Heavy Metals in Fertilizers: A Historical Analysis of Contamination Trends (1960-2025)

This preprint presents a comprehensive historical analysis of heavy metal contamination associated with synthetic fertilizer production and use from 1960 to 2025, examined through a food metallomics lens. While much of the existing literature on fertilizer contamination and heavy metals is metal-specific, regionally constrained, or cross-sectional in nature, this review reconstructs a continuous, multi-decade source-term history across fertilizer classes, regulatory eras, and agricultural regions, addressing a critical gap in long-range exposure context. Drawing on fertilizer production records, regulatory histories, environmental monitoring studies, and food-chain exposure data, the work traces how cadmium, lead, chromium, nickel, and related metals entered agricultural systems at scale during the Green Revolution and subsequently accumulated in soils, crops, and human diets. Phosphate fertilizers are identified as the dominant long-term source of cadmium loading in agricultural soils, while urea fertilizers contribute a distinct, mechanistically important nickel burden linked to industrial scaling, raw material sourcing, and nitrogen fertilizer chemistry. Although regulatory interventions implemented since the early 2000s have reduced heavy metal concentrations in newly manufactured fertilizers, the analysis demonstrates that legacy contamination persists due to long soil residence times, slow desorption kinetics, and continued crop uptake. By explicitly linking fertilizer industrialization timelines to later soil accumulation, food-chain transfer, and dietary exposure, the review highlights the causal importance of historical inputs in shaping present-day exposure profiles and health risk. This work is intended to serve as a historical reference framework for food metallomics research, environmental health studies, dietary exposure assessment, and regulatory or standards-based analyses. It does not present new primary measurements, toxicokinetic modeling, or microbial functional analyses; rather, it establishes the upstream contamination history necessary to contextualize contemporary food metal data, microbiome-associated exposure hypotheses, and policy discussions surrounding fertilizer quality, soil safety, and food security.