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The Chemistry and Fertility of Soils under Tropical Weeds 89

and earthworms. Several workers suggest that soil microorganisms produce
enzyme more significantly in the soil environment (Duxbury and Tate III, 1981; Ross
and Cairns, 1982; Frankenberger, Jr. and Dick, 1983; Jha et al., 1992; Joner and
Jakobsen, 1995; Vinotha et al., 2000). Some of the soil enzymes also involve
directly in the release of important nutrient elements such as N, P, and S. Some of
these soil enzymes are phosphatases, proteases, and arylsulfatase. These enzymes
work on soil organic matter decomposition to release ions of N, P, and S,
respectively, that are available to root absorption.
Carbon is released in the form of CO 2 as organic matters containing N, P, or S
decays accelerated by the presence of their respective enzyme, i.e. proteases,
phosphatases, and arylsulfatase. Carbondioxide molecules produced in the
reaction are finally consumed by plants, and along with water molecules absorbed
from soils, the CO 2 molecules are transformed into new organic matters. This
means that all the above enzymes participate in the C-cycle.
Phosphatases involve in the destruction of organic matters containing P in
their structures called Organic P. The released P in the soil environment will then
eventually re-absorbed by plant roots and re-utilized to produce new organic P.
The chain of transformation from Organic P to inorganic P by decomposition
process and absorption of inorganic P through its re-synthesis in plants to produce
new organic matters and back to the decomposition of organic matters to produce
inorganic P is called Phosphorus Cycle or P-Cycle. The P-Cycle is previously shown
in Fig. 1.7. The enzymatic reaction of organic P decomposition catalyzed by
phosphatases is previously depicted in Eq. 1.4.
The cycle of sulfur or S-Cycle, P-Cycle, and N-Cycle in the soil – plant system is
-
2-
2-
just similar to the C-Cycles. Sulfur, P, and N in the form of SO 4 , HPO 4 , and NO 3 in
soil water are absorbed by vegetation roots and are transported to particular plant
parts to form proteins. Since S is needed to form wax in plants, S is reduced to
form – S – S – bonds or – S – H functional group of proteins. Protein-S will then
enter the food chain through consumptions by animals and humans. The residues
of plants, animals, and humans will enter the soil system and will encounter
decomposition process to form inorganic form of S available to plant root
2-
absorption such as SO 4 . The S-Cycle is previously depicted in Fig. 1.8.
2-
The destruction of organic S not available to plants to inorganic SO 4 readily
available to plant root absorption involves enzyme arylsulfatase produced by soil
microorganisms. The enzymatic reaction in the decomposition of organic S is
shown in Eq. 6.2 as follows (Tabatabai, 1982).

Abdul Kadir Salam and Nanik Sriyani – 2019