Chemistry and physics of submerged soil

Presented by
J.Anandhan,
2015672001

CONTENTS
1. Introduction
2. Kinds of submerged soils
3. Characteristics of submerged soils
4. Electro chemical changes in submerged soils
5. Chemical transformations of submerged soils
6. Mineral equilibria in submerged soils
7. References

INTRODUCTION
 Applications in geochemistry, agriculture, limnology,
oceanography and pollution
 72% of the earth's surface is covered by submerged soils and
sediments
 It is suitable of wet soils for crops, aquatic lives, marine plants
and sinks for many nutrients
 Acts as reservoir for many nutrients

KINDS
a. WATERLOGGED (GLEY) SOILS
b. MARSH SOILS
c. PADDY SOILS
d. SUBAQUATIC SOILS

A. WATERLOGGED (GLEY) SOILS
 saturated with water for a sufficiently long time annually
 Forms horizons like:
(a) a partially oxidized A horizon high in organic matter
(b) a mottled zone
(c) a permanently reduced zone with bluish green colour
(Robinson, 1949)

B. MARSH SOILS
Freshwater marsh
 occur on the fringes of lakes and the networks of streams
that feed them
 In this the G horizon is blue or green
 Types,
• Upland (pH 3.5-4.5)
• Lowland (pH 5.0-6.0)
• Transitional

Saltwater marsh
 marshes are found in estuaries, deltas and tidal flats
 it is green if iron silicates are present and dark grey if
pyrites are the main iron minerals

Fresh water marshes
salt water marshes

C. PADDY SOILS
 Developed by cultivation practises of paddy (includes
puddling, levelling and water stagnation)
 When irrigated soil undergoes reduction and turns dark grey.
 Fe, Mn, Si and P become more soluble and diffuse to the
surface
 Moves by diffusion and mass flow to the roots and to the
subsoil. When Fe2+ and Mn2+ reach the oxygenated surface, the
surface of rice roots, or the oxidized zone below the plough
sole they are oxidized and precipitated along with silica and
phosphate

 It is Sandwiched between the oxidized surface layer and the zone
of Fe and Mn illuviation.
 The root zone of rice with reddish-brown streaks along root
channels.
 When the land is drained at harvest, almost the entire profile
above the water table is reoxidized, giving it a highly mottled
appearance.
 Precipitation in the plough layer is not pedologically of any
consequence because ploughing and puddling redistribute the
deposits

 Downward movement of Fe and Mn causes loss of these
elements from the topsoil. The eluviated Fe and Mn, along with
some phosphate, are deposited below the plow sole to
produce an iron-rich B1r horizon overlying a manganese-rich
Bmn horizon.
 Reduction eluviation and oxidative illuviation as the soil
forming processes characteristic of paddy soils and have
proposed the new term "Aquorizem" at the Great Soil Group
level to define soils which have the sequence of reductive
eluviation/oxidative illuviation. (Kyuma and Kawaguchi
(1966) )
 A well developed paddy soil has the horizon sequence
Apg,/Birg/ B2g/G (Kanno (1957))

D. SUBAQUATIC SOILS
 Formed from river, lake, and ocean sediments.
 Formed by,
• the sediments are formed from soil components
• typical soil-forming processes such as hydrolysis, oxidation-
reduction, precipitation, synthesis, and exchange of matter
• deep sea sediments contain OM and a living bacterial flora

CHARACTERS OF SUBAQUATIC SOILS
 the bacteria in lake and ocean sediments are similar to
those in soils
 the metabolism of subaquatic sediments is similar to
those of submerged soils
 the uppermost layers show ’A’ horizon differentiation
distinct from physical stratification
 sediments differ in texture, composition, clay mineralogy,
organic matter content, and oxidation-reduction level

CHARACTERISTICS OF SUBMERGED SOILS
A. Absence of Molecular Oxygen
B. Oxidized Mud-Water Interface
C. Exchanges between Mud and Water
D. Presence of Marsh Plants
E. Soil Reduction

