Cadmium accumulations and bioavailability in soils from long-term phosphorus fertilization

Soils from citrus groves that had been fertilized with the equivalent of approximately 175 kg P/ha per year [as treble superphosphate (TSP)] over a 36-year period were sampled and analyzed for total P, Cd, and Zn as well as water-soluble P and Cd. A P fertilization rate of 175 kg/ha (about 400 kg P₂O₅/ha) is much higher than normal field rates, which are typically about 30 kg/ha. Concentrations of total Cd in surface soil were highly correlated (r = 0.89) with concentrations of total P. The concentrations of Cd in surface soil receiving broadcast P for 36 years averaged 1.0 μg/g, and were considerably greater than those of the controls, which showed a mean concentration of 0.07 μ/g. Phosphorus in soil saturation paste extracts ranged from 0.10 μg/ml (controls) to 8.87 μ/gml in P ferilized soils. Water-soluble saturation extract Cd ranged from 0.008 μg/ml in controls of 0.017 μg/ml in fertilized soils, and was not well correlated with water-soluble P. Cadmium levels in barley (Hordeum vulgare var. U.C. 566) grain and leaves grown in the field on soil subject to lon-germ heavy P fertilization were not elevated above the levels in barley grown on the control soil. Swiss chard (Beta vulgaris vas. cicla) was grown in the greenhouse on the above surface soils collected from the field. Although Cd levels averaging 1.6 μg/g in plant tissue were significantly elevated over those on the control soil (0.26 μg/gm), no yield depression was observed. Long-term heavy application of TSP caused measurable increases in total Cd of surface soil, but the accumulated Cd burden per year was much lower than levels reported from application of sewage sludge. Cadmium accumulating in soils as a result of P fertilization seemed to be less available to Swiss chard than Cd accumulations resulting from applications of sewage sludge reported in the literature.