Geoscience Reference
In-Depth Information
8
Cambisols, Luvisols and
Planosols
According to the WRB, Cambisols cover 1500 million ha in the world,
Luvisols cover 500 or 600 million ha and Planosols, 130 million ha.
Also, on the soil map of the world produced by the Americans on the
scale of 1:10 million, Western Europe is variegated with Inceptisols
(approximately Cambisols) and Alfisols (Luvisols). These are very
important soils for a large part of humanity. But only the Luvisols and
Planosols will be discussed here. Cambisols, being less differentiated,
are less important at the scientific level and will only be mentioned.
8.1
FUNDAMENTAL MECHANISMS OF DEVELOPMENT
In the temperate environment, soils are mostly under the influence of
three fundamental mechanisms of development that must be reviewed
before the relative importance of each can be understood.
8.1.1 Weathering
In the soils with particles observable under the microscope in thin section
(sands, silts), many authors have noticed that the primary minerals are
weathered to clay. This pertains particularly to fresh concentrations not
organized into lamellae (Feijtel
et al.
1988). When time has not done its
job, as for example in the case of Cambisols, this is often manifested
in a slight tendency to increase in clay content towards the top of the
profile, in keeping with a stronger weathering. At the same time, the
clay minerals originally present in the rock (chlorite, illite) give place
gradually to neoformed minerals of the smectite type. This weathering
has for corollary the extraction of iron (Fe
2+
) from crystalline structures
and its oxidation to Fe
3+
on contact with the oxygen of the air. Goethite
