Page 58 - Enzymes in Tropical Soils

46 Enzymes In Tropical Soils

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kinetics of H 2 PO 4 ion from organic P progresses relatively slower compared to that
from soil solids through desorption and/or dissolution processes. The role of
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organic P in availing H 2 PO 4 in soil water is greatly dependent on the soil
biochemical reaction related to the presence of phosphatase produced by soil
microorganisms, earthworms, and plant roots.

1.2 H 3 PO 4 H 2 PO 4 - HPO 4 PO 4
0
2-
3-
Mole Fraction of Total P 0.8
1
0.6
0.4

0.2
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
pH

Fig. 4.7. The distribution of P species in soil solution as a function of pH
(After Lindsay, 1979).

Based on the pH for analysis, phosphatases are of two types: acid and
alkaline phosphatase. Acid phosphatase in general dominates acid soils, while
alkaline phosphatase dominates alkaline soils (Trasar-Cepeda and Gil-Sotres, 1980;
1987; Rojo et al., 1990; Garcia et al., 1993). The activity of phosphatase in general
increases with the increase in soil pH until a maximum value (a pH value in which
the phosphatase activity is maximum) and decreases with the increase in soil pH
above the optimum pH. This enzyme profile is a consequence of reversible
reaction involving ionization and deionization of acidic and basic sites on enzyme
proteins (Frankenerger, Jr. and Johanson, 1982). The optimum pH is at pH below 7
for acid phosphatase and above pH 7 for alkaline phosphatase. Frankenberger and
Johanson (1982) used pH 6.5 to determine acid phosphatase. Pang and Kolenko
(1986) used pH 7 for the determination of acid phosphatase. The pH 11.0 is
commonly used for the determination of alkaline phosphatase (Frankenberger and
Johanson, 1982; Tabatabai, 1982). From his investigation using some soils from

Abdul Kadir Salam - 2014