(2)
where Pnc is the NCPI of soil HMs;Pi, max is the maximum SFPI of certain HM in the soil; n is the number of soil HMs determined; evaluation: the standard classification of Pnc is the same as the SFPI.
3) Impact index of comprehensive quality (IICQ )
The IICQ takes into account multiple evaluation indicators such as ”soil geological background value”, ”pollutant limit value”, ”HM ion impulse-valence effect” and other evaluation indicators, eliminating the defects of the traditional ”soil standard management method” (Wilson et al., 2010; Wang et al., 2016). It is more objective to evaluate the cumulative risk of HMs in farmland soil, which can be calculated as follows: