Compost is the end product resulting from the controlled biological decomposition of organic material from a feedstock into a stable, humus-like product that has many environmental benefits. Composting is a natural process that is managed to optimize the conditions for decomposing microbes to thrive. This generally involves providing air and moisture, and achieving sufficient temperatures to ensure weed seeds, invasive pests, and pathogens are destroyed. A wide range of material (feedstock) may be composted, such as yard trimmings, wood chips, vegetable scraps, paper products, manures and biosolids. Compost may be applied to the top of the soil or incorporated into the soil (tilling).
Finished compost bears little resemblance to the original materials from which it was created. Mature compost is normally dark brown in color and should have an even texture and a pleasant, earthy aroma. Compost products may vary since the properties of any given compost depend on the nature of the starting feedstocks and the conditions under which they were processed. CalRecycle requires all permitted and otherwise authorized facilities to test their finished compost to ensure it meets specific requirements and is safe for humans and the environment. Fully mature compost can break down no further and may be safely applied directly to existing landscapes and sensitive garden plants.
Mulch is a layer of material applied on top of soil. Examples of material that can be used as mulch include wood chips, grass clippings, leaves, straw, cardboard, newspaper, rocks, and even shredded tires. Benefits of applying mulch include reducing erosion and weeds and increasing water retention and soil vitality. Whenever possible, look for mulch that has been through a sanitization process to kill weed seeds and pests.
Sheet mulching is a process in which multiple layers of materials are used to provide an impermeable barrier to weeds while allowing water to soak through. The mulch layers break down slowly, adding nutrients to the soil over time.
- BioCycle magazine operates the Find a Composter website that includes composting sites throughout North America.
Be a wise compost consumer. Not all compost is created equal. Composters who are under pressure to move materials from their site may sell materials that are not fully composted. If this is the case, the compost may need to sit for a few weeks before it can be used without damaging plants. Warning signs that a compost is immature and not ready for use include:
- High temperature: The compost should not be steaming or significantly warmer than ambient conditions.
- Odor: Strong smells of ammonia or other odorous or unpleasant compounds indicate that the product is not mature.
- Visible feedstock: You should not be able to see green materials, bits of food, or chunks of dirt.
- Visible contamination: A small amount of plastic or glass contamination is normal when feedstocks are from urban environments. CalRecycle regulations allow up to 0.05 percent contamination by weight in any sample. This can be hard to estimate, but you can be the judge of how much is too much based on where the compost will be used, whether children will have contact with the product, etc.
- Woodiness: Compost that is too woody may be great for mulch but not ideal for a planting medium.
Packaging and labeling: Compost that is suitable for use in organic production in California must be registered as an Organic Input Material with the California Department of Food and Agriculture and may use the CDFA Organic logo. Such products may also contain an approval from the Organic Materials Review Institute, but this is no longer required in California.
Compost products that make claims for nutrient values must have a CDFA fertilizer label. Compost products that make no claims for organic production or for nutrient values may legally be sold without registration.
The U.S. Composting Council’s Seal of Testing Assurance (STA) is a voluntary quality-control protocol that uses a suite of physical, chemical, and biological tests performed by a group of independent, certified labs.