Availability of applied heavy metals as a function of type of soil material and metal source

For this investigation, we applied the heavy metals Cd, Cu, Ni, and Zn, as sulfate salts and as lime-stabilized, digested Washington, D.C., sewage sludge, to surface and subsurface horizon materials of six Maryland soils in the greenhouse. Rates of metal addition were equivalent to field rates of 224 metric tons (t)/ha of the sludge. Soil metal levels were monitored by extraction with DTPA (diethylenetriaminepenta-acetic acid buffered at pH 7.3). Corn (Zea mays L.) was grown for 30 d, 13 mo after treatment application, and the tissue was analyzed for metals. The mean final pH values across all soil materials were 6.3 and 6.2 for the metal-salt-treated and sludge-treated soil materials, respectively, and 6.4 for the control treatment.Compared with limed and fertilized controls, yields of plant material were generally enhanced by sludge and suppressed by metal salts. Tissue levels of both Cd and Zn were considerably elevated, compared with the limed control, by both metal salts and sludge, but more by the metal salts than by the sludge. In general the sludge did not elevate tissue Cu and Ni, whereas metal salts slightly elevated tissue Cu and Ni with some soil materials. DTPA-extractable levels of metals were elevated by both metal salts and sludge, compared with the limed control.High-CEC soil materials and materials higher in iron and manganese “oxides” generally gave lower DTPA-extractable metal values with both metal salts and sludge and the lowest levels of tissue metals with metal salt treatment. However, this relationship did not hold for tissue metal levels with sludge-treated soil materials.An interaction between type of soil material and source of metal on plant tissue metal levels was observed. Essentially inert soil materials, such as the Evesboro (Typic Quartzipsamment), tended to produce tissue with the highest metal levels when metal-salt-treated, but with the lowest metal levels when sludged. Conversely, reactive soil materials, such as the Myers-ville (a fine loamy Ultic Hapludalf quite rich in vermiculite and iron and manganese “oxides”), tended to produce tissue with relatively lower levels of metals than other materials when metal-salt-treated, but with relatively high levels when sludged.Thus, although the data from the metal-salt-amended soil materials suggested that sandy, lower CEC soils are less suitable for heavy-metal-bearing wastes than finer higher CEC soils, tissue metal levels from the sludge-amended soil materials did not agree with this suggestion.