3: Restore Disturbed Soils
Disturbed soils (i.e. any soil in a town, city, or other historically populated area) tend to get compacted by vehicular or, in the case of clayey soils, even foot traffic. Compaction reduces spaces in the soil where water can infiltrate, which has several impacts to water quality & availability .
- Rain falling on compacted soil can no longer be absorbed and conveyed downhill in the soil, but instead runs off, carrying pollutants with it.
- Plant establishment depends on roots’ access to air and water in spaces within the soil (voids).
- Landowners often respond to unhealthy plants by applying chemicals like pesticides, herbicides, and fertilizers and by watering more.
In studies done by Washington State University Extension Services, simply ripping/disturbing, without folding in compost, allows the soil to slump back into a compacted state. These studies found that when disturbed soils were properly amended with compost and finished in lawn or shrubs in a landscape area, the areas responded to rainfall events as if they were 50% - 80% forested, respectively, and remained permeable (allow water to soak in) over time. This method of soil restoration in developed areas is recommended to regain greater stormwater functions, provide increased treatment of pollutants and sediments, and minimize the need for some landscaping chemicals, thus reducing pollution through prevention . The practice of amending these soils is a simple, cost-effective practice for restoring and preserving the long-term permeability of compacted soils. The practice is also a great way to conserve water.
Real estate agents estimate that the landscape is about 5-10% of the total sales price of a house. This practice can cost as little as $1,000/lot during construction . Long-term irrigation demand can be cut by 50% with a payback period of 3-7 years, which is a selling point for landowners. Stockpiling topsoil for use later as an ingredient in the compost amendment can save money in hauling costs.
Where to Amend
Compost amended soils should be used anywhere soils have been disturbed and where a future landscape area is proposed. Compost amendment should not be performed under tree canopies or other established landscape areas to be preserved since the tilling process will damage roots. Any kind of soil will benefit from compost amendment, but in particular, watersheds with clay will benefit the most.
For all proposed landscape areas inside the disturbed area (clearings, parking, roads, pathways, re-grading), till compost into the top few inches of native soils. The ideal organic content is 10% for landscaped beds. A lower target rate for turf areas of 5% is recommended since a higher organic content could make mowing more difficult. The document "Building Soil " includes great detail on methods to calculate the appropriate depth of compost to use to achieve these percentages, but the following guidance is a method that can easily applied, without calculations or lab testing, to any kind of soil (sandy, clay, silty, etc).
To amend proposed landscaped beds:
- Till or scarify soil 12” deep.
- Place 3” of compost and till into 5” of soil (a total amended depth of about 9.5”, for a settled depth of about 8”).
- Rake beds to smooth and remove surface rocks larger than 2” in diameter.
- Mulch planting beds with 2 – 3” of organic mulch.
To amend proposed turf areas that will be mowed:
- Till or scarify 12” deep.
- Place 1.75” of composted material and roto-till into 6.25” of soil (a total amended depth of about 9.5”, for a settled depth of about 8”).
- Water or roll to compact to 85% of the maximum density there would be in dry conditions. Rake to a level surface and remove surface woody debris and rocks larger than 1” diameter.
To amend native soils for rain gardens and vegetated filter strip areas, see “4 Build a Rain Garden” and “2 Disconnect Impervious Areas”.
When amending soils, care should be taken to ensure that compost is clean and free of weeds, pollutants, or other harmful materials that may impact plant health and water quality.
Organic compost should have the following properties:
- Weed seed and pollutant free.
- 100% of material shall pass a ½” screen.
- pH between 5.5 and 7.0. If the pH isn't quite right, it may be lowered by adding iron sulfate and sulfur or raised by adding lime or recycled, ground gypsum board.
- Carbon nitrogen ratio of 35:1.
- Organic matter content between 40 and 50%.
- Fully decomposed. Earthy is good. Avoid compost that smells like ammonia.
Organic compost may consist of the following:
- Mushroom Compost. The used bedding material from commercial mushroom production.
- Local nursery or garden supply’s stock of organic compost. There is US Compost Council Seal of Testing Assured (STA) compost. Visit http://compostingcouncil.org/participants to find a participating supplier near you. The STA program is no guarantee of quality, only that the compost has been tested and those test results are available for review.
Organic compost may NOT be:
- Composted Yard Debris. This is because excessive pollutants, mostly herbicides, pesticides, and fertilizers, have historically been found in these materials. “Cides” can kill beneficial soil life, reduce stormwater benefits, and increase maintenance.
- Peat Moss. Peat moss is extracted from wetlands; this has negative impacts on the watershed from which the peat moss was removed.
Compost amended areas are considered self managing. Grading plans should show a 2% minimum slope away from buildings for a minimum distance of 10’ in landscape areas to ensure adequate drainage during large storms, which are expected to generate runoff. This is a common rule of thumb and shouldn't change the grading design from that of a conventional stormwater approach. If compost amended landscape areas drain to a structured outlet such as an area drain and pipe, size the infrastructure to adequately convey even the very large, perhaps infrequent, storm flows safely away.
Soils should be amended at the end of construction or at least at the completion of concrete work. Protect areas from compaction and erosion afterward with fencing and signage as needed . Minimize erosion by covering soil with mulch and planting right away.
Maintenance of compost amended soils is the same as any landscape area. It should be possible to irrigate less and reduce or eliminate the use of fertilizers, herbicides and pesticides. Keep soil in landscaped garden areas covered with 2-4" of compost by mulching once a year. Aerate turf areas and top-dress with fine mulch .
Stormwater Management Manual for Western Washington: http://www.ecy.wa.gov/programs/wq/stormwater/manual.html
Permitting varies, so check with your local jurisdiction’s building or development services department to find out what codes may apply to your project. If information in this guidance conflicts with your jurisdiction’s requirements or approach, then follow their guidance instead.
"Building Soil: Guidelines and Resources For Implementing Soil Quality and Depth BMP T5.13." Stormwater Management Manual. 2010.
Washington Organic Recycling Council. Soils for Salmon. <http://www.soilsforsalmon.org/>.
Figures 1-6: Maria Cahill, Green Girl Land Development Solutions
Figure 7: Eric Rosewell, Depave
Figure 8: Sid Scott
Figure 1-6 licensed for reuse (Creative Commons), others used with permission.
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©2012. Five Counties Salmonid Conservation Program and Green Girl Land Development Solutions. This publication may be photocopied or reprinted in its entirety or in portions for noncommercial purposes. This publication is available in an accessible format on the 5C web site at http://www.5counties.org/docs.htm. If future documents are based on this document, credit should be given with the following wording: “Portions of this document are adapted from [“Restore Disturbed Soils”, link to original that will appear on my publications webpage] ©2012, Five Counties Salmonid Conservation Program and Green Girl Land Development Solutions. No risk or liability by either of these organizations shall be assumed for information offered in this document.”
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