Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
% rock fragments in profile = (100 - % cobbles and stone) * % gravels in sample
For example, a soil is estimated to have 25 percent cobbles and stones from observing road
cuts and from several soil pits. Sieving shows that 50 percent of the sieved soil is composed of
gravels. The soil would be composed of 25 percent cobbles and stones, 37.5 percent gravels
((100 – 25) * 0.50), and 37 percent soil.
Mitigating for High Rock Content
Rock Removal
Screening rock fragments from the soil will increase the available water- holding capacity of a soil.
The greatest benefit from screening is with soils that are very high in cobble and stone,
where the reduction in volume of rock in the soil would be significant. One type of screen is
the “grizzly feeder” which acts as a giant sieve to sort out rock of any size depending on the
screen opening widths. Screened soils have the greatest benefit where soils are shallow or a
good ground cover is required (e.g., grasses and forbs).
Incorporate Compost
Compost incorporated in the soil at high rates will increase the water-holding capacity of a
rocky soil (see Section 10.1.5, Organic Matter Amendments). Depending on the size of the
coarse fragments, incorporation can be difficult.
Surface Apply Compost
A more practical method to mitigate for rocky soils is to apply composts to the soil surface
without mixing. When surface applied, composts can be good growing media for seeds of
grasses and forbs (see Section 10.1.3, Mulches). At rates greater than 3 inches applied to the
surface, seeds germinate well and establish into seedlings that can access moisture and nutrients
not only from the compost, but also some moisture from the rocky soil below the compost.
Be aware that on steep slopes, if the site is is not prepared correctly, the layer between the
compost and soil can become a slip plane on slopes when compost becomes saturated with
water (see Section 5.7 Slope Stability).
Apply Topsoil
If topsoil is available, it can also be applied over a rocky soil (see Section 10.1.4, Topsoil). Topsoil
will have to be placed deep enough to compensate for the quantity of rock in the soil being
covered. On steep slopes, preparation of the site prior to application of topsoil is important
to avoid slope stability issures (Section 5.7 Stability).
Planting Islands
On very rocky sites, rocky soils can be mitigated by focusing mitigating measures into planting
islands (see Section 10.1.8, Topographic Enhancements). Where topsoil, compost, or screened
soil is limited, this material can be concentrated in mounds, pockets, or benches strategically
located throughout a revegetation.
Figure 5-14 | Soil sieves for
estimating rock content
The number 10 sieve (2 mm opening)
on the right separates soil particles (C)
from rock particles (B and A). The 3/4
inch sieve on the left separates the fine
and medium gravels (B) from the coarse
gravels (A).
5.3.3 SOIL STRUCTURE
Just as soils are composed of many-sized mineral particles, they are also composed of different-sized
voids (also referred to as pores) whose influence is responsible for water movement,
water storage, air flow, and root penetration. Small pores (micropores) strongly influence the
moisture-holding capacity of soils, while large pores (macropores) are responsible for water
movement, air flow, and root penetration. The arrangement of large pores is called soil structure.
It is qualitatively observed as cracks, channels, aggregates, crumbs, and clods in the soil, and
described by alternative terms such as friability and tilth. Water flow and root penetration
depend on good soil structure. If soil structure is poor or compacted, roots are less able to
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
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