Podzolic Soils - Major Soil Groups of the World - page 329

The amount of pseudo-amorphous materials (proto-imogolites,
imogolites, allophanes—cf. Chap. 10) is very variable: from 0 to 100 per
cent of the clay fraction.
In thin section, the structure of the B horizons appears to be of three
major types (Righi 1975; Boudot and Bruckert 1978; Schwartz et al . 1986;
Buurman and Jongmans 2005):
￿ A flocculent microstructure formed of aggregates of size a few
fractions of a mm. These microaggregates are polymorphic . They
contain root debris, coprolithic materials (faecal pellets), fulvic
acids, fine-clay particles. This structure often characterizes the
Bs horizons situated at shallow depths in soils that have retained
some biological activity, particularly mountain soils and soils of
the boreal zone.
￿ A complete filling up with fine material (organic matter, iron,
aluminium) of the spaces between the skeleton grains. This
is then a homogeneous mineral-organic paste described as
monomorphic . It cracks on drying. Sometimes it constitutes a
cement provoking induration of the horizon that then becomes a
humic alios or ortstein . This second type of structure particularly
characterizes the deep B horizons of tropical Podzols.
￿ Lastly, metal-organic coatings can cover the skeleton grains. This
organization could be derived from the preceding one by drying,
that is to say, it will not necessarily come from a redistribution of
material from one horizon to the other. Figure 11.4 shows that a
very tiny amount of iron-organic substances, very small in mass
quantitatively, suffices to coat the skeleton grains and brightly
colour the horizon.
Fig. 11.4 Iron-organic coatings (or pseudo-coatings?) around sand grains (quartz and or-
thoclase) in the Bs horizon of a Podzol on gneiss. Pilat Massif, France. Photo : author.