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History & Overview
Soils for Salmon
In native forests in the Puget Sound region, up to 50 percent of the rain that falls returns to the sky through evaporation and transpiration from plants, compared to 30 percent returned from suburban residential areas, and less than 15 percent from impervious surfaces like roads, roofs and driveways.
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Native forest soils also allow up to 35 percent of our annual rainfall to be detained and infiltrated into the groundwater. This reduces damaging peak storm flows in streams and providing more flow during the dry summer months.
Suburban residential areas, where soils have been stripped and compacted and most of the forest has been removed, detain and infiltrate less than 16 percent of rainfall. Of course, impervious surfaces detain none at all.
Restoring living soils with compost
One way to restore some of the forest’s functions in urbanized areas is to restore soils by tilling in lots of compost (2 to 4 inches of compost per 8 inches of soil, depending on soil type).
This practice can significantly improve detention/infiltration and reduce storm runoff from lawn and landscape areas, especially on the sand, clay, or compacted glacial till soils that are common in this region.
Other benefits of compost
Compost-amended soils also filter and break down or immobilize urban pollutants such as hydrocarbons and heavy metals from cars, and pesticides or soluble fertilizers applied to landscapes, keeping them from reaching streams.
By improving soil fertility and plant pest resistance, compost also greatly reduces the need for pesticides or synthetic fertilizers. And by improving soil moisture retention and plant rooting depth, compost greatly reduces summer irrigation needs, which can allow us to leave more water in our rivers for fish.
How compost improves soil and plant life
How does compost improve soil structure, fertility, bio-filtration and plant vigor? Compost provides food and homes for the incredibly diverse web of tiny creatures that make up the soil ecosystem.
In turn, these organisms aggregate soil particles to create soil structure and pore spaces from the micro up to the macro scale. They break down organic pollutants and bind heavy metals. They recycle nutrients endlessly and make them available to plants. Plus, these organisms crowd out and sometimes feed on the pests and diseases that attack plants -- creating naturally healthier, more attractive landscapes that are easier to maintain.
So, to say this another way: organic matter + soil organisms + time create soil structure, bio-filtration, fertility, & stormwater detention
Soil: discover another world
How do soil organisms create soil structure? Dr. Elaine Ingham of Oregon State University says, "An analogy to building a brick house is useful here. To build bricks, straw and sand have to stick together. Then the bricks are held together with mortar to form walls. The house has structure when the walls are arranged in certain patterns.
Different organism groups in the soil foodweb do the same for soil structure. Bacteria glue the clays, silts and sands together into microaggregates. Microaggregates are bound together by fungal hyphae, root hairs and roots. The structure of the rooms are made by the arthropods, insects and earthworms. Only when all the organisms are present and active can roots and water move into the soil with ease."
Building food for plants
The soil foodweb is fueled by the primary production of plants, which provide the living and dead organic matter that supports the other creatures.
Plants actively give off large amounts of carbohydrates, produced by photosynthesis, from their roots. These materials support both bacteria and the essential symbiotic fungi that form a net around plant root hairs, protecting them from disease-causing fungi and greatly extending their reach for nutrients.
The bacteria and fungi, along with the organic matter they produce, are the storehouse for most of the nutrients in the soil. They are eaten by protozoa, nematodes, and microarthropods, whose wastes provide the soluble nutrients that plants need in small but steady amounts, which is optimal for plant health.
Many species of larger creatures, up to and including earthworms, eat these smaller soil organisms, create the larger aggregates and pore spaces in soil, and also contribute their waste nutrients to be recycled into the storehouse by bacteria and fungi or used by plants.
Breaking down chemicals
The multitude of biochemical pathways used by this diverse soil life also enables it to break down hydrocarbon and pesticide pollutants, bind heavy metals into immobile forms, and convert excess soluble fertilizers into complex stored organic forms before they can run off into streams – this is known as "biofiltration" or "bioremediation". All of this activity arises naturally in soils with adequate organic matter.
Where adequate organic matter is missing, as are many urban soils, organic matter levels can be restored by incorporating compost into the soil, which also inoculates the soil with many of the bacteria and fungi that form the base of the soil food web.
High-quality compost
The quality of the compost used for soil restoration is important. Composts should have passed through a hot (150°F for 3 days) aerobic process and have a sweet, earthy smell. If buying, ask suppliers for a compost proven to meet the Washington Department of Ecology’s guidelines for "Grade A" compost. Mature composts settle less, provide stable nutrient sources, and also provide higher levels of beneficial organisms.
References and web resources
Beyerlein, Douglas, and Joseph Brascher. "Traditional Alternatives: Will More Detention Work?" in Salmon in the City, proceedings of conference in Mt. Vernon, WA, 1998, p. 45. Download under "Links" at the Center for Water and Watershed Studies web site, or request copy from Washington State University at (253) 445-4575.
Kolsti, Kyle F., Burges, Stephen J., and Bruce W. Jensen. Hydrologic Response of Residential-Scale Lawns on Till Containing Various Amounts of Compost Amendment. Univ. of WA Center for Urban Water Resources, for Washington Department of Ecology, 1995, pages 1-88. Copies available from UW Engineering Professional Programs at (206) 543-5539.
Chollak, Tracy, and Paul Rosenfeld. Guidelines for Landscaping with Compost-Amended Soils. City of Redmond Public Works, 1998, p. I.1-I.4. Download from the City of Redmond website.
McDonald, David K. Ecologically Sound Lawn Care for the Pacific Northwest: Findings from the Scientific Literature and Recommendations from Turf Professionals. City of Seattle Public Utilities, 1999, pp. 15-18. Download from this web site (link below), or request at (206) 684-7560.
Coleman, David C., and D. A. Crossley, Jr. Fundamentals of Soil Ecology. San Diego; Academic Press (Harcourt Brace & Company), 1996, pp. 12-167.
Ingham, Elaine R. "The Soil Foodweb". 1998, p. 2. on the Soil Food Web website. This web site provides a number of resources for understanding soil ecology in lay terms.
Related links
Ecologically Sound Lawn Care Report (PDF)
Links to other sites
King County: Soils for Salmon
Washington Organic Recycling Council: current information on the Soils for Salmon campaign
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