4.2.2 Design

Design submittals for amended soils should include plans, work sheets, and specifications including:
  • All site areas to be protected.
  • Soil areas to be disturbed and restored.
  • Any soil areas previously disturbed and intended for restoration.
  • Proposed site access and construction circulation routes.
  • Site areas intended for staging and material storage (to be restored post-construction).
  • Measures proposed to minimize compaction and restore compacted areas post-construction.
  • Specific soil amendment details for all landscaped areas listed by type and use.

These design submittals should be coordinated with the project Temporary Erosion and Sediment Control Plan and Storm Water Pollution Prevention Plan.

To determine the amendment requirements for disturbed soils the following general steps should be followed:
  • Visit the site.
  • Conduct soil sampling.
  • Review site grading and landscape areas.
  • Select amendments options based on soils analysis and proposed planting plans.
  • Specify topsoil, mulch, and compost or other locally available, suitable materials to be used for amending soils.
  • Calculate quantities.

To calculate soil and amendment needs, Simple Amendment Rates or Custom Rates can be used, as described below. For both approaches, the following general design goals apply (Ecology, 2012b), unless specified otherwise based on local conditions and plant choices:
  • A target organic matter content of 6 to 8 percent by dry weight for all non-turf planting areas.
  • A target organic matter content of 3 to 5 percent for turf areas.
  • pH between 6.0 and 8.0 or as specified for particular plant choices.

Note that some experimentation may be needed to determine optimum organic matter content and best sources of materials available for amending soils in eastern Washington.

Simple Amendment Rates
The simplest way to calculate soil and amendment needs is to use these pre-approved rates:
  • Planting Beds: 6-8 percent organic content using 3 inches of compost amended to an 8-inch depth or a topsoil mix containing 35-40 percent compost by volume.
  • Turf Areas: 3 to 5 percent organic matter content using 1.75 inches of compost amended to an 8-inch depth or a topsoil mix containing 20 to 25 percent compost by volume.

Calculating Custom Rates

The target organic matter content may be achieved by using the pre-approved rates outlined above or by calculating a custom rate. In many instances the organic matter content of existing site soils may be relatively good, or compatible with the natural site conditions, and therefore not require as much amendment. In some instances, calculating a site-specific amendment rate may result in more efficient use of amendments and significant cost savings. This may be particularly true for large sites due to economies of scale.

A spreadsheet that performs these calculations is available on the Soils for Salmon website at: www.soilsforsalmon.org/excel/Compost_Calculator.xls. For information on how to use this spreadsheet, see Building Soil: Guidelines & Resources for Implementing Soil Quality and Depth BMP T5.13 in the Ecology Stormwater Management Manual for Western Washington, pages 10-12 and 18-19, available online at www.SoilsforSalmon.org or www.BuildingSoil.org. This spreadsheet allows the user to enter the following inputs:
  • Soil bulk density (pound [lb]/cubic yard [cy] dry weight).
  • Initial organic matter (%).
  • Final target soil organic matter (%).
  • Compost bulk density (lb/cy dry weight).
  • Compost organic matter (%).
  • Depth compost is to be incorporated (inches).

Generally, on-site soils should be tested for bulk density and initial organic matter. Based on these measurements and input goals on final target soil organic matter and compost bulk density and organic matter, the spreadsheet calculates the compost application rate (CR) to be applied (in units of inches).

Another easy-to-use calculator developed by King County is available at: http://your.kingcounty.gov/solidwaste/compost_calculator.htm. Compost application rates can be calculated using the following equation:

CR = D x                        SBD (SOM% - FOM%)                     
                  SBD (SOM% - FOM%) – CBD (COM% - FOM%)

CR = compost application rate (inches).
D = finished depth of incorporated compost (inches).
SBD = soil bulk density (lb/cy dry weight).
SOM% = initial soil organic matter (%).
FOM% = final target soil organic matter (target will be 5% or 10% depending on turf or landscape area) (%).
CBD = compost bulk density (lb/cy dry weight).
COM% = compost organic matter (%).

Compost material should be mature and derived from organic waste materials including plant debris, bio-solids, or wood wastes that meet the functional requirements and intent of the organic soil amendment specification.