6.04_Topsoiling

Practice Standards and Specifications
6.04
0Preserving and using topsoil to enhance final site stabilization with
Definition g g p
vegetation.
Purpose To provide a suitable growth medium for vegetation.
Conditions Where Where a sufficient supply of quality topsoil is available.
Practice Applies
Where the subsoil or areas of existing surface soil present the following
problems:
• The structure, pH, or nutrient balance of the available soil cannot be
amended by reasonable means to provide an adequate growth medium for
the desired vegetation,
• The soil is too shallow to provide adequate rooting depth or will not
supply necessary moisture and nutrients for growth of desired vegetation,
and
• The soil contains substances toxic to the desired vegetation.
Where high -quality turf or ornamental plants are desired.
Where slopes are 2:1 or flatter.
Planning Topsoil is the surface layer of the soil profile, generally characterized as darker
than the subsoil due to enrichment with organic matter. It is the major zone of
Considerations root development and biological activity. Microorganisms that enhance plant
growth thrive in this layer. Topsoil can usually be differentiated from subsoil
by texture as well as color. Clay content usually increases in the subsoil. In
North Carolina, were subsoils are often high in clay, the topsoil layer may be
significantly coarser in texture. The depth of topsoil may be quite variable.
On severely eroded sites it may be gone entirely.
Advantages of topsoil include its high organic -matter content and friable
consistence (soil aggregates can be crushed with only moderate pressure),
and its available water -holding capacity and nutrient content. Most often it is
superior to subsoil in these characteristics. The texture and friability of topsoil
are usually much more conducive to seedling emergence and root growth.
In addition to being a better growth medium, topsoil is often less erodible than
subsoils, and the coarser texture of topsoil increases infiltration capacity and
reduces runoff.
Although topsoil may provide improved growth medium, there may be
disadvantages, too. Stripping, stockpiling, hauling, and spreading topsoil, or
importing topsoil, may not be cost-effective. Handling may be difficult if
large amounts of branches or rocks are present, or if the terrain is too rough.
Most topsoil contains weed seeds, which compete with desirable species.
In site planning, compare the options of topsoiling with preparing a seedbed
in the available subsoil. The clay content of many subsoils retains moisture.
When properly limed and fertilized, subsoils may provide a satisfactory growth
medium, which is generally free of weed seeds.
6.04.1

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Topsoiling is normally recommended where ornamental plants or high -
maintenance turf will be grown. It may also be required to establish vegetation
on shallow soils, soils containing potentially toxic materials, stony soils, and
soils of critically low pH (high acidity).
If topsoiling is to be used, consider the following:
• quality and amount of topsoil, and
• location for a stabilized stockpile that will not erode, block drainage, or
interfere with work on the site.
Bonding —if topsoil and subsoil are not properly bonded, water will not
infiltrate the soil profile evenly, and it will be difficult to establish vegetation.
Do not apply topsoil to slopes steeper than 2:1 to avoid slippage, nor to a
subsoil of highly contrasting texture. Sandy topsoil over clay subsoil is a
particularly poor combination especially on steep slopes. Water may creep
along the junction between the soil layers and cause the topsoil to slough.
Construction MATERIALS
Determine whether the quality and quantity of available topsoil justifies
Specifications selective handling. Quality topsoil has the following characteristics:
Texture —loam, sandy loam, and silt loam are best; sandy clay loam, silty clay
loam, clay loam, and loamy sand are fair. Do not use heavy clay and organic
soils such as peat or muck as topsoil
Organic matter content —(sometimes referred to as "humic matter") should
be greater than 1.5% by weight.
Acidity —pH should be greater than 3.6 before liming, and liming is required
if it is less than 6.0.
Soluble salts —should be less than 500 ppm.
Sodium —sodium adsorption ratio should be less than 12.
The depth of material meeting the above qualifications should be at least 2
inches. Soil factors such as rock fragments, slope, depth to water table, and
layer thickness affect the ease of excavation and spreading of topsoil.
Generally, the upper part of the soil, which is richest in organic matter, is
most desirable; however, material excavated from deeper layers may be worth
storing if it meets the other criteria listed above.
Organic soils such as mucks and peats do not make good topsoil. They can be
identified by their extremely light weight when dry.
STRIPPING
Strip topsoil only from those areas that will be disturbed by excavation, filling,
roadbuilding, or compaction by equipment. A 4-6 inch stripping depth is
common, but depth varies depending on the site. Determine depth of stripping
6.04.2

Practice Standards and Specifications
by taking soil cores at several locations within each area to be stripped. Topsoil
depth generally varies along a gradient from hilltop to toe of the slope. Put
sediment basins, diversions, and other controls into place before stripping.
STOCKPILING
Select stockpile location to avoid slopes, natural drainageways, and traffic
routes. On large sites, respreading is easier and more economical when topsoil
is stockpiled in small piles located near areas where they will be used.
Sediment barriers —Use sediment fences or other barriers where necessary
to retain sediment.
Temporary seeding —Protect topsoil stockpiles by temporarily seeding as
soon as possible, no more than 21 calendar days after the formation of the
stockpile (Practice 6.10, Temporary Seeding).
Permanent vegetation —If stockpiles will not be used within 90 days they
must be stabilized with permanent vegetation to control erosion and weed
growth (Practice 6.11, Permanent Seeding).
SITE PREPARATION
Before spreading topsoil, establish erosion and sedimentation control practices
such as diversions, berms, dikes, waterways, and sediment basins.
Grading —Maintain grades on the areas to be topsoiled according to the
approved plan and do not alter them by adding topsoil.
Limit of subsoil —Where the pH of the existing subsoil is 6.0 or less, or the
soil is composed of heavy clays, incorporate agricultural limestone in amounts
recommended by soil tests or specified for the seeding mixture to be used
(Practice 6.11, Permanent Seeding). Incorporate lime to a depth of at least 2
inches by disking.
Roughening —Immediately prior to spreading the topsoil, loosen the subgrade
by disking or scarifying to a depth of at least 4 inches, to ensure bonding of the
topsoil and subsoil. If no amendments have been incorporated, loosen the soil
to a depth of at least 6 inches before spreading topsoil.
SPREADING TOPSOIL
Uniformly distribute topsoil to a minimum compacted depth of 2 inches on
3:1 slopes and 4 inches on flatter slopes. To determine the volume of topsoil
required for application to various depths, use Table 6.04a. Do not spread
topsoil while it is frozen or muddy or when the subgrade is wet or frozen.
Correct any irregularities in the surface that result from topsoiling or other
operations to prevent the formation of depressions or water pockets.
Compact the topsoil enough to ensure good contact with the underlying
soil, but avoid excessive compaction, as it increases runoff and inhibits
seed germination. Light packing with a roller is recommended where high -
maintenance turf is to be established.
Rev. 6/06 6.04.3

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Table 6.04a
Cubic Yards of Topsoil
Required for Application to
Various Depths
Depth
Per 1,000
Per
(Inches)
Sq. ft.
Acre
1
3.1
134
2
6.2
268
3
9.3
403
4
12.4
536
5
15.5
670
6
18.6
804
On slopes and areas that will not be mowed, the surface may be left rough after
spreading topsoil. A disk may be used to promote bonding at the interface
between the topsoil and subsoil.
After topsoil application, follow procedures for seedbed preparation, taking
care to avoid excessive mixing of topsoil into the subsoil.
References Site Preparation
6.03, Surface Roughening
Surface Stabilization
6.10, Temporary Seeding
6.11, Permanent Seeding
Chapter 3, Vegetative Considerations
6.04.4