Page 21 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 9

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SO 4 concentrations in soil water. The rates of dissolution of soil solids is greatly
dependent on the kind of minerals, K sp , and the concentration of all species forming
the minerals (Bohn et al., 1985). It is believed that complexation and chelating
processes by organic matters can also precipitate heavy metals if the molecular
weights of the organic matters are relatively high (Keeney and Wildung, 1986;
Kabata-Pendias and Pendias, 1992).
High supplying rates of free ions can also be performed by desorption
processes of exchangeable elements. Desorption is also a response to lowering
concentrations of particular free ions in soil water. The process may take place due
to absorption of free ions by plant roots or leaching by percolating water. The
rates of desorption processes are determined by K values and preferences of soil
colloids towards free ions (Lyotropic Series). The lower the K value the easier the
adsorbed elements to be freed, meaning that to release the exchangeable
elements we have to decrease the concentrations of free ions. Because the plant
absorption will usually decrease the concentrations of free ions, the release of
exchangeable ions in the soil system will be easier.
The release of exchangeable elements is in general also affected by the
preferences of the soil colloids towards particular free ions. Therefore, the ionic
charges, radius, and concentration are very important in the release of
exchangeable elements to form free ions. Because the negative charges of soils
also control preferences, all factors influencing the soil negative charges are also
important. Among these factors are CEC, which indicates the amounts of soil
colloid negative charges, and soil pH, that influence the soil CEC. The soil CEC in
general increases with the increase in soil pH, particularly in soils with high
amounts of 1:1 clay minerals, sesquioxides, and organic matters (Salam, 2012).
Some other processes supply free ions relatively slowly. These include the
weathering of soil minerals and the decomposition of organic matters. Weathering
of soil minerals is controlled by several factors, among which: mineral structural
and chemical composition, soil moisture content, and soil pH. As mentioned
previously, minerals with structural weaknesses and particular chemical
compositions such as easily oxydizable elements like Fe and Mn, will weather and
release free ions more easily. The weathering of soil minerals needs also the
+
presence of water and H ion, which performs as an attacking agent in the
+
weathering process. The concentration of H ions is indicated by soil pH.
Nutrient elements in the structure of minerals is the most slowly available to
supply free ions. The elements can be absorbed by plant roots after encountering
weathering process and releasing nutrient elements in the form of free ions. The
rates of soil mineral weathering are controlled by various factors. Some soil

Abdul Kadir Salam - 2014