A question frequently asked at Bay-Friendly Landscape Maintenance certification classes is, “How is compost supposed to smell?” While “smell” is a nebulous measure of odors that can be both bad and good, the underlying question here concerns compost quality.

An example of this involves a recent news story from Pleasanton, CA where local police were called to investigate the source of a stink when telephone complaints started pouring into the police station. The offending odor was quickly traced to compost.

With the number of garden and landscape pros making the shift to using compost and compost-based products increasing, I decided it was time to clear the air about stinky compost.

In short, if the compost you receive stinks, BEWARE!

On-site visual and so-called “olfactory” inspection of compost should be considered your last line of defense in the quality assurance process. I say last line because sight and smell alone are not sufficiently reliable methods for determining the suitability of a compost product. You should rely upon testing evidence to confirm what your eyes see and nose smells.

A good rule of thumb here is that a mature (note the use of “mature”) compost product should not contain visual evidence of feedstock materials (leaves, grass, food scraps) and should have an earthy smell. A mature compost product should never smell like garbage, manure, or ammonia.

Making compost is a lot like making hash. The quality of the ingredients and how they’re prepared dramatically influences the appeal of the final product. Making compost is a dynamic process. It involves both science and the artfully applied skill of the producer. Whether a producer has successfully employed both science and art is determined by testing in what the Woods End Laboratories folks, the gurus of compost, call the “Completed State.” Testing at the Completed State includes nutrient value, elimination of pathogens, stability, and maturity among other parameters.

What should be obvious here is that you should purchase only compost products that have been tested, and for which you have thoroughly reviewed the test results.

A compost producer’s identification with a testing program such as the U.S. Composting Council – Seal of Testing Assurance, or listing by the Organic Materials Review Institute (OMRI) while valuable and an indicator of a producer that strives for quality, it is NOT a guarantee. You must perform your due diligence by making sure that the producer guarantees the product and can provide you with the laboratory test results to back it up.

It’s been my experience that people often mistake “finished” compost to be stable and mature. This is NOT the case. Finished compost indicates completion of decomposition. Like a fine wine or cheese, finished compost needs additional time to cure to become stable and mature. While the composting of feedstock materials can be accomplished in 60 days or less, “curing” of compost may take three times as long to complete. Testing compost for both stability and maturity is your best defense against angry telephone calls about the release of a nasty odor.

Summarized below are indicators of compost stability and maturity from the CalRecycle website that I think you’ll find useful.

Indicators of compost stability

1. Temperature. if the temperature of the compost is more than 15 degrees F (8 degrees C) higher than the ambient air, the compost is still fairly unstable.

2. Respiration Rate. The rate of oxygen utilization represents the extent of biological activity. For horticultural applications, < 20 mg O2 / Kg compost dry solids/hour is considered stable. For field applications, < 100 mg O2 / Kg compost dry solids/hour is considered stable. The Solvita test, available from Wood’s End Laboratories, is a quick test for respiration rate and also measures ammonia content. CO2 production may also be used to assess respiration rate. Less than 5 mg CO2 carbon/g compost carbon/day is considered stable and is usually suitable for seeds. Greater than 20 mg CO2 carbon/g compost carbon/day may be fairly unstable. Composts that are cold, dry, or very salty may not respire even though they are not stable.

3. Compost Processing. Generally, compost made by the aerobic windrow method should be processed for a minimum of 60 to 90 days to achieve “finished” compost. “Finished” means usable, but not fully stable. Compost should be processed a minimum of 90 to 120 days to be considered “stable.” This is sometimes referred to as being “cured.” Some experts believe that compost should cure for six months before use.

4. Carbon:Nitrogen (C:N) ratio. The C:N ratio decreases as compost becomes more mature or stable. Consequently, the C:N is sometimes used as an indicator of compost stability. However, for this ratio to be meaningful, you need to know the C:N ratio at the beginning and the end of the compost process. Ideally, the C:N should be approximately 30:1 at the beginning of the compost process. If the C:N ratio is low at the beginning of the compost process, a low C:N at the end of the process may not be a meaningful indicator of compost stability. Assuming the beginning C:N is approximately 30:1, the C:N of a moderately stable finished compost will be between 15:1 and 20:1. A very stable compost will have a C:N between 10:1 and 14:1 at the end of the composting process. A final C:N ratio above 20:1 may not readily release nitrogen. A final C:N of greater than or equal to 30:1 is thought to inhibit mineralization of nitrogen and may actually tie up nitrogen from the soil.

5. Visual/Olfactory Inspection. Although not a reliable method, one can do a cursory assessment of a compost by look and smell. In general, a mature compost will not contain recognizable feedstock material and should smell like rich soil. It should not smell foul or of ammonia.


Indicators of compost maturity

1. Seed Germination. Growers may want to perform a germination test using the seeds they will be planting. The following Web site includes guidelines for conducting your own seed germination tests: www.compostinfo.com/tutorial/MaturityTests.htm. Many labs will also perform seed germination tests.

2. Maturity Index. Some labs will assign a maturity index to compost based upon both the germination rate and the root tissue growth compared to a control.

Okay. Let’s all take a breath of fresh air.

Now…. Compost! Compost! Compost! But, please use the good stuff!

Additional information on compost quality may be obtained from the following:

Internet Sites

• CalRecycle: http://www.calrecycle.ca.gov/organics/products/Quality/Needs.htm
• U.S. Composting Council: www.compostingcouncil.org
• Wood’s End Laboratories: www.woodsend.org

• Field Guide to Compost Use
U.S. Composting Council, (440) 989-2748.
• Compost Quality Guidelines
Organic Ag Advisors and BBC Laboratories, Inc. (530) 292-3619.
• Recommended Test Methods for the Examination of Compost and Composting
U.S. Composting Council, (440) 989-2748.
• Compost–A Guide for Evaluating and Using Compost Materials as Soil Amendments
William Darlington, Soil and Plant Laboratories, Inc.:http://www.soilandplantlaboratory.com/pdf/articles/CompostAGuideForUsing.pdf