A. ABSENCE OF MOLECULAR OXYGEN
 Gas exchange between soil and air is drastically reduced
 O2 and other atmospheric gases can enter the soil only by
molecular diffusion in the interstitial water is 10,000 times
slower than diffusion in gas-filled pores
 Within a few hours of soil submergence, microorganisms use
up the oxygen present in the water or trapped in the soil and
render a submerged soil practically devoid of molecular
oxygen

Oxygen moves through water layer
Soil layer with no oxygen (anaerobic)
Thin aerobic soil layer

B. OXIDIZED MUD-WATER INTERFACE
 Concentration of O2 may be high in the surface layer which is a
few millimeters thick and in contact with oxygenated water
 Below the surface layer, the O2 concentration drops abruptly to
practically zero
 The chemical and microbiological regimes in the surface layer
resemble those in aerobic soils

C. EXCHANGES BETWEEN MUD AND WATER
 The presence of this oxygenated surface layer in lake and ocean
muds is of the most ecological importance because it acts as a
sink for phosphate and other plant nutrients and as a chemical
barrier to the passage of certain plant nutrients from the mud to
the water
 The surface may use up oxygen faster than it receives it, undergo
reduction and release large amounts of nutrients from the lake
mud into the water
 In summer, some lakes undergo thermal differentiation into
three layers:
• Epilimnion
• Thermocline
• Hypolimnion

 The epilimnion is the surface layer of warm water 10-20 m
deep which because of mixing by wind action, is uniform in
temperature and is saturated with atmospheric O2 from top to
bottom. (Mortimer, 1949).
 Immediately below this is the thermocline, a layer in which
there is a rapid fall in temperature with depth. In this, the
concentration of O2 is relatively constant in lakes poor in plant
nutrients (oligotrophic lakes), but it decreases with depth in
lakes rich in plant nutrients (eutrophic lakes) (Ruttner, 1963).
 The hypolimnion is the layer of cold stagnant water practically
isolated from the epilimnion, except for solids, both organic
and inorganic, that sink through it and accumulate on the mud
surface. Bacteria in the surface layer use the O2 in it to oxidize
the organic matter.

D. PRESENCE OF MARSH PLANTS
 Plants growing in submerged soils have two adaptations that
enable the roots to ward off toxic reduction products,
accumulate nutrients, and grow in an O2 -free medium: O2
transport from the aerial parts and anaerobic respiration
 It has been known for quite some time that the roots of marsh
plants receive their oxygen from the aerial parts (shoot, air
roots or stilt roots) through gas spaces connecting these
organs

E. SOIL REDUCTION
 The most important chemical difference submerged soil is in a
reduced state.
 Except for the thin, brown, oxidized layer at the surface (and
sometimes an oxidized zone in the subsoil), a submerged soil
is grey or greenish, has a low oxidation-reduction potential,
and contains the reduced counterparts of NO2-, SO4
2-, Mn4+,
Fe3+, and CO2, NH4
+, H2S, Mn2+, Fe2+, and CH4

27
OXIDATION AND REDUCTION IN AN AEROBIC
SOIL
 Organic matter in soil gives
up 4 electrons (e-) which are
received by O2. As a result, O2
is reduced.
 Hydrogen ions (H+) react
with the reduced O2 to form
water (H2O).
4 e- + O2 + 4 H+→ 2 H2O

28
OXIDATION AND REDUCTION IN AN ANAEROBIC SOIL
Electrons (e-) from organic matter
in soil are accepted by nitrate
(NO3
-) instead of O2.
 Nitrogen (N) in NO3
- is reduced;
the N compound becomes nitrogen
gas (N2)
 Hydrogen ions (H+) react with
oxygen from NO3
- to produce H2O.
10 e- + 2 NO3
- + 12 H+→ 1 N2 + 6 H2O

