Purpose in a green infrastructure soil: Drainage
Suggested volume: 60% of total mix
What to look for: Coarse, gravelly sand without too many smaller particles (fines) as these may clog pores.
Sand generally refers to the coarse-textured (less than 2-millimeter) mineral fraction of soil. Sand’s main job in a green stormwater or "bioswale" soil mix is to provide high infiltration rates and resist compaction. Sand has a low surface area and hence a low “cation exchange capacity” (CEC), meaning it does very little to filter contaminants such as metal pollutants and plant nutrients. The large pore space found in sandy soil is also poor at holding water. This makes for good drainage, but can also make for sad-looking, nutrient- and water-starved plants in the garden.
It’s best to select a coarse, gravelly sand for your soil mixture so that water can infiltrate at the desired rate (approximately one inch per hour). Washed, concrete sand is often recommended as it is coarse, clean, and a recycled product available locally in most areas. While sand may make up the majority of these mixes—frequently 60% or more by volume—compost is the real workhorse of any bioswale stormwater treatment system.