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

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.