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.