Page 109 - Ilmu Tanah

96 The Chemistry and Fertility of Soils under Tropical Weeds

Because the resulting energy will be much lower when the soil microorganisms
use electron acceptors other than O 2 , the use of organic matter as a source of
energy for the living things in soils must be acompanied by implementing a good
mechanism in availing O 2 into the soil system. One of the most important
mechanisms to do this is by increasing the soil porosity.
Soil porosity indicates the part of soil volume not occupied by soil solids and
may be expressed indirectly by the soil bulk density calculated by the following
equation (Hillel, 1980; Singer and Munns, 1987) (Eq. 6.7).

= ....... Eq. 6.7

Table 6.3. Primary and secondary electron acceptors in
the soil system.

PRIMARY ELECTRON ACCEPTOR:

+
-
1 O 2 + 4e + 4 H  2 H 2 O
(the only electron acceptor that can be used by plant roots;
energy produced is highest)
SECONDARY ELECTRON ACCEPTORS:
If O 2 is absence in soils due to diffusion limitation; energy produced will
- - 2-
be lower; products will be toxic like NH 3 and NO 2 > NO 3 , H 2 S > SO 4 , N 2
and N 2 O volatilized)
- +
1 FeOOH + e + 3 H  2 H 2 O
+
2+
-
2 2 MnO 1.75 + 3e + 7 H  2 Mn + 3.5 H 2 O
+
-
2-
2-
3 SO 4 + 8e + 8H  S + 4 H 2 O
-
-
+
4 NO 3 + 5e + 6 H  ½ N 2 + 3 H 2 O
- +
5 N 2 O + 2e + 2 H  N 2 + H 2 O
+
-
6 H + e  ½ H 2
7 No Acceptors: organic molecules will be rearranged with a lower
energy. For example, carbohydrates will be converted to C 2 H 5 OH
or CH 3 or CO 2
After Bohn et al. (1985)

Abdul Kadir Salam and Nanik Sriyani – 2019