www.urban-forestry.com 31
increase growth rates, improve soil permeability for storm-
water management, increase soil carbon stores, and help
meet soil restoration requirements for projects seeking
SITES certification (Sustainable Sites Initiative,
sustain-
ablesites.org). Urban foresters can use SPR to rehabilitate
soils damaged by construction and development and set
them on the path to long-term recovery. The procedure is
relatively simple—deep mixing of high quality compost with
existing soil followed by planting trees or shrubs.
Documented Benefits of SPR
Virginia Tech research has documented that in controlled
studies, SPR can dramatically increase canopy growth of
shade trees (by as much as 84% after 7 years), reduce
soil bulk density (by anywhere from 0.19 to 0.57 g/
cm3), increase soil permeability by as much as 6-11
times, increase stable soil carbon stores, and possibly
result in deeper rooting. The end result is, of course,
dependent upon all aspects of the plant × environment
interaction including species, soil fertility, climate, and
rainfall. Furthermore, the magnitude of improvements over
untreated soil will strongly depend upon just how bad
conditions were to start with—the worse the starting soil
conditions are, the greater effect you can expect. Links to
peer-reviewed research are available at
urbanforestry.frec.
vt.edu/SRES that can give insight into the conditions and
outcomes of these trials as well as help make informed
estimates of outcomes that can reasonably be expected
in a given situation.
Stormwater management is a driving force in tree planting
and green infrastructure management. In many areas,
compost incorporation such as SPR can be used as a
best management practice (BMP) or qualify projects for
some type of stormwater credit. However, the approach to
stormwater management is somewhat different than that
used in bioinfiltration cells. First, SPR is best used where
there is sufficient space—broad medians, roadside plant-
ings, landscapes, lawns, or parks. Rather than collecting
stormwater from a large area in a confined location, SPR
improves the performance of existing soil to receive rain-
fall, thus reducing runoff from the entire area. Second,
water movement through the soil is dependent upon the
least permeable layer. If the surface seals, it won't matter
how permeable lower regions are. Consequently, surface
permeability must be maintained by restricting pedestrian
traffic and using mulches or groundcovers. The good news
is that woody plant roots will help maintain paths for water
to move through the soil profile and into deep horizons
below the root zone.
After construction this area will be planted with trees, shrubs, and turf. Heavy grading and a confined building site have resulted in
severe soil compaction that is a prime candidate for SPR. Photo by Susan D. Day
