Micromorphology of argillic horizons
The micromorphological expression of an argillic horizon depends to a large extent on the texture of the soil, as this determines not only the size of the particles which can be transported, but also influences the size of the pores through which the illuviated particles move; it also decisively affects the stability of the illuvial deposits formed. Three general cases can be distinguished: soils with sandy, loamy and clayey texture.
Sandy argillic horizons
In sandy soils, the porosity is mainly made up of fairly continuous simple packing voids, through which various sized particles can pass. In the argillic horizons of these soils, the mineral grains are coated with and fairly bridged with illuvial clay.
Many argillic horizons with a sandy texture have a microstructure ranging from grains with coatings but lacking bridges and soils with both coatings and bridges.
In most cases, these coatings have an intense interference colour, although there is a mixture of particles of different sizes. In any event, the illuvial accumulations in these soils always have a very distinctive clear boundary with regard to the sandy matrix of the horizon, making it easy to identify the illuvial origin of the deposits.
Additionally, as these horizons have low dynamics, the clay coatings are very stable. To sum up, the sandy argillic horizons are characterised by some very distinctive and permanent features which are fairly uniformly distributed and from which it is easy to recognise their illuvial origin.
Horizontes argíllicos francos
In soils with a loamy texture, the translocated clay can be identified by the fact that the surface of the aggregates and the walls of the macropores are coated by clay skins. When there are no aggregates, the translocated clay establishes itself in channels and cracks. The difference from the soils with a sandy texture is thus obvious.
In these soils with a medium texture, the contrast in the distribution of the size of particle between the illuvial clayey wrapping and the adjacent less clayey matrix is very clear and this, together with the strong birefringence of the illuvial skins makes them easy to recognise. All the classic features of clay skins (strong orientation, strong textural contrast, abrupt boundaries and laminations are apparent in these clay coatings which appear in the soils with medium textures. These illuvial deposits are rarely uniformly distributed throughout the horizon.
As many argillic horizons in soils with medium textures are subject to a high biological activity and other forms of pedoturbation, disruption of the clay coatings is a very common feature. The degradation of the clay coatings makes them difficult to identify in the field, but they are clearly recognisable under the microscope as they still show the remains of their strong initial orientation and have a very fine texture.
Occasionally, they may be confused with separations of plasma associated with contraction and expansion, if there exists a highly developed fabric, but the presence of laminations, abrupt boundaries and strong orientations can be the identifying features of the illuvial process.
Clayey argillic horizons
In soils with clayey textures, the illuvial arcilanes are generally difficult to identify, both in the field and under the microscope. We can be reasonably sure that translocation has taken place, but most illuvial clay skins are difficult to recognise, mainly for the following reasons:
1 ) As the matrix is clayey, it is not possible to detect the textural contrast between this and the illuvial clay.
2) Owing to the very texture of these horizons, they are particularly prone to severe expansion and contraction. This dynamic behaviour means that the illuvial clay coatings have a very short existence, as they suffer severe fragmentation accompanied by internal disorganisation of their constituent particles and are rapidly displaced within the soil matrix. In this way, either they disintegrate totally or if they remain, they are very difficult to distinguish, particularly due to the fact that clayey soils show a strongly birefringent fabric, with highly developed plasmic orientations, which have features very similar to those of illuvial clay. Additionally, the slickensides along the cracks can give the impression that they are illuvial clay coatings surrounding the pores.
These reasons account for why the identification of translocated clay in clayey soils is difficult and even, occasionally, impossible. In these cases, the deepest horizons should be examined. At deeper levels, the clay skins tend to be more stable and remain on the surface of the aggregates due to the fact that the structural dynamism of the horizon is lower.
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