A simple titration method to determine elemental sulfur required for compost acidification

D.M. Sullivan, R.C. Costello, S.B. Andrews
Development of high-value horticultural markets for compost requires improved characterization of compost properties. Compost pH is typically high (7 to 8) relative to the requirements of many plants. We developed a protocol to determine the acidity required to reduce compost pH to 5 for use with acid-loving plants. The same protocol may also have application to pH adjustment in soil-less media that include compost. The amount of elemental S (S°) required depends upon initial compost pH and its pH buffering capacity. Compost response to acidification was determined by equilibration of compost with dilute sulfuric acid (H2SO4; 0.1 to 1.0 mol H+ kg‑1 compost) at 22°C. After a 72-h equilibration period, the decrease in compost pH with acid addition was linear vs. acid addition rate. Compost buffering capacity, the amount of H+ required for a one unit pH decrease, ranged from 0.17 to 0.28 mol H+ kg‑1 across all composts (n=28). The quantity of S° required to acidify compost to pH 5 averaged 7 to 8 g S° kg‑1 for composts derived from municipal yard debris, deciduous tree leaves, or horse stall waste, and 12 g kg‑1 for composts from dairy solids or peppermint biomass after oil extraction.
Sullivan, D.M., Costello, R.C. and Andrews, S.B. (2019). A simple titration method to determine elemental sulfur required for compost acidification. Acta Hortic. 1266, 15-20
DOI: 10.17660/ActaHortic.2019.1266.3
substrate, titration, acidity, pH buffering capacity

Acta Horticulturae