2017 ASHS Annual Conference

Compost typically has an alkaline pH of 7.5 to 8.5. Continual application of compost can cause soil pH to rise above the desired range. Furthermore, if a soil has a pH above the suitable range, alkaline soil amendments should not be used. Three experiments were conducted to determine the amount of elemental sulfur (S) required to create acidic compost. In the first experiment, finished compost created from wood chips, horse manure, green plant material and food waste was amended with pelleted S (90% S) at six rates (0, 0.9, 1.8, 3.5, 5.3 and 7.1 g per L). Treatments were in 2.5 L containers and were watered once a week. Compost pH was measured once per week over 150 days, and there were minimal treatment differences for the first 30 days. Compost pH then began to decrease in all treatments receiving more than 0.9 grams of S, and this decrease continue for 60 days. At 90 days, the pH ranged from 5.5 to 8.2 and increased with decreasing S rates. In experiment two, pelleted S was blended at four rates (0, 2.3, 4.6, and 9.2 g per L) with compost feedstocks prior to the thermophilic composting process. Each treatment consisted of approximately ten cubic meters of feedstock material. At the end of the three-month composting process, the respective compost pH was 6.9, 6.0, 5.2 and 2.9. Microbial analysis of the finished compost indicated the highest rate of S suppressed populations of bacteria and actinomycetes. The third experiment was similar to the second with S rates of 0, 4.6, and 6.9 g per L. The resulting compost pH values were 8.01, 5.39, and 3.55, respectively. Results indicate that combining S with compost is an effective way to lower compost pH. If S can be blended with the feedstocks prior to the composting process rather than finished compost, reaction time to lower pH can be reduced or eliminated.