Ferralsols and other Soils of the Hot Regions - Major Soil Groups of the World - page 148

become synonymous with tropical soil in the expression 'lateritic soil'. It
has, therefore, been abandoned in favour of other terms, chiefly Ferralsol
(Ferrallitic soil) and Plinthosols (hardened soil, from Greek plinthos =
brick), as well as Alisols , Lixisols , Acrisols , which will be defined later.
5.1.1 Principal Characteristics of Ferralsols
The most typical Ferralsols have, on the surface or close to it, a
material ( Ferralic horizon of the WRB or Oxic horizon of the Americans)
that exhibits the following features in the top one or more metres
(Eschenbrenner 1988):
￿ reddish colour imparted by iron oxides;
￿ low bulk density and high porosity;
￿ good micro-crumb structure in the horizons near the surface;
they contain pellets 50-300 µ m in diameter, composed of kaolinite
and ferric hydroxides enclosing quartz grains (Chauvel et al. 1977;
Balbino 2001); they result from biological (termites) activity; these
pseudo-sands are destroyed when kneaded for a time with a little
water between the thumb and index finger; the material becomes
plastic and reveals its clayey nature;
￿ very high clay content (generally greater than 60%);
￿ predominantly clay minerals of the kaolinite family ( low-activity
clays ); presence of hydroxides of aluminium and oxides and
hydroxides of iron;
￿ low cation exchange capacity [<16 cmol (+) kg -1 clay] related to
the presence of kaolinite, which has an electrically neutral layer
and only edge charges;
￿ high anion exchange capacity at the pH of the soil because
of presence of oxides and hydroxides that have amphoteric
properties; the AEC can be 4 or 5 cmol (-) kg -1 clay at pH 4
(Becquer et al . 2001);
￿ pH (in water) of 4.5 to 6.5;
￿ low water reserves: <1 mm per cm soil and sometimes 10 times
lower;
￿ almost complete weathering of primary minerals (feldspars,
micas, amphiboles, pyroxenes, peridots…);
￿ retention of a high proportion of quartz;