First illuviation (typic)In the illuviation of clay, two phases can be distinguished.
First illuviation (typic)
The first phase is called primary illuviation (which may be considered as the normal illuviation). It occurs typically in soils developed over unconsolidated original sedimentary material with good drainage conditions in slightly humified, neutral or slightly acid and biologically active mediums. Under these conditions, if the climatic characteristics are suitable, the clay particles stick to the iron, forming stable complexes which can migrate together to deeper layers.
This phase of illuviation is characterised micromorphologically by the existence of reddish or yellowish clay coatings (depending on whether they contain a lot or a little iron, respectively); these colours also appear with crossed nicols. They show strong orientations and are homogenous, microlaminations only being seen as a result of variations in the iron content which clearly show the successive phases of accumulation.
The development of this illuviation leads to the formation of non-hydromorphic illimerized (clay illuviated) soils (luvisols, acrisols, etc.)
Hydromorphic illuviation
The other illuvial phase, which may be develop independently or as a result of the first, is known as secondary or hydromorphic. During the previous phase, the soil presented a good structure, in very fine, stable blocks, but as the illuviation of the clay develops to its maximum, the clay accumulated in the pores is so great that these become completely full; the porosity therefore decreases drastically, the structure begins to deteriorate and hydromorphic properties begin to appear. The medium becomes increasingly acid and the biological activity diminishes, meaning that the soluble organic compounds are more stable. A certain reduction is produced and thus part of the ferric iron is changed into ferrous iron which, as it is now soluble, separates from the clay. Now the clay mixes with the organic material and forms a very stable complex which can migrate, despite the acidity of the medium, to much deeper zones than in the previous phase.
This phase is characterised by the presence of white or greenish-grey clay coatings which are more heterogenous and contain a considerable amount of silt and organic matter which impede the orientation of the clay particles.
This process leads to the formation of hydromorphic illimerized soils.
Requisites: scarce porosity, structure in degradation, strong acidity, low biological activity, anaerobic conditions.