 A change in chemistry results in a
change of soil color
• bright colors indicate a well-drained soil
• submerged soils change to a gray or
blue-green color (often referred to as
gley)
• Reddish-yellowish brown colors are an
indication of iron oxides in a well-
drained environment
• Submergence causes iron to be reduced
resulting in a different iron form and the
gley color
Well-drained soil
profile
Reduced soil profile

1. OXIDATION-REDUCTION POTENTIAL
 Oxidation-reduction is a chemical reaction in which electrons
are transferred from a donor to an acceptor.
 The source of electrons for biological reductions is organic
matter.
 Redox potential (Eh) is a quantitative measure of the tendency
of a given system to oxidize or reduce susceptible substances.
Eh is positive and high in strongly oxidizing systems; Negative
in negative and low in strongly reducing systems

 Change in free energy
 Redox potential is measured using following
equation,
 Where,
• Eh = Redox potential
• Eo = Eh at where Oxi and Red are equal
• F = Faruday’s constant

MEASUREMENT OF REDOX POTENTIAL
Redox meter Platinum electrode

Reaction sequence following submergence
Reaction sequence after draining
Chemical Reduction Sequence of
Submergence
O2
N2
Mn2+
Fe2+
NO3
-
MnO2
Fe3+
CO2
CH4
SO4
-2
H2S
H2O
Slightly
Reduced
Moderately
Reduced
Strongly
Reduced
Oxidized

Redox potential of various compounds under submergence
Patrick (1964), and Turner and Patrick (1968

ELECTROCHEMICAL CHANGES IN SUBMERGED
SOILS
Submerging a soil brings about a variety of electrochemical
changes. These include,
 (a) a decrease in redox potential,
 (b) an increase in pH of acid soils and a decrease in pH of
alkaline soils,
 (c) changes in specific conductance and ionic strength,
 (d) drastic shifts in mineral equilibria,
 (e) cation and anion exchange reactions,
 (f) sorption and desorption of ions.

A. REDOX POTENTIAL
 The low potentials (0.2 to -0.4 V) of submerged soils and
sediments reflect this reduced state.
 The high potentials (0.8 to 0.3 V) of aerobic media, their
oxidized condition.

1. SUBMERGED SOILS AND MUDS
 When an aerobic soil is submerged, its Eh decreases during the first
few days and reaches a minimum (-0.42 V ).
 Then it increases, attains a maximum, and decreases again
asymptotically to a value characteristic of the soil, after 8-12 weeks of
submergence
 The presence of native or added organic matter sharpens and hastens
the first minimum, nitrate abolishes it (0.2 V). The rapid initial
decrease of Eh is apparently due to the release of reducing substances
accompanying oxygen depletion before Mn(IV) and Fe(III) oxide
hydrates can mobilize their buffer capacity
 The course, rate, and magnitude of the Eh decrease depend on the
kind and amount of organic matter, the nature, and content of
electron acceptors, temperature, and the duration of submergence
(Ponnamperuma, 1955, 1965; Motomura,1962; Yamane and Sato, 1968).

B. PH
Decrease of pH in first few days of submergence,
then it reaches minimum and increases to a
stable value (6.7 – 7.2)

C. SPECIFIC CONDUCTANCE
 The specific conductance of depends on the kind and
concentration of ions present.
 Ionic strength (I) = ½  CiZi
 Where, Ci= concentration of ions (mol/lit)
Zi = valence of ions
 Under reduced condition ionic strength was equal to 16 times
the specific conductance (k) in mhos/cm at 25°C

CHEMICAL TRANSFORMATIONS IN SUBMERGED
SOILS
A. Carbon
B. Nitrogen
C. Iron
D. Manganese
E. Sulfur
F. Phosphorus
G. Silicon
H. Trace Elements

