5.4.1
5.4.1.1 BMP Description
Post Construction Soil Quality and Depth (BMP T5.13 in Volume V, Chapter 11 of the Ecology Manual) requires that site soils shall meet minimum quality and depth requirement at project completion. Requirements may be achieved by either retaining and protecting undisturbed soil or restoring the soil (e.g., amending with compost) in disturbed areas.
5.4.1.2 Performance Mechanism
Naturally occurring (undisturbed) soil and vegetation provide important stormwater functions including water infiltration; nutrient, sediment, and pollutant adsorption; sediment and pollutant biofiltration; water interflow storage and transmission; and pollutant decomposition. These functions are largely lost when development strips away native soil and vegetation and replaces it with minimal topsoil and sod. Not only are these important stormwater functions lost, but such landscapes themselves become pollution generating pervious surfaces (PGPS) due to increased use of pesticides, fertilizers and other landscaping and household/industrial chemicals, the concentration of pet wastes, and pollutants that accompany roadside litter.
Establishing soil quality and depth regains greater stormwater functions in the post-development landscape, provides increased treatment of pollutants and sediments that result from development and habitation, and minimizes the need for some landscaping chemicals, thus reducing pollution through prevention.
5.4.1.3 Applications and Limitations
Establishing a minimum soil quality and depth is not the same as preservation of naturally occurring soil and vegetation. However, establishing a minimum soil quality and depth will provide improved onsite management of stormwater flow and water quality.
Soil organic matter can be attained through numerous materials such as compost, composted woody material, biosolids, and forest product residuals. It is important that the materials used for this BMP be appropriate and beneficial to the plant cover to be established. Likewise, it is important that imported topsoil improves soil conditions and does not have an excessive percent of clay fines. This BMP can be considered infeasible on till soil with slopes greater than 33%.
Soil amendments can also be used to help achieve on-site stormwater management and flow control standards when integrated into a dispersion BMP. See Table II-5.6 for a summary of minimum requirements that can be met, partly or in full, through the use of this BMP. Refer to Vol II–5.4.4 Dispersion BMPs for additional information on dispersion BMPs.
| BMP | MR #7: Flow Control | ||||||
|---|---|---|---|---|---|---|---|
|
List |
LID Performance Standard |
Basic |
Enhanced |
Oil Control |
Phosphorus |
||
|
Post Construction Soil Quality and Depth |
X |
X |
|
X | |||
5.4.1.4 Site Considerations
At project completion, meet soil amendment requirements for all areas subject to clearing, grading, or compaction that have not been covered by a hard surface, incorporated into a BMP, or engineered as structural fill or slope. Only the areas where existing vegetation and/or soil are disturbed or compacted are required to be restored.
5.4.1.5 Design Information
The soil quality design requirements shall be met by using one of the following four options. Refer to the Building Soil manual (Stenn et al. 2018) or website (www.buildingsoil.org) for additional details on these options:
-
Retain and Protect Undisturbed Soil:
-
Leave undisturbed vegetation and soil and protect from compaction by fencing and keeping materials storage and equipment off these areas during construction.
-
For all areas where soil or vegetation are disturbed, use option 2, 3, or 4.
-
-
Amend Soil:
Amend existing site topsoil or subsoil either at default “pre-approved” rates, or at custom calculated rates to meet the soil quality guidelines based on engineering tests of the soil and amendment. The default pre-approved rates are:
-
In planting beds: place 3 inches of compost and till in to an 8‑inch depth.
-
In turf areas: place 1.75 inches of compost and till in to an 8‑inch depth.
-
Scarify (loosen) subsoil 4 inches below amended layer to produce a 12‑inch depth of un-compacted soil.
-
After planting: apply 2 to 4 inches of arborist wood chip or compost mulch to planting beds. Coarse bark mulch may be used but has lower benefits to plants and soil. Do not use fine bark because it can seal the soil surface.
-
-
Stockpile Soil:
-
Stockpile existing topsoil during grading and replace it prior to planting. Amend stockpiled topsoil if needed to meet the organic matter or depth requirements either at the default “pre-approved” rate or at a custom calculated rate (see the Building Soil manual (Stenn et al. 2018) or website (www.buildingsoil.org), for custom calculation method). Scarify subsoil and mulch planting beds, as described in option 2.
-
-
Import Soil:
-
Import topsoil mix of sufficient organic content and depth to meet the requirements. Imported soils shall not contain excessive clay or silt fines (more than 5% passing the No. 200 sieve) because that could restrict stormwater infiltration. The default pre-approved rates for imported topsoils are:
-
For planting beds: use a mix by volume of 35% compost with 65% mineral soil to achieve the requirement of a minimum 8% (target 10%) organic matter by loss-on-ignition test.
-
For turf areas: use a mix by volume of 20% compost with 80% mineral soil to achieve the requirement of a minimum 4% (target 5%) organic matter by loss-on-ignition test.
-
Scarify subsoil and mulch planting beds, as described in option 2.
-
-
More than one method may be used on different portions of the same site.
Areas meeting the design guidelines listed above and in BMP T5.13 in Volume V, Chapter 11 of the Ecology Manual can be modeled as “Pasture” rather than “Lawn” in an approved runoff model.
5.4.1.6 Minimum Construction Requirements
-
Establish soil quality and depth toward the end of construction and once established, protect from compaction, such as from large machinery use, and from erosion.
-
Plant vegetation and mulch the amended soil area after installation.
-
Leave plant debris or its equivalent on the soil surface to replenish organic matter.
-
Reduce and adjust, where possible, the use of irrigation, fertilizers, herbicides and pesticides, rather than continuing to implement formerly established practices.
5.4.1.7 Operations and Maintenance Requirements
Key maintenance considerations for compost-amended soils include the replenishment of soil media as needed (as a result of erosion) and addressing compacted, poorly draining soils. Site uses shall protect vegetation and avoid compaction.
The full benefits of compost-amended soils are realized when desired soil media depths are maintained, and soil compaction is minimized. Care shall be taken to prevent compaction of soils via vehicular loads and/or excessive foot traffic, especially during wet conditions.
After installation, plant vegetation and mulch the amended soil area. Plant debris and its equivalent should be left on the soil surface to replenish organic matter. The use of irrigation, fertilizers, herbicide and pesticides should be reduced or adjusted, where possible, rather than continued implementation of formerly established practices.