oxide can precipitate outside the bulk soil. In the case of the iron mottles in the plow
layer soil, phosphorus is also a significant constituent, as shown in Sect. 5.3.
5.3 Vivianite
As an iron phosphate mineral under reducing conditions, vivianite [Fe 3 (PO 4 ) 2 ・
6H 2 O] has been found in sediments or organic soils under reducing conditions
(Rothe et al. 2016). Here we introduce the formation and dissolution of vivianite
in the plow layer soil of paddy fields. Vivianite is formed and dissolved according to
the redox conditions of the soil. Vivianite is formed significantly in ordinarily
P-enriched paddy field soils, whereas it is not formed in Andisol paddy fields.
5.3.1 Detection of Vivianite in Paddy Field Soil
Vivianite has previously been reported in the deep horizon, 1 m from the surface, of
paddy field soil with high ground water level (Ito 1975). This site of vivianite
presence is too deep to be related to rice cultivation, i.e., P fertilizer application in
the paddy field. Subsequently, Wada et al. (1977) reported a vivianite-like material
on rice roots using an optical microscope. Their finding was directly related to rice
plants. Their observation was later confirmed using X-ray microdiffraction and
SEM-EDX analyses on rice roots grown in pots (Nanzyo et al. 2010) and in ordinary
paddy fields (Nanzyo et al. 2013). Figure 5.11a shows vivianite crystal aggregates
formed on rice roots. The color is blue–green, which is due to partial oxidation of
iron after exposure to air. Regarding the color change of vivianite after exposure to
air, see Fig. 5.20. The powder XRD pattern obtained from the vivianite aggregates
(Fig. 5.11b) by the microdiffraction method is significantly different compared to
that obtained from a rice root with almost no vivianite crystals. The XRD pattern
shown in Fig. 5.11b is identical to the reference XRD pattern for vivianite reported
by Lehr et al. (1967) (Fig. 5.11c). Hence, the XRD analyses show that vivianite is
dominant in the crystal aggregates (Fig. 5.11). Important conditions required to
detect vivianite on rice roots are (i) lowland paddy field soils (not Andisols),
(ii) duration of approximately one-and-a-half months after transplanting a rice
seedling under continuous submergence, (iii) plant-available P level of plow layer
soil higher than approximately 0.1 g P 2 O 5 kg
À1 as determined by the Truog method,
(iv) washing of rice roots to remove soil soon after sampling, (v) air-drying of
washed roots, and (vi) magnifying glass with magnification factor of greater than
30 times.
The elemental composition of a vivianite crystal aggregate was examined
(Fig. 5.12). From the crystal aggregate (Fig. 5.12a), dashed square b was selected,
and the corresponding EDX spectrum (Fig. 5.12b) showed that the crystal aggregate
110
5 Inorganic Soil Constituents Sensitive to Varying Redox Conditions
layer soil, phosphorus is also a significant constituent, as shown in Sect. 5.3.
5.3 Vivianite
As an iron phosphate mineral under reducing conditions, vivianite [Fe 3 (PO 4 ) 2 ・
6H 2 O] has been found in sediments or organic soils under reducing conditions
(Rothe et al. 2016). Here we introduce the formation and dissolution of vivianite
in the plow layer soil of paddy fields. Vivianite is formed and dissolved according to
the redox conditions of the soil. Vivianite is formed significantly in ordinarily
P-enriched paddy field soils, whereas it is not formed in Andisol paddy fields.
5.3.1 Detection of Vivianite in Paddy Field Soil
Vivianite has previously been reported in the deep horizon, 1 m from the surface, of
paddy field soil with high ground water level (Ito 1975). This site of vivianite
presence is too deep to be related to rice cultivation, i.e., P fertilizer application in
the paddy field. Subsequently, Wada et al. (1977) reported a vivianite-like material
on rice roots using an optical microscope. Their finding was directly related to rice
plants. Their observation was later confirmed using X-ray microdiffraction and
SEM-EDX analyses on rice roots grown in pots (Nanzyo et al. 2010) and in ordinary
paddy fields (Nanzyo et al. 2013). Figure 5.11a shows vivianite crystal aggregates
formed on rice roots. The color is blue–green, which is due to partial oxidation of
iron after exposure to air. Regarding the color change of vivianite after exposure to
air, see Fig. 5.20. The powder XRD pattern obtained from the vivianite aggregates
(Fig. 5.11b) by the microdiffraction method is significantly different compared to
that obtained from a rice root with almost no vivianite crystals. The XRD pattern
shown in Fig. 5.11b is identical to the reference XRD pattern for vivianite reported
by Lehr et al. (1967) (Fig. 5.11c). Hence, the XRD analyses show that vivianite is
dominant in the crystal aggregates (Fig. 5.11). Important conditions required to
detect vivianite on rice roots are (i) lowland paddy field soils (not Andisols),
(ii) duration of approximately one-and-a-half months after transplanting a rice
seedling under continuous submergence, (iii) plant-available P level of plow layer
soil higher than approximately 0.1 g P 2 O 5 kg
À1 as determined by the Truog method,
(iv) washing of rice roots to remove soil soon after sampling, (v) air-drying of
washed roots, and (vi) magnifying glass with magnification factor of greater than
30 times.
The elemental composition of a vivianite crystal aggregate was examined
(Fig. 5.12). From the crystal aggregate (Fig. 5.12a), dashed square b was selected,
and the corresponding EDX spectrum (Fig. 5.12b) showed that the crystal aggregate
110
5 Inorganic Soil Constituents Sensitive to Varying Redox Conditions