However, in the second batch, plants grown in the soils added sludge contain more heavy metals than those
harvested in the first batch, and the metal content shows a tendency to increase with higher portions of sludge
addition. Chinese cabbage plants grown in soil with 1:1 sludge addition have the highest metal contents; 12.0
mg/kg Cu that is 12 times the amount in the control set, 28.9 mg/kg Cr that is 16.8 times the amount in the
control set, 6.7 mg/kg nickel that is 7.3 times the amount in control set, 141.9 mg/kg Zn that is 7.6 times the
amount in control set, 7.0 mg/kg Cd that is 21.3 times the amount in control set, and 10.9 mg/kg lead that is
17.4 times the amount in control set. Both the cadmium and lead concentrations are 35 times the current
limitations of 0.2 ppm cadmium and 0.3 ppm contained in Chinese cabbage. In the third batch, zinc
concentration in the plant grown in the soil with 1:1 sludge addition further increases to 368.4 mg/kg whereas
all other metal contents are lower than those in the second batch. The plants grown in sludge with 1:5 sludge
addition and in the control set have lower metal contents than those grown in the first batch indicating that
except zinc that is continuously released because of its high concentration in the biological sludge, the release
of all other metals has reached the maximum level. As shown in Figure 2, most metals exist in the sludge
mostly in Fe-Mn oxides, organic bonding form or residual form that are not easily absorbed by the plant.
Based on the analyses of heavy metals bonding forms, the metals contained in the sludge have not been
dissolved in large quantities so that the plants in the first batch are taller with increasing sludge addition as
shown in Figure 3. Hence, the adverse impact of heavy metals on plant growth is not seen in the first batch
(Figure 4).
Results of all three batches show that the cabbage grown in both control without sludge addition and in soil
with 1:5 sludge addition do not show much difference in metal content. Fig. 3 shows that except in the second
batch that the cabbage plant height for the soil with 1:5 sludge addition is slightly shorter, in the first and third
batches, the plants grown in the soil with 1:5 sludge addition are obviously taller than the control set.
Therefore, addition of adequate quantities of sludge to farm land has definite benefits, and as shown by the
results obtained in this study, the sludge addition of 1:5 (sludge/soil ratio) or less is not expected to cause
problems of high heavy metal concentrations in the crop.ection headings should be left justified, with the first
letter capitalized and numbered consecutively, starting with the Introduction. Sub-section headings should be