Roadside Revegetation

PLANNING PHASE TWO: ASSESS SITE
penetrate the soil to access water. Soil structure is important for other soil functions such as
air flow, drainage, permeability, infiltration, and essential habitat for most soil organisms. Soils
with good structure are typically very productive.
Soil structure is significantly reduced by operating heavy equipment over soils. The pressure
applied by heavy equipment compacts the macropores, reducing soil volume and increasing
soil density. This impact is called soil compaction (Figure 5-15). The effects of soil compaction
on tree growth are well documented (Poff 1996). Trees growing on highly compacted soils have
far less root, stem, and leaf production than those growing on non-compacted sites. Studies
have shown a linear relationship between the increase in surface soil bulk density and decrease
in height growth of young Douglas-fir and ponderosa pine trees (Froehlich and McNabb 1984).
It should be assumed that soils will be highly compacted after construction due to the use of
heavy equipment. In addition to reducing the potential of a construction site to grow vegetation,
compaction also increases runoff and sediment during rainstorm events, which can impact
water quality. On sites where summer rainfall is limiting, there will also be less water entering
the soil, reducing the amount of water available for plant growth.
Compaction can occur several feet below the soil surface, depending on soil texture, moisture,
and the type and weight of equipment being operated. Very compacted soil layers can
significantly reduce or eliminate root penetration. Where compacted layers occur, downward
water movement is restricted and water may saturate the soil layers above the compacted layer.
The resulting saturated soil conditions can be very restrictive to root growth because of the
lack of oxygen and the propensity for higher incidence of disease (Steinfeld and Landis 1990)
and seedling mortality (Figure 5-16). Compacted layers will naturally recover to their original
porosity through root penetration, animal activity, and freeze-thaw events, but recovery can
take 20 to 70 years (Wert and Thomas 1981; Froehlich and others 1983).
Soil Structure – How to Assess
It is easy to qualitatively differentiate good soil structure from compacted soil, but measuring
it quantitatively can be difficult. Indirect field tests to quantify soil structure include bulk
density and penetrometer tests.
The bulk density test measures the dry weight of a standard volume of soil. If the soil has a
high porosity, the bulk density values will be low; if the soil is compacted, the bulk density will
be high. In this method, a cylindrical tube is driven into the soil with a portable bulk density
sampler and a soil core is removed (Figure 5-17). The soil is shaved evenly on both ends so
that the soil is exactly the shape and volume of the cylinder. The soil is then removed from
the cylinder, oven-dried, and weighed.
Bulk Density = weight of soil (g) / cylinder volume (cc)
Bulk density values of a disturbed site must be related back to the bulk density of an adjacent
reference site to make the values meaningful. Remaining within a 15 percent increase in bulk
density over reference site values is ideal. Unfortunately, the bulk density method is time
consuming and cannot be conducted on soils with high rock fragments.
A less quantitative, but more practical, method of measuring soil porosity is with a soil penetrometer.
This equipment measures soil strength instead of density. Compacted soils have
greater strength, and greater resistance to penetration by a penetrometer, than non- compacted
soils. Several types of penetrometers can be purchased for field work – penetrometers that
measure the resistance as a continuous pressure is applied to the probe and penetrometers
(impact penetrometers) that measure the number of blows of a hammer to drive the penetrometer
into a specified depth. A monitoring procedure for assessing compaction using an
impact penetrometer has been developed by the NRCS (Herrick and others 2005b). The most
practical and economical field method for assessing compaction, however, is simply using a
long shovel, as shown in Figure 5-18. In this method, a site is traversed and, at predetermined
Figure 5-15 | Compacted soil
Compacted soils are created by heavy
equipment operating over soil. The
large pore spaces are compressed and
the impacted soils often form a platy
structure as shown in this photograph.
Figure 5-16 | Poor draining
soils due to soil compaction
Compacted soils drain very slowly,
as the puddles on the surface of the
obliterated road in this photograph
indicate. During rainfall or snowmelt,
soils can stay saturated for days and
even weeks. Establishing seedlings
during this period can be very difficult
because roots cannot survive when soils
are poorly drained. Seedlings shown
in this photograph were dead within
three months.
Figure 5-17 | A soil core is used
to determine bulk density and
water-holding capacity
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
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