Page 115 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 103

The shifting of soil pH may be caused by some natural or anthropogenic
acidification or alkalination processes, that decreases or increases the soil pH.
Among the natural soil acidification process is the formation of carbonic acid as a
result of the reaction between water and carbondioxyde molecules produced in
the respiration of hydrocarbon in plant roots, soil macroorganisms, and soil
microorganisms. The reaction is shown chemically in Eq. 7.1 as follows:

-
+
CO 2 + H 2 O  H 2 CO 3  H + HCO 3 ....... Eq. 7.1.

The resulting carbonic acid is not stable in the soil system, therefore, it easily
+ +
dissociates to produce H ions. The increase in H concentration 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 system, and so the higher the decrease in soil pH.
The above acidification is very common in the rhizosphere soils. Yang et al.
(1996) reported 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., 1999e).
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
Amaranth (Amaranthus tricolor L.). The pH in the root zones of these vegetations
were 6.25, 5.03, 5.01, and 5.64, respectively. The significant difference in the pH
measurement (0.64 – 1.24 units) indicates that Alang-alang posseses an
extraordinary adaptability. Its adaptability is also observed by the higher activities
of phosphatases in Alang-alang root zones than those in the other vegetations
(Table 7.1).
One example of anthropogenic acidification is related to the use of Urea in
agricultural fields. Urea is an important commercial source of N commonly used,
particularly in Indonesia. Since it contains one of the major plant nutriens, the use
of Urea is huge, and in greater amount than those of other major nutrients such as
P and K. However, the use of Urea may cause acidification as seen chemically in
Eq. 7.2 as follows:

NH 2 CONH 2 + H 2 O  CO 2 + 2 NH 3 ....... Eq. 7.2.

Abdul Kadir Salam - 2014