(1)

where P_i is the SFPI of a certain HM;C_si is the determined concentration of the respective HM, mg·kg^-1; C_st is the permitted limit of the respective HM in the soil, mg·kg^-1.

2) Nemeiro comprehensive pollution index (NCPI)

The NCPI considers the average and the highest value of the SFPI to highlight the impact (pollution risk rate) of the highest contributing pollutant on the environmental quality of farmland soil (Duan et al., 2021). It can be calculated as follows: