Page 19 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 7

and adsorption by soil particle surfaces. This releasing process (Decomplexation) is
determined by an equilibrium constant (K) for complexation. The higher the K
value the more difficult for the main cation to be released. Some common free
ions and their complex ions in the soil water system are listed in Table 1.2.

Table 1.1. Several mechanisms releasing free ions along with their
controlling factors (Salam, 2012).

No. Mechanisms Sources Dominant Controlling
Factors
1 Decomplexation Complex Ions K*
2 Dissolution Precipitates Ksp*
3 Desorption Exchangeable Ions K*, CEC, pH, Preference,
4 Weathering Soil Mineral pH, Soil Water, and Mineral
Structures Structure, Composition, and
Chemistry
5 Decomposition Soil Organic pH, Soil Water, Enzymatic
Structures Activities, Temperature, C/N
Ratios, Microbial Populations
and Activities

*K is an equilibrium constant related to a particular chemical reaction; K sp is an
equilibrium constant related to dissolution of a precipitate.

The major cations and the minor cations show different behaviors towards
complexation processes. The differences are subjected to the affinities of elements
towards ligands. The concentrations of free ions and their related complex ions of
some elements are listed in Table 1.3. It is obvious that the free ions of the major
2+
+
+
2+
cations Ca , Mg , Na , and K are about 70 – 90% of their respective total soluble
2+
concentrations in soil solution. The concentration of free Zn is about 40% of the
total soluble Zn. Copper shows a very high affinity towards ligand; only 2% of the
2+
total soluble Cu is in the form of free Cu (Ellis and Knezek, 1982). This
2+ 2+
phenomenon indicates that Cu is more strongly complexed compared to Zn and
also compared to the soil major cations.
In addition to those in the forms of complex ions, part of nutrient elements
are present in the forms of precipitates. The dissolution of precipitates is
controlled by their equilibrium constants (K sp ) and solubility products, which are
the product of all product species concentrations. For example:

Abdul Kadir Salam - 2014