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The Chemistry and Fertility of Soils under Tropical Weeds 139

primary minerals, and organics which are mostly in equilibrium among each other.
The existing equilibria between these various forms of heavy metals in the soil
environment are shown in Fig. 8.1. Free ionic form of heavy metals in the soil-
water is the central of heavy metal dynamics due to several reasons: (a) the
availabilities of heavy metal elements to plant root absorption and heavy metal
toxicities are related to this form (Allen et al., 1980; Checkai et al., 1987a; 1987b;
Hernandez-Soriano et al., 2012), (b) the rates of heavy metal movement and
leaching in the soil system are related to this form, (c) all chemical mechanisms
controlling the concentrations of heavy metals (compelexation-decomplexation,
chelation-dechelation, precipitation-dissolution, and adsoption-desorption
processes) in the soil environment are also related to this form (Bowman and
O’Connors, 1982; Sanders, 1982; Salam and Helmke, 1998; Hernandez-Soriano et
al., 2012).
Free ion of heavy metals is in a direct relationship with their absorption by
plant roots. The free ionic form is also in chemical equilibria with the processes of
chelation-dechelation, complexation-decomplexation and precipitation-dissolution,
as well as adsorption-desorption processes. The status of free ionic forms of heavy
metals is also greatly affected by the emission of heavy metals from industrial
wastes, fertilizers, and pesticides. Numerous reports shows that these materials
contribute great amounts of heavy metals in the soil environment (Lagerwerff,
1982; Kardoz et al., 1986; Leung, 1988; Hegstrom and West, 1989; Alloway, 1990b;
Davies, 1990; Kiekens, 1990; Rivai, 1990; Dowdy et al., 1991; Boon and Soltanpour,
1992; Jing and Logan, 1992; Wang et al., 1992; Herrero and Martin, 1993; Sweet et
al., 1993; Cabrera et al., 1994; Nicholson et al., 1994; Tsoumbaris and Tsoukali-
Papadopoulou, 1994; Schuhmacher et al., 1994; Bilski and Alva, 1995; Flegal and
Smith, 1995; Vile et al., 1995; Gimeno-Garcia et al., 1996; Salam et al., 1996; Yeh et
al., 1996; Salam et al., 1997a; Juracek and Ziegler, 2006; Biasioli et al., 2007; Benke
et al., 2008; Berenguer et al., 2008; Lin et al., 2008; Hobara et al., 2009; Benn et al.,
2010; Cakmak et al., 2010; Kien et al., 2010; Wang et al., 2010; and Tu et al., 2012).
Similarly, the status of free ionic of heavy metals in soil water is also affected by
absorption by plant roots and/or leaching by percolating water, which may cause
the decrease in heavy metal free ionic form concentration in the soil water.
Fig. 8.1 clearly shows that the heavy metals in complexes, chelates, adsorbed
sites, and/or precipitates will buffer the concentrations of free ionic heavy metals
based on the equilibration principles (Lindsay, 1979). This suggests that the related
equilibrium constants are of great importance. Whenever the concentration of
free ionic heavy metals in soil water is lowered by some mechanisms such as plant

Abdul Kadir Salam and Nanik Sriyani – 2019