Predicting Compost Stability

Abstract

Sixteen composts consisting of 14 commercial samples, one immature yard waste compost and one raw material of mostly grass clippings were evaluated for their stability. In a three-day incubation test, the commercial composts yielded from 9 to 99 mg CO₂/kg/ hr, with a mean CO₂ production rate of 61 mg CO₂/kg/hr. By contrast, the immature compost produced 684 mg CO₂/kg/hr and the raw material, 1,433. The low CO₂ production rates of the commercial composts along with dark brown color and lack of unpleasant odors in moist conditions indicated that these composts were indeed stable. We devised a quick chemical test to predict compost stability. Several compost properties were measured: (i) total (C, N and C:N), (ii) water-soluble (C, N and C:N), (iii) NaOH-soluble C, humic and fulvic acids, and optical absorbances at 465 nm (E4) and 665 nm (E6) of the NaOH-soluble fractions. Water-soluble fraction, particularly water-soluble C and the C:N ratio, best separated stable from unfinished composts. However, water-soluble organic N in some stable composts was less than 0.01 g/kg, making measurement difficult. Thus, alternatively a ratio of water-soluble C:total organic N ≤0.70, based on Mean (0.32) + 2*SD (0.19), is suggested as a predictor of compost stability. Also, compost stability can be predicted by NaOH-soluble C:water-soluble C ≥6.0, but not by (humic acid:fulvic acid) or (E4:E6) of any NaOH-soluble fractions.