Chapter 5
Inorganic Soil Constituents Sensitive
to Varying Redox Conditions
Abstract Inorganic soil constituents sensitive to varying redox conditions, such as
hydrated iron oxide, vivianite, siderite, iron (II) sulfides, and jarosite, are analyzed
using optical and electron microscopes, energy dispersive X-ray spectroscopy (EDX),
and X-ray diffraction (XRD). Many of these minerals are sourced from paddy field
soils, which undergo reducing and oxidizing conditions in the plow layer every year.
Iron mottles formed at the soil redox interface in the presence of reducing and
oxidizing conditions provide significant visual evidence of varying redox conditions
in soil. Polished sections were used to examine the elemental distributions and
morphological properties of the mottles. One type of iron mottles is formed around
rice roots by oxygen diffusion from the roots. They are cylindrical in form and
include soil matrix minerals. Other type of iron mottles is formed on the surfaces of
irregular or vesicular pores by oxygen diffusion through soil pores after drainage.
These mottles contain few soil matrix minerals. In association with iron, the distribution of phosphate is strongly affected by changes in redox conditions in paddy field
soils with low active Al content.
5.1 Introduction
Oxidizing conditions are common in surface soils in an oxygen-containing atmosphere. Under oxidizing conditions, the major factors affecting dissolution and
precipitation reactions in soil are the pH, temperature, and elemental composition
of the soil. For example, Ca carbonate dissolves in acid soils, whereas it precipitates
in alkaline soils. However, under submergence or high ground water level, reducing
conditions develop in soil associated with microbial activity. For example, the
solubility of iron is very low in neutral and alkaline soils, whereas a part of iron
dissolves under reducing conditions. Thus, changes in redox conditions affect the
chemical forms of elements, including redox-sensitive elements and related elements
(Ponnamperuma 1972). The morphological properties of soil profiles are also
affected by these redox reactions (Vepraskas and Craft 2016).
Reduced soils are typically characterized by the reduction of iron, as included in
the diagnostic criteria for redoximorphic features in the United States Department of
© The Author(s) 2018
M. Nanzyo, H. Kanno, Inorganic Constituents in Soil,
https://doi.org/10.1007/978-981-13-1214-4_5
97
Inorganic Soil Constituents Sensitive
to Varying Redox Conditions
Abstract Inorganic soil constituents sensitive to varying redox conditions, such as
hydrated iron oxide, vivianite, siderite, iron (II) sulfides, and jarosite, are analyzed
using optical and electron microscopes, energy dispersive X-ray spectroscopy (EDX),
and X-ray diffraction (XRD). Many of these minerals are sourced from paddy field
soils, which undergo reducing and oxidizing conditions in the plow layer every year.
Iron mottles formed at the soil redox interface in the presence of reducing and
oxidizing conditions provide significant visual evidence of varying redox conditions
in soil. Polished sections were used to examine the elemental distributions and
morphological properties of the mottles. One type of iron mottles is formed around
rice roots by oxygen diffusion from the roots. They are cylindrical in form and
include soil matrix minerals. Other type of iron mottles is formed on the surfaces of
irregular or vesicular pores by oxygen diffusion through soil pores after drainage.
These mottles contain few soil matrix minerals. In association with iron, the distribution of phosphate is strongly affected by changes in redox conditions in paddy field
soils with low active Al content.
5.1 Introduction
Oxidizing conditions are common in surface soils in an oxygen-containing atmosphere. Under oxidizing conditions, the major factors affecting dissolution and
precipitation reactions in soil are the pH, temperature, and elemental composition
of the soil. For example, Ca carbonate dissolves in acid soils, whereas it precipitates
in alkaline soils. However, under submergence or high ground water level, reducing
conditions develop in soil associated with microbial activity. For example, the
solubility of iron is very low in neutral and alkaline soils, whereas a part of iron
dissolves under reducing conditions. Thus, changes in redox conditions affect the
chemical forms of elements, including redox-sensitive elements and related elements
(Ponnamperuma 1972). The morphological properties of soil profiles are also
affected by these redox reactions (Vepraskas and Craft 2016).
Reduced soils are typically characterized by the reduction of iron, as included in
the diagnostic criteria for redoximorphic features in the United States Department of
© The Author(s) 2018
M. Nanzyo, H. Kanno, Inorganic Constituents in Soil,
https://doi.org/10.1007/978-981-13-1214-4_5
97