Cambisols, Luvisols and Planosols - Major Soil Groups of the World - page 239

is formed. A general brunification results, very clearly seen in section,
in quarries, where it penetrates to a few decimetres or metres. This
brunification was noticed by all the pioneers of pedology. Ramann, in
his topic Bodenkunde published in 1905, used the term Braunerde (brown
earth). Murgoci of Romania, in 1909, observed the Brauner Waldböden ,
a term translated by Anglophones to Brown Forest Soils (Tavernier and
Smith 1957).
For Duchaufour (1983) the association of iron with clay and organic
matter leads to clay-humus-iron complexes that confer a distinct, stable,
fragmentary structure on the material.
The solubility curves of ferric iron as function of pH (Chap. 3, Fig. 3.7)
show that Fe 3+ appears in noticeable quantities in the soil solution only
if the pH drops below 6. It can then be adsorbed on the clay minerals.
Thus, in calcareous environment, decarbonatation necessarily precedes
brunification. Contrarily, this brunification can be direct on acid igneous
rocks or even on basalts.

8.1.2 Illuviation of Clay
Vertical transport of clay minerals in water suspension is observed. It
results in enrichment of the intermediate horizons of the profile with
clay (B = illuvial horizons) and depletion of the horizons above (E =
eluvial). The terms clay illuviation , clay translocation and argilluviation are
synonymous. They characterize Luvisols in particular. Traditionally the
term leaching is reserved for removal of basic cations. Planosols resemble
the preceding soils but in them the contrast in texture between E and
B is particularly strong. The transition from the one to the other is very
abrupt, over a few cm at the most.

Scientists have identified two types of argilluviation.

'Primary' argilluviation pertains to a brown clay that is bound to iron.
It is typically seen in the pores lined with coatings, the birefringence
of which reveals a strong particle orientation (see Fig. 1.4). Taken at
low magnification, Figure 8.1 shows a pore gradually being filled up
with clay.
Some soil aggregates have shiny faces. It was thought that their faces
were coated with clay. We say clay skins and, more often, clay coatings .
But these shiny faces can also be pressure faces resulting from shrink-
swell activity without there being translocation of clay in the soil. To
create them, it is generally enough if the aggregate surface is smoothed
with a knife blade using gentle pressure.

Primary argilluviation