Raw Materials for Humic Substance Production: A Review of Sources, Suitability and Assessment Approaches

Resource-efficient conversion of biomass and organic residues into humic substances represents an important pathway for waste valorization within the circular bioeconomy. This review systematizes feedstock sources for humic substance extraction and compares their characteristics, sustainability, processing requirements, and assessment approaches. The scientific literature was analyzed together with regulatory and statistical sources relevant to feedstock classification and resource availability. The review synthesized evidence from 156 sources and shows that humic substance potential cannot be evaluated from resource volume or origin alone; it also depends on humification, organic-matter stability, physicochemical composition, contamination risks, and extraction technology. Primary resources such as peat and sapropel provide naturally humified fractions but are constrained by slow regeneration and environmental considerations. Secondary resources, including agricultural and forestry residues, fresh manure, and food waste, require flow- and coefficient-based assessment and may undergo further processing or stabilization. Tertiary resources, such as compost, digestate, treated sewage sludge, and processed or stabilized manure, are evaluated by linking material flows with quality, humification, safety, and recovery-yield indicators. For selected feedstocks, reported humic-substance content varied substantially, ranging from 2–20% dry matter in sapropel and 15–40% dry matter in peat, with different source-specific ranges reported for other organic materials. Because these values also depend on the analytical and extraction procedures applied, they should not be interpreted as directly comparable without harmonized analytical and reporting criteria. The quantitative assessment framework distinguishes theoretical, technical, and sustainably available resource potential and incorporates residue-to-product ratios, availability coefficients, material composition, and humic-substance content or recovery factors. The proposed differentiated framework supports more consistent comparison of renewable and waste-derived feedstocks and provides a methodological basis for assessing resource suitability, while highlighting the need for standardized analytical procedures, uncertainty assessment, contaminant-fate evaluation, and validation using regional or national case studies.