Plastic in compost: Prevalence and potential input into agricultural and horticultural soils

To maintain and improve soil fertility, compost application is a widely recommended practice. We hypothesized that this practice is, however, also a main entry path for plastic into soil. Hence, we i) quantified the prevalence of plastic in eight composts from different composting plants and hardware stores to derive estimations about related plastic inputs into soil, and ii) characterized the properties of these plastic residues in regard to size and shape for further risk assessment. Plastic remains were analyzed via density separation (ZnCl₂) and light microscopy. Testing this method recovered 80 ± 29% of spiked plastic items. Applying this method revealed that all composts contained plastic particles in detectable amounts, with contents ranging from 12 ± 8 to 46 ± 8 particles kg^-1, corresponding to calculated plastic weights of 0.05 ± 0.08 to 1.36 ± 0.59 g kg^-1. Because of this high variability, an a-priori discrimination of plastic loads between compost types cannot be achieved. Upscaling these loads to common recommendations in composting practice, which range from 7 to 35 t compost ha^-1, suggest that compost application to agricultural fields goes along with plastic loads of 84,000 to 1,610,000 plastic items ha^-1 per year (a), respectively, amounting to 0.34 to 47.53 kg plastic ha^-1 a^-1. Large potential inputs should thus also occur for horticultural soils, where application rates of compost usually vary between 6.48 and 19.44 t ha^-1, therewith resulting in a minimum plastic contamination of 77,770 plastic items and 0.31 kg plastic ha^-1 a^-1, but a maximum amount of up to 894,240 plastic items and 26.4 kg plastic ha^-1 a^-1. We conclude that compost application must be considered as potential source of plastic for both agricultural and horticultural soils, and technical solutions are needed to minimize these contamination risks while continuing this practice as important option to secure soil health.