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Enzymes in Tropical Soils 139

9.4 Regulating Heavy Metals Levels in Soils

The relationship between various forms of heavy metals in the soil
environment is depicted in Fig. 9.7. Free ionic forms of heavy metals in the soil
water system are definitely the central of heavy metal dynamics. This is due to
several reasons: (a) the availabilities of heavy metal elements to plant root
absorption is related to this forms (Allen et al., 1980; Checkai et al., 1987a; 1987b,
(b) the rates of heavy metal movement and leaching in the soil system is related to
this forms; (c) all chemical mechanisms controlling the solubilities of heavy metals
(compelexation/decomplexation, precipitation/dissolution, and adsoption/
desorption processes) in the soil water system are related to this forms (Bowman
and O’Connors, 1982; Sanders, 1982; Salam and Helmke, 1998).

Wastes
Organics Plants
Fertilizers
Minerals Pesticides
Adsorbed

Complexes Free Ions Chelates

Leaching
Precipitates

Fig. 9.7. The relationships between forms of heavy metals in the soil environment
(Adapted from Salam, 1997a).

Whenever the concentration of free ionic heavy metals in soil water is
lowered by some mechanisms such as plant root absorption or leaching, the heavy
metals in complexes, chelates, adsorbed sites, or precipitates will be released to
compensate the respective equilibrium constants through equilibrium processes.
On the other hand, if the concentration of free ionic heavy metals in soil water is
concentrated enough due to external addition, parts of the free ionic heavy metals
will be complexed, chelated, adsorbed, or precipitated, depending on the

Abdul Kadir Salam - 2014