Solid organic matter (OM) is the most abundant soil organic matter and widely distributed. It controls the cycling of carbon and nutrients. Here we show that solid OM reduces Ag ions to metallic silver nanoparticles (AgNPs) under natural solar irradiation. The reduction kinetics include a nuclei formation rate constant of 2.49×10−3 h−1 and surface autocatalytic reduction rate constant at 4.47×10-2 mM−1 h−1. We further demonstrate through electron spin resonance measurements that superoxide radicals, produced from quinone-like groups within solid OM under irradiation, are a driving force in AgNPs formation. Our estimates indicate that solid OM contributes to about 11–31 % of AgNPs occurred naturally in soils. This provides a fresh insight into the sources of AgNPs in terrestrial environment and facilitates an unbiased assessment of fate and impacts of engineered AgNPs. The reduction role of solid OM is likely widespread within surface environment and can be expected to significantly influence the biogeochemical cycling of Ag and other contaminants that are reactive towards quinone-like groups.