August 10, 2022

The fate of plastic use in agriculture: the state of agricultural soils

Written by: Sam Cusworth.  The use of plastics in agriculture is well established. Within the UK, plastics are used for a range of agricultural applications, ranging from propagation trays to greenhouse covers. However, the use of plastic in the UK agricultural system is poorly documented. Existing estimates are outdated by a decade and are crudely assessed. Similarly, the waste management of plastics in agriculture is poorly understood and relatively unmapped, particularly in the UK.

Perhaps, the least understood aspect of plastic use in agriculture is the pollution hazard manifested from widespread use. There is little understanding about the degradation of agriplastics whilst in use, and the transport and fate of microplastics in the terrestrial environment.

Here, we examine the load of microplastic concentrations in soils from plastic applications used in crop production. Here, we focus on the carrot and potato crop due to the widespread growth across the UK. The typical plastic applications used on these crops in the UK include fleece, mulches, nets and irrigation drip tape.

The state of agricultural soils in the UK

Here, we present the results of our ongoing study. We have taken samples from 99 sites across the UK where either carrots or potatoes are grown, with or without plastic crop covers. These sites were equally spread across the UK, covering a range of growing regions, climate gradients and soil types.

Overall, there was a significant difference in microplastic concentrations between agricultural soils that used and did not use plastic applications for crop production, 4806 counts kg¯¹ and 2690 counts kg¯¹, respectively. Per crop, these trends were consistent. Where plastic was and was not used on the carrot crop across the UK, microplastic concentrations were 4900 counts kg¯¹ and 2620 counts kg¯¹, respectively. Similarly, agricultural soils where potatoes were grown with plastic applications had a microplastic load of 4720 counts kg¯¹, compared to soils where no plastic was used (2750 counts kg¯¹).

The results from this research suggest that plastic applications used in crop production, particularly mulch films, represented a significant source of pollution. Interestingly, the load of microplastics in soils where plastic applications have not been used for over a decade were significant (2690 counts kg¯¹). This suggests that sources of microplastics to agricultural soils are non-exclusive to plastic applications used directly on the soil. It is thought that plastic-coated fertilisers, sewage sludge, processed biosolids and other soil amendments are significant sources of microplastics. Supplementary to this, it is thought that microplastics are persistent particles, capable of remaining in the environment in excess of 100 years. Considering this, soils where plastic applications were used over a decade ago, even after a period of quiescence, could contain a significant amount of microplastics. The effect of microplastics in agricultural systems are relatively unknown. How may microplastics affect agricultural productivity? Soil health? Nutrient availability? Microbial communities?

Granted, there are caveats to this study. Due to the inherent variability of microplastic concentrations within the environment, owing to the localised point sources of pollution, it is unsuitable to extrapolate these values across a scale greater than a field. Future research must identify the type of polymer present in the soil. Determining this will help to identify the application(s) responsible for the microplastic pollution in agricultural soils.

The next step

This leaves us with an array of avenues to explore. From these results we are conducting experiments which aim to address the following questions: How does the concentration of microplastics in soil change over time? What mechanisms are responsible for determining the concentration of microplastics at different soil depths? Understanding these questions is key to understanding the legacy of agricultural plastic use.

Using novel methods to identify and quantify the microplastics across a historical timeline of soils, we aim to lay the foundation for a currently unknown, yet integral aspect of plastic pollution. This research is in collaboration with Rothamsted Research and is bound to yield impactful results.

We are close to harvesting results from a two-year mesocosm experiment based at Hazelrigg Weather Station. Here we explore the affect of crop type and soil type on the migration of microplastics through the soil profile. We are excited to report the results back from both of these experiments.