The source and distribution of cadmium in soils on a regularly fertilised hill-country farm

Cadmium (Cd) accumulation in New Zealand pastoral soils is often attributed to application of Cd-containing phosphatic fertiliser. Other sources may, however, contribute to the Cd load of the topsoil, e.g. atmospheric deposition and residues from the clearing and burning of native bush prior to the establishment of pasture. A study was conducted to evaluate the source of Cd to pastoral soils by comparing Cd loads to a 30 cm depth in 2 soil types at 2 pedologically matched sites: one fertilised and the other unfertilised. The Cd load in the fertilised soils (420 and 482 g/ha) was markedly greater than in their unfertilised counterparts (16 and 189 g/ha). A review of local pollution sources discounted atmospheric deposition as a potential source of this additional Cd. Estimates of Cd concentrations in the above-ground biomass of the native bush indicated a Cd store of probably <10 g/ha. Cadmium stores in the native bush and additions of Cd to the topsoil following clearing and burning of native bush were unlikely to account for differences in Cd load between the fertilised and unfertilised sites. Estimates of Cd additions from phosphatic fertiliser comfortably accounted for the difference, and indicated that fertiliser was the main Cd source for these soils.