Eluviación / iluviación

Two processes can be distinguished in the translocation of substances: an initial mobilisation and loss of materials, which develops in the surface horizons and is known as eluviation; and a second process which represents the deposition and immobilisation, with a gain in substances in the subsurface horizons which is known as illuviation; in both cases, water is always the means of transport.

From the above, it can be seen that this process can be divided into three stages: first dispersion, then transport and finally accumulation.


First stage: mobilisation of the clay

It seems to be accepted that in order for a considerable mobilisation of clay to occur, there has to be a prior dispersion. The dispersion happens as a result of the mutual repulsion experienced by the clay particles as a result of the existence of electric charges, fundamentally negative, located on their surface.

The main factors which affect dispersion (and which will thus affect mobilisation) are: the type of clay minerals, the particle size, pH, the types of cations present (absorbed by the clays), the electrolyte concentration in the soil solution and the organic matter content.


Second stage: transport

As an identical or very similar mineralogy for clays in both eluvial and illuvial horizons is observed, we can assume that the clays are transported as solid particles in suspension, without suffering any important transformation at this stage.

The factors involved in this phase are: the quantity of water and the porosity of the soil.


Third stage: accumulation

The accumulation may occur either because of a detention in the infiltration of the suspension, or through flocculation. The causes are due to mechanical paralyzation of the gravity water front at a certain depth, because the macroporosity is reduced to a minimum), biological (by biodegradation of the organic substances which are contained in the clay) and physico-chemical (a rise in the electrolyte concentration because of a change in the pH or enriched levels of iron and aluminium compounds that would flocculate the clays.


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