UPTAKE OF CADMIUM BY LETTUCE (Lactuca sativa) AS INFLUENCED BY ITS ADDITION TO A SOIL AS INORGANIC FORMS OR IN SEWAGE SLUDGE

Abstract

The effect of forms of added Cd in a soil on the uptake of Cd by lettuce (Lactuca sativa) was studied in the greenhouse. For two levels of added Cd (2.5 ppm and 5.0 ppm) and for two successive crops grown on the same soil, the average Cd concentration of plants grown in soil treated with inorganic forms was five times greater than that of plants from soils treated with sewage sludged containing Cd at the same concentrations. Forms of inorganic Cd had some effect on Cd uptake; lower amounts were taken up from soils treated with Mn oxide-Cd precipitates than from Cd precipitates with Fe and Al hydroxides and with lime. The dry matter weight of the first lettuce crop was higher at 2.5 ppm levels of Cd additions than at 5.0 ppm levels of Cd. The differences were significant for one inorganic treatment and for both sludge treatments. In the second crop of lettuce, the differences were smaller between the two levels of Cd for inorganic treatments; however, for two sludge treatments the yields were greater and statistically significant at 5.0 ppm Cd as compared to 2.5 ppm Cd. A significant correlation coefficient (r) was obtained between the DTPA-extractable Cd and the Cd content of lettuce plants for the six inorganic treatments. However, when sludge treatments were included along with inorganic treatments, the value of the correlation coefficient (r) was reduced. Caution is advised in employing the DTPA extraction of soils as a measure of Cd uptake by plants when Cd is incorporated from heterogeneous sources.