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The Chemistry and Fertility of Soils under Tropical Weeds 91

bacteria may fix atmospheric N. This N is eventually available to plants after the
bacteria are dead and decayed.
The protein-N is not available to plants and, therefore, plants cannot utilize it
before this protein-N is converted into inorganic forms that are available to plants.
Various soil microorganisms decompose and detach the protein-N in the form of
NH 3 , which will encounter an ammonification process upon reaction with water to
+
+
form NH 4 . Ammonium is easily absorbed by plant roots. However, NH 4 usually
cannot suffice the plant need of N except for paddy plants.
Ammonium ions are then oxydized by particular bacteria through a two step
+
-
reactions called nitrification. This reactions change of NH 4 to NO 2 with
- -
Nitrosomonas as the oxydizing agent and of NO 2 to NO 3 with Nitrobacter as the
oxydizing agent (Bartlett, 1981). These reactions follow Eq. 6.2 and Eq. 6.3.

+ - +
2 NH 4 + 3 O 2  2 NO 2 + 4 H + 2 H 2 O ....... Eq. 6.2

-
-
2 NO 2 + O 2  2 NO 3 ....... Eq. 6.3

The above reactions can be summarized as shown by Eq. 6.4 as follows.

+ - +
2 NH 4 + 2 O 2  NO 3 + 2 H + H 2 O ....... Eq. 6.4
This reaction shows that the nitrification process needs the presence of enough O 2 .
The nitrification does not occur in waterlogged soils because the absence of O 2 .
This reaction also suggests that the soil porosity controls the nitrification process.
Eq.6.4 also shows that the nitrification acidify soils as indicated by the production
+
of H ions.
Wu et al. (2011) report that the Ammonia Oxidizing Bacteria sharply increased
about 50 times in soils fertilized with long-term N. This indicates that nitrification
process occurs faster in the presence of N fertilizers. For example, the presence of
+
Urea increases NH 3 which very quickly becomes NH 4 upon hydrolysis. The
decomposition of urea that produces NH 3 progresses as shown in Eq. 6.5 as follows
(Tabatabai,1982).

NH 2 CONH 2 + H 2 O  CO 2 + 2 NH 3 ....... Eq. 6.5

+
The production of NH 3 and, hence NH 4 , is higher in the presence of enough water
and is accelerated by the presence of enzyme Urease, the activity of which was
reported to increase with the increase in applied Urea-N (Kumar and Wagenet,
Abdul Kadir Salam and Nanik Sriyani – 2019