We all know what it is, but to what extent? Let’s take a moment to learn more about compost.

Stability and maturity are terms often used to characterize compost, yet opinions about what these terms mean vary widely. The term “stable” typically refers to a compost that is not undergoing rapid decomposition and whose nutrients are slowly released into the soil. The term “mature” typically refers to the degree of completeness of the composting process. In mature compost, raw feedstocks have sufficiently decomposed for 60 to 90 days under controlled moisture and aeration conditions and cured for another 30 days to result in a stable product that is non-phytotoxic to plants. Immature compost may contain one or more growth-inhibiting compounds (e.g., short-chain organic acids that are phytotoxic to seedlings), weed seeds, or other undesirable characteristics.

Compost stability and maturity are difficult to assess by simple sight or smell. In general, though, mature compost will not contain recognizable feedstock material, should smell like rich soil, and should not smell foul or ammonia-like.

Indicators of Compost Stability

Composting Pile Temperature. In moderate climates, if the temperature of the composting pile is more than 15°F (8°C) higher than the ambient air, the compost may be unstable.

Respiration Rate. The rate of oxygen utilization and/or carbon dioxide release may be used to assess respiration rate, a measure of biological activity. The Solvita test, available from Woods End Research Laboratory, is a quick test for respiration rate. However, composts that are cold or dry or that have a high salinity content may not respire even though they are unstable.

Length of Composting Process. In general, compost from an aerobic windrow should be processed for a minimum of 60 to 90 days, although even after this time the compost may still be unstable. Most compost should be actively processed 90 to 120 days to be considered stable. This additional processing time results in compost that is sometimes referred to as being “cured.” Some experts believe that compost should be cured for six months before use.

Carbon:Nitrogen (C:N) ratio. The C:N ratio usually decreases during the composting process and consequently is sometimes used to indicate compost stability. However, for this ratio to be used meaningfully, you need to know relative C:N ratios at the beginning and end of the composting process. Ideally, C:N should be approximately 30:1 at the beginning. If the C:N ratio is low at the beginning (e.g., around 10:1 or 15:1), then a low C:N at the end may not indicate stability. Assuming the beginning C:N is approximately 30:1, the C:N of stable compost at the end of the process will be between 10:1 and 20:1, with the most stable composts falling in the lower end of this range. A final C:N above 20:1 may indicate a compost that will not readily release nitrogen, while a final C:N above 30:1 may indicate a compost that will inhibit nitrogen mineralization and tie up nitrogen from the soil.

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