Relative Humidity and Matric Potential Constraints on Composting Microbial Activity

Composting has attracted increasing attention as a sustainable management strategy for organic residuals. Moisture management in composting is critical, but also challenging as different materials have different optimal moisture levels. Matric potential has shown promise as a more general measure of moisture constraints, but is difficult to measure with current technology. This study developed a low cost alternative measurement strategy, using relative humidity sensors and known humidity, water activity, and matric potential relationships. Bench-scale measurements were used to relate compost moisture content to water activity and matric potential. Matric potential ranged from –180 MPa (-1800 bars) at 9% moisture (w.b.) to >-1.5 MPa (-15 bars) at moisture contents above 35% (w.b.). At these moisture contents above 35% (w.b.) water activity is consistently above 0.98. Sensor accuracy was limited in this moderate to high moisture range (±3% at > 90 %RH), and calibration with saturated salt solutions reduced but did not eliminate the resulting scatter in the data. Some hysterisis was observed during rewetting of samples, although additional data are needed to better define this effect.