Investigating matric potential measurements as a means of determining optimal moisture levels for high-rate composting
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The objectives of this investigation were to add to the limited body of knowledge regarding matric potential curves in organic media, and to determine if matric potential, a common soil measurement, can be used as an indicator of the optimal moisture for achieving maximum composting rates. A hog manure solids and wood shavings (HW) mixture, and a dairy manure, paper mill sludge and seed processing screenings (DPS) mixture were each was composted in pilot-scale bioreactors. Temperature, exhaust 02 and C02 concentration, and gas mass flow were observed. Compost samples were collected from the bioreactors over the course of each experiment for bench-scale testing of matric potential and C02 respiration. Matric potential was measured using pressure plates and pressure cells. In neither of the trials did a universal matric potential range yielding the highest respiration rates emerge. Compost matric potential curves conformed to the shape seen in common soil water potential curves, but expanded over a much larger scale of negative pressures. At -15 bars (-15,000 cm water), considered to be the permanent wilting point for many plants, moisture content did not fall below 43% (wet basis). In the majority of sample sets, the earliest samples held the higher moisture content, the oldest samples had the next highest, and those collected in between held the lowest moisture content. These results along with the CO2 respiration data may support a hypothesis that optimum moisture content increases with time as the composting process breaks down particles into smaller components. Further studies including the measurement of CO2 respiration rates during bench-scale matric potential measurements, and over a broader moisture content range may offer more conclusive results. Additional testing and research is recommended to validate whether or not pressure plates are viable for testing matric potential in compost.