The science of greenroof soil mixtures is
very important to the market, especially in Germany where the major companies
have PhD's in soil sciences either running the company or placed in high levels
of management. Most companies have developed proprietary mixes and have patented their various
soil substrate formulas.
Since 1995, the German
Forschungsgesellschaft Landschaftsbau Landschaftsentwicklung e.V. (The
Landscaping and Landscape Development and Research Society) has highly
researched various areas concerning greenroof design, including the
"Vegetation Support Course" breaking the materials groups and types down
into Soil Mixtures; Aggregate Mixtures; Substrate Boards; and Vegetation
Matting categories in their Guidelines for the Planning, Execution and
Upkeep of Green-Roof Sites (English Release, 2002).
American Society for the Testing of Materials
(ASTM) Green Roof Task Force passed the following ASTM standard regarding
E2396-05 Standard Test Method for Saturated Water Permeability of
Granular Drainage Media [Falling-Head Method] for Green Roof Systems
Stay on top of things by
reading the ASTM Updates column by Ralph Velasquez here.
Read the extremely
informative July 2004 Greenroofs.com Guest Feature Article entitled
"Don't Call it Dirt!" from landscape architect and growth media expert
Chuck Friedrich, ASLA, RLA,
Chuck goes into great detail on the necessary properties for a good
The growing medium or soil substrate can be
selected from several engineered factory mixes designed by the various
single-source greenroof suppliers or they can be custom designed by a soil
expert. This option is usually less expensive.
By using a
mixture of native soil upgraded with organic or mineral additives (peat, humus,
wood chips, sand, lava, or expanded clay), it is possible to achieve optimum
water retention, permeability, density and erosion control necessary to support the
greenroof vegetation. But although
sometimes successful in smaller projects, it is not advisable to use ordinary
garden soil, as degeneration often results from compacting and acidification (Hendriks
and Hooker, 1994).
Most current experts frown
on using topsoil at all for fear of introducing unwanted properties. Generally, if you are mixing your own
substrate, a good guideline is approximately 75-80% inorganic (i.e., expanded slate
or crushed clay) to 20-25% organic (humus + some clean topsoil). This will
provide essential drainage and soil air capacity, and sufficient organic
nutrients for the shallow-rooted plants. Just remember that your weight
requirements will be higher with a greater concentration of topsoil, and weeds
and certain pathogens are always possible when ordinary garden topsoil is added.
Certainly, it is recommended to consult with a growth media expert in your area
to determine the correct mixture for your project.
The thinner the
engineered soil layer, the higher the physical demands on the plants. Some of the problems
in the past have been the result of too shallow soil depths, resulting in root damage
from heat and frost fluctuations. Some greenroof companies will suggest a
minimum 2 3/4" soil substrate depth, but again, a little higher depth of
say at least 3" (or higher) is recommended for do-it-yourselfers who are
mixing their own soil mix.
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