Environmental Engineering Reference
In-Depth Information
weathering products of clay minerals, oxides and hydroxides of iron and aluminium
('sesquioxides'), and silica. In the ensuing hot and dry season the non-crystalline
(amorphous) iron and aluminium oxides become dehydrated and crystallize to form
crystalline oxides; where the soil retains some moisture only partial dehydration takes
place, and the browner hydrated oxides of iron are formed ( goethite , α FeO.OH, and
lepidocrocite , γ FeO.OH). Where dehydration is complete (dryer soil climate, well
drained profile, porous parent material) the iron oxides take the form of anhydrous
haematite (Fe 2 O 3 ) which imparts a strong red colour to the soil. As this chemical reaction
is irreversible, the development of a red hue will increase with time, and thus the degree
of reddening can be used as an indicator of the age of a soil. In 1853 in Italy these red
soils were first called terra rossa and the designation has remained ever since. The whole
set of processes producing them is termed rubefaction (reddening) (see Colour Plate 24
between pp. 400 and 401). Figure 26.5 shows three profiles which are widespread in the
Mediterranean region and where the
Figure 26.5 The soil profile features of Brown
Mediterranean, Red Mediterranean and alluvial soils.
relative imprint of weathering, leaching, argillation and rubefaction varies. The Brown
Mediterranean soil (FAO Calcic Luvisol; terra fusca ) is characteristic of more humid
sites (higher rainfall, cooler summers, higher elevation, impervious parent material). By
Search WWH ::




Custom Search