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

  • Amend soil between May 1 and October 1.
  • Avoid plowing or tilling within the drip line of trees to be retained.
  • Test soil pH and adjust (its range should be between 6.0 and 8.5).
  • Do not exceed soluble salt content above 500 parts per million.
  • Rototill compost into soil to a depth of at least 8 inches.

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.

  • Plan site development to leave native vegetation undisturbed to the greatest extent possible.
  • Fence off areas of native vegetation to protect it from construction activities and disturbance (e.g., parking construction equipment).
  • Protect from invasive species establishment.

Option 2:

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

  • Apply a layer of compost to existing soil at the amendment rate of 2.5 inches. Use MN DOT 3890 grade 2-certified compost (this compost shall not contain any biosolid/mixed municipal compost/animal manure components).
  • Retain copies of compost test results and receipts for compost delivered to the site, as they will be used during inspection to verify soil requirements have been met.
  • Rototill compost into soil to a depth of at least 8 inches. Note that tilling to this depth will require repeated passes with a large machine, such as a tractor-mounted or heavy rear-tine rototiller.

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.

  • Import and apply a topsoil mix with 8% to 13% soil organic matter, which should contain 30% to 40% compost by volume and clean sand or sandy soil.
  • Apply to a depth of 8 inches, and ensure that soil pH is suitable for the proposed plants.
  • Ask topsoil suppliers for test results of their product to verify the material contains the desired organic matter content and pH. Retain copies of topsoil test results and receipts for topsoil delivered to the site, as they will be used during inspection to verify that the soil requirements have been met.
  • For best results, till compacted subsoil at least 2 inches deep, if possible, before applying topsoil mix, and/or rototill some of the newly applied topsoil into the subsoil.
  • Use topsoil specified in MN DOT 3877 Topsoil Material if suitable.

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.

  • Remove duff layer and topsoil, and stockpile separately in an approved location prior to grading. Cover soil and duff piles with a woven weed barrier that sheds moisture yet allows air flow.
  • Reapply soil to landscape areas to a minimum 8 inch depth after grading and other disturbances are completed.
  • Plow or till compacted subsoil at least 2 inches deep before replacing stockpiled topsoil, and/or rototill some of the replaced topsoil into the subsoil.
  • Apply a 2-inch layer of stockpiled duff as mulch after planting.

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.

  • Remove soil and stockpile in an approved location prior to grading. Cover soil with a woven weed barrier that sheds moisture yet allows air flow.
  • Reapply stockpiled soil to landscape areas to a minimum 6-inch depth after grading and other disturbances are completed.
  • Plow or till compacted subsoil at least 2 inches deep before replacing stockpiled topsoil, and/or rototill some of the replaced topsoil into the subsoil.
  • Apply a layer of compost to the reapplied soil at the amendment rate of 2.5 inches. Use MN DOT 3890 grade 2-certified compost (this compost shall not contain any biosolid/mixed municipal compost/animal manure components).
  • Retain copies of compost test results and receipts for compost delivered to the site, as they will be used during inspection to verify the soil requirements have been met.
  • Rototill compost into soil to a depth of at least 8 inches. Note that tilling to this depth will require repeated passes with a large machine, such as a tractor-mounted or heavy rear-tine rototiller.

Adopted from King County Department of Development & Environmental Services DDES Land Use Services Division, 2005, Washington State: http://your.kingcounty.gov/ddes/forms/ls-inf-SoilPost-ConStd.pdfThis link leads to an external site.