Agriculture – Soil Taxonomy (USDA-ST) (Soil Survey Staff 1999) and gleyic
properties in the World Reference Base for Soil Resources (WRB) (IUSS Working
Group WRB 2015). Iron is one of the major elements in soil, and it strongly affects
soil color and mottle formation.
In relation to redox reactions, the soil pH also changes. For example, pH increases
with reduction of hydrated iron (III) oxide, and decreases with oxidation of ferrous
iron (iron(II)). A pH–pE diagram can be used as a method to describe the chemical
stability of minerals under varying redox conditions in soil, and is recommended for
further study (Stumm and Morgan 1996; Kyuma 2004).
5.1.1 Alternating Oxidized and Reducing Conditions
in Paddy Field Soils
Paddy field soils are an example of soil with varying redox conditions. The total
paddy-field land area comprises irrigated, rain-fed lowland, and rain-fed upland
paddy fields covering 93, 52, and 15 million ha (Global Rice Science Partnership
2013), respectively. More than 90% of the paddy fields in the world are distributed in
Asia. Elsewhere, paddy fields are distributed in temperate, subtropical, and tropical
areas where enough water is available. The rain-fed lowland paddy field area
includes that covered by deep-water rice. Although significant areas of irrigated
paddy fields are also grown in rotation with a range of other crops, lowland paddy
field soils experience relatively reducing conditions when rice is grown under
submergence. Merits of submergence are (i) high rice yield, (ii) weed control, (iii)
an increase in phosphate availability under reducing conditions, (iv) supply of
micronutrients as solutes in irrigation water, and (v) high N-fixation ability compared to ordinary uplands (Kyuma 2004). This chapter focuses on paddy field soils,
and lowland soils with high ground water level in relation to their varying redox
conditions.
Reducing conditions are caused by microbial activity. Three typical requirements
for the development of reducing conditions are (i) submergence of soil in water to
restrict oxygen diffusion, (ii) appropriate temperature for microbial activity, and (iii)
carbon source for microbes. Oxidizing forms of C, N, Mn, Fe, and S are also
important, and nearly-neutral soil pH is preferable for microbial activity. In contrast,
when reduced soil is exposed to air due to drainage or ground water level fall, the soil
becomes oxidized.
Major elements affected by varying redox conditions in soil are C, N, Mn, Fe, and
S. The behavior of contaminant elements in soil, such as Cd, Cu, and As, is also
affected by varying redox conditions. Sulfide precipitation of Cd and some other
heavy metals is possible under reducing conditions. The oxidation numbers of Cu
and As change with redox potential.
Figure 5.1 illustrates changes in the color of soil and rice roots as a result of redox
reactions. An Ap horizon (plow layer) soil (Udifluvent, according to the USDA-ST),
which was sampled from a paddy field, was used. The oxalate-extractable Fe (Fe o ),
98
5 Inorganic Soil Constituents Sensitive to Varying Redox Conditions
properties in the World Reference Base for Soil Resources (WRB) (IUSS Working
Group WRB 2015). Iron is one of the major elements in soil, and it strongly affects
soil color and mottle formation.
In relation to redox reactions, the soil pH also changes. For example, pH increases
with reduction of hydrated iron (III) oxide, and decreases with oxidation of ferrous
iron (iron(II)). A pH–pE diagram can be used as a method to describe the chemical
stability of minerals under varying redox conditions in soil, and is recommended for
further study (Stumm and Morgan 1996; Kyuma 2004).
5.1.1 Alternating Oxidized and Reducing Conditions
in Paddy Field Soils
Paddy field soils are an example of soil with varying redox conditions. The total
paddy-field land area comprises irrigated, rain-fed lowland, and rain-fed upland
paddy fields covering 93, 52, and 15 million ha (Global Rice Science Partnership
2013), respectively. More than 90% of the paddy fields in the world are distributed in
Asia. Elsewhere, paddy fields are distributed in temperate, subtropical, and tropical
areas where enough water is available. The rain-fed lowland paddy field area
includes that covered by deep-water rice. Although significant areas of irrigated
paddy fields are also grown in rotation with a range of other crops, lowland paddy
field soils experience relatively reducing conditions when rice is grown under
submergence. Merits of submergence are (i) high rice yield, (ii) weed control, (iii)
an increase in phosphate availability under reducing conditions, (iv) supply of
micronutrients as solutes in irrigation water, and (v) high N-fixation ability compared to ordinary uplands (Kyuma 2004). This chapter focuses on paddy field soils,
and lowland soils with high ground water level in relation to their varying redox
conditions.
Reducing conditions are caused by microbial activity. Three typical requirements
for the development of reducing conditions are (i) submergence of soil in water to
restrict oxygen diffusion, (ii) appropriate temperature for microbial activity, and (iii)
carbon source for microbes. Oxidizing forms of C, N, Mn, Fe, and S are also
important, and nearly-neutral soil pH is preferable for microbial activity. In contrast,
when reduced soil is exposed to air due to drainage or ground water level fall, the soil
becomes oxidized.
Major elements affected by varying redox conditions in soil are C, N, Mn, Fe, and
S. The behavior of contaminant elements in soil, such as Cd, Cu, and As, is also
affected by varying redox conditions. Sulfide precipitation of Cd and some other
heavy metals is possible under reducing conditions. The oxidation numbers of Cu
and As change with redox potential.
Figure 5.1 illustrates changes in the color of soil and rice roots as a result of redox
reactions. An Ap horizon (plow layer) soil (Udifluvent, according to the USDA-ST),
which was sampled from a paddy field, was used. The oxalate-extractable Fe (Fe o ),
98
5 Inorganic Soil Constituents Sensitive to Varying Redox Conditions