Progress in Additives That Promote Humification During Agricultural Waste Composting

Aerobic composting converts agricultural waste into stable, humus-rich products, and the application of exogenous additives is an efficient strategy to enhance humification. This systematic review synthesized studies published between 2023 and 2026 on additives that promote humification during agricultural waste composting. Based on the distribution of the retrieved literature, additives are categorized into inorganic additives, organic additives, biological strategies, and composite systems, and the effects and mechanisms of each category are systematically discussed. Iron-based additives achieve the highest humic acid (HA) increases of 82–267% through Fenton-like redox catalysis. Clay minerals and biochar produce moderate HA enhancements of 25–163% via physical structuring and surface adsorption with broader applicability. Small-molecule precursors and exogenous humic substances achieve HA gains exceeding 100% at sub-percent doses. Biological strategies provide self-sustaining catalytic activity but are sensitive to environmental conditions. Composite additives, the largest category, generally outperform single additives through functional complementarity, though antagonistic effects have also been documented. Cross-study patterns suggest that different feedstocks respond preferentially to distinct additive types, though systematic experimental validation is lacking. Critical gaps between laboratory findings and practical application are identified, including the predominance of small-scale studies, the absence of techno-economic analysis, and the unassessed environmental fate of metal-based additives. Future research priorities include pilot-scale validation under industrial conditions, the establishment of standardized humification metrics, and long-term field monitoring.