a Alloway, B.J., in Heavy Metals in Soils, Alloway, B.J. (Ed.), Blackie and Sons, Glasgow, U.K. 1990; based largely on Bowen, H.J.M., The Environmental Chemistry of the Elements, Academic Press, London, 1979. With permission.

Usually, lime application reduces uptake of Zn and Ni more than Cd [83-85]; no decrease in availability is noticed in the case of Mo and Se [85]. Metal uptake in response to liming may also vary among plant species [86,87]. Liming decreased more Cd uptake by lettuce and carrot than by potatoes and peanuts [74]. Some evidence indicates that pH of about 6 is high enough to regulate metal uptake [88]. Increase of pH to 6.5, recommended for controlling trace elements in the food chain, does not seem to be necessary [89]. This finding is of practical importance because liming of acidic soils to pH 6.5 is often costly and can require a considerable amount of time.

The effect of other soil properties on element uptake by plants is less evident than that of pH, and the results are often conflicting. Hinesly et al. [91] conducted a study to determine the effect of CEC on Cd uptake by corn. The soil CEC inversely affected Cd uptake by corn when the metal was applied as a soluble salt, but not when Cd was supplied as a constituent of sewage sludge. These experiments were further confirmed in greenhouse studies conducted by Korcak and Fanning [92].

Some trace elements exhibit affinity for soil organic matter (OM), which has the CEC property and chelating ability. Therefore, addition of sewage sludge, peat, or plant residues can bind trace elements in soil. On the other hand, because of chelating ability, OM is viewed as a source of soluble complexing agents for trace elements. The binding ability of organic matter is not permanent; it is generally agreed that the OM level in soil must eventually return to a value not much greater than that of the original soil [38].

Trends in metal availability as a function of metal content in soils can be described by three models: (1) linear (constant partitioning model); (2) plateau (saturation model); and (3) the Lang-muir sorption model (Figure 31.3) [10]. Usually, uptake of trace elements by plant tops does not occur in linear response to concentrations of the metal in soils, except at a low range of concentrations [10,38,92]. Uptake of trace elements by plants becomes less efficient at higher metal loading in soil, and the plateau relationship is used to describe this saturation effect [93]. Soils have a finite capacity to immobilize trace elements by adsorption or precipitation. When this protective potential is exceeded, a Langmuir type of relationship is expected (Figure 31.3). This relationship is found sometimes for trace elements added to soil in soluble salt forms [94]. Under more realistic field conditions, when trace elements are introduced to soil with sewage sludges or with industrial wastes,

FIGURE 31.3 Metal availability as a function of metal content of soils. (From Dudka, S. and Miller, W.P., J. Environ. Sci. Health B, 34(4): 681-708, 1999.

the plateau model prevails because it best describes the plant response to increased metal concentrations in the soil [10].

The simple soil-plant relationship of plant element uptake is often modified by environmental, plant, and soil factors [95]. As a result, only a small proportion of elemental variability in plants can be explained by element concentrations in soils. Plants can accumulate trace elements, in their tissues due to their ability to adapt to various environmental conditions [96-98]. Plant uptake of elements from the soil solution requires positional availability to the plant root. The element must be moved to the root through diffusion or mass flow or the root must grow to the element [99]. This transfer requires that the element move through a solution phase. Therefore, water solubility and a variety of complexation, chelation, and other chemical reactions controlled by pH become important in regulating element availability [100].

Except for a few special cases, plant tissue concentrations do not positively correlate with total element content of untreated soils [101]. The element concentrations in plant tissues vary greatly, depending on species and cultivate, plant organ, age, and environmental conditions [102-107]. The extent of increase in trace element concentration above control for crops grown on metal-contaminated soils is strongly affected by crop species [108]. Studies conducted in England report [109] the response of many crop species grown in the same experiment on two soils contaminated with long-term sludge application. The relative crop uptake removes factors other than crop species (or cultivate). Leafy vegetables and, interestingly, wheat grains had the highest relative increased uptake of Cd [110,111].

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