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The Chemistry and Fertility of Soils under Tropical Weeds 75

acids, CO 2 , and enzymes. By these reasons, weeds may release more structural
mineral and organic matter nutrient elements such as those reported by Salam
+
(1989; 2019). The production of H by weeds with higher root-to-shoot ratios is
reported higher than those with lower root-to-shoot ratios (Salam et al., 2019).

5.1 Effects on Soil pH

Soil pH is one of the two most important environmental variables along with
the soil E, which is a measure of the electron concentration in the soil environment.
+
As a measure of H concentration, soil pH may increase and decrease dependent of
+
the dynamic of H ions. The shifting of soil pH is caused by some natural or
anthropogenic acidification or alkalization processes. Among the natural soil
acidification process is the formation of carbonic acid as a result of the reaction
between water and carbondioxide molecules produced in the respiration of
hydrocarbon in plant roots, soil macroorganisms, and soil microorganisms. The
reaction is shown chemically in Eq. 5.1 as follows:

+ -
CO 2 + H 2 O  H 2 CO 3  H + HCO 3 ....... Eq. 5.1.

The resulting carbonic acid is not stable in the soil environment; therefore, it easily
+ +
dissociates to produce H ions. The increase in H concentration in the soil water
will acidify soils and decrease the soil pH. The more the CO 2 produced during the
+
respiration, the more H is produced in the soil environment, and so the higher the
decrease in soil pH.
The above acidification is very common in the rhizosphere soils. Yang et al.
(1996) report that the soil pH in the soybean rhizosphere of Alfisols was lower than
that in the bulk soil in the extent of 0.07 – 0.65 units. The acidification of the soil
+
rhizosphere is also intensified by the excretion of H ions by plant roots during
absorption of nutrient cations to preserve the electrical balance in the soil system
(Tisdale et al., 1985). Long-term soil culture with cassava (Mannihot utilisima) was
also reported to lower the soil pH (Salam et al., 1999b).
Acidification of the rhizosphere is dependent on the types of vegetations.
Salam et al. (1997e) observed in Oxisol Gedongmeneng that the root zones of
Alang-alang (Imperata cylindrica L.) showed a higher pH value than those of
pigweed (Amaranthus spinosus L.), Green Kyllinga (Cyperus kyllingia L.), and
Abdul Kadir Salam and Nanik Sriyani – 2019