ABSTRACT
Synthesis/discussion ƒ 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’. De Oliveira 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.
