The organic layer (mulch) provides a medium for biological growth, decomposition of organic material, adsorption and bonding of heavy metals. Fresh bark mulch should be used when possible to maximize nitrogen retention. If aged mulch is used, use the shredded type instead of the "chip" variety to minimize floating action. The organic or mulch layer on the surface of the soil has several physical and biological functions. The surface layer acts as a filter for pollutants in the runoff; protects the soil from drying and eroding; and simulates the leaf litter in a forest community. The organic or mulch layer provides an environment for microorganisms to degrade petroleum-based solvents and other pollutants. The University of Maryland laboratory and field experiments have shown that the mulch layer provides a mechanism for extensive heavy metal capture through organic complexing.  The mulch layer should not exceed 3" in depth. Plants must be in contact with the soil to grow, and too much mulch can restrict oxygen flow to roots. In addition, mulch should not be mounded around the base of plants since this encourages damage from pests and diseases.

CONTINUE TO Planting Soil and Filter Media

The above information was derived from the Prince George's County Bioretention Manual.

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Last modified: 10/23/09