Page 103 - Ilmu Tanah

90 The Chemistry and Fertility of Soils under Tropical Weeds

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+
2-
R—OSO 3 + H 2 O  R—OH + H + SO 4 ....... Eq. 6.1

Eq. 6.1 clearly shows that the decomposition of organic S also needs the presence
of water molecules. Therefore, moistening the organic S is important to release S
from the organic materials. Unlike the decomposition of organic P, the
decomposition of organic S will acidify the soil system because this reaction
+
produces H ions. The soil containing organic S may become more acidic with time
of decomposition.
Like C, N is originated from atmosphere (Fig. 1.9). The atmosphere actually
contains approximately 78% N 2 which is not available to vegetation. Unlike C,
which is directly absorbed by plants as CO 2 through leaf or stem stomata, N can be
absorbed by plant roots in a particular form after encountering relatively long
processes. Atmospheric N initially enters the bodies of bacteria and particular
vegetation roots in the soil – plant system through a process called N Fixation. The
bacteria involved in the fixation are called Nitrogen Fixing Bacteria (Rendig and
Taylor, 1989). Among these bacteria there is Rhizobium sp. that live symbiotically
with the roots of legumes.
+
-
In the soil environment N is finally transformed to be NH 4 and NO 3 , which are
available for vegetation root absorption and are utilized by plants to form various
structural and functional substances. These structural and functional N in plant
tissues and in microorganisms are finally changed into inorganic N that can be
absorbed by plant roots after encountering decomposition processes. A part of the
inorganic N also volatilizes into the atmosphere, particularly in a reductive soil
environment. All the processes of N transformation from N 2 in the atmosphere to
-
+
NH 4 and NO 3 in soil water, and then their absorption by plant roots and
transformation to be structural and functional substances in plant tissues and
microorganisms, and then their ultimate decomposition back to form inorganic N
+ -
(NH 4 and NO 3 ) that are available to vegetation roots build a cycle. This cycle is
called Nitrogen Cycle or N-Cycle as shown previously in Fig. 1.9.
The organisms that can fix the atmospheric N is the Nitrogen Fixing Bacteria;
so-called due to their ability to fix and incoorporate atmospheric N into their
bodies. A part of these bacteria such as Rhizobium sp. lives symbiotically with
particular vegetation roots. Legumes and peanuts are two of these particular
plants. In this symbiosis, vegetation supplies nutrients and water needed by
bacteria, while bacteria supply N for plants. In addition to these bacteria that live
symbiotically, there are also free nitrogen fixing bacteria in soils living outside of
plant root nodules. Among these bacteria are Azotobacter and Azospillum. These

Abdul Kadir Salam and Nanik Sriyani – 2019