Measurement of plant-available cadmium in New Zealand soils

Abstract

Several methods of measuring plant-available cadmium (Cd) were compared using soils that had accumulated Cd under normal New Zealand agricultural practices (low total Cd concentrations, and phosphatic fertiliser as the dominant Cd source). The study encompassed 9 New Zealand soils with different Cd input histories. Cadmium was extracted from these soils by demineralised water, 0.05 M Na₂EDTA, 1 M NH₄OAC (pH 7), 0.01 M CaCl₂, and 0.05 M CaCl₂ and quantified by differential pulse anodic stripping voltammetry (DP-ASV) and graphite furnace atomic absorption spectrophotometry (AAS). The DP-ASV measures the free Cd ion and that associated with labile complexes, but not large organic Cd complexes. Extractable Cd levels were compared with those which are plant-available, as determined by pot studies (lettuce). The 0.01 M CaCl₂-extractable Cd measured by AAS and 0.05 M CaCl₂-extractable Cd measured by DP-ASV gave the best estimate of plant availability of Cd.