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Microplastics are an environmental issue of global concern. Although they have been found in a range of environments worldwide, their contamination in the terrestrial environment is poorly understood, particularly in relation to their source and fate. One source of microplastics in soils of particular concern is biosolids. Biosolids are the solid by-product of the wastewater treatment and are commonly spread on agricultural land as a fertilizer, indicating a potential route for microplastics into terrestrial soils. The aim of this thesis was therefore to broaden the understanding of microplastic contamination in agricultural soils in relation to biosolid application. The lack of suitable methods for microplastic detection and quantification is a major obstacle for determining their concentrations in soil environments. Therefore, an experiment was carried out to determine the best methods for microplastic extraction based on soil characteristics. The efficiency of organic matter removal methods was measured. Soils with a range of particle size distribution and organic matter content were spiked with a variety of microplastic types and density separation methods were tested. The optimal organic removal method was found to be hydrogen peroxide with organic removal rates up to 93%. The recovery efficiency of microplastics was variable across polymer types. Overall, canola oil was shown to be the best method for density separation, however, efficiency was dependent on the amount of organic matter in the soil. This outcome highlights the importance of including matrix-specific calibration in future studies considering a wide range of microplastic types, to avoid underestimation of microplastic contamination. To understand the sources and fate of microplastics in agricultural soils, these tailored methods were used to extract microplastics from samples collected from agricultural soils in the River Test catchment area in the UK. Soils were collected from fields which had historical biosolid application and these were compared to a similar set of fields which had never received biosolid application during summer and winter. The mean microplastic concentration was high in both the biosolid treated fields (874 MP/kg) and the untreated fields (664 MP/kg) and a wide variety of polymers were found across sites. There was a lack of significant difference between treated and untreated soils suggesting the influence of other sources and environmental processes. Additionally, soil samples were collected from five separate fields over the course of a year, before and after biosolid application. Microplastic contamination was ubiquitous across these fields up to a maximum concentration of 7950 MP/kg. Despite previous reports of high concentrations of microplastics in biosolids, their concentrations in soils did not significantly increase after application of biosolids. This suggests that biosolids may not be the key influencing factor in microplastic soil concentrations and transport out of soil systems is likely through horizontal (run off) and lateral (percolation) routes. Agricultural soils may thus be acting as a vector for microplastics to freshwater systems and the wider environment. Overall, the results of this thesis suggest that biosolids, whilst are likely a contributor, are not the sole source of microplastics in agricultural soils. The importance of additional sources and pathways are explored, and the complexities of the soil environment are considered, suggesting the highly dynamic nature of soil environment may determine the variability in microplastic concentrations. The research presented here significantly increased the understanding of microplastic sources and fate in agricultural soil systems while highlighting directions for future soil microplastic research.