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 professional association FLL - 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).

The American Society for the Testing of Materials (ASTM) Green Roof Task Force passed the following ASTM standard regarding growth media:

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, here.  Chuck goes into great detail on the necessary properties for a good greenroof medium.

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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