The use of plastic much films has been fundamental to promoting food production in many regions of the world. However, concern is growing about the progressive accumulation of plastic residues in soil after crop harvest and its subsequent impact on soil health and potential to enter the food chain. Although biodegradable films have been developed to prevent these problems, it is still unclear whether they are environmentally benign. Here we evaluated the physical and chemical breakdown of four commercial poly(butylene adipate-co-terephthalate) based biodegradable mulch films (BMF1, BMF2, BMF3 and BMF4) in an agricultural soil over a 26-month period. Based on visual examination, degradation followed the series BMF4 > BMF1, BMF2 > BMF3. Importantly, microplastic residues (fragments <5 mm) still remained in the soil of all 4 plastic types after 2 years, suggesting that they are likely to accumulate over time if used on an annual basis. Viscosimetry, Fourier transform infrared (FTIR) spectroscopy and Thermogravimetric Analysis (TGA) were used to characterise the breakdown process. Our results indicated that the degradation of the mulch film after burial in agricultural soil may be linked to the nature of the polymer but also to its manufacturing formulation. Although the peak changes of polyester in the infrared spectrum were not distinct, the plastic films showed other signs of degradation including a reduction in intrinsic viscosity after burial in soil. The different degradation rates of BMF1 and BMF2 at the molecular level may be due to the different CaCO3 contents. In conclusion, under field conditions, we show that slight variations in the formulations of commercial biodegradable mulch films leads to very different persistence rates in soil. Further, we conclude that their slow rate of degradation will ultimately lead to their progressive accumulation in soil if used repeatedly