When using compost with constructed soil media, the volumetric respiration behavior is the key to good performance and does not necessarily correlate with gravimetric respiration.

The compost industry may be promoting stability and maturity tests for compost that are neither current with published science nor consistent with emerging soil markets,- this in a new report by Woods End Labs presented to Sustainable Sites Initiative.  The report examines the origins of commonly accepted compost stability guidelines in the USA and Europe, highlighting the near complete absence of calibration of these tests to relevant horticultural traits. The investigation casts significant doubt as to whether common compost tests are appropriate for soil building, container media and constructed soils.”If you compare so-called “stable” composts to natural soil systems, the discrepancy in biological properties is enormous”, reports principal author Dr. Brinton. Inappropriate compost stability tests may be harmful and could set back the burgeoning interest in areas such as amending disturbed, depleted and manufactured soil ecosystems. “Skills are involved in sourcing materials suitably stable to foster and sustain healthy growing environments”. The paper recollects the dramatic failure of Biosphere 2 (Arizona) due to excessive CO2 production from compost-amended soil. In a recent Manhattan project unstable compost caused a street to be cordoned-off to allow ammonia to dissipate.

Woods End scientists propose reporting stability as CO2 or O2 exchange per unit volume (g/liter) as an alternative to traditional gravimetric (weight-based) approaches. The new approach represents the behavior of the whole growing media environment, rather than the absolute properties of the organic matter. The Solvita® test was designed to do this as a field test and provides a different measure of stability than other lab tests since it integrates respiration with bulk-density. Test results show that volume respiration may actually increase with age while apparent gravimetric stability appears to be declining. “Which unit is useful depends on the application”. The report presents charts and tables comparing USDA values for soil respiration to norms for compost and this reveals that stability values accepted in the compost industry, can be 20 to 100 times higher than those encountered in natural soil-ecosystems.

In proposing volumetric respiration, which is the exchange of CO2 for O2 in a standard volume (1-liter) Woods End points out that other useful volumetric tests have been available, such as the Dewar self-heating test and the closed-cress test, but none have been given serious attention in the USA.  “Volumetric tests need to be re-appreciated for their practical application to high-end horticultural applications”.  The essay is available at STABILITY.pdf.