Why Build Healthy Soil?

Soil performs valuable functions: nourishing plants, absorbing and cleaning stormwater. These functions are often degraded during development when soil is removed or compacted. Restoring healthy soil is essential to protecting our waterways and salmon, and our way of life in the Puget Sound region.

Soil Best Practices:

New Construction
  • Retain and protect native topsoil & vegetation where practical
  • Restore disturbed soils by tilling 2-3" of compost into upper 8" of soil
  • Loosen compacted subsoil, if needed, by ripping to 12" depth
  • Mulch landscape beds after planting
Existing Landscapes
  • Till in compost when re-landscaping
  • Mulch beds with organic mulches
  • Topdress turf with compost

On this page:

The Soil-Water-Salmon Connection

In native forests around Puget Sound, soil and plants absorb most rainfall and minimize stormwater runoff. 50% of the rain that falls returns to the sky as "evapo-transpiration" (as tree roots pull water from the soil and it evaporates from leaves and needles). Almost all the rest filters slowly into and through the soil, to feed streams year ’round. But typical development practices remove forests and topsoil, degrading the land's ability to hold and recycle rainwater. After typical development, only 15-30% of rain evaporates, while most rushes swiftly off roofs, roads, and compacted soil (known as "impervious surfaces"). This erodes streams, causes flooding, and carries pollution and sediment, damaging essential habitat for salmon and other aquatic life.

Native Soil
- rain soaks in

Click to enlarge - Native Soil

Click image to enlarge.

After Development
- rain rushes off

Click to enlarge - Disturbed Soil

Click image to enlarge.

Native soils and forests are disappearing rapidly in the Puget Sound region. Conventional development practice has been to strip topsoil during grading, compact the whole site with heavy equipment during construction, and replace only 1-2 inches of soil over the compacted subsoil before landscaping.

The impacts on salmon, water quality, and streams are well-documented. Not only are winter peak storm flows much higher, but summer stream flows are much lower because groundwater is non being recharged. Scientists at the University of Washington and elsewhere have documented significant loss of stream health with the first 5-10% of impervious area constructed in watersheds under conventional development practices.

UW Stormwater Trials
- till soil, no compost

Click to enlarge - Without Compost

Glacial till soil typical in developing areas. High runoff, and poor turf quality.

With Compost
- less runoff, better turf

Click to enlarge - With Compost

Same soil with 30% compost added. Up to 50% less runoff, and turf is still healthier 4 years later.

Salmon in the City (PDF 350K on University of Washington website) Scientific papers presented at this 1999 conference highlighted the need to change development practices, especially to restore soil hydrology functions on every site.

How Compost and Other Organic Amendments Restore Soil Functions

Soil functions are created by soil life. The billions of organisms in the soil are fueled by soil organic matter. They create soil structure and pore spaces, which provide stormwater infiltration, drainage, and moisture-holding capacity. They also create soil fertility and resistance to erosion, protect plants against diseases, and break down urban pollutants like oil, grease, metals, fertilizers, and pesticides.

Soil Biology Primer (on USDA Natural Resources Conservation Service website) illustrates how the soil ecosystem creates these essential functions, in agriculture, forest, and urban landscapes.

Compost amendment restarts the soil ecosystem – restoring functions in soils disturbed during development. Tilling in compost immediately reduces soil compaction and increases stormwater infiltration rates. The compost feeds and creates habitat for beneficial soil organisms – restoring the ecosystem in degraded soils, so they begin to function again more like native soils.

Soils for Salmon: Integrating Stormwater, Water Supply, and Solid Waste Issues in New Development and Existing Landscapes (PDF 660K) reviews the science behind soil restoration, and opportunities to change soil practices in this region.

Protecting native soil, and restoring disturbed soils with compost, is essential to restoring salmon and the health of our waterways in the Puget Sound region. Soil restoration is the foundation of the Low Impact Development (LID) practices being adopted around Puget Sound to cost-effectively manage stormwater and restore the health of our waterways.

The Relationship Between Soil and Water: How Soil Amendments and Compost Can Aid in Salmon Recovery (PDF 970K) illustrates the need for soil "Best Management Practices", and describes the Soils for Salmon initiative of the Washington Organic Recycling Council.

Benefits of Soil Best Practices

For Builders, Developers, and Landscape Contractors
  • Better landscape appearance = happier customers, which sells the next job
  • Better erosion control
  • Better stormwater management
  • Easier, faster planting in amended soil
  • Much better plant survival (fewer callbacks for dying plants and lawns)
For Property Owners
  • Easier, cheaper landscape maintenance
  • Up to 50% less summer irrigation needs
  • Lower fertilizer needs
  • Much less pesticides and herbicides needed – easier, and healthier for families
  • Better landscape appearance, all year ’round
For Our Waterways, Salmon, and Other Wildlife
  • Reduced stormwater runoff = less erosion and sediment in salmon streams
  • Increased groundwater recharge = better summer stream flows
  • Less demand for irrigation water supply = more water in rivers for wildlife
  • Bio-filtration of urban pollutants (running through compost-amended soil filters out 60-95% of contaminants)
  • Less need for fertilizers and pesticides, so less washing off into streams
--> Continue to How To: soil best management practices, tools, and specifications
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