This image shows a compost delivery to a project site
Compost delivery to a project site

Land development including landscaping practices damage soil structure and function by removing or compacting topsoil. These practices can impact water resources by decreasing infiltration, increasing erosion, impairing fish habitat, and increasing the need for permanent stormwater management.

These practices also create chemically dependent landscapes which are difficult and expensive to maintain and contribute to polluted runoff. Soil compaction also reduces the water retention capacity of soil which requires additional irrigation and increased public water supply demand.

This fact sheet provides guidance on soil amendment practices and implementation of soil amendment standards and ordinances. For more information on compost, see the section on turf.

Benefits / pollution reduction

Compost, an organic material, absorbs and infiltrates rainwater, reduces flooding and soil erosion and filters out pollutants typically associated with stormwater runoff. Compost also stores water and nutrients for plants to use during drought conditions, promoting healthy plants and better looking lawns that require less irrigation, pesticides and fertilizers. In addition, healthy amended soils that require less irrigation reduce municipal water demand.

Program development and implementation

Programs developed to provide volume control through soil amendments may include Municipal Separate Storm Sewer System (MS4) standards and/or ordinances. Soil amendment guidelines as well as guidelines for standards and ordinance development are identified below. The program is ultimately dependent upon several factors including the MS4’s available resources, extent of development and/or redevelopment opportunities, and character of its soil and stormwater runoff.

Awareness campaigns

Awareness campaigns inform the public, public employees, businesses, property owners, and elected officials of the negative effects of soil compaction and the benefits of soil amendments. Efforts can also contribute to generating acceptance of a new ordinance and encouraging individuals and organizations to implement soil amendments on a voluntary basis.

  • Brochures. Develop informative brochures, and guidance for specific audiences such as developers, businesses, homeowners and local development permitting authorities.
  • Signage at MS4 installations. Locate signage at parks and government buildings identifying compost-amended sites and the associated functions and benefits.
  • Workshops and seminars. Workshops and seminars can be used to provide the technical assistance that developers, city staff and consultants will need in order to meet a new soil amendment ordinance.

Soil amendment application guidelines

This image shows rototilling compost into the soil.
Rototilling compost into the soil.

Design variants are summarized below to provide guidance appropriate for implementing soil amendments within various site constraints and conditions. A good design approach will likely apply a combination of techniques at a single site based on the local conditions.

  • General guidance Unless soils are native and can be left undisturbed, the following guidance applies to techniques implemented:
    • Minimum final soil depth of 8 inches
    • Avoiding plowing or tilling within drip line of trees
    • Soil pH testing, and if necessary, adjusting proposed suite of plants
  • Undisturbed native soil Areas of the site that do not need to be disturbed should be identified to protect areas of native vegetation. Fence off these areas to protect them from compaction during the construction phase.
  • Amend existing soil in-place Where the soil has been compacted or the organic layer (e.g. forest duff or upper soil horizon) removed, the simplest way to restore soil quality is to rototill compost into the existing soil. Apply a 2.5-inch deep layer of compost to the existing soil. Rototill compost into the soil to a depth of at least 8 inches. Tilling to this depth will require repeated passes with a large machine, such as a tractor-mounted or heavy rear-tine rototiller.
  • Import topsoil mix: Where subsoil is too rocky, compacted or poorly drained to amend effectively, a topsoil mix with 8 to 13 percent soil organic matter can be imported and placed on the surface. The topsoil mix should contain 30 to 40 percent compost by volume and clean sand or sandy soil to promote adequate drainage. The soil depth should be 8 inches and the pH suitable for proposed plants. Ask topsoil suppliers for test results of their product to verify the material contains the desired organic matter content and pH. For best results, plow or till compacted subsoil at least 2 inches deep before applying topsoil mix and/or rototill some of the newly applied topsoil into the subsoil.
  • Native soil: Sites that contain original, undisturbed native soils (most often applicable to forested land) may be stockpiled and reapplied without compost amendments after grading or other construction disturbances are completed. Remove forest duff layer and topsoil and stockpile separately prior to grading. Cover soil and duff piles with woven weed barrier (available from nursery supply stores) that sheds moisture yet allows air flow. Reapply topsoil to landscape areas to a minimum 8-inch depth after grading and other disturbances are completed. For best results, 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.
  • Disturbed soil: Stockpile topsoil, reapply and amend in place. This design variant is only applicable to sites where the soil is not the original, undisturbed native soil. Topsoil and forest duff excavated for structures and paved areas or removed before site grading can be stockpiled and reapplied after grading and amended.

