Abstract The co-pyrolysis of Single-use low density polyethylene (LDPE) and Eucalyptus biomass (EuBm) can be considered as a sustainable waste management technique to produce viable byproducts. This study elucidates the effects of variable temperatures (300–600 °C), residence times (90–150 minutes), and proportions of LDPE (0.25, 0.33 (w/w)) on physicochemical characteristics of LDPE - EuBm char composites. The interference of liquified polymer coating on the surface with degradation of biomass could be the reason for low nutrient extractability of chars synthesized at 300 and 400 °C. These chars were rich in volatile matter (> 68 %) and their pores were filled with partially pyrolyzed products. Interestingly however, substantial changes in properties were observed at 500 °C due to the likely synergetic effect between the feeds. The highest plant-extractable concentrations of major nutrients (Na, K, Ca, Mg, NO3−, PO43-), electrical conductivity (4.73 mS/cm), and cation exchange capacity (50.5 Cmolc/kg) of char were observed at this temperature. The optimization through regression modeling identified 524 °C, 118 min, and 31 % (w/w) of LDPE as optimal process parameters to obtain char suitable for application in soil. Soil incubation test fortified the benefits of char to soil with 3.5 times improvement in soil fertility index at 5 % (w/w) rate of application.