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. Beware of where the topsoil comes from, and what vegetation was on site before disturbance, invasive plant seeds may be included and could cause problems for establishing native plants, landscaped areas, or grasses. . Topsoil from the site will contain mycorrhizal bacteria that are necessary for healthy root growth and nutrient transfer. These native mycorrhiza are acclimated to the site and will provide optimum conditions for establishing grasses. Use com- mercially available mycorrhiza products when using off -site topsoil. Design and Installation Specifications Meet the following requirements for disturbed areas that will be developed as lawn or landscaped areas at the completed project site: . Maximize the depth of the topsoil wherever possible to provide the maximum pos- sible infiltration capacity and beneficial growth medium. Topsoil shall have: • A minimum depth of 8-inches. Scarify subsoils below the topsoil layer at least 4-inches with some incorporation of the upper material to avoid stratified lay- ers, where feasible. Ripping or re -structuring the subgrade may also provide additional benefits regarding the overall infiltration and interflow dynamics of the soil system. • A minimum organic content of 10% dry weight in planting beds, and 5% organic matter content in turf areas. Incorporate organic amendments to a minimum 8-inch depth except where tree roots or other natural features limit the depth of incorporation. • A pH between 6.0 and 8.0 or matching the pH of the undisturbed soil. • If blended topsoil is imported, then fines should be limited to 25 percent passing through a 200 sieve. o Mulch planting beds with 2 inches of organic material Accomplish the required organic content, depth, and pH by returning native topsoil to the site, importing topsoil of sufficient organic content, and/or incorporating organic amendments. When using the option of incorporating amendments to meet the organic content requirement, use compost that meets the compost specification for Bioretention (See BMP T7.30: Bioretention Cells, Swales, and Planter Boxes 959 , with the exception that the compost may have up to 35% biosolids or manure. Sections three through seven of the document entitled, Guidelines and Resources for Implementing Soil Quality and Depth BMP T5.13 in WDOE Stormwater Man- agement Manual for Western Washington, provides useful guidance for imple- menting whichever option is chosen. It includes guidance for pre -approved default strategies and guidance for custom strategies. Check with your local jurisdiction 2014 Stormwater Management Manual for Westem Washington Volume 11- Chapter 4 - Page 298