SOIL FORMATION - Soil Ecology - page 172

Podzolisation is often associated with the presence of a fluctuating perched watertable
created by the blockage of pores in the deeper horizons by translocated clay minerals
(Guillet et al ., 1979).

Andosolisation
This process is characteristic of soils forming on eruptive materials containing volcanic
glass or readily-weatherable fine-grained crystalline materials. A major feature is the
presence of active aluminium, either amorphous or associated with allophane. In this
process, the precursors of humic compounds are rapidly insolubilised by active aluminium
giving substantial protection against mineralisation. In turn, this protection reduces
the production of crystalline clay minerals and only small amounts of halloysite are
normally produced. Andosolisation is accompanied by a continuing loss of basic cations
and silica (Duchaufour, 1982, 1997).

Vertisolisation
Vertisols are soils with high clay contents that form under conditions of contrasting
seasonal climates, notably in the tropics and sub-tropics and characteristically in
base-rich locations with restricted drainage. The clays typical of these soils are largely
neoformed smectites (Section I.1.1.1.2) which shrink markedly and deeply on drying to
form polygonal patterns of cracks and swell on rehydration. The consequence of this
dehydration:rehydration process is a churning or a turnover of the upper profile leading
to its homogenisation. This occurs through materials falling into the cracks that form
during dry periods; on rehydration, the centres of the polygons formed are lifted upwards
to accommodate the extra materials deposited at depth. This churning gives rise to
characteristic structures (slickensides) in the soils formed where blocks of soil slide
against each other.

Ferrallitisation
Ferrallitisation is characterised by the contrasting apparent downward movement of silica
and the accumulation of sesquioxides. Such processes are characteristic of conditions
of active tropical soil weathering. As occurs during podzolisation, the original clays are
broken down and poorly crystallised kaolinite or the aluminous mineral gibbsite are
recrystallised. Active alteration and translocation result in a significant lowering of
the soil surface associated with the loss of materials (Chauvel, 1976; Lucas et al., 1996).

Additional processes
Three additional processes are recognised which may occasionally superimpose their
effects on those listed above:
(i) Gleying, movements of iron and manganese and their accumulation in reduced
forms under conditions of impeded drainage and low redox potential.
(ii) Rubefication, the in situ weathering of iron oxides and their transformation into
deep red, poorly-hydrated ferric oxides haematite) under climates with

marked seasonal contrasts.
(iii) Solonisation (or alkalinisation), the leaching and illuvial accumulation of clays
and sesquioxides in the B horizons of alkaline soils (Duchaufour, 1997).