Development and Validation of an Integrated Quality Index for Organic Waste-Derived Compost Using Fertility, Physicochemical, and Potentially Toxic Element Parameters

Compost quality assessment remains fragmented globally, with most standards evaluating parameters in isolation rather than providing a unified quality score. This study developed and validated a Quality Criteria Index (QCI) model comprising three composite sub-indexes to holistically quantify organic waste-derived compost quality. The Fertility Criteria Index integrates plant nutrients. The General Criteria Index assesses physicochemical properties and contaminants. The Toxic Criteria Index covers potentially toxic elements. The model was validated using compost samples from 20 commercial-scale Sri Lankan producers. International applicability was assessed using data extracted from 20 peer-reviewed studies across 20 countries. The Shapiro–Wilk test confirmed that model residuals were normally distributed. Multiple linear regression confirmed a model fit: R2 = 0.939 for the Fertility Criteria Index, R2 = 0.875 for the General Criteria Index, and R2 = 0.919 for the Toxic Criteria Index. Most predictors showed low multicollinearity, and all indices showed good correlation between the actual and predicted values, confirming the validity of the model. The Wilcoxon rank-sum test revealed no significant differences between Sri Lankan and international compost index values. Based on the index values, a three-tier classification system is proposed: First Class, Second Class, and Rejected Compost. This index-based approach provides a single, interpretable quality metric for producers, regulators, and end-users and offers a framework for revising national compost standards. The final benefits of this approach include sustainable waste management and sustainable soil quality in agricultural systems.