47
FORM OF COMPOUNDS IN AERATED AND
SUBMERGED SOIL
Element Aerated soil
(Oxidized)
Submerged soil
(Reduced)
Oxygen (O) Oxygen gas (O2) Water (H2O)
Nitrogen (N) Nitrate ion (NO3
-) Nitrogen gas (N2)
Manganese (Mn) Manganese IV ion (Mn4+) Manganese II ion (Mn2+)
Iron (Fe) Iron III ion (Fe3+) Iron II ion (Fe2+)
Sulfur (S) Sulfate ion (SO4
2-) Hydrogen sulfide (H2S)
Carbon (C) Carbon dioxide (CO2) Methane (CH4)

A. CARBON
 The two main transformations of carbon in
nature are photosynthesis and respiration. On
the balance between these two processes
depend (a) the amount of organic matter that
accumulates in soils and sediments, and
 (b) the quality of streams, lakes, and estuaries.
In submerged soils, respiration
(decomposition of organic matter) is the main
transformation

1. DECOMPOSITION OF ORGANIC MATTER
 In well drained soils aerobic microbes will decompose OM to
form CO2, NO3
-, SO4
2-.
 Under submerged condition anaerobic microbes will
decompose OM to produce CO2, H2, CH4, NH4
+, amines,
mercaptans, H2S, and partially humified residues

2. PYRUVIC ACID METABOLISM
 This will occur in both aerobic and submerged conditions.
 The precursor is sugars like glucose
C6H12O6 + 2ATP + 2NAD+ 2CH3COCOOH + 4ATP + 2NADH + 8H+
(Pyruvic acid)
 Under submerged condition Pyruvic acid will transforms,
 (a) reduction to lactic acid,
 (b) decarboxylation to CO2 and CH3CHO
 (c) dissimilation to lactic, butyric and acetic acids and CO2,
 (d) cleavage to acetic, formic acids, H2, and CO2,
 (c) carboxylation to oxaloacetic acid
 (f) condensation with itself or acetaldehyde to give
acetylmethylcarbinol
Werkman and Schlenk (1951),

2. KINETICS OF CO2
 1 to 3 tons of CO2 are produced in the ploughed layer of 1 ha of
a soil during the first few weeks of submergence (IRRI, 1964).
 Being chemically active, it forms HCOO-, HCO3
- and insoluble
CO3
2-.
 The excess accumulates as gas.
 The partial pressure of CO2 in a soil increases after
submergence, reaches a peak of 0.2-0.8 atm 1-3 weeks later
and declines to a fairly stable value of 0.05-0.2 atm
 The decline in Pco2 after 1-4 weeks of submergence is due to
escape, leaching, removal as insoluble CO3
2-, dilution by CH4
produced during the decomposition of organic acids, and
bacterial reduction of CO2to CH4

4. KINETICS OF VOLATILE ORGANIC ACIDS
 The main organic acids found in anaerobic soils and sewage are
formic, acetic, propionic, and butyric acids.
 When a soil is submerged, the concentration of volatile organic
acids increases, reaches a peak value of 10-40 mmol/lit in 1-2
weeks and then declines to less than 1 mmol/lit a few weeks
later.
 Soils high in native or added organic matter produce high
concentrations of acids (Motomura, 1962).
 Low temperature retards acid formation slightly, but acid
destruction markedly.
 Thus organic acids persist longer in cold soils than in warm
soils.
 Ammonium sulphate appears to increase acetic acid formation
but suppresses the formation of propionic and butyric acids

5. METHANE FERMENTATION
 Methane is the typical end product of the anaerobic
decomposition of organic matter.
 Some of the methane is oxidized bacterially at the surface of
paddy soils (Harrison and Aiyer, 1913, 1915) and in the
oxygenated strata of lakes (Hutchinson, 1957).
 Methane formation is ecologically important because it helps
the disposal of large amounts of organic matter sedimented in
lakes.

 Methane is produced by a small group of obligate anaerobes
(like Methansarcina inethanica).
 Methane bacteria function best at temperatures above 30°C,
but most abundant in natural anaerobic waters, produces
methane even at 50°C (Ruttner, 1963).
 Methane bacteria are highly substrate specific and can
metabolize only a small number of simple organic and
inorganic substances, usually the products of fermentation.

