Purpose in bioswale soil mixes: Provides growing medium for plants
Suggested volume: Depends on the amount of organic matter and fines in topsoil and other amendments
What to look for: Loamy topsoils with adequate organic matter and minimal clay will provide some water filtration and support plants without hindering infiltration rates.
Topsoils, particularly native topsoils found on site, can be used in a bioswale soil mix. Incorporating native soils into a soil mix reduces the need for hauling away excess soil and purchasing additional amendments.
Topsoil will contain a specific mixture of sand, silt, clay, and organic matter depending on the soil type. The amount of topsoil to add is generally determined by the total organic matter, clay, or silt components desired in the final soil mix, as these components can hinder infiltration by clogging pore spaces.
Like organic matter, the silt and clay particles in native soils have much higher surface areas—and hence cation exchange capacity—than sand. Depending on its chemical composition, topsoil may provide nutrients and assist with binding metals, as well as hold water for use by plants, but not to the same degree as the organic matter in compost.
In addition, topsoil is not teeming with microbes to the same extent as organic matter, and thus does little to aggregate soil and remove organic pollutants.