Cadmium uptake by crops estimated from soil total Cd and pH

Although it is well established that Cd uptake by crops is dependent on soil total Cd and soil pH, actual prediction of uptake based simultaneously on these two soil parameters has not generally been tested. In the present study, reanalyzing available data on the uptake of cadmium by lettuce, Swiss chard, and corn leaves from sewage-sludge-amended soils showed that an equation of general form: log (Cd)(crop) = a + b log (soil Cd) - c (soil pH) which includes the predictive factors of soil pH and total soil Cd only, was reasonably predictive of above-ground plant tissue Cd concentration, (Cd)crop, although the coefficients a, b, and c varied depending on the particular soil, climate, and crop type. Nevertheless, this equation allows a more direct comparison of Cd bioavailability in sewage-sludge-treated and uncontaminated soils because, in many experiments, soil pH is very different in the treated and control plots. After allowing for pH effects, the data fail to show consistently that Cd applied to soils in sewage sludge is less bioavailable than native Cd or Cd from other sources of contamination. The strongest evidence for a protective effect of sewage sludge in limiting Cd uptake by leafy crops was for low Cd loadings and nonacid soil conditions. Overall, however, long-term bioavailability depended primarily on the soil pH and Cd concentration in the soil. Although other factors, particularly soil organic matter content, soil texture, and mineralogy, probably affect the bioavailability of Cd in different soils, the lack of reported data sets including these soil parameters has hindered testing of their significance in controlling plant uptake.