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Flooded soils.pptx
1. Chemistry of Flooded Soils
• Flooded soils are submerged soils used to grow
rice; also known as paddy soils
• Profile:
– the plow layer is a puddled system which results from
plowing and harrowing while the soil is wet
– underneath is a hardpan which
– prevents or minimizes the percolation of water into
the subsoil
• Effect of flooding on soil pH and redox potential:
– pH value of acid soils increase on submergence
whereas the opposite occurs in calcareous soils and
sodic soils.
– For most soils the fairly stable pH attained after
several weeks of submergence is between 6.5 to 7.0
2.
3. Reduction of several soil redox
systems in rice soils
• As O2 is depleted from the soil, NO3
- is utilized by
facultative anaerobes and is rapidly reduced
• The reduction of Mn (IV) to Mn (II) takes place
during reduction of O2 and NO3
-
• Reduction of Fe(III) only occurs after O2 and NO3
-
are consumed; carried out by facultative
anaerobes
• Reduction of SO4
2- to S2- is carried out by true
anaerobic bacteria.
• CH4 is not produced in soils until most of the
sulfate has been reduced
4. Summary of the major reduction
processes involved in flooded soils:
O2 H2O
NO3
- N2
Mn4+ Mn2+
Fe3+ Fe2+
SO4
2- H2S
CO2 CH4
5. Difference in chemistry of lowland
and upland soils
• Upland or dryland soils are grown to such
crops as as corn, vegetables, etc .
– aerobic most of the time
– the nutrient elements exist generally in their
oxidized state: NO3
–, H2PO4
–, SO4
2-, Fe3+,
Mn4+ and CO2
– aerobic soil is brown, yellowish brown or
reddish brown.
– organic matter decomposes with CO2 as a
major product.
6. Difference in chemistry of lowland
and upland soils
• Lowland or paddy soils are usually grown to
rice
– continuously submerged are anaerobic except for
a thin oxidized layer at the surface
– nutrients are also in reduced forms like NH4
+,
H2S, Mn2+, Fe2+ and CH4
– submerged soil is dark gray or bluish gray
– the major products of organic matter
decomposition are methane (CH4), hydrogen
`sulfide (H2S), organic acids, alcohols and
ketones.
