Washington’s “Soils for Salmon” – Compost Amended Soil

Soils for Salmon is an initiative established to promote guidelines, best management practices (BMPs), and policy change in an effort to preserve the overall environmental health of waterways in Washington State. Developed by the Washington Organic Recycling Council (WORC), the program was founded in 1999 to protect Western Washington’s Puget Sound by educating the public about the soil to water connection. The program drives landscapers, builders, developers, and citizens to use low impact development (LID) BMPs, that reduce stormwater runoff and pollution, while limiting water usage demands. Such methods, like amending soil with compost, help restore soil ecosystems and maintain microbiological life that are integral components to the region’s economic and environmental well-being. Recognizing the connection between degraded watersheds and soil health, the state of Washington adopted BMPs suggested by Soils for Salmon in the early 2000s.


WA State Policy

BMP T5.13, Post-Construction Soil Quality and Depth,” found in Washington State Department of Ecology’s Stormwater Management Manual for Western Washington (Volume V: Runoff Treatment BMP’s, Chapter 5, BMP T5.13) is the guideline driving soil policy change. This stormwater manual serves as the main reference document from which local jurisdictions must base amendments of their codes. Since its initial publication, local governments in western Washington (west of the Cascade Mountains) have been required to use this state manual to implement regulations that comply with state water quality goals/standards.

Key points of BMP T5.13, including the use of compost, are outlined below:

  • Naturally occurring (undisturbed) soil and vegetation provide important stormwater functions including: water infiltration; nutrient, sediment, and pollutant adsorption; sediment and pollutant biofiltration; water interflow storage and transmission; and pollutant decomposition. These functions are largely lost when development strips away native soil and vegetation and replaces it with minimal topsoil and sod.
  • Establishing a minimum soil quality and depth will provide improved onsite management of stormwater flow and water quality.
  • Soil organic matter can be attained through numerous materials such as compost, composted woody material, biosolids, and forest product residuals.
  • Soil retention. The duff layer and native topsoil should be retained in an undisturbed state to the maximum extent practicable. In any areas requiring grading remove and stockpile the duff layer and topsoil on site in a designated, controlled area, not adjacent to public resources and critical areas, to be reapplied to other portions of the site where feasible.
  • Soil quality. All areas subject to clearing and grading that have not been covered by impervious surface, incorporated into a drainage facility or engineered as structural fill or slope shall, at project completion, demonstrate the following:
  • A topsoil layer with a minimum organic matter content of 10% dry weight in planting beds, and 5% organic matter content in turf areas, and a pH from 6.0 to 8.0 or matching the pH of the original undisturbed soil. The topsoil layer shall have a minimum depth of 8 inches except where tree roots limit the depth of incorporation of amendments needed to meet the criteria. Subsoils below the topsoil layer should be scarified at least 4 inches with some incorporation of the upper material to avoid stratified layers, where feasible.
  • Planting beds must be mulched with 2 inches of organic material.
  • Quality of compost and other materials used to meet the organic content requirements:
    • The organic content for “pre-approved” amendment rates can be met only using compost that meets the compost specification for Bioretention (BMP T7.30), with the exception that the compost may have up to 35% biosolids or manure. The compost must also have an organic matter content of 40% to 65%, and a carbon to nitrogen ratio below 25:1.
    • Calculated amendment rates may be met through use of composted materials as defined above; or other organic materials amended to meet the carbon to nitrogen ratio requirements, and not exceeding the contaminant limits identified in Table 220-B of WAC 173-350-220.


Early Adopters

Western Washington jurisdictions all drain directly into Puget Sound, so these municipalities have been the main geographic area of focus. Local governments such as King County and the City of Seattle adopted these practices early on as regulatory code.

King County Code § 16.82 – Clearing and Grading Regulations

King County, Washington requires conservation and/or restoration measures compliant with the Stormwater Management Manual for Western Washington (e.g. minimum organic matter content) for all permitted as well as non-permitted construction sites:

  • Replaced topsoil must have an organic matter content of 5% dry weight for turf applications and 10% for planting beds.
  • Replaced soils must also be a minimum of 8 inches thick, unless the soil’s moisture holding capacity native to the site can be demonstrated to require a different thickness.
  • Soil amendments must take place between May 1 and October 1, when soils are the driest and less likely to compact.

