Page 21 - Ilmu Tanah Book

8 The Chemistry and Fertility of Soils under Tropical Weeds

1.2 H 3 PO 4 H 2 PO 4 HPO 4 PO 4
-
0
2-
3-
Mole Fraction of Total P 0.8
1
0.6

0.4
0.2
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
pH

Fig. 1.5. The dependence of P species concentration on soil pH
(After Lindsay, 1979).

The soil pH also greatly influences the precipitation/dissolution reaction as
shown by the precipitation/dissolution of Fe species where Fe precipitation
-
intensifies with the increase in soil pH due to the increase in OH ion
concentrations. The precipitation of heavy metal cations may follow Eq. 1.1.
(Salam, 2017).

2+
Log (M ) = X – 2 pH ……. Eq. 1.1

-14
where X = log K sp – 2 log (10 ) which is specific for a particular heavy metal
cations. This equation show that pH is determining in precipitation/dissolution
processes. Scientific reports (Lindsay, 1979; Workman and Lindsay, 1990; Ma and
Lindsay, 1990; El-Falaky et al., 1991) show this relationship for several heavy metal
cations as shown in Table 1.2. The precipitation occurs at low pH when the
concentration of heavy metal is high and conversely it occurs at high pH when the
concentration of heavy metal is low (Fig. 1.6).
The dependence of precipitation/dissolution reaction on pH is shown by the
weathering of some soil minerals such as the dissolution of K-feldspars as shown by
Eq. 1.2. It is clear that the release of K from K-feldspars is dependent on soil pH.
The amount releases may be greater at low pH. Salam (1989) reports the pH
dependence of soil mineral dissolution from an Ultisol from West Java Indonesia

Abdul Kadir Salam and Nanik Sriyani – 2019