Remove soil and stockpile prior to grading. Cover soil with woven weed barrier (available from nursery supply stores) that sheds moisture yet allows air flow. Reapply stockpiled soil to landscape areas to a minimum 8-inch depth after grading and other disturbances are completed. In some cases, purchasing additional topsoil will be needed to achieve the 8-inch depth. Plow or till compacted subsoil at least 2 inches deep before replacing stockpiled soil, and/or rototill some of the replaced soil into the subsoil. Apply a layer of compost to the reapplied soil at a depth of 2.5 inches. Rototill compost into the soil to a depth of at least 8 inches. Tilling to this depth will require repeated passes with a large machine, such as a tractor or heavy rear-tine rototiller.

  • Scarification: The Minnesota Stormwater Manual recommends plowing or tilling (scarifying) compacted subsoil more than the 2 inches recommended in the above applications. For high-traffic areas, the recommended depth of scarification is 10 inches. For all other areas within the construction limits, the recommended depth of scarification is four inches.
  • Planting areas vs. turf areas: The Minnesota Stormwater Manual recommends a greater depth of compost, 3 inches, for planting areas than for turf areas which may be adequately amended with only 1.75 inches of compost. In all cases, the recommended minimum depth of the resulting topsoil layer with the incorporated compost is 8 inches.

Compost specifications

When purchasing compost to be incorporated into the soil as a volume control soil amendment, look for specifications presented in the following table.

Compost Specifications
Link to this table

Parameter Parameter Definition Range
Source material/ Nutrient content Typically biosolids/animal manure, source separated compostable materials or sorted yard wastes Nitrogen: 0.5 – 3

Phosphorus: 0.5 – 1.5

Potassium: 0.5 – 1
Maturity Level of completeness of the composting process Seed emergence and seed vigor = minimum 80% relative to positive control
Stability Biological activity in the composted material CO2 evolution rate: < 8 milligrams CO2-C per grams organic matter per day
pH Acidity/alkalinity 5.5 – 8.5
Soluble salts The amount of soluble ions in a solution of compost and water Varies widely according to source materials for compost, but should be < 10 deciSiemen per meter (millimhos per centimeter)
Organic matter The amount of carbon-based materials 30-65% dry weight basis
Particle size Size of particles Pass through 1-inch screen or less
Biological contaminants Pathogens (disease causing organisms) and weed seeds Meet or exceed US EPA Class A standards, 40 CFR Section 503.32(2) levels
Physical contaminants Man-made materials (like pieces of plastic or glass) that do not compose, also called ‘inerts’ < 1% dry weight basis
Trace metals Elements that can be toxic to humans, animals or plants Meet or exceed standards for Class I compost set in Minn.R. 7035.2836, Subp. 6, (A)

Soil amendment ordinances

Introduce regulations whereby property owners and developers are required to provide soil amendments to any development or redevelopment site. King County, Washington, may have been the first local government to institute a clearing and grading ordinance that includes soil amendment requirements. The ordinance was first introduced in 2005 and was updated in December 2008. It serves as a good starting point for an MS4 ordinance. the ordinance includes the following language: “The topsoil layer shall be a minimum of eight inches thick, unless the applicant demonstrates that a different thickness will provide conditions equivalent to the soil moisture-holding capacity native to the site. The topsoil layer shall have an organic matter content of between five to ten percent dry weight and a pH suitable for the proposed landscape plants. When feasible, subsoils below the topsoil layer should be scarified at least four inches with some incorporation of the upper material to avoid stratified layers. Compost used to achieve the required soil organic matter content must meet the definition of "composted materials" in WAC 173-350-220.”

Rice Creek Watershed District water quality and volume control rules are designed to account for loss of infiltration due to soil compaction during construction. As an incentive for soil amendments, the water quality and volume control benefits of compost amended soils are given credit in the rules. The District provides a corresponding soil amendment guidelines worksheet for permit applicants.

Monitoring and assessment

The MS4 could engage in documenting the effectiveness of its soil amendment standards by conducting monitoring to see what water quality and other benefits are accomplished. Findings could provide feedback for standards/ordinance revisions.

Maintenance considerations

Compost amended sites are maintained no differently than sites that have not been amended. However, less watering and fertilizer may be required, as well as less runoff management.

Typical cost

Amending with compost is often the most economical way to uncompact and bring soils up to the desired soil organic matter content. On sites with the original, undisturbed, native soil and where space permits, stockpiling and reapplying topsoil may be less costly. Importing topsoil usually costs more than amending existing soil, although it may be easier where subsoil conditions make cultivation difficult. Reductions in the need for irrigation and fertilizer can provide payback for up front costs in the range of 2 to 7 years. Implementation of amended soils can also result in a cost savings due to reduced detention ponding requirements.

The adoption of a soil amendment ordinance requires an investment in training for the plan reviewer, the consultant, and possibly the public. MS4s must also consider the cost of enforcement, including staff and equipment requirements. Awareness campaign costs are determined by the type of materials produced and the method of distribution selected. Signs at city buffer installations may initially have a higher cost than printed materials, but can serve as a more effective tool for increasing public understanding.

This page was last edited on 22 November 2022, at 18:36.