
When stockpiling topsoil, mound soil no higher than
1.3 m (4 feet) high for less than 1 year. Cover to
prevent soil erosion and contamination by weeds.
Mitigate construction-related soil compaction in
vegetation restoration areas by ripping the soil to
loosen its structure. After final slope grading and
prior to placement, cut slopes should be cross-ripped
horizontal to the slope to assist in anchoring the
topsoil. The spacing of the ripping shanks should be
one meter (three feet) and penetration should not exceed
300 mm (12 inches) in depth. Where embankments are
constructed, offsetting lifts of material to create
an uneven surface prior to topsoil placement should
be considered. Smooth slopes are not acceptable.
Following construction, stockpiled topsoil should
be uniformly redistributed (placement) to a depth of
150 mm (six-inches). Placed topsoil should be cat tracked
vertically to the slope to compact the topsoil and
to create horizontal pockets (safe sites) to hold seed
and water.
Leave the topsoil surface in a roughened condition
to reduce erosion and facilitate establishment of permanent
vegetation. The roughening establishes safe sites for
seed to germinate and grow.
Where quantities of topsoil are limited, it is recommended
to cover the more critically disturbed areas to the
proper depth, rather than cover all areas. If necessary,
the more favorable sites may be left without topsoil.
Approved compost and/or manufactured organic soil
amendments can be added to the topsoil to increase
the organic content of the soil and assist in rebuilding
soil microorganism populations. Topsoil can be added
to rock mulch for added slope protection, to reduce
the potential of erosion, and to enhance vegetative
growth.
Do not mix organic material such as wood bark or
fiber, grass hay or grain straw in topsoil unless nitrogen
fertilizer is included (organic material uses nitrogen
to break down and decompose the fibers).
Compost derived from livestock or green urban waste
(trees, leaves, lawn clippings) is far superior to
noncomposted manure or wood fiber.
Mix compost with surface soil to reduce drying of
seeds in arid climates. A literature review by Montana
DOT found that compost is rarely mixed with surface
soil, which can be accomplished with a compost blower.
MDT explored various equipment and methods to incorporate
compost and then evaluated vegetation condition and
erosion condition over three years. The research indicated
that compost blowers work well, and that compost applied
as a blanket is more effective at excluding weeds.[N]
Periodically, and after each storm event or snow
melt, inspect, repair, and reseed if necessary to control
erosion and loss of topsoil.
Procedures for periodic maintenance apply to both
temporary soil stabilization or permanent seeding application.
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