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Enzymes in Tropical Soils 141

Salam et al., 1998a; 1998e; 1998h; 1998h; Salam et al., 1999c; 1999f; 1999g;
1999h; 1999i; Tokunaga et al., 2003; Brown et al., 2004; Salam et al., 2005;
Stehouwer et al., 2006; Schroder et al., 2008; Brown et al., 2009; Kukier et al.,
2010; Salam and Ginanjar, 2014). Addition of lime may enhance the soil pH and,
thereby, may also increase the negative charges on soil colloids (Fig. 9.8) or the soil
capacity to adsorb cations or cation exchange capacity (CEC) (Fig. 9.9) and decrease
the solubility of heavy metals (Helling et al., 1964; Udo et al., 1970; Singh and
Sekhon, 1977; Trehan and Sekhon, 1977; Cavallaro and McBride, 1980; Kuo and
Baker, 1980; Tyler and McBride, 1982; Sumner et al., 1991; Berthelson et al., 1994;
He and Singh, 1994; Salam et al., 1995b; Tack et al., 1996; Salam and Helmke, 1998;
Salam et al., 1998g; Salam et al., 1999c; Choi et al., 1999; Bang and Hesterberg,
2004; Quaghebeur et al., 2005; Gagnon et al., 2013). Adams et al. (2004) reported
that the wheat and barley Cd accumulation was dependent on the soil total Cd and
pH and could be predicted by the model in Eq. 9.1.

Log (Grain Cd) = a + b log (Soil Cd) – c (Soil pH) ....... Eq. 9.1

where a, b, and c are contrants.

POINT OF ZERO CHARGE/PZC Soil A Soil B
CEC = O, Charges (+) = Charges (-)
Neg. Charges (CEC)

pH
Pos. Charges (AEC)

Extremely Low pH Increases Colloid
Positive Charges

Fig. 9.8. Changes in soil colloid charges as a function of soil pH
(Salam, 2012)

Abdul Kadir Salam - 2014