Page 97 - Ilmu Tanah Book

84 The Chemistry and Fertility of Soils under Tropical Weeds

Cepeda and Gol-Sotres, 1988; Rojo et al., 1990; Trasar-Cepeda et al., 1991). Using
two series of soils from The United States of America i.e. Aridic Calcixeroll and
Umbric Dystrochrept, Rojo et al. (1990) varied the pH and measured the
phosphatase activity. These soils show different optimum pH values. Aridic
Calcixerol shows an optimum pH at about pH 10.5 while Umbric Dystrocherept
shows an optimum pH at pH 5.5, similar to those reported by Salam et al. (1998b).
Rojo et al. (1990) conclude that the alkaline phosphatase dominates alkaline soils
like Aridic Calcixeroll whose pH is 8.4 and acid phosphatase dominates acid soils
like Umbric Dystrochrept whose pH is 4.3. Differences in the optimum pH of these
soils and also of Indonesian soils are depicted in Fig. 4.4. A similar pattern was also
reported for urease (Gianfreda et al., 1992). The activity of urease increases with
the increase in soil pH until the pH value reaches 7.0 and then decreases.
In conclusion, the presence of weeds may directly and indirectly influence
the activities of soil enzymes. Weeds may directly excrete various enzymes into the
soil environment and thereby, as pointed out by soil workers, directly increase the
activities of enzymes in soil. Weeds may also change the rhizosphere chemical and
physical environment like soil pH and thereby stimulate the main enzyme
producers to develop their populations and activities including producing soil
enzymes. The changes in the soil chemical environment may also affect the
activities of soil enzymes like phosphatases and ureases.

Key Questions

1. What is root excretion? What compounds are excreted by roots? What are the
purposes of root excretion?
2. How does the plant root excretion lower the soil pH? Explain!
3. Explain the relationship between the root excretion and the enhancement of nutrient
release from soil minerals and soil organic matters!
4. What are the differences between weeds and other vegetation in affecting the soil
chemical properties? Explain Why!
5. How do plant roots enhance the soil mineral weathering and organic matter
decomposition?
6. Explain how the evolved CO 2 around plant roots acidify the soil environment and
increase the nutrient element release from soil solids!
7. Explain the various organic acids in the soil environment! Explain their effects on soil
pH and the resulted enhancement of nutrient element release from soil solids!
8. Why does the long-term culture of soils with cassava lower soil pH? Explain!

Abdul Kadir Salam and Nanik Sriyani – 2019