Clay eluviation
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Fig.1 The process of clay eluviation |
(
Source: Bridges 1997, p. 35.) |
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Clay eluviation is the loss in suspension of material from a
horizon and refers to purely mechanical washing of fine particles.
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Finely dispersed humus and clay particles can move as colloidal
suspensions from upper (eluvial horizon) to illuvial horizons lower in the
profile. Clay is transported through the coarse pores with percolating soil
water.
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The process takes place where rainfall exceeds
evapotranspiration seasonally. The process appears to be encouraged by a
climate where period of desiccation results in the soil shrinking and cracking.
As the soil dries, suspended material is deposited from the solution along
pores.
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Pedologically, clay eluviation is one of the most important
translocation processes and results in the development of a B horizon enriched
in clay. This horizon is termed argillic horizon, Bt or textural B
horizon.
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Fig.2 Argillic horizon |
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The Bt contains often clay coatings on subsurface soil
structures and often has a prismatic or or sub-angular blocky
structure.
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Note: clay eluviation horizons can also be formed by the
process of weathering but then lack the structured form of clay cutanes (clay
skins).
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Fig.3 Clay skins |
(
Source: Bridges, 1997.) |
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Clay eluviation consists of 3 major processes:
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Mobilization of clay in the upper horizon
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Transportation in the lower (illuvial horizon)
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Immobilization of the transported clay
1. Mobilization of clay in the upper
horizon
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The dispersion and mobilization of clay in the upper horizon is
pH-dependent.
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At pH values higher than 6.5, a high Ca-saturation in soils
prevails and Ca-clay-bridges are formed that stabilizes clay aggregates.
[Exception: Na is the dominant ion in the solution].
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Between pH 5.5 and 6.5, the concentration of dissolved
Al3+ ions and divalent cations in the soil is
marginal/negligible leading to the resolution of the bridges. As a result, clay
disperses and no longer is flocculated.
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At pH lower 5, a higher Al-concentration prevails in the soil
solution and Al-clay-bridges are formed. Clay is aggregated (is in flocculated
state).
2. Transportation in the lower (illuvial horizon)
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The transportation of the dispersed clay particles in the lower
horizon (illuvial horizon) requires pores and voids which have a diameter of at
least 20 µm.
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The process appears to be encouraged by a climate where period
of desiccation results in the soil shrinking and cracking. This is primarily
found in soils containing smectites (2:1 layer clay mineral that from cracks
during the dry season).
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Further informations
( here)
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3. Immobilization of the transported clay
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Through flocculation due to an increase of ion concentration in
the soil solution; e.g. the appearance of Ca2+-ions
in a carbonate-rich horizon in the subsoil.
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Through Filtration: as the soil dries, the suspended material
is deposited from the soil solution on the sides of the peds and along pores
resulting in a strong orientation as clay cutanes/skins (see fig.2).
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Further reading:
Clay
eluviation/illuviation processes including the micromorphology of argillic
horizons
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