Assessment of zinc phytoavailability by diffusive gradients in thin films.

Assessment of Zn phytoavailability can be predicted with routine soil extractants, but these methods generally do not perform well across a wide range of soils. The newly developed technique of diffuse gradients in thin films (DGT) has been employed to determine phytoavailable Cu concentrations, but its suitability for determining plant available Zn concentrations has not been evaluated. A greenhouse study was conducted to assess the phytotoxicity thresholds and the phytoavailability of Zn to sorghum-sudan (Sorghum vulgare var. sudanese) grass by DGT, compared with CaCl2 extraction. A range of phytoavailable Zn concentrations was created by amending sand with ZnSO4 or with two different Zn mine wastes. Plant nutrients were added as Hoagland solution. In general, increasing Zn concentrations in the sand mixtures increased Zn adsorption by DGT and decreased the sorghum-sudan yield. A critical value for 90% of the control yield was chosen as an indicator of Zn toxicity. Critical values of DGT Zn, CaCl2-extractable Zn, and plant tissue Zn were similar statistically across the three Zn sources. The performances of DGT and CaCl2 extraction for assessing Zn phytoavailability were similar. Shoot and root Zn concentrations of sorghum-sudan grass exceeded 500 mg kg(-1) for many treatments. Calcium-to-Zn ratios for shoots were <32, suggesting Zn phytotoxicity. The data suggested that Zn phytotoxicity can be induced with mine wastes, although further evaluation is needed to establish a link between mine waste and Zn phytotoxicity.