MEASUREMENT OF PLANT-AVAILABLE CADMIUM IN NEW-ZEALAND SOILS

Several methods of measuring plant-available cadmium (Cd) were compare d using soils that had accumulated Cd under normal New Zealand agricul tural practices (low total Cd concentrations, and phosphatic fertilise r as the dominant Cd source). The study encompassed 9 New Zealand soil s with different Cd input histories. Cadmium was extracted from these soils by demineralised water, 0.05 M Na(2)EDTA, 1 M NH(4)OAC (pH 7), 0 .01 M CaCl2, and 0.05 M CaCl2 and quantified by differential pulse ano dic stripping voltammetry (DP-ASV) and graphite furnace atomic absorpt ion spectrophotometry (AAS). The DP-ASV measures the free Cd ion and t hat associated with labile complexes, but not large organic Cd complex es. Extractable Cd levels were compared with those which are plant-ava ilable, as determined by pot studies (lettuce). The 0.01 M CaCl2-extra ctable Cd measured by AAS and 0.05 M CaCl2-extractable Cd measured by DP-ASV gave the best estimate of plant availability of Cd.