Guidelines for Writing Compost or Mulch Procurement Specifications
Organic materials such as lawn clippings, landscape trimmings and food scraps make up over 35 percent of disposed materials in California. However, these materials can be made into compost and mulch and used in many environmentally beneficial ways. State and local government agencies can play a vital role in increasing demand for these products.
Mulch. While there is currently no statutory or regulatory definition for mulch, it is commonly defined as a soil covering used to control weeds or erosion; retain moisture in soil; and insulate soil from cold weather. It is also used for aesthetic purposes. Organic materials commonly used for mulch include wood chips, ground up landscape trimmings, shredded bark, coarse compost material, straw, and shredded paper. Non-organic materials include crushed concrete and brick, stones and gravel, lava rock, and plastic film.
Compost. California statute (Public Resources Code, Section 42231) defines compost as the product resulting from the controlled biological decomposition of organic wastes that are source separated from the municipal solid waste stream. Compost feedstock materials include yard and landscape trimmings, agricultural crop residues, paper pulp, food scraps, wood chips, manure, and bio-solids. Compost is used for enhancing soil structure and building organic matter content; adding nutrients to soil; controlling weeds and dust; and retaining moisture in soil.
Mulch is used in the following applications:
- Moisture retention in soil
- Weed suppression
- Erosion control
- Dust abatement
- Landscape area top-dressing/decoration
- Hiking and equestrian trail maintenance
End-use specifications center on application rates. For example, the City of San Diego's Mulch Guide includes the following instructions for use of mulch to suppress weeds:
"Spread 1-inch minus or 4-inch minus length (pieces less than 1 inch or less than 4 inches) mulch evenly over bare soil to a depth of 3-4 inches. Replenish annually."
However, the main considerations for a mulch procurement specification concern physical properties. You should write your mulch specification to define the properties you want, for example:
- Color (e.g., brown; red; black; green; natural wood).
- Particle size and consistency (e.g., less than one, two, or three inches).
- Contamination with plastic; glass; metal; rock; paper (e.g. none visible for glass, metal, rock and plastic; and, for paper, less than 0.5 percent by weight or volume).
For a brief description of values and other considerations associated with particle size, contaminants, and other characteristics, please see the compost specification elements table.
To help keep materials out of landfills, be sure that the material type you specify is an organic mulch product or appropriate material from construction and demolition. For example, the recovered materials listed below are often specified:
- Wood chips
- Ground up yard trimmings
- Crushed brick or cement
The Southern California Chapter of the American Public Works Association has developed a mulch specification for use in public works application that includes elements of both procurement and end-use. It specifies the particle size and material type (essentially coarse compost material), and also specifies application rates.
We recommend that you inquire with local suppliers before you write up your final specification for purchase order. Based on the information they provide, you may be able to adjust your specification in a way that satisfies your basic needs.
The physical or chemical properties to include in your specification will depend on the end use you have in mind. Compost is typically used in the following ways:
- Slow release fertilizer
- Turf grass top dressing
- Landscape mulch
- Soil amendment (to build organic matter)
- Mulch for erosion control
- Mulch for dust control
To help you make the right choice for your end use, some suppliers will provide you with recommended end-use application rates and information on key characteristics that you may want to include in your specification. For example, a Davis Street SMaRT brochure states that "When top dressing a lawn, it is best to add compost 4 times per year. Add 1/8 inch if you mulch mow: If you do not mulch mow, add 1/4 inch." Recommended supplier specifications for this end use might include feedstock, organic matter content, carbon to nitrogen ratio, and pH.
CalTrans has developed a detailed procurement specification for use of coarse compost material for an erosion control application. This specification was developed to minimize potential impacts associated with weed seed germination, leaching of pollutants into ground and surface waters, odor problems, unaesthetic appearance due to litter contamination, and nitrogen depletion from soils.
When writing a procurement specification, keep things simple and specify a characteristic only if it makes a difference for your purpose. A typical specification usually mentions no more than six characteristics.