Plants need water, oxygen, nutrients and a stable rooting substrate to thrive. Most bioretention soils use some combination of sand and compost sourced on a project by project basis to provide these elements.
A primary challenge with such a non-standardized approach is the lack of chemical and physical consistency of materials and therefore wide variation in hydraulic conductivity and pollutant removal capabilities.
That's why we developed and standardized a consistently high performance biofiltration soil mixture for use in the UrbanGreen BioFilter. The major component is sand, which is inert and provides a porous yet stable soil structure.
Organic content is added in the form of pelletized composted leaf media to support biotic activity in the root zone and to increase pollutant sorption. Unlike conventional compost which tends to rapidly degrade both chemically and physically, the compost pellets are comparatively stable and are uniformly sized which preserves the infiltration rate.
Pumice, a highly porous volcanic material is added for its high void volume.
Finally starch based water absorbing crystals are added to the soil to aid in water retention between storms. The resulting soil supports plant growth, a healthy root zone community and provides highly effective high-rate stormwater filtration.