Fig.1 The process of clay eluviation

( Source: Bridges 1997, p. 35.)

• Clay eluviation is the loss in suspension of material from a horizon and refers to purely mechanical washing of fine particles.
  
• 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.
  
• 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.
  
• 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.
  

  Fig.2 Argillic horizon

• The Bt contains often clay coatings on subsurface soil structures and often has a prismatic or or sub-angular blocky structure.
  
• Note: clay eluviation horizons can also be formed by the process of weathering but then lack the structured form of clay cutanes (clay skins).

  Fig.3 Clay skins

  ( Source: Bridges, 1997.)

  
  

• Clay eluviation consists of 3 major processes:
  

1. Mobilization of clay in the upper horizon
   
2. Transportation in the lower (illuvial horizon)
   
3. Immobilization of the transported clay
   

1. Mobilization of clay in the upper horizon

• The dispersion and mobilization of clay in the upper horizon is pH-dependent.
  
• 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].
  
• 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.
  
• 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)

• 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.
  
• 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).
  
• Further informations ( here) .
  

3. Immobilization of the transported clay

• 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.
  
• 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).
  
• Further reading:  Clay eluviation/illuviation processes including the micromorphology of argillic horizons