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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.
 
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.
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====Compost Specifications====
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When purchasing compost to be incorporated into the soil as a volume control soil amendment, look for specifications presented in the following table, adapted from Table 1 of Chapter 12-3 Runoff Volume Minimization of the Minnesota Stormwater Manual.
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[[Compost Specifications]]

Revision as of 14:52, 14 March 2017

Volume Control Using Compost Materials / Soil Amendments

Soil amendment techniques, standards and ordinances

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.

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 & Implementation

Programs developed to provide volume control through soil amendments may include 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.

This image shows a compost delivery to a project site
symbol for Stormwater Disconnection in MIDS calculator Unrestricted flickr ® Image

Soil Amendment Application Guidelines

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. There are soil and compost calculator worksheets in the Additional Resources section.

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.

This image shows rototilling compost into the soil.
Rototilling compost into the soil Unrestricted flickr ® Image

Import topsoil mix

Where subsoil is too rocky, compacted or poorly drained to amend effectively, a topsoil mix with 8-13 percent soil organic matter can be imported and placed on the surface. The topsoil mix should contain 30-40 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, adapted from Table 1 of Chapter 12-3 Runoff Volume Minimization of the Minnesota Stormwater Manual.

Compost Specifications

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