Page 57 - Enzymes in Tropical Soils
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Enzymes in Tropical Soils 45
However, the presence of water molecule is also determined by the need to
provide O 2 in soils. Oxygen molecule is needed by soil macroorganims,
microorganisms, and plant roots to conduct respiration in the decomposition of
organic matters for energy. The optimum condition for satisfying this requirement
is the soil field capacity where water and gas are both available in soil pores. The
presence of excessive water will also inhibit the process as a result of less O 2 .
The availability of phosphorus in soils is not as simple as those released by
organic matters. The resulted inorganic phosphorus as the primary and secondary
orthophosphate ions in soils are also greatly affected by other forms of inorganic
phosphorus and soil properties like soil pH and specific adsorptive sites. In soil
0 - 2- 3-
water, phosphorus is present as H 3 PO 4 , H 2 PO 4 , HPO 4 , PO 4 , and various forms of
0
their complexes. In pure water, H 3 PO 4 is dominant at pH ranging below its pk 1 or
-
pH 2.15; H 2 PO 4 is dominant at pH ranging from pk 1 (pH 2.15) to pk 2 (pH 7.20);
2- 3-
HPO 4 is dominant at pH ranging from pk 2 (pH 7.20) to pk 3 (pH12.35), and PO 4 is
dominant at pH more than pk 3 (pH 12.35) (Fig. 4.7) where k 1 through k 3 are the
constants for the following dissociation reaction in Eq. 4.2 through Reaction in Eq.
4.4 (Lindsay, 1979). The dominant form of phosphates in soil water is probably
-
H 2 PO 4 due to the fact that soil pH usually ranges from pH 4 to pH 7.
0
-
H 3 PO 4 H 2 PO 4 k 1 = 10 -2.15 ....... Eq. 4.2
2-
-
H 2 PO 4 HPO 4 k 2 = 10 -7.20 ....... Eq. 4.3
2- 3- -12.35
HPO 4 PO 4 k 3 = 10 ....... Eq. 4.4
-
The primary orthophosphate (H 2 PO 4 ) in soil water is in equilibrium with P
on soil solids, i.e. P precipitates and adsorbed P (Lindsay, 1979), meaning that the
-
concentration of H 2 PO 4 is controlled by adsorption/desorption and/or precipitation
-
and/or dissolution processes. Consequently, the concentration of H 2 PO 4 is pH-
dependent because the soil P adsorption capacity and the solubilities of P
-
precipitates are pH-dependent. Therefore, the concentration of H 2 PO 4 is
dynamically controlled by soil solids and the equilibrium is pH dependent (Salam et
al., 1997d).
The orthophosphate released from organic matters may also influence the
-
concentration of H 2 PO 4 -. The released phosphate ion will enter the soil water and
will eventually affects the equilibrium and participate in the adsorption/desorption
and precipitation/dissolution processes controlled by the respective equilbrium
constant (Salam et al., 1997d). As suggested by Salam et al. (1997d), the release
Abdul Kadir Salam - 2014
