In this diagram, a solid phase with a higher log {A i }/{Fe
2+
} value is considered to be
more stable than those with lower values, where {A i } shows the formal activities of
the solid phase in the same way as the activities of the solutes. Uncorrected equilibrium constants (ionic strength ¼ 0) were used for simplicity. The concentrations of
total dissolved phosphorus (P T ), sulfur (S T ), and inorganic carbon (C T ) were tentatively assumed to be 10
À6
, 10
À6
, and 10
À3 mol L
À1
, respectively. These values may
vary widely with soil conditions and biological activity (Kirk 2004; Kyuma 2004),
which may affect the results. Furthermore, the reported solubility product value for
vivianite can vary by several orders of magnitude (10
À32
–10
À36
) (Sadiq and Lindsay
1979; Stumm and Morgan 1996). This discussion about the relationship of these
ferrous salts is made under these assumptions. The stability of FeS (amorphous) is the
highest of the four salts. However, since the labile S content in the studied soil was
Fig. 5.12 Elemental composition of vivianite on a rice root. (a) SEM image of vivianite crystal
aggregates formed on a rice root, (b) EDX spectrum for dashed square (b) shown in (a), (c, and d) P
and Fe element maps of the crystal aggregates, respectively
112
5 Inorganic Soil Constituents Sensitive to Varying Redox Conditions
2+
} value is considered to be
more stable than those with lower values, where {A i } shows the formal activities of
the solid phase in the same way as the activities of the solutes. Uncorrected equilibrium constants (ionic strength ¼ 0) were used for simplicity. The concentrations of
total dissolved phosphorus (P T ), sulfur (S T ), and inorganic carbon (C T ) were tentatively assumed to be 10
À6
, 10
À6
, and 10
À3 mol L
À1
, respectively. These values may
vary widely with soil conditions and biological activity (Kirk 2004; Kyuma 2004),
which may affect the results. Furthermore, the reported solubility product value for
vivianite can vary by several orders of magnitude (10
À32
–10
À36
) (Sadiq and Lindsay
1979; Stumm and Morgan 1996). This discussion about the relationship of these
ferrous salts is made under these assumptions. The stability of FeS (amorphous) is the
highest of the four salts. However, since the labile S content in the studied soil was
Fig. 5.12 Elemental composition of vivianite on a rice root. (a) SEM image of vivianite crystal
aggregates formed on a rice root, (b) EDX spectrum for dashed square (b) shown in (a), (c, and d) P
and Fe element maps of the crystal aggregates, respectively
112
5 Inorganic Soil Constituents Sensitive to Varying Redox Conditions