Geoscience Reference
In-Depth Information
Synthesis/discussion
To summarize, we feel that, starting from various kinds of materials
in temperate climate, weathering leads in the first phase to massive
formation of clay in the B horizon and, in the second phase, to
disappearance of the clay and accumulation of residual silts and sands.
But these two phases more or less coexist in the B. In the same profile,
at the same time, some clay minerals are formed (micro-sites) and others
destroyed (in the pores accessible to drainage water). The E horizon
develops when the stock of primary minerals is exhausted and clay
destruction surpasses clay formation.
In fact, the question of transport versus weathering/degradation
has been exercising minds in the discipline for decades, although
some investigators of high repute have concluded after discussion that
argilluviation is preponderant. We should cite in particular Bullock
et
al
. (1974), de Coninck
et al.
(1976), Jamagne (1978) and van Ranst and
de Coninck (2002). We are of a different opinion (Bornand 1978; Legros
1982) and our point of view is also popular. We quote from Gunal
and Ransom (2006) in support of our view: '
In-situ weathering of biotite
was one of the reasons for the clay increase in all soil profiles. In all the soils
studied, the clay increase and cause of clay orientation cannot be attributed
to a single genetic process or event. Both illuviation and shrink-swell activity
were involved in orientation of the clay
'. D e O l i v e i r a
et al
. (2008) said: '
The
micromorphological descriptions indicate no evidence to support that clay
illuviation played an important role in the formation of the textural gradient
of these soils
' (i.e. Luvisols and Planosols of Brazil). Lastly, we give the
viewpoint of Sauer
et al
. (2009): '
As preferential flow and leaching along
the cracks continues, the tongues increase in length and width, progressively
consuming the prisms between the cracks in the upper Bt horizon
'.
Our model clearly explains the low mass of coatings in the B, the
considerable loss of material measured by the authors (soil collapse) and
the presence of papules behind the depletion front (residues of B). It is
coherent with the observations presented above, which also show the
almost simultaneous formation and destruction of clays in the profiles.
Certainly, the discontinuities occurring in mature soils can have other
origins (Phillips 2004). But it is important to note that weathering in
the temperate zone leads naturally to large textural differences in the
profiles. Invoking other causes is justified only if their involvement has
been demonstrated.
