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. Studies in Maryland have shown newspaper mulch to be an ideal source of organics. In North Carolina, peat moss 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.