Page 79 - Ilmu Tanah

66 The Chemistry and Fertility of Soils under Tropical Weeds

optimum pH, in which the activity of phosphatase is highest, the activity declines to
an asymptotic value at higher pH. The cause of this phenomenon is unclear. The
negative effect of high pH on the populations and activities of particular enzyme
producers is probable.
The increasing pattern of phosphatase activities at low pH and decreasing
pattern at higher pH above the optimum pH were also previously reported by
several workers (Frankenberger and Johanson, 1982; Malcolm, 1983; Trasar-
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 showed different optimum pH values. Aridic
Calcixerol showed an optimum pH at about pH 10.5 while Umbric Dystrocherept
showed 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 dominated
alkaline soils like Aridic Calcixeroll whose pH was 8.4 and acid phosphatase
dominated acid soils like Umbric Dystrochrept whose pH was 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 increased with the increase in soil pH until the pH value reached 7.0 and
then decreased.

120
Phosphatase Activity (in %) 80 Acid Soil
100

(Indonesia)
60
Acid Soil (US)
40
20
0 Alkalie Soil (US)
3 4 5 6 7 8 9 10 11 12
Soil pH

Fig. 4.4. The differences in the optimum pH between acid and alkaline soils
(Redrawn from Rojo et al., 1990 and Salam et al., 1998b).

Abdul Kadir Salam and Nanik Sriyani – 2019