Maturity cannot be described by a single property. Some laboratories assign a maturity index to compost based upon germination rate, root tissue growth, and other factors. The California Compost Quality Council (CCQC) developed a numerical Maturity Index that uses standard laboratory tests to rate compost maturity. To qualify as “mature” or “very mature,” a compost must have a C:N ratio of less than or equal to 25 and pass two additional tests performed concurrently from each group:

• Group A tests, which indirectly measure the degree of organic matter decomposition, include carbon dioxide release or respiration; oxygen demand; and Dewar self-heating test.
• Group B tests, which measure chemical characteristics of the product (some of which can be toxic to plants) include ammonium nitrate ratio; ammonia concentration; volatile organic acids concentration; and plant bioassays.

www.CCQC.org publications describe how to apply the index and interpret test results to determine whether a compost is very mature, mature, or immature, and provide general guidelines on best uses of composts based on the Maturity Index rating.

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