B. NITROGEN
In submerged soils, the main transformations are
1. Accumulation of ammonia,
2. Denitrification,
3. Nitrogen fixation.

1. ACCUMULATION OF AMMONIA
 Ammonia production in submerged soils follows a roughly
asymptotic course and the kinetics of ammonia release can be
described by
log (A-y) = log A – ct
Where, A = mean maximum NH4-N concentration
y = actual concentration ‘t’ days after submergence
c = parameter depending on the soil.
(Ponnamperuma, 1965)

2. DENITRIFICATION
 Nitrate undergoes two transformations in submerged soils:
 assimilation or reduction of NO3
- with incorporation of the
products into cell substance
 dissimilation or nitrate respiration in which NO3
- functions
as an alternative to O2 as an electron acceptor
 Rate of denitrification increases with temperature up to 60°C.
 Denitrification will occurs at below the redox potential of 350
mv
 Denitrification is slow in high OM soils (OM provides C, H and
O2 to microbes )
 Alternate wetting and drying increases denitrification loss

3. N2 FIXATION
 BNF is reduction of N2 to NH3.
 It requires high electron activity or low pE
pE = - log ae
Where ae = activity of e-
 Microbes help in BNF are Nostoc, Anabaena, Ocillatoria,
Tolypothrix, Calothrix, Phormidium and some algae species
 Slight alkaline and high P will increase the N- fixation
 They fix as much as 22 kg /ha of N2

N transformations in aerobic vs anerobic

C. IRON
 The reduction of iron has important chemical consequences:
 (a) the concentration of water-soluble iron (Fe2+)increases;
 (b) pH increases
 (c) cations are displaced from exchange sites
 (d) the solubility of phosphorus and silica increases and
 (e) new minerals are formed.

 In acid soils high in OM and Fe will increases to 600 ppm
within 1-6 weeks after submergence
Fe2O4.nH2O Fe3+ Fe2+ (Clay)
 Fe2+ diffuses and mass flow to the surface of soil and also to
plant roots where oxidise and forms precipitates under the
plough sole
 Grey colour mottles due to FeS2
 Paddy soils contains hydrated magnetite (Fe2O4.nH2O) along
with some hydrtrolilite (FeS.nH2O)

D. MANGANESE
 In submerged soils Mn2+ availability is increased by conversion
of Mn(IV) oxides into Mn(II) ions or carbonates
 These Mn2+ ions moves to the oxygenated interfaces in soils by
mass flow and diffusion
 When co2 concentrations in soil increases Mn2+ precipitated as
MnCO3

E. SULPHUR
In aerated soils,
1. Elemental S is converted into SO4
2-, sulphides and
organic sulphur compounds
2. Reduction of SO4
2- and incorporation into plant
tissues as elemental S.
In submerged soils,
1. SO4
2- to sulphide ,
2. Other S containing compounds into H2S (forms bad
ordous )
3. And used by S reducing microbes like Desulfovibrio

F. PHOSPHORUS
 Phosphorus in valence states from +5 to -3
 The forms are phosphite, hypophosphite, phosphine and
phosphate in anaerobic media.
 Soils having forms like,
(a) iron(III) and aluminum phosphates (in acid soils)
(b) phosphates adsorbed or co-precipitated with Fe(IlI) and
Mn(IV) hydrous oxides
(c) phosphates held by anion exchange on clay and hydrous
oxides,
(d) calcium phosphates (in neutral & alkali soils)
(e) organic phosphates.
 The increase in concentration of water-soluble P on soil
submergence
(Stumm and Morgan, 1970)

