Page 58 - Enzymes in Tropical Soils
P. 58

46     Enzymes In Tropical Soils


                                        -
                           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
                                                   -
                           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
   53   54   55   56   57   58   59   60   61   62   63