the dashed square areas (d) and (e), respectively, shown in Fig. 5.23a. These results
suggest that the outer part of the siderite rod mainly consists of aluminosilicates
accompanied by hydrated iron oxide (Fig. 5.23d), and that the central part consists of
siderite (Fig. 5.23e). The central pore was almost entirely filled with siderite.
Figure 5.23f shows an optical micrograph of a similar rod. Although it has the
central pore, the color of the cross-section is dominantly grayish. Several similar
rods were separated from the same horizon, of which some were light brown and
others were grayish in color (Fig. 5.23g). These rods appear to be formed from root
iron plaque (Figs. 5.23h and 5.6), although the latter is more accurately cylindrical.
5.5 Pyrite and Related Sulfur-Containing Inorganic
Constituents
Sulfur is an essential element of organisms. It is sensitive to redox reactions in soil
and exists as both inorganic and organic constituents in soil. In this section, two
sulfides and one sulfate are introduced. Although gypsum is also an important sulfate
in soil, it is mainly introduced in Sect. 6.2.
5.5.1 Noncrystalline Iron(II) Sulfide
Dark-colored areas are sometimes found in submerged soil, particularly when
sulfate-containing fertilizers are applied. Figure 5.24a shows rice cultivation with a
large amount of gypsum in a glass container, such that roots can be observed
occasionally, although the glass container was covered with a dark sheet to avoid
growth of photosynthetic organisms on the inner surface. Figure 5.24b shows a
close-up photograph of roots at the middle depth of the container. The thick roots are
mostly brown, whereas thin lateral roots are mostly black. If there is no significant
difference in organic matter supply to microorganisms between the thick and thin
roots, the air supply to the thin roots is probably more restrictive than that to the thick
roots (Trolldenier 1977). The deeply dark color of the fine roots suggests diffusion of
iron and sulfur to the thin roots due to the low solubility of the precipitate.
Figure 5.24c shows a cross-section of the soil with roots. The upper 4–5 cm is an
oxidizing layer. Roots are distributed more along the glass container than in the bulk
soil. Below the oxidizing layer, the color of both the reducing layer and roots in the
soil is dark.
The dark-colored material is noncrystalline ferrous sulfide. After the roots were
washed, they were freeze-dried. An optical micrograph (Fig. 5.25a) shows that the
thick roots are dark brown, and the thin lateral roots are dark. EDX spectra obtained
from the dashed outlined areas of a lateral thin root (Fig. 5.25c) and the thick root
124
5 Inorganic Soil Constituents Sensitive to Varying Redox Conditions
suggest that the outer part of the siderite rod mainly consists of aluminosilicates
accompanied by hydrated iron oxide (Fig. 5.23d), and that the central part consists of
siderite (Fig. 5.23e). The central pore was almost entirely filled with siderite.
Figure 5.23f shows an optical micrograph of a similar rod. Although it has the
central pore, the color of the cross-section is dominantly grayish. Several similar
rods were separated from the same horizon, of which some were light brown and
others were grayish in color (Fig. 5.23g). These rods appear to be formed from root
iron plaque (Figs. 5.23h and 5.6), although the latter is more accurately cylindrical.
5.5 Pyrite and Related Sulfur-Containing Inorganic
Constituents
Sulfur is an essential element of organisms. It is sensitive to redox reactions in soil
and exists as both inorganic and organic constituents in soil. In this section, two
sulfides and one sulfate are introduced. Although gypsum is also an important sulfate
in soil, it is mainly introduced in Sect. 6.2.
5.5.1 Noncrystalline Iron(II) Sulfide
Dark-colored areas are sometimes found in submerged soil, particularly when
sulfate-containing fertilizers are applied. Figure 5.24a shows rice cultivation with a
large amount of gypsum in a glass container, such that roots can be observed
occasionally, although the glass container was covered with a dark sheet to avoid
growth of photosynthetic organisms on the inner surface. Figure 5.24b shows a
close-up photograph of roots at the middle depth of the container. The thick roots are
mostly brown, whereas thin lateral roots are mostly black. If there is no significant
difference in organic matter supply to microorganisms between the thick and thin
roots, the air supply to the thin roots is probably more restrictive than that to the thick
roots (Trolldenier 1977). The deeply dark color of the fine roots suggests diffusion of
iron and sulfur to the thin roots due to the low solubility of the precipitate.
Figure 5.24c shows a cross-section of the soil with roots. The upper 4–5 cm is an
oxidizing layer. Roots are distributed more along the glass container than in the bulk
soil. Below the oxidizing layer, the color of both the reducing layer and roots in the
soil is dark.
The dark-colored material is noncrystalline ferrous sulfide. After the roots were
washed, they were freeze-dried. An optical micrograph (Fig. 5.25a) shows that the
thick roots are dark brown, and the thin lateral roots are dark. EDX spectra obtained
from the dashed outlined areas of a lateral thin root (Fig. 5.25c) and the thick root
124
5 Inorganic Soil Constituents Sensitive to Varying Redox Conditions