Information on the levels of various metals present in farmland is given in the first column of Table 13.1. Depending on the history of the land, various studies have shown that the toxic metal levels can vary over a wide range, for example lead concentrations between 3 and 710 mg kg−1 have been found against a maximum acceptable level for farmland of 100 mg kg−1. While some of this metal is naturally occurring, i.e. may have a geological origin, the remainder represents manmade pollution such as airborne pollutants and the use of agrochemicals or sewage sludge as a fertiliser.