TEMPORAL CHANGES IN CADMIUM, THALLIUM, AND VANADIUM MOBILITY IN SOIL AND PHYTOAVAILABILITY UNDER FIELD CONDITIONS

A field study was conducted over a 30 mo period to examine movement of Cd, Tl, and V through the profile of a Coastal Plain soil (Typic Kand iudult) and the availability of these trace metals to bush bean (Phase olus vulgaris is L.) plants. The metals were applied to field plots as dissolved salts and mixed into the surface 7.5 cm. The greatest conce ntration of all three metals was observed in the surface soils, with a steep decrease occurring down to the 7.5 to 15 cm depth. Thallium was the most mobile of the three metals; approximately 15% of the applied Tl and <3% of the applied Cd and V moved below the surface 7.5-cm reg ion during the 30-mo experiment. Extractable concentrations of all thr ee metals in the surface soils decreased significantly (P less than or equal to 0.05) during the initial 18 mo after treatment. No further d ecrease occurred between 18 and 30 mo. The presence of Al- and Fe-oxid es and small amounts of clay minerals and organic matter in this highl y-weathered, low cation-exchange soil were likely responsible for the retention of the trace metals. Bioavailability, as measured by concent rations and total amounts of metals in root and aboveground tissues of plants, did not change significantly between 18 and 30 mo. These data suggest that bioavailability of Cd, Tl, and V decreased over time as a result of transformation of these elements into unavailable forms an d not to leaching. These changes in bioavailability occurred soon afte r application, becoming negligible after 18 mo.