Soil Media Mix

Selection of the appropriate fill soil media for a bioretention area is important to ensure adequate drainage, reduce pollutant loads, and support plant growth.

The following "recipe" for a bioretention soil media, or fill-soil mix, works best:

• 85 to 88 percent sand. A washed, medium sand is sufficient. A USGA greens mix is not necessary and can be costly.
• 8 to 12 percent fines. Fines include both clay and silt.
• 12% to obtain 1 in/hr infiltration rate for nitrogen removal
• 8% to obtain 2 in/hr infiltration rate for phosphorus, metal, and other pollutant removal
• 3 to 5 percent organic matter. pine bark fines has been successfully used.

Soil Chemistry

To support plant growth while removing phosphorus from runoff, the fill soil must have a P-index between 10 and 30. If the bioretention area is not designed to reduce phosphorus in runoff, a P-Index for the fill soil of 25 to 40 is recommended. For additional information on the soil P-index, visit About the P-Index / P-Index Conversion. In addition to having a low P-Index, it is best for fill media to have a relatively high cation exchange capacity (CEC). Higher CECs describe soils that have a greater ability to capture and retain phosphorus. While a minimum CEC has yet to be established, CECs exceeding 10 are expected to work relatively well at removing target pollutants in bioretention systems.