To provide a suitable growth medium for final site stabilization with vegetation. While not a permanent cover practice in itself, topsoiling is an integral component of providing permanent cover in those areas where there is an unsuitable soil surface for plant growth. Native soils and disturbed soils that have been organically amended not only retain much more stormwater, but they also serve as effective biofilters for urban pollutants and, by supporting more vigorous plant growth, reduce the amount of water, fertilizer, and pesticides needed to support installed landscapes. Topsoil does not include any subsoils, only the material from the top several inches, including organic debris.
Use this BMP in conjunction with other BMPs such as seeding, mulching, or sodding. This BMP is functionally equivalent to BMP L613 - Post-Construction Soil Quality and Depth which is required per Minimum Requirement 5 - Onsite Stormwater Management for disturbed areas that will be developed as lawn or landscaped areas at the completed project.
Permanent landscaped areas shall contain healthy topsoil that reduces the need for fertilizers, improves overall topsoil quality, provides for better vegetal health and vitality, improves hydrologic characteristics, and reduces the need for irrigation.
Leave native soils and the duff layer undisturbed to the maximum extent practicable.
To the maximum extent practical, native soils disturbed during clearing and grading shall be restored to a condition equal to or better than the original site condition’s moisture-holding capacity. Use onsite native topsoil, incorporate amendments into onsite soil, or import blended topsoil to meet this requirement.
Topsoiling is a required procedure when establishing vegetation on shallow soils, and soils of critically low pH (high acid) levels.
Stripping of the existing, properly functioning soil system and vegetation for the purpose of topsoiling during construction is not acceptable. If an existing soil system is functioning properly, it shall be preserved in its undisturbed and uncompacted condition.
Depending on where the topsoil comes from, or what vegetation was onsite before disturbance, invasive plant seeds may be included and could cause problems for establishing native plants, landscaped areas, or grasses.
Topsoil from the site will contain mycorrhizal bacteria that are necessary for healthy root growth and nutrient transfer. These native mycorrhiza are acclimated to the site and will provide optimum conditions for establishing grasses. Commercially available mycorrhiza products should be used when topsoil is brought in from offsite.
1.13.3 Design and Installation Specifications
If topsoiling is to be done, the following items should be considered:
A minimum depth of 8-inches. Scarify subsoils below the topsoil layer at least 4-inches with some incorporation of the upper material to avoid stratified layers, where feasible. Ripping or restructuring the subgrade may also provide additional benefits regarding the overall infiltration and interflow dynamics of the soil system.
A minimum organic content of 10% dry weight in planting beds, and 5% organic matter content in turf areas. Incorporate organic amendments to a minimum 8-inch except where tree roots or other natural features limit the depth of incorporation.
A pH between 6.0 and 8.0 or matching the pH of the undisturbed soil.
To obtain a topsoil meeting the above specifications return native topsoil to the site, import topsoil of sufficient organic content, and/or incorporate organic amendments:
To meet the organic content requirements, the compost shall:
Meet the definition for “composted material” per WAC 173-350-100 and comply with standards in WAC 173-350-220, except the feedstock may contain biosolids or manure feedstocks.
Have no visible water or dust during handling.
Have soil organic matter content of 40% to 65%.
Have a carbon to nitrogen ratio below 25:1. Carbon to nitrogen ratio may be as high as 35:1 for plantings composed entirely of plants native to the Puget Sound Lowlands region.
City of Tacoma TAGRO Topsoil Mix can be used as an alternative to the compost component in BMP C125 - Compost.
For till soils use a mixture of approximately two parts soil to one part compost. This equates to 4 inches of compost mixed to a depth of 12 inches in till soils. Increasing the concentration of compost beyond this level can have negative effects on vegetal health, while decreasing the concentrations can reduce the benefits of amended soils.
Mulch planting beds with 2” of organic material.
If blended topsoil is imported, fines should be limited to 25 percent passing through a 200 sieve.
The final composition and construction of the soil system will result in a natural selection or favoring of certain plant species over time. For example, recent practices have shown that incorporation of topsoil may favor grasses, while layering with mildly acidic, high-carbon amendments may favor more woody vegetation.
Locate the topsoil stockpile so it meets specifications and does not interfere with work on the site. It may be possible to locate more than one pile in proximity to areas where topsoil will be used.
Allow sufficient time in scheduling for topsoil to be spread prior to seeding, sodding, or planting.
Do not place topsoil while in a frozen or muddy condition, when the subgrade is excessively wet, or when conditions exist that may otherwise be detrimental to proper grading or proposed sodding or seeding.
Care must be taken not to apply topsoil over subsoil if the two soils have contrasting textures. Sandy topsoil over clayey subsoil is a particularly poor combination, as water creeps along the junction between the soil layers and causes the topsoil to slough.
If topsoil and subsoil are not properly bonded, water will not infiltrate the soil profile evenly and it will be difficult to establish vegetation. The best method to prevent a lack of bonding is to work the topsoil into the layer below for a depth of at least 6 inches.
Field exploration of the site shall be made to determine if there is surface soil of sufficient quantity and quality to justify stripping. Topsoil shall be friable and loamy (loam, sandy loam, silt loam, sandy clay loam, clay loam). Areas of natural groundwater recharge should be avoided.
Confine stripping to the immediate construction area. A 4- to 6- inch stripping depth is common, but depth may vary depending on the particular soil. Place all surface runoff control structures in place prior to stripping.
Stockpile topsoil in the following manner:
Side slopes of the stockpile shall not exceed 2:1.
Surround all topsoil stockpiles between October 1 and April 30 with an interceptor dike with gravel outlet and silt fence. Between May 1 and September 30, install an interceptor dike with gravel outlet and silt fence if the stockpile will remain in place for a longer period of time than active construction grading.
Complete erosion control seeding or covering with clear plastic or other mulching materials of stockpiles within 2 days (October 1 through April 30) or 7 days (May 1 through September 30) of the formation of the stockpile. Do not cover native topsoil stockpiles with plastic.
Topsoil shall not be placed while in a frozen or muddy condition, when the subgrade is excessively wet, or when conditions exist that may otherwise be detrimental to proper grading or proposed sodding or seeding.
Maintain previously established grades on the areas to be topsoiled according to the approved plan.
When native topsoil is to be stockpiled and reused, the following should apply to ensure that the mycorrhizal bacterial, earthworms, and other beneficial organisms will not be destroyed:
Topsoil is to be re-installed within 4 to 6 weeks;
Topsoil is not to become saturated with water;
Inspect stockpiles regularly, especially after large storm events. Stabilize any areas that have eroded.
Establish soil quality and depth toward the end of the construction. Once established, protect from compaction and erosion.
Plant and mulch soil 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.