Description.  This work shall consist of incorporating compost within the root zone to improve soil quality, plant viability and soil hydraulic conductivity. (See Comment 1)

Materials.  Compost shall be mature, stable, weed free, and produced by aerobic decomposition of organic matter.  Compost feedstock may include, but is not limited to: agricultural, food or industrial residuals; class A biosolids as defined in the EPA CFR Title 40, Part 503; yard trimmings, or source-separated municipal solid waste.  The product must not contain any visible refuse or other physical contaminants, substances toxic to plants, or over 5% sand, silt, clay or rock material by dry weight.  The product shall possess no objectionable odors.  The product must meet all applicable USEPA CFR, Title 40, Part 503 Standards for Class A biosolids.  The moisture level shall be such that no visible water or dust is produced when handling the material.

Testing.   The results of compost analysis shall be provided by the compost supplier.  Maturity testing should be performed on site after the compost is delivered.  Compost maturity test kits can be ordered from a number of commercial sources.  Before delivery of the compost, the supplier must provide the following documentation:

• feedstock percentage in the final compost product
• a statement that the compost meets federal and state health and safety regulations
• a statement that the composting process has met time and temperature requirements
• a copy of the lab analysis, less than four months old, performed by a Seal of Testing Assurance Certified Laboratory verifying that the compost meets the physical requirements as described.

Table 1.  Physical Requirements for Compost

Recommended compost testing methodologies and sampling procedures are provided in Test methods for the Examination of Composting and Compost (TMECC)¹, and Standard Methods for the Examination of Water and Wastewater².

Soil evaluation.  Prior to soil amendment, existing soils must be sampled and evaluated to determine amendment quantities and plan the amending process.  In addition to compost, soil analysis may reveal the need for other soil amendments, such as lime, gypsum and specific nutrients.  Contact your local Cooperative Extension for soil testing information. (See Comment 2)

Determining compost quantity.  Compost shall uniformly be applied over the entire area to a depth of two (2) inches, and incorporated into the soil to a minimum depth of six (6) inches.  Where tree roots or other natural features limit the maximum depth of incorporation, compost quantities should be adjusted.  Required volume of compost may be estimated using the following approximation:  one (1) inch compost spread over 1000 square feet = three (3) cubic yards.  The Designer may specify different compost application rates depending upon soil conditions.

Compost Application Procedure.

1.      Rototill to a depth of 6-8”.  If the soil is too dense for a rototiller, the soil should first be broken up into large aggregates using a soil ripper. 

2.      Clear surface of obstructions.  The soil surface shall be reasonably free of large clods, roots, stones greater than 2 inches, and other material which will interfere with planting and subsequent site maintenance.

3.      Distribute compost evenly to a depth of two inches over the soil surface.  For small areas, compost may be spread by hand with a shovel and raked evenly over the soil.  For larger areas, use a tractor-mounted spreader or other similar device.

4.      Spread lime and nutrients as indicated by soil testing.

5.      Rerototill several times in perpendicular directions to incorporate compost and other soil amendments

6.      Water thoroughly.  Allow soil to settle for one week.  If compost is immature, extend settling period to two to five weeks. 

7.      Fine grade

8.      Hand roll

Alternatives for deep compaction.  If soil is compacted deeper than 6 inches, alternative techniques, such as trenching, may yield better results.

Slope limitations.  On steep slopes, increasing soil moisture could potentially cause soil instability.  In general, compost amendment is not recommended for slopes steeper than 3:1 (horizontal:vertical)³. Special care should be taken during installation on steep slopes.  Installation should be performed during dry weather, and early enough in the year to permit the establishment of vegetation before the onset of winter. 

Tree and shrub considerations.  Care must be taken around existing trees and shrubs to prevent root damage during soil amendment.  Soil disturbance should be avoided entirely within the tree’s drip line (the outer limits of the branches).  A tree’s roots may extend well beyond the drip line.  Some trees and shrubs will recover from a certain amount of root disturbance.  It is important to consult with a professional arborist for guidelines regarding specific tree and shrub species.

Seeding/ vegetation.   Vegetation should be installed immediately after compost application. Follow published guidelines for seeding, sodding or planting.

References.

¹Test Methods for the Examination of Composting and Compost. 2002. USDA and U.S. Composting Council. 

²Standard Methods for the Examination of Water and Wastewater. 1992. 18th Edition, American Public Health Association, 1015 Fifteenth Street, NW, Washington , DC 20005.

³Chollak, T. and Rosenfeld, R.  1998.  Guidelines for Landscaping with Compost-amended Soils.  Prepared for City of Redmond Public Works, Redmond, WA. 

Available at http://depts.washington.edu/cwws.

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