Bioretention cells are needed when the percolation rates are not adequate enough to infiltrate at .50" per hour.
The engineered soil mix used in a bioretention cell is 60% washed concrete sand, 25% compost, and 15% topsoil. Several surrounding states are using an engineered soil without topsoil to avoid clay content, which they have found to plug the soil surface. They use 50-80% sand with the balance being compost and avoiding soil in the mix.
The Iowa Storm Water Management Manual provides approved design guidelines.
Bioretention Cell Design Worksheet