Agriculture Reference
In-Depth Information
through and across it. Depending on the alkalinity of the water entering and
leaving, the soil may be enriched with bases or depleted. Large quantities of
bases are liberated in soil reduction following flooding and may be leached.
The balance will depend on the topological and hydrological situation of the
soil, and in general soils low in the landscape will accumulate bases and those
higher will be depleted. There may be biological fixation of bases, for example
by aquatic snails in fields receiving base-rich interflow water or irrigation. For
example, after 15 years of intensive irrigation of ricefields at the International Rice
Research Institute (Laguna, Philippines) with base-rich water (4mmol
c
L
−
1
Na
+
and 1mmol
c
L
−
1
Mg
2
+
), there was a marked accumulation of CaCO
3
in snail
shells and the aerobic soil pH increased from 5.6-6.0 to 6.5-7.0 (Moormann and
van Breemen, 1978). The reverse process—decalcification—may also occur. For
example, in the calcareous soils of the Ganges and Megha sediments, Bangladesh,
where the ricefields are rainfed and the rainwater tends to be acid, Brammer
(1971) reported losses of 1% of the CaCO
3
in 25 years from sediments that
originally contained 5-10% CaCO
3
. The calcite is dissolved by acids in the rain
and CO
2
formed during soil reduction, and Ca
(
HCO
3
)
2
is leached out of the soil.
The second factor is the changing redox state of the soil. Generally iron is
the most abundant redox species present. Table 3.11 shows total iron contents
of a range of rice soils across Asia. From 20 to 80% of the iron is present
as free Fe(III) oxides and often from 1 to 20%—and sometimes as much as
90%—of the free Fe(III) is reduced to soluble and exchangeable Fe(II) fol-
lowing submergence (see for example Figure 2.8). Some of the exchangeable
Fe(II) is subsequently reprecipitated as mixed Fe(II)Fe(III) compounds of uncer-
tain composition. There may also be reduction of structural Fe(III) to Fe(II)
within clay minerals. These changes take place over a matter of weeks. Upon
subsequent drying and re-oxidation, the exchangeable and amorphous Fe(II) are
rapidly converted to ferric hydroxides, initially in amorphous forms that recrys-
tallize only very slowly (Figure 3.8). As a result, amorphous ferric hydroxides
and similar materials tend to accumulate in the soil at the expense of more stable
Table 3.11
Total iron contents (mg Fe g
−
1
) of rice soils
Country
Mean
Min.
Max.
n
Bangladesh
40.1
8.0
66.1
53
Burma
39.7
5.9
65.5
16
Cambodia
32.2
0.0
80.1
16
India
72.3
9.0
117.6
73
Indonesia
20.8
1.7
50.3
44
Malaysia
20.8
1.9
50.3
41
Philippines
54.1
28.8
86.7
54
Sri Lanka
37.4
5.0
149.6
33
Thailand
25.2
0.0
90.0
80
Japan
42.0
—
—
155
Source
: Kyuma (1978).