Soil Restoration Options

  • Option 1 – Leave native soil undisturbed, and protect from compaction during construction.
  • Option 2 – Amend existing soil in place by rototilling compost into compacted soils or when the forest duff or topsoil has been removed. Apply a layer of compost to existing soil at the pre-approved amendment rate of 1.75 inches for turf areas, or 3 inches for planting beds. Rototill compost into soil to a depth of at least 8 inches.
  • Option 3 – Import topsoil mix with a lab test demonstrating a soil organic matter content of 5% (turf) or 10% (planting beds). The imported topsoil mix must be at a depth of 8 inches.
  • Option 4 – Stockpile site soil, reapply, and amend in place. After all landscape disturbances by the construction site are finished, a minimum 8 inch depth of soil must be achieved, even if this require the purchase of additional topsoil.

King County requires developers to retain receipts of compost delivered on-site as part of the inspection process to verify code compliance. The county also provides an online compost calculator to assist projects.


City of Seattle’s Stormwater Code, Vol. 3, Chapter 4 – Green Stormwater Infrastructure

The City of Seattle revised its 2016 version of the Stormwater Code and Manual with updated regulations that took effect on July 1, 2021, and are jointly administered by Seattle Public Utilities and the Department of Construction and Inspections. Under the latest version of the code, proposed alterations to a land area are evaluated for site characteristics and project type, which then trigger a minimum requirement standard for soil amendment. For all five project types–residential, trails, parcel-based, and roadways–Sections 22.805.030 through 22.805.060 of the Seattle Municipal Code includes the following minimum requirement:

“Soil Amendment. Retain and protect undisturbed soil in areas not being developed, and prior to completion of the project, amend all new, replaced, and disturbed topsoil (including construction lay-down areas) with organic matter to the extent required by and in compliance with the rules promulgated by the Director.”

The “rules promulgated by the Director” refers back to the five volume Seattle Stormwater Manual, which requires all areas that have been subjected to clearing and grading to meet post-construction soil quality and depth requirements, as summarized below:

  • A topsoil layer with a minimum 8 inch depth and an organic matter content of 5% (turf) or 10% (planting beds), as quantified by a loss-on-ignition test
  • 4 inches of scarified subsoil—for a finished, un-compacted depth of 12 inches
  • Mulch planting beds with 2 to 4 inches of organic material, such as wood chips, shredded leaves, compost, etc. after planting is completed
  • Use compost that meets organic content requirements of WAC 173-350, Section 220 (i.e. 40 to 65 percent organic matter content, with a carbon nitrogen ratio below 25:1)
    • One exception is if all plants used are native to Puget Sound Lowlands region, in which case the carbon to nitrogen ratio may be as high as 35:1.
Soil Amend - Seattle lyrs
Image courtesy of Seattle Public Utilities, from Seattle’s Stormwater Manual Vol. 3, Chp. 5, p.5-4

Seattle also provides compliance options for areas disturbed during construction. Seattle’s manual acknowledges that several of these options may be appropriate for the same site, and individual methods may be applied to different areas within the same site.

  • Option 1 – Retain and protect undisturbed soil
  • Option 2 – Amend site soil at custom calculated rate or at “pre-approved” rates:
    • Turf areas: 1.75 inches of compost tilled in at an 8 inch depth
    • Planting beds: 3 inches of compost tilled in at an 8 inch depth; then apply an additional 2 to 4 inches of wood chip or compost mulch
  • Option 3 – Stockpile soil, replace it prior to planting; amend, if necessary, to meet the organic matter requirements as outlined in option 2
  • Option 4 – Import soil that meets the minimum of 5% organic matter by loss-on-ignition test for turf, or 10% organic matter by loss-on-ignition test for planting beds


Impact at the National Level

The Soils for Salmon model has not only reshaped and influenced the regulations in the State of Washington and the Pacific Northwest, the program has also influenced design and construction practices on a national level. For instance, the Sustainable Sites’ program criteria is to eventually be incorporated into the US Green Building Council’s LEED building standards.


More Information


Original post from July 30, 2012
Updated July 13, 2021

Avatar photo
Follow Brenda Platt:
Brenda Platt

Brenda Platt directs ILSR's Composting for Community project.