Soil pH is probably the most widely recognized factor affecting heavy metal availability. With the exception of molybdenum, arsenic, and selenium, heavy metal retention increases with a pH increase and thus availability decreases. This is recorded in experiments measuring plant uptake or quantifying metal adsorption onto solid phases and metal extractability. In the first type of experiment, pH values of acidic soils increase with the effect of lime-stabilized sludge and are found to have depressed metal uptake by plants (e.g., Basta and Sloan ), or lime is applied to natural and metal-contaminated soils to improve soil pH and similar uptake effects are found (e.g., Oliver et al. ). In the second type of experiment, several solid phases, pure (oxides, clays, etc.) or natural soils, are used to evaluate the adsorption strength of heavy metals in a variety of pH values (e.g., Salam and Helmke ; Elliott et al. ; Yong and Phadungchewit ; Gray et al. ) (see also Figure 3.2).
These trends do not affect different heavy metals in a similar way. It is found that "mobile" elements such as Ni, Cd, and Zn are much more sensitive in changing their availability status with pH than other elements, such as Pb and Cu, which are more strongly retained by humus . However, some data suggest that the positive pH effect on metal availability is not permanent, especially in high buffer capacity soils, because soils tend to return to initial pH over time. Ca2+ ions introduced to soil through liming may even compete with other metals for the limited adsorption sites, causing adverse effects . Also, at certain periods of time, metal availability may have a dramatic peak, due to a temporary pH depression, even if pH values before and after that period are well controlled. Such periods may occur during early summer when the intense increase in soil microbial biomass may cause a rapid sludge-borne organic matter decomposition, which, in turn, tends to reduce soil pH due to the release of weak organic acids.
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