An electrical conductivity (EC) measure is typically used as an indicator of compost quality and generally assumed to be one of the major limiting factors for many compost applications. Compost end use specifications typically do not require reported values for soluble ions that are known to influence EC readings, (e.g., sodium, chloride, ammonium-nitrogen, sulfates, potassium, calcium, magnesium, etc.), even though compost is a complex matrix and concentrations of soluble ions in finished compost vary with feedstock selection and compost maturity.

We evaluated analytical data from more than 600 compost samples to deconvolute EC readings using total and soluble ion data. Lab test methods employed include TMECC (Test Methods for the Examination of Composting and Compost) Method 04.10-B Electrical Conductivity 1:5 Slurry Method (Mass Basis), TMECC Method 04.04-A Total Potassium, TMECC Method 04.02-C Ammonium Nitrogen Determination (Ion Chromatograph), TMECC 04.05 Secondary and Micro-Nutrient Content, and TMECC Method 04.11-A 1:5 Slurry pH.

Statistical analysis revealed that Na, Cl, K, NH4-N and SO4-S correlate most closely with EC. We then constructed an interactive tool based on a multivariate regression model to assist composters and end users to more accurately interpret reported EC values.