Plastics in soils: Precaution is key

Every year, more than 19,000 tonnes of plastics end up in the soil in Germany. Little is known about the consequences, but a scientific assessment suggests that rapid and comprehensive countermeasures should be taken.

Plastic waste in the landscape has long since become a "familiar" sight in many places. Yet such plastic waste, carelessly discarded or deliberately dumped on roadsides, is one of the main pathways through which soils become contaminated with plastic. Especially when it is reduced to microplastics and accumulates permanently below the earth's surface, it can have negative consequences.

The questions of how much plastic gets into the soils in the first place, where it comes from and what dangers it poses were the focus of the webinar "Plastic in Soils" at the end of January 2022. A team led by Jürgen Bertling from Fraunhofer UMSICHT, coordinator of the "Plastic in the Environment" project PlastikBudget, determined which emission sources are significant for agricultural soils. These include production materials such as foils used in asparagus cultivation, plant pots or planting aids such as clips and wires. Seeds, fertilisers and so-called soil conditioners are also much used. These are all frequently coated with polymers. This is to ensure that seeds germinate at a certain time or that nutrients are released into the soil over a longer period of time.

Another important source is compost, which often contains plastic – partly due to mishandling, and littering, i.e. plastic waste that gets into the environment, for example through tourism or large outdoor events. Plastic is also frequently found in sewage sludge used for fertiliser. According to the research team's analyses, the total emission of plastics into agricultural soils in Germany is about 19,000 tonnes per year. 19 percent of this is caused by agriculture itself, mainly in the form of coated fertilisers (about three quarters) and foils (about one quarter, most of it from fodder production). The remaining 81 per cent of total plastic emissions to soils consist of sewage sludge (54 per cent) and drifting plastic waste (38 per cent), while composts and digestate account for 8 per cent.

The researchers did not take their own measurements, but used existing data. The estimates determined are conservative, Jürgen Bertling emphasised - it is possible that the quantities of plastic emitted into the soil are much greater. The distribution of the inputs is very heterogeneous: It ranges from a few grams to several hundred kilograms per hectare and year. The team's findings are worrying: "We assume that the input of plastics leads to a devaluation of our agricultural soils," says Jürgen Bertling. Soils consisting of 0.1 percent plastic are considered devalued. At the current input dynamics, the soils could therefore be completely devalued in 20 to 100 years. "Since the use of plastics is currently increasing, this could even accelerate considerably," warns the researcher.

A remedy could be an improved collection of plastics, for example from foils, the use of more degradable plastics and legally defined degradation times. Such measures should be additionally combined with an educational campaign for farms that aims to inform farmers about possibilities and obligations to reduce emissions and consumption, about how to avoid the use of plastics and, should that not be possible, about how to recycle used plastics. "We know little about the damage, yet in the spirit of precaution we should work towards reducing plastic emissions into the soil in the future," said Jürgen Bertling.

Dr. Annegret Biegel-Engler, head of the department "Measures for Soil Protection" at the Federal Environment Agency, took a closer look at the impact of plastic in soils and whether this could become problematic. Generally, micro- or nanoplastics cannot be removed from soils. Plastics often contain additives such as plasticisers, flame retardants or UV-protective agents. Degradation in the environment is very slow: plastic parts are broken down in various ways, first to micro, then to nano size, for example by physical influences such as abrasion or UV radiation. The amount of time it takes for plastic to actually degrade in the soil depends on the type of plastic, particle size and environmental conditions.

Microplastics in soil can have different effects: they reduce the density and thus the water-holding capacity, an effect that is often desired and brought about in horticulture, for example by introducing polystyrene beads. In laboratory experiments at high microplastic concentrations, effects on the lifespan, growth, digestion and reproduction of soil organisms have been observed. Chemical additives, when ingested by small organisms, can potentially be harmful to their health. "It is still difficult for us to make a general assessment, but there are indications that microplastics could be a relevant, long-term stress factor for the soil ecosystem. For precautionary reasons, action should therefore be taken, especially since microplastics in soils can also shift to other ecosystems, such as aquatic ones," Annegret Biegel-Engler summarised. The biochemist pointed to other pathways of microplastics into soils, such as riding surfaces and artificial turf sports fields, and the dismantling of wind turbines. In order to reduce the input of plastics into soils, various legal requirements and measures should be used in Germany and the EU. Already in force in 2021 is a far-reaching ban on the use of single-use plastic products in the European Union. Stricter microplastic limits should apply to materials placed on or in soils (e.g. composts, sewage sludge). To this end, the EU Fertiliser Regulation and the Sewage Sludge Directive, as well as waste legislation, should be amended. In addition, the microplastic content of soils should be examined and, if necessary, precautionary values should be anchored in the Federal Soil Protection Ordinance.

Written by Wiebke Peters