Page 18 - Enzymes in Tropical Soils

6 Enzymes In Tropical Soils

Unfortunately, not all forms of nutrient elements in soils can be directly absorbed
by plant roots. Only free ions are directly absorbed by plant roots (Lindsay, 1979;
Stumm and Morgan, 1981). Therefore, the presence of free ions in the soil system
may determine the amounts of nutrient elements available to plants.
The concentrations of free ions in soil water system are in equilibrium with
some other compounds such as complex ions, exchangeable ions, preciptates, and
soil primary minerals. The supplies of nutrient elements from these compounds to
provide free ions available for plant absorption may take place through several
mechanisms such as decomplexation, desorption, dissolution, and weathering. By
these processes, the concentrations of free ions in soil water are maintained at the
levels controlled by their respective equilibrium constants. These mechanisms
along with their several dominant controlling factors are listed in Table 1.1.
Complex ions are soluble, but are not avaliable for direct absorption by plant
roots. To be available by direct plant root absorption, complex ions must release
their main elements to be free ions. This process is called decomplexation and is
0
controlled by an equilibrium constant (K) (Table 1.1). For example, CuCO 3 and
0 2+ 2+
MgCO 3 are complex ions containing Cu and Mg , respectively. Even though
2+
these compounds are soluble in soil water, plant roots can not absorb their Cu
2+
and Mg . To be able to absorb these elements, their complex forms must release
their respective main cations as follow:

0 2+ 2-
CuCO 3  Cu + CO 3 , K ....... Eq. 1.1
2+
2-
0
MgCO 3  Mg + CO 3 , K ....... Eq. 1.2

The decomplexation processes will continue as long as the product of the
concentrations of free ions involved in the respective chemical reactions are below
the value of their respective K.
Conversely, at high concentrations, the free ions will combine with their
related ligands to form complex ions. With the increase in concentrations of the
main free ions and their related ligands, the complex ions may precipitate. The
precipitation process may decrease the availability of the related nutrient
elements. Precipitates are secondary minerals, which are one form of nutrient
element in soils that are moderately available to plants because they have to
dissolve to free the main elements to be available for plant root absorption.
Various complex ions are presence in the soil-water system in equilibrium
with free ions. Complex ions are the most easiest to supply free ions through
equilibrium reaction whenever the concentrations of free ions decreases in
response to absorption by plant roots or some other processes such as leaching

Abdul Kadir Salam - 2014