FROM WASTE TO FERTILIZER OR SOIL AMENDMENT: A PERFORMANCE-BASED CRITICAL REVIEW OF AGRONOMIC EFFECTIVENESS, SAFETY, PROCESSING, AND CIRCULAR-ECONOMY READINESS

Abstract

Waste-derived fertilizers and soil amendments are increasingly proposed as responses to fertilizer supply risk, waste-disposal pressure, and the environmental burdens of conventional nutrient production. This structured critical review evaluates evidence published from January 2020 through August 2026 on the conversion and agricultural use of municipal, agro-industrial, industrial, mining, metallurgical, and construction-associated residues. The review compares biosolids and composts, digestates, recovered ammonium products, struvite and other recycled phosphorus salts, sewage-sludge ash, biochar and hydrochar, organomineral formulations, steel slag, phosphogypsum, rock powders, and construction-derived mineral fines. A performance-first framework separates total nutrient content from plant-available nutrient delivery and distinguishes defined fertilizers, hybrid fertilizers, and soil amendments or remineralizers. Evidence is appraised separately for agronomic effectiveness, safety, process maturity, product engineering, and techno-economic/life-cycle robustness, so that field performance is not conflated with technology scale. The strongest direct-fertilizer evidence is associated with compositionally defined recovered nutrients, particularly struvite and selected recycled nutrient salts, whereas standardized digestate fractions and organomineral products show conditional readiness that depends on composition control and management. Mineral residues and construction-derived fines generally function more defensibly as liming, calcium, magnesium, silicon, sulfur, or remineralizing amendments unless plant-available nutrient delivery is demonstrated. Across product classes, decisive constraints include nutrient speciation and release kinetics, feedstock variability, metals and emerging contaminants, pathogens, microplastics, source-specific radionuclides, product handling, long-term accumulation, and inconsistent analytical and regulatory frameworks. The review proposes an ordinal, evidence-based readiness framework and identifies priorities for multi-season field validation, standardized nutrient-availability testing, contaminant bioavailability, manufacturing quality assurance, predictive modeling, and full-scale integration.