Ferralsols and other Soils of the Hot Regions - Major Soil Groups of the World - page 168

In Soil Taxonomy, the Ferralsols become Oxisols.

5.4.3 Other Tropical Soils
In the intertropical zone, there are soils geographically associated with
Ferralsols without forming part of them because they do not have the
characteristic 'Ferralic' horizon. For want of anything better, because
studies are still lacking, the WRB distinguishes them by two of their
chemical characteristics: type of clay and base saturation percentage
(Table 5.7):

Table 5.7 Different types of tropical soils with clay-enriched horizons at depth
(according to WRB).

Base satn ≈ Base satn

50% << 50%

2/1 clay mineral present (smectite) Nitisols Alisols

Acrisols

Exclusively 1/1 clay mineral (kaolinite) Lixisols

Nitisols (earlier Nitosols) are found in very dry climates. They are less
mature than Ferralsols and do not exhibit thick mottled altérites at depth,
so characteristic of Ferralsols. They are very red and form on basic rocks
(volcanic rocks, limestone). In the field, we often see intergrades between
Ferralsols and Nitisols and, sometimes, sequences mixing the two types.
Nitisols are very abundant in East Africa, in Central America, in the
Caribbean islands and in part of Asia. They contain a small amount of
2/1 clay minerals.
Alisols also have 2/1 clay minerals, but are strongly desaturated.
Aluminium freed by the acidity can be trapped in the interlayer position
(2/1/1 clay mineral). When they are red, they can be grouped with the
degraded red Mediterranean soils of France.
Lixisols have 1/1 clay minerals, but a higher base saturation. They
can be grouped with soils earlier classified as Tropical Ferruginous soils
in the French classification.
Acrisols contain 1/1 clay minerals, often large quantities of gibbsite
and low base saturation. They are acid and depleted of clay, at least in
the surface layer. They are close to the Ultisols of USA and correspond to
the 'Yellow Latosols' of the Brazilian classification. Thus they represent
a very mature extreme of tropical soils. They are found in high-rainfall
environments (more than 2500 mm y -1 ), are widespread in South
America, Amazonia and Colombia for example (Gaviria 1993), and are
also seen in Southeast Asia. They do not have a cuirass, but it could have