Page 124 - Enzymes in Tropical Soils

112 Enzymes In Tropical Soils

increases with the increase in soil pH until an optimum pH is reached and then
decreases with the increase in soil pH (Frankenberger and Johanson, 1982; Trasar-
Cepeda and Carballas, 1991; Sakai and Tadano, 1993; Huang et al., 1995; Salam et
al., 1998d)
The value of pH for the determination of phosphatases is usually the
optimum pH. Optimum pH for phosphatase is a pH in which the activity of
phosphatase is maximum. The optimum pH is different between soils, dependent
on which phosphatase is dominant in a particular soil. Acid phosphatase is
dominant in acid soils while alkaline phosphatase is dominant in alkaline soils.
Salam et al. (1998d) showed that the soils from West Lampung Sumatra Indonesia
are dominated by acid phosphatase (Fig. 7.2).
The optimum pH for acid phosphatase is various, dependent on the
locations and types of land-uses, ranging from 5.0 to 7.0. Therefore, the use of pH
6.5 for the determination of acid phosphatases in these soils may result in the
activity of phosphatase either lower or higher than those measured at their exact
optimum values. For example, if pH 6.5 is used for the determination of
phosphatase activity in soil from the primary forest of Bukit Ringgis (Fig. 7.2), the
measured activity of phosphatase will be definitely lower than it should be if it is
measured at its optimum pH of 5.0. The activity of phosphatase at pH 6.5 is about
-1 -1
400 and at pH 5.0 (Optimum pH) is about 500 mg p-Nitrophenol g h .
As mentioned previously, phosphatases are divided into two groups, acid
phosphatase and alkaline phosphatase. The difference is based on the
measurement technique related to buffered pH, of which for acid soils is in the
range of 4 – 6 (Juma and Tabatabai, 1978). Activity of acid phosphatase is
determined at pH < 7 (Trasar-Cepeda and Gil-Sotres, 1987; 1988; Garcia et al.,
1993), i.e. with pH 6.5 (Frankenberger and Johanson, 1982) and pH 7.0 (Pang and
Kolenko, 1986). The activity of alkaline phosphatase is measured at pH above
neutral or > 7 (Trasar-Cepeda and Gil-Sotres, 1987; 1988; Garcia et al., 1993) and in
general at pH 11.0 (Frankenberger and Johanson, 1982; Tabatabai, 1982).

7.3 The Shifting of Optimum pH

Optimum pH is not static but changes with time and land-use conversion.
Salam et al. (1998d) reported that the optimum pH for the maximum activity of
phosphatase in general shifted to higher values by forest clearing and conversion to

Abdul Kadir Salam - 2014