Geoscience Reference
In-Depth Information
climate and uniform vegetation we can study the role of relief in
differentiation of the soils. We shall see in Chapter 2 some situations
that allow these monofactorial studies in the field.
1.2.2 Laboratory Investigations
The thin-section technique was established by geologists. The method
consists of slicing rocks to sections as thin as a few hundredths of a
millimetre, at which thickness they are transparent and can be studied
under the microscope. It was adapted for soils by Henri Lagatu and
Auguste Delage of France. These scientists introduced a hardener to
prevent crumbling of the naturally soft soil. From then on a new world
became accessible to the soil scientist. The work earned the authors a
gold medal at the Paris World Fair of 1900.
Following this,
micromorphology
became a branch of the discipline of
pedology. It led to description of the aggregates that combined particles
of various sizes (sands, silts, clays) in soils. This in turn gave rise to the
publication of numerous papers using a new vocabulary, quite esoteric,
sad to say. In the twenty years between 1970 and 1990, it was fashionable
to learn to talk jargon by using the new language. But descriptions in
science are, most of the time, of interest only if they can be interpreted
in terms of soil processes. Yet, unfortunately, this has not always been
the case. Thus micromorphology has suffered a sort of disaffection. The
pendulum has certainly swung too far. It is a pity we are today deprived
of an always enriching direct method of study (Fig. 1.4).
Thin sections
The study of the surfaces of sand grains enables identification of polished
faces (flowing water), of dulling (wind), of dissolution and corrosion
(soil solution) or even precipitated forms. In Rwanda, it was shown
that the surface of quartz grains was different at the top, in the middle
and at the bottom of slopes in relation to the intensity of dissolution in
accordance with topographic position (Marcelino
et al
. 1999).
Examination of sands
1.2.3 Chemical Analysis
Consideration of this question is not in our objectives or resources,
although analytical data have to be used in presentations and
demonstrations. Specialized topics on the topic are available (Baize
1993).
