**3.2 Chromium**

Chromium is naturally present in soils. Its content depends on the content of the parent rock in which it is found. Generally speaking, the average content in the earth's continental crust is 35 mg.kg−1. In soil, the migration and speciation of chromium are influenced by many factors such as the valence state of chromium ions, soil pH, redox potential, soil organic matter and the concentration of manganese dioxide in the soil [20–22]. The maximum allowable chromium content in agricultural soils is 100 mg.kg−1 [22, 23].
