Uptake of copper, nickel and zinc by crops growing in contaminated soils

A series of three replicated pot trials is reported in which various crops were grown in soils having enhanced concentrations of copper, nickel and zinc. Concentrations of these elements in the tops of plants harvested at their most sensitive stage were compared with ‘total’ and ‘extractable’ concentrations in soil and with concentrations in soil solutions. There was little difference between the relationships of ‘total’ and ‘extractable’ soil metal and concentrations in plant tissue. In general, the correlation between concentrations of metals in soils and plants was unpredictable. Plants differed in their efficiency of uptake of elements; lettuce assimilated more than the other crops tested (barley, rape and ryegrass). Similarly, soil concentrations of the elements required to achieve toxic thresholds in plant tops increased in the order lettuce, ryegrass, rape and barley. Measurements made with conventional extractants of copper, nickel and zinc in soils can be of value in predicting plant uptake and hence toxicity, only if appropriate calibration curves plotting extractable soil metal against plant uptake are at hand for the particular soils and plants under consideration. Mild extracts are more sensitive to the soil properties, especially pH value, which determine plant uptake and results with metals in soil solution were promising, especially for zinc. Nevertheless, soil analyses for copper, nickel and zinc are not always closely associated with their likely toxicity to crops.