1. Sandy clay (pH= 7.6)
14. Clay(pH= 4.6)
25. Sandy loam(pH= 4.8)
26. Clay loam(pH= 7.6)
27. Clay(pH= 6.6)

G. SILICON
 In soils occurs as crystalline and amorphous silica
 Also as silicates, adsorbed or co-precipitated with hydrous
oxides of Al, Fe(III)and Mn(IV), and also dissolved in the soil
solution.
 Dissolved silica is present as monomeric Si(OH)4.
 The concentration of Si(OH)4, in equilibrium with amorphous
silica at 25°C is 120-140 ppm as SiO2, and is independent of pH
2 to 9.
 Submergence will slightly increases (due to release by Fe3+
ions and higher CO2 concentration) and then decreases the Si
concentrations ( decrease in Pco2).

H. TRACE ELEMENTS
 Submergence will increase availability of Co, Cu and Zn.
 Increase in pH of acid soils lower the solubility of nutrients
due to release of Sulphide which forms precipitates
 The elements in reduced layer will moves towards to the
oxidised layer

MINERAL EQUILIBRIA IN SUBMERGED SOILS
A. Redox Systems
B. Carbonate Systems

A. REDOX SYSTEMS
 Reduction sequences as follows under submerged condition of
soil,
O2, NO3
-, Mn4+, Fe3+, SO4
2-, CO2, N2 and H+.
 These each are associated with H+ ions and Electrons.
 They includes systems like,
1. The O2 – H2O system
2. The N2 system
3. The Mn system
4. The Fe system
5. The sulphur system

B. CARBONATE SYSTEMS
It includes,
(a) high concentrations of CO2
(b) the presence of the divalent cations, Fe2+, Mn2+, Ca2+ and
Mg2+ in most soils, CaCO3 in calcareous soils and NaHCO3 in
sodic soils
(c) intimate contact between solid, solution, and gas phases
(d) virtual isolation of the system from the surroundings.
Thus sodic soils behave like NaHCO3, calcareous soils like
CaCO3, ferruginous soils like Fe3O4nH20, and manganiferrous
soils likeMnCO3 when submerged and equilibrated with CO2.
(Ponnamperuma et al., 1969)

 It includes the systems like,
1. The Na2CO3 – H2O – CO2 system
2. The CaCO3 – H2O – CO2 system
3. The MnCO4 – H2O – CO2 system
4. The FeCO3 – H2O – CO2 system

 Drastically retards gas exchange between soil and air
 Stabilizes soil temperature
 Causes swelling of colloids
 Destroys aggregates
 Reduces permeability
EFFECTS OF FLOODING

RETARDATION OF GAS EXCHANGE
 Oxygen deficiency - The moment a soil is flooded, its oxygen
supply is virtually cut off. Oxygen can enter the soil only by
molecular diffusion in the interstitial water. The process is
10,000 times slower than in gas-filled pores.
 Thus the oxygen diffusion rate suddenly decreases when a soil
reaches saturation by water. Within a few hours of flooding,
microorganisms use up the oxygen present in the water or
trapped in the soil and render a submerged soil practically
devoid of molecular oxygen.
 A flooded soil, however, is not uniformly devoid of oxygen: the
oxygen concentration may be high in the surface layer which is
a few millimeters thick and in contact with oxygenated water.

 The thickness of the layer represents a balance between
diffusion from the flood water and oxygen consumption by the
soil.
 It increases in thickness as the crop matures. Below the
surface layer, the oxygen concentration drops abruptly to
practically zero.
 The brown color of the oxygenated layer, its chemical
properties, and its oxidation-reduction potential undergo a
similar abrupt change with depth in submerged soils. The root
zone of rice is practically free of molecular oxygen
(Ponnamperuma 1972).
RETARDATION OF GAS EXCHANGE

ACCUMULATION OF CARBON
DIOXIDE
 The presence of a layer of water also drastically cuts down the
escape of soil gases. Carbon dioxide, methane, hydrogen, and
nitrogen produced in the soil tend to accumulate, build up
pressure, and escape as bubbles.
 The partial pressure of carbon dioxide in a soil increases after
submergence and reaches a peak of 0.2 – 0.8 bars 1-3 weeks
later. Carbon dioxide injury to rice may occur on acid soils low
in iron, in organic soils, and cold soils.

