Central Puget Sound
Green Roof Project
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Soil mixes are the heart of the water retention we hope to achieve. the challenge is to get a soil mix that will sustain plants, be light enough to allow 3" - 6" of depth without undue structural loading, and retain stormwater long enough to significantly detain the run-off surge into storm drains.
The constituency of the soil mix will vary in our tests. We are currently looking at purlite, vermiculite, encapsulated polystyrene, coconut fibre and bark, crushed lava, peat, plus more common soil components. Issues such as peat being considered old growth vegetation, the cost of certain components, their availability, and their long- or short- term ecological footprint are under consideration.
Here is data for the some of the soil mixes used in our current demonstration projects:
| Soil Mix | Dry Weight | Saturated Weight | Formula |
| #1 | 30% organic matter (15% organic compost, 14% coconut husk) 1% decayed leaf matter or live ground soil, 70% aggregate (30% pumice, 40% perlite) | ||
| #2 | 10% organic compost, 1% decayed leaf matter, 30% fine bark, 15% coarse bark, 15% coarse perlite, 10% pumice, 20% coir | ||
| #3 | 15% compost, 35% fine bark, 25% coconut fiber, 25% perlite; added fertilizer, bloodmeal, bonemeal, regular and dolomitic limes | ||
| #4 | 40lbs./cu. ft. | 60% pumice, 20% perlite, 10% lava rock, 10% compost |
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