Investigating plastic in organic fertilizers: A 2-year comparative study

The application of organic fertilizers such as compost and sewage sludge to agricultural soils has emerged as a significant pathway for introducing (micro)plastics into terrestrial environments, where they may leach into groundwater, be ingested by soil organisms, or enter the food chain through plant uptake. The impact of plastic contamination introduced into soils via different types of organic fertilizers remains insufficiently quantified, particularly concerning the quantity, polymer composition, and particle size distribution of plastics. While research has highlighted the presence of microplastics in organic waste products, a comprehensive evaluation comparing various fertilizer types is lacking. This study investigates plastic contamination within seven organic fertilizers (e.g. green waste and biowaste composts, etc.) by analyzing polymer types, particle sizes, surface areas, and mass distribution in two consecutive years. While plastics were the primary focus of detailed characterization, the presence of glass and metal was also recorded to provide a broader context of foreign matter contamination. Three out of seven organic fertilizers (dry chicken manure, digested pig manure, and mixed digestate) and straws remained visually free of foreign matter in both years. Each visual-isolated particle of the remaining fertilizer piles was manually analyzed. Attenuated-total reflectance with Fourier transform infrared spectroscopy (ATR-FTIR) and siMPle were used for identification, showing that 82 % of the foreign substances were plastic. The particle number, mass, and size distribution were further contextualized by incorporating pile surfaces and application rates to calculate plastic loads per square meter (m²). Key findings reveal that biowaste compost of 2023 exhibited the highest plastic mass (1.51 g/m²), while green waste compost and sewage sludge of the same year exhibited lower contamination levels (0.069 and 0.125 g/m²). However, variability between 2022 and 2023 was pronounced, complicating definitive conclusions about generally higher abundances of foreign matter. These results underscore the necessity for further long-term research to establish guidance on surveilling abundant foreign matter and incorporate this data into regulatory frameworks. Implications suggest that the quality of input materials and processing procedures plays a vital role in the particle mass and count of foreign matter, requiring stricter monitoring protocols to reduce the environmental impact of plastics in organic fertilizers.