Relative Accumulation of Heavy Metals by Different Crops
High plant uptake
Low plant uptake
Sources: From Alloway, B.J., in Heavy Metals in Soils, Allo-way, B.J., Ed., Blackie Academic and Professional, London, 1995, 38-57; and Kloke, A., Saurbeck, D.R., and Vetter, H., in Changing Metal Cycles and Human Health, Nriagu, J., Ed., Springer-Verlag, Berlin, 1994.
• Soils with low sorptive capacities (low organic matter and clay contents, acidic pH conditions)
• High rainfall or irrigation water rates
Many research efforts have been put into performing laboratory experiments with soil columns or monitoring metal behavior in soil profiles in areas with history in sludge application. As Camobreco et al.  pointed out, measuring metal movement in repacked soil columns is different from measuring it in the field under "real" conditions. The reason is that metal movement more often occurs through soil macropores or cracks. This leads to the preferential flow of the water and, subsequently, to the metals that it carries; this fragile soil structure is destroyed when soil is sampled and taken to the laboratory to be set up for a column experiment . Thus, metal mobility in the profile is very often severely underestimated, and results seem very promising when conclusions rely only on laboratory trials. However, column experiments are useful because they provide a picture based on carefully controlled parameters.
Although it is often assumed that downward movement of heavy metals is not a significant process, there is a disagreement in the literature on that matter. For example, Davis et al.  applied 40 t ha-1 sludge and found heavy metal movement only down to 10 cm depth, after a residual time of 3 years. El-Hassanin et al.  found that Cd, Zn, and Pb only moved in the surface soil layer; similarly, Higgins  found that no heavy metal movement to the B horizon was evident. Chang et al.  reported that, 4 years after the termination of sewage sludge application, heavy metals were deposited only in the 0- to 15-cm upper soil layer, and Harmsen  found that Cd and Pb did not accumulate below 40 cm of the surface in a heavily contaminated Zn smelter area. Dowdy et al.  also reported that heavy metals did not move significantly below the incorporation zone (or plow layer). The same was concluded by Williams et al. , Emmerich et al. , and Miner et al. .
In contrast to this, data have also been published suggesting that metals can move below the incorporation zone, under certain conditions; such works many times concern long periods of sludge application. Works of this kind are very important and should be taken seriously under consideration before sludge utilization to agricultural soils is to be generalized. Antoniadis and Alloway  studied an area in central England where sewage sludge had been applied in a farm for decades at normal rates. They found that Ni was significantly higher than the control even at a depth of 80 cm; Zn had been moved to 60 cm and Cd to 40 cm. Lead was the only metal found higher than
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