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The Chemistry and Fertility of Soils under Tropical Weeds 87

and organic matter (organic matter is about 1.7 times of organic C), as well as
organic N, organic P, and organic S.
Organic C, N, P, and S are released into the soil environment by two
mechanisms, through direct and indirect mechanisms. First, organic C is
continuously released through weed rooting system as organic acids. In addition to
acidify the ambient soils, the release of organic acids may surely increase the soil
organic C. Second, organic C, N, P, and S can be enhanced indirectly through the
decomposition of dead weed parts like dead roots and shoots. The hydrocarbon
composed by weeds through photosynthesis may encounter a reverse reaction so-
called decomposition that may produce more stable organic matters containing C
-
and CO 2 that may react with water forming HCO 3 and also inorganic N, P, and S. All
- 2- 2-
organic matters may finally decompose producing CO 2 , NO 3 , HPO 4 , and SO 4 and
complete the nutrient-cycles. Competitive and progressive weeds may contribute
significantly to the increase in soil organic C and organic matter and probably to the
soil nutrients..

6.1 Nutrient Cycles

The influence of weeds on soil C, N, P, and S is closely related to the cycles of
these nutrient elements in the soil environment (Fig. 6.1, Fig. 1.7, Fig. 1.8, and Fig.
1.9). The so-called C-cycle is the transformation of organic matters to their more
simple constituents that are available for plant root absorption continued by the
absorption of these constituents by plant roots and the following formation of
organic substances through photosynthesis in the plant-leaf chlorophylls (Fig. 6.1).
The destruction of organic matters into their constituents that are available to
plant involved microorganisms called decomposers. Decomposers are of great
importance in the C-Cycle that also involve Producers (plants with chlorophylls),
Consumers (animals and humans), and Decomposers (microorganisms). Without
Decomposers, organic matters contributed by vegetation like weeds may
accumulate in the environment causing various problems because the cycle is not
completed.
Soil microorganisms are small in size and are present in soils in great numbers.
The general rule is that the smaller the sizes of the microorganisms the greater
their numbers (Singer and Munns, 1987). The smaller size also causes the more
significant role in the environment. One gram of soil may contain around one
Abdul Kadir Salam and Nanik Sriyani – 2019