From LID SWM Planning and Design Guide
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  • Topsoil may be material that was stripped from the project site and stored in stockpiles for re-use, or material imported to the site from a supplier provided the physical and chemical characteristics are within acceptable ranges.
  • Topsoil shall be in compliance with Ontario Regulation 153/04 Record of Site Condition standards for soil quality or as amended through Ontario Management of Excess Soil - A Guide for Best Management Practices.
  • Soil laboratory reports shall certify the material to be suitable for re-use on residential, parkland, institutional, industrial, commercial, or community landscapes for the germination of seeds and the support of vegetative growth.

The factors to consider in determining a suitable soil mix for a vegetated stormwater practice include the following:

Specify that topsoil must be friable, neither heavy clay nor of a very light sandy nature. An example of sandy loam topsoil is 60 % sand, 25 % silt, 10 % clay, organic matter 5 % and pH value of 6 - 7.5. Topsoil must be capable of sustaining vigorous plant growth and to be free from subsoil, roots, vegetation, debris, toxic materials and stone over 50 mm diameter. Specify that topsoil sample must be provided to the consultant for testing and analysis including herbicide or atrazine content. All topsoil supplied must conform to a sample provided. Minimum topsoil depth is 150 mm for turf areas and ranging to 1.25 m for perennials, shrubs and trees.

Avoid creating a "bathtub" effect where isolated areas of coarser soils or loamy sands are surrounded by fine soils or clay. The design engineer and landscape architect should review both subsoil and topsoil materials for compatibility of compactness and texture in planting areas of the LID Practice. Methods, such as soil amendment and preparing a larger planting bed, may be required to prevent discontinuous soil layers. In bioretention practices with a loamy sand soil media placed in clay soil textures, an underdrain is necessary.