STABILIZATION OF SOIL TEMPERATURE
 The effects of flooding on soil temperature follow from three
important thermal properties of water – its high specific heat,
high latent heat of vaporization, and a higher thermal
conductivity than soil material.
 The high specific heat prevents violent temperature
fluctuation. The high heat of vaporization tends to keep
flooded soils cooler than dry soils.

 The cooling effect is used to reduce high temperature injury in
hot locations while the stabilizing effect is used to prevent low
temperature injury at night.
 Standing water markedly influences the microclimate of the
first 50 cm above the soil surface (Nagai 1958). Introduction of
cold or warm water into the field rapidly changes the
temperature of soil, air, and plants.
 Low soil temperatures retard mineralization of organic
nitrogen and phosphorus and favor the accumulation of
carbon dioxide, organic acids, and excess water-soluble iron in
flooded soils (Cho and Ponnamperuma 1971, Ponnamperuma
1976).

 The adverse effects of low soil and water temperatures are
present in tropical soils above 1000 m and in soils irrigated by
cold, mountain streams.
 Low water temperature reduces germination, retards growth,
and depresses grain yield.
 Grain yield is reduced if the water temperature is less than 17
degrees C during the active tillering and meiotic stages.
 Rice with panicle primordia above the water level suffered less
yield reduction (IRRI 1980).

SWELLING OF COLLOIDS
 When a dry soil is flooded, soil colloids absorb water and
swell. The rate of water sorption and volume increase of
mineral soils depend on the clay content, type of clay mineral,
and the nature of the adsorbed cations. Swelling is usually
complete in one to three days.
 The higher the clay content the greater the swelling. The
expanding-lattice type of clays (montmorillonite and
beidellite) swell more than the fixed-lattice type (kaolinite and
halloysite). Sodium clays swell more than calcium and
potassium clays.

 When a puddled soil is dried, it shrinks and the decrease in
volume equals the volume of water lost.
 Deep cracks are common in puddled rice fields after draining
and drying.
 They cause heavy loss of water by percolation during
reflooding (Wickham and Singh 1978).
SWELLING OF COLLOIDS

CONSISTENCY
 As the moisture content of a soil increases the cohesion of
water films around soil particles causes them to stick together
rendering the soil plastic. At this moisture content soils are
easily puddled.
 At higher moisture contents (as in flooded soils), cohesion
decreases rapidly, making tillage easy. But penetration
increases and soil strength decreases, rendering the use of
heavy machinery impractical on flooded soils.

DESTRUCTION OF SOIL AGGREGATES
 When a dry soil is flooded, the aggregates become saturated
with water. During the process, internal air pressure disrupts
the aggregates (Baver et al 1972). Swelling of colloids and
dissolution of cementing agents, such as iron oxide, further
decrease aggregate stability.
 Sodic soils show marked aggregate breakdown on flooding,
whereas soils high in iron and aluminum oxides and organic
matter suffer little aggregate destruction (Sanchez 1976). On
soil drying and oxidation, reaggregation occurs through soil
cracking and cementing by higher oxides of iron.

REDUCTION OF PERMEABILITY
(PERCOLATION RATE)
 Flooding decreases percolation rate in soils of low
permeability even without puddling. This has been attributed
to dispersion of soil particles, swelling, aggregate destruction,
and clogging of pores by microbial slime.
 In porous, non-swelling soils, flooding (by providing a greater
head of water) increases percolation (Wickham and Singh
1978).

REFERENCES
1. The Chemistry of Submerged Soils, F.N. Ponnamperuma,
Advances in Agronomy, Vol. 24
2. Physical changes in flooded soils, F.N. Ponnamperuma

Chemistry and physics of submerged soil

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