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The Chemistry and Fertility of Soils under Tropical Weeds 65

nutrient medium. Frankenberger and Johanson (1982) also show the dependence
of other enzymes i.e. urease and phosphodiesterase activities on soil pH. The
activities of soil enzymes may change accordingly as a result of ionization and
deionization of the functional sites of the enzyme proteins with the increase in soil
pH.
This behavior was clearly demonstrated by phosphatases in tropical soils.
Salam et al. (1998b) treated tropical soil samples of some deforested locations in
West Lampung, Indonesia, at a series of buffered pH of 3 to 12. The results are
shown in Fig. 4.3. Similar patterns were also documented by Trasar-Cepeda et al.
(1991) and Nakas et al. (1987). Ekenler and Tabatabai (2003) suggest that acid
phosphatase was the most sensitive and arylsulfatase the least sensitive to changes
in soil pH.

500
Act. of Phosphatase (mg p-Nitrofenol g-1 h-1) 300 Primary Forest
400

Secondary Forest
200
Coffee Plantation
100

0 Cultivated Land
3 4 5 6 7 8 9 10 11 12
pH

Fig. 4.3. The relationship between phosphatase activity and pH in soils of different land-
uses from West Lampung Indonesia (Adapted from Salam et al., 1998b).

Salam (2014) argues that the lower activity of phosphatase at low pH is
+
probably due to the ionization of H ion on the functional groups of enzyme
proteins. As the soil pH rises, the ionization of enzyme functional groups
progresses and so does the activity of phosphatase. At this range of soil pH, the
activity of phosphatase increases with the increase in soil pH until the related

Abdul Kadir Salam and Nanik Sriyani – 2019