oxyhydroxides. Under oxidizing conditions, the Fe o /dithionite-extractable iron (Fe d )
values of paddy field plow layer soils are higher than 0.3, suggesting that the major
form of iron is poorly crystalline ferrihydrite due to yearly repetition of reduction and
oxidation, i.e., dissolution and precipitation (Childs et al. 1991). Although
lepidocrocite was identified in the iron mottles (Kojima 1971), lepidocrocite is partly
soluble in acid oxalate solution (dark) (Schwertmann 1973; Fonseca and da Silva
1998). A significant form of ferrous iron is exchangeable or acetate-extractable Fe
2+
(Kyuma 2004), and other forms of ferrous iron may include vivianite, siderite, and
noncrystalline ferrous sulfide.
Figure 5.5a shows a profile of a poorly drained paddy field soil. The plow layer
soil is gray in color, suggesting that the bulk soil is still under reducing conditions
although irrigation had been stopped more than one month before, and the rice
harvest was finished. As the texture of this soil is fine (Togami et al. 2017), with the
major clay mineral being montmorillonite (Fig. 3.8), the soil is poorly drained. This
soil profile contains two major types of iron mottles. The first type is brown-colored
iron mottles, which resemble a coating of irregular or vesicular pore surfaces, as
shown in Fig. 5.5b. Since the pore surface coating-like mottles are found in the plow
layer soil, they were formed after puddling in the spring of that year. During the next
rice cultivation, the mottles will be reduced and dissolved again. The second type is
cylindrical iron mottles, which resemble root iron plaque. Since the layer rich in
these mottles is deeper than 15 cm, the iron plaque-like mottles may have been
formed by hygrophytes other than cultivated rice. A few small iron plaque-like
mottles can also be found in the plow layer soil. Iron minerals included in the root
iron plaque are suggested to be ferrihydrite, lepidocrocite, and others (Kahn et al.
2016). A possible reason for the remaining brown, iron plaque-like mottles in the
reduced subsoil may be depletion of easily decomposable organic matter in this soil
horizon. The properties of these mottles were further examined microscopically.
The cylindrical iron mottles, shown in Fig. 5.6, are well developed and can be
separated from a clod. Figure 5.6a shows a cross-section of an air-dried clod, which
contains root iron plaque-like mottles of different sizes and colors. The browncolored larger mottles can be dug out from a field-moist clod, as shown in
Fig. 5.6b, and were probably iron plaque formed around the roots of former
vegetation. Figure 5.6c shows the gently washed iron plaque. It contains sand-size
particles in the brown-colored area. The color distribution pattern is concentric; from
the outside inward the color changes from yellowish brown to brown and gradually
to light brown. Figure 5.6d shows a longitudinal section of the cylindrical iron mottle
(Fig. 5.6c). The sand-sized particles and color distribution pattern observed in
Fig. 5.6c can be ascertained in Fig. 5.6d. The distribution of sand-sized particles
within the cylindrical iron mottle (Fig. 5.6c, d) suggests that oxygen transported
through the plant roots diffused outside of the roots, oxidized ferrous iron around the
roots, and precipitated hydrated iron oxides in the reduced soil environment. Over
time, more ferrous iron probably diffused from the soil matrix and concentrated
around the iron plaque, increasing its thickness. Similar and larger cylindrical root
5.2 Hydrated Iron Oxide
105
values of paddy field plow layer soils are higher than 0.3, suggesting that the major
form of iron is poorly crystalline ferrihydrite due to yearly repetition of reduction and
oxidation, i.e., dissolution and precipitation (Childs et al. 1991). Although
lepidocrocite was identified in the iron mottles (Kojima 1971), lepidocrocite is partly
soluble in acid oxalate solution (dark) (Schwertmann 1973; Fonseca and da Silva
1998). A significant form of ferrous iron is exchangeable or acetate-extractable Fe
2+
(Kyuma 2004), and other forms of ferrous iron may include vivianite, siderite, and
noncrystalline ferrous sulfide.
Figure 5.5a shows a profile of a poorly drained paddy field soil. The plow layer
soil is gray in color, suggesting that the bulk soil is still under reducing conditions
although irrigation had been stopped more than one month before, and the rice
harvest was finished. As the texture of this soil is fine (Togami et al. 2017), with the
major clay mineral being montmorillonite (Fig. 3.8), the soil is poorly drained. This
soil profile contains two major types of iron mottles. The first type is brown-colored
iron mottles, which resemble a coating of irregular or vesicular pore surfaces, as
shown in Fig. 5.5b. Since the pore surface coating-like mottles are found in the plow
layer soil, they were formed after puddling in the spring of that year. During the next
rice cultivation, the mottles will be reduced and dissolved again. The second type is
cylindrical iron mottles, which resemble root iron plaque. Since the layer rich in
these mottles is deeper than 15 cm, the iron plaque-like mottles may have been
formed by hygrophytes other than cultivated rice. A few small iron plaque-like
mottles can also be found in the plow layer soil. Iron minerals included in the root
iron plaque are suggested to be ferrihydrite, lepidocrocite, and others (Kahn et al.
2016). A possible reason for the remaining brown, iron plaque-like mottles in the
reduced subsoil may be depletion of easily decomposable organic matter in this soil
horizon. The properties of these mottles were further examined microscopically.
The cylindrical iron mottles, shown in Fig. 5.6, are well developed and can be
separated from a clod. Figure 5.6a shows a cross-section of an air-dried clod, which
contains root iron plaque-like mottles of different sizes and colors. The browncolored larger mottles can be dug out from a field-moist clod, as shown in
Fig. 5.6b, and were probably iron plaque formed around the roots of former
vegetation. Figure 5.6c shows the gently washed iron plaque. It contains sand-size
particles in the brown-colored area. The color distribution pattern is concentric; from
the outside inward the color changes from yellowish brown to brown and gradually
to light brown. Figure 5.6d shows a longitudinal section of the cylindrical iron mottle
(Fig. 5.6c). The sand-sized particles and color distribution pattern observed in
Fig. 5.6c can be ascertained in Fig. 5.6d. The distribution of sand-sized particles
within the cylindrical iron mottle (Fig. 5.6c, d) suggests that oxygen transported
through the plant roots diffused outside of the roots, oxidized ferrous iron around the
roots, and precipitated hydrated iron oxides in the reduced soil environment. Over
time, more ferrous iron probably diffused from the soil matrix and concentrated
around the iron plaque, increasing its thickness. Similar and larger cylindrical root
5.2 Hydrated Iron Oxide
105