Purpose in a green infrastructure, or "bioswale," soil: Filter pollutants and nutrients, and hold water in soil for plants and microbes

Suggested volume: 30 to 40% by volume of total mix

What to look for: Homogeneous, dark compost that smells earthy. Beware of composts with lots of woody debris, trash, or with a sour or ammonia smell.

What is Compost?

Man shoveling compost

Compost is manmade organic matter. It’s the result of recycling carbon-containing wastes such as food scraps, yard debris, biosolids, and manure. These wastes are termed “feedstocks” when used as ingredients for making compost.

In aerobic composting, micro- and macro-organisms break down the carbon-containing matter, creating a dark, homogeneous substance from a heterogeneous mixture of feedstocks. While the same process occurs in nature, composting accelerates decomposition by providing the ideal mix of air and moisture to foster a microbial feeding frenzy. 

The organic matter that compost adds to a bioswale soil is the chemically and biologically active part of these systems. Organic matter in compost is extremely chemically reactive with a large electrically charged surface area per unit volume. This means organic matter can pull metals, organic chemicals, and nutrients out of solution (water). These bound chemicals can then be absorbed onto soil surfaces, broken down by soil microbes, or used for growth by plants and microbes.

Organic matter in bioswale soil mixes:

  • Binds contaminants such as copper, zinc, and hydrocarbons that are potentially harmful to aquatic life.
  • Absorbs and slowly releases water, providing moisture for healthy plant growth, regulating the rate at which groundwater recharges streams, and preventing flooding.
  • Provides the microbial environment necessary for the breakdown of organic contaminants such as pesticides, herbicides, and hydrocarbons.
  • Speeds soil aggregation to prevent compaction and erosion.
  • Supplies the necessary nutrients for plant growth, resulting in systems that provide habitat for wildlife, reduced erosion, increased soil pore space, air filtration, and aesthetic appeal. 
  • Reduces maintenance of systems by supporting plant growth without the use of irrigation and synthetic fertilizers.

Composts Vary

As feedstocks and decomposition processes vary depending on the season, local resources, and composting method, it’s important to keep in mind that the resulting product will also vary. Such variations include the amount of available nutrients and metals in the compost, the maturity and stability of the finished product, and the final carbon to nitrogen or C:N ratio.

For example, composted biosolids will contain more phosphorus, copper, and zinc than compost made mostly of yard debris. While these elements can be potential pollutants, it’s important to know that they are often bound tightly to the organic matter and other compounds (e.g., iron and aluminum oxides) that are present in feedstocks, particularly biosolids. These pollutants tend to stay put as a result. But compost may also contain some trash, which can clog bioswales, harm wildlife, and result in aesthetically displeasing gardens. 

Choosing a Compost

Prior to choosing a compost, you should evaluate which compost will be best under the given circumstances. General criteria for evaluating compost include: the C:N ratio, pH, stability, maturity, and whether the compost is contaminated with trash or weeds.

Chemical attributes of the compost may also be specified by local standards, and some municipalities have vendor lists for suppliers of approved materials. The U.S. Composting Council also sponsors a compost testing program. Composts that have received the Seal of Testing Assurance, or STA, stamp are generally suitable materials for use in green infrastructure ( Below are requirements for compost used in Seattle, WA, bioswales set forth by Seattle Public Utilities:

  • C:N: ratios between 20:1 and 45:1, depending on type of compost and plants used
  • pH: 6.0 to 8.5
  • Maturity: Solvita score of 5 or 6, depending on compost type
  • Stability: Carbon dioxide evolution rate below 7
  • Contamination: No more the 1% contamination by volume