Natural resource planning tools


Best Management Practices for Preservation and Restoration of Soil

The goal of BMPs for soil is to first preserve natural soil and then restore soil compromised by development to as close to natural as possible. Natural undisturbed soil is rich in organic material; it absorbs rainwater and supports dense healthy plant growth. It provides important storm-water management functions including efficient water infiltration and storage, adsorption of excess nutrients, filtration of sediments, biological decomposition of pollutants, and moderation of peak flows and temperatures. Healthy soil that supports vigorous plant growth intercepts rainfall, returning some of it to the atmosphere through evaporation and transpiration. Healthy soil also stores water and nutrients for plants to use in dry times.

Except for areas that will be covered by impervious surface or have been incorporated into a storm-water bioretention facility, areas that have been cleared and graded must have soil moisture-holding capacity restored to that of the original, undisturbed soil to the maximum extent practicable. Areas that have been compacted or had the topsoil or duff layer removed should be amended by first loosening compacted subsoil, then adding compost to stockpiled site topsoil or imported suitable topsoil, or through other techniques that can mitigate for lost moisture-holding capacity.

Four soil treatment options guide postconstruction soil restoration. They can be used individually or in combination so that they work best for the specific situation.

General Requirements

Option 1:

Leave native soil undisturbed, and protect from compaction during construction. Note: This option is only applicable to sites that have the original, undisturbed soil native to the site. This is land that is being left undisturbed in the current project.

This option is the most economical and best for the environment, but not always feasible.

Option 2:

Amend existing soil in place. Where the soil has been compacted or topsoil has been removed, rototill compost into the existing soil.

Option 3:

Import topsoil mix that contains between 8% and 13% dry weight soil organic matter (SOM). Where subsoil is too rocky, compacted, or poorly drained to amend effectively, a topsoil mix with 8% to 13% soil organic matter should be imported. "Manufactured" topsoil mixes must be certified weed free.

Option 4a:

Stockpile site duff and topsoil, and reapply after grading and construction. Note: This option is only applicable where original, undisturbed soil native to the site must be disturbed.

Option 4b:

Stockpile site soils, reapply, and amend in place. Note: This option is only applicable to sites where the soil is not the original, undisturbed soil native to the site.

Adopted from King County Department of Development & Environmental Services This link leads to an external site.DDES Land Use Services Division, 2005, Washington State.