Protect Groundwater Resources

To ensure adequate water quality treatment, healthy soil infiltrating not too fast and not too slow is needed for a depth of 18”. When the infiltration rate of your native soil exceeds 12 inches/hour, groundwater resources may be at risk, so native soils should be amended or replaced for a depth of 18”. Amending soils to drain slower can be tricky and potentially expensive. Many designers send a sample of the native soil to a laboratory and request a “recipe” of what to mix in and in what quantities. A more cost-effective approach to lab testing is to replace the first 18” of native soil with an engineered soil mix. (See “Amended Planting Soil for specifications on materials, mixing and placement.)


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Amended Planting Soil Specifications

Amended planting soil (aka engineered soil, 3-way mix, bioretention soil mix) is a mix of loamy soil, gravelly sand, and compost. There are a number of landscape material suppliers in Oregon that can provide you with a suitable amended planting soil (aka 3-way mix, bioretention soil mix). If your supplier has a mix that meets the City of Portland’s requirements, this mix will be equivalent to the mix that meets the specifications below. If your supplier does not have a mix that meets the City of Portland’s requirements, share the following specifications with your supplier to see if they have an equivalent product.


Care should be taken to ensure that compost is clean and free of weeds, pollutants, or other deleterious materials that may impact plant health and water quality.

Organic compost should have the following properties:

  • Weed seed and pollutant free.
  • 100% should pass a 1/2-inch 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 percent.
  • Fully composed. 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.
  • US Compost Council Seal of Testing Assured (STA) compost. Visit 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 the designer’s review.

Organic compost may NOT be:

  • Composted Yard Debris. 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.

Gravelly Sand

Gravelly sand should be free of organic material, contaminants, and hazardous materials, and should conform to the following gradation, which you can compare against the gradation provided by your quarry’s material:

U.S. Sieve Size Percent Passing
2-inch 100
3/4-inch 70-100
1/4-inch 50-80
No. 40 15-40
No. 200 0-3


Mix soil and amendments to a homogeneous (i.e. all the same) consistency. Do not mix compost, sand, and native soil in the rain or wet conditions. Even in dry weather, soils and amendments themselves should not be overly wet.


Store stockpiles of organic soil mix in a manner that prevents them from becoming wet from rain, stormwater runoff, or other sources of water, or contaminated by fine soil or other undesirable materials. All stockpiles of mixed soil material should be protected and covered.


Place amended soil mix in thickness (i.e. lifts) between 9 and 12 inches in loose thickness. Spray water over the entire lift to accelerate settlement. After all lifts have been placed, grade soil to finish grades as specified on the plans and spray a final time to ensure that soil will not slump to an unacceptable finished grade after the first few rainfall events. Do not over compact soil mix with mechanical equipment after placement; following the construction steps above, soils have already been water compacted.

Engineered Soil Mix

Engineered Soil Mix (aka 3-way mix or bioretention soil mix) should have the following properties:

  • Free of contaminants & hazardous materials
  • 60% Loamy sand
  • 40% organic compost
  • Organic content matter from 8-10% by weight
  • Cation exchange capacity (CEC) greater than 5 millequivalents/100 grams of dry soil
  • 2 – 5% mineral fines
  • Conform to the following gradation:
U.S. Sieve Size Percent Passing
3/8-inch 100
#4 95-100
#10 75-90
#40 25-40
#100 4-10
#200 2-5
  • Minimum long-term hydraulic conductivity of 1 inch/hour per ASTM D2434 at 85% compaction per ASTM D2668
  • Meet specifications above for organic compost, mixing, storage, & placement.

Post-Construction Facility Infiltration Testing

To test a recently constructed or existing bioretention facility:

1.  Wet the surface of the rain garden with a sprinkler or hose until saturated.

  • Small rain gardens or cells separated by check dams (<100 square-feet in surface) area can be tested full-scale.
  • Large rain gardens, vegetated filter strips and swales can utilize isolated falling head tests (minimum 2 per 100 square- feet of area). For how to perform a falling head test, see “The Oregon Rain Garden Guide" under Step 3: Assess Soil.

2.  Fill the testing area to a depth of 4-inches and track the time it takes to completely draw down.

3.  Repeat test 3 times. If the water in any of the tests fails to draw down in less than an hour (i.e. infiltration rate = 4 inches/hour), add compost and gravelly sand to the mix and re-till. (If the facility is existing, remove plants before tilling soil, set aside, mix and re-till, then replace plants.)

4.  Repeat this procedure until favorable test results are achieved.

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