Agriculture Reference
In-Depth Information
In tropical environments, weathering is intense and as much as 57 % of the original
material, mainly comprising bases and silica, may be leached or eluviated out compared
with 24
%
under equivalent temperate-climate conditions (Lelong and Souchier, 1972).
Primary minerals disappear while sesquioxides accumulate and low activity clays such
as kaolinite are formed. These clays may be further degraded in the upper horizons and
nutrients leached out. Depending on their final nutrient status, these soils are defined as
unsaturated ferrallitic soils (ultisols and oxisols; Soil Survey Staff, 1999).
Under drier conditions, weathering is less complete and a proportion of the primary
minerals may persist leading to the formation of ferruginous soils (alfisols; Soil Survey
Staff, 1999) which have a higher nutrient status than the ferrallitic soils.
As discussed above (Section II.3.3.1), vertisols are predominantly formed in areas of
low relief and from basic parent materials in climates with strongly differentiated wet
and dry seasons (Millot, 1979). Where unexploited, most have appreciable nutrient
reserves. Such soils are widespread in Australia, Africa, India and in parts of Central
and South America.
4.3
Pedogenetic processes in cold and temperate climates
Soil processes differ very strongly below and above the Arctic treeline and in the ice-free
areas of Antarctica. The latter two areas are underlain by frozen ground and northwards
of the tundra are subject to an increasing aridity, also characteristic of the Antarctic climate.
4.3.1
COLD CLIMATE PROCESSES
In going northward from the Arctic treeline, the tundra supervenes with its continuous
cover of vegetation (Ugolini, 1986a) and this progressively gives way to the sub-polar
desert, the polar desert and finally the cold desert. This latter zone may occur in
northern Greenland but its major representation is in the ice-free areas of Antarctica
(Campbell and Claridge, 1992). North of the tundra zone, the environment becomes
increasingly arid and the vegetation becomes increasingly discontinuous with
a commensurate reduction in its influence on soil processes. Finally, in the cold desert,
the vegetation is restricted to a few scattered mosses and lichens in favourable locations.
To the north of the Arctic treeline, the soils are largely frozen or underlain by permafrost.
These are all subject to the substantial physical effects of wind erosion and
cryoturbation including the patterned ground common to the polar and cold desert
soils (Ugolini, 1986b).
Because of the ice melting during the summer thaw, most soils in the tundra are very
wet. They typically have a surface horizon of decaying vegetation up to 15 cm thick
overlying 10-30 cm of light olive to olive-gray very wet silt loam overlying very dark
grey permanently frozen silt loam. Physical processes associated with freezing and
thawing dominate the landscape while reduction, gleying and the very slow
decomposition of organic matter dominate the pedogenetic processes (Campbell and
Claridge, 1992). In the lesser areas of dry tundra, acid Arctic Brown soils (inceptisols)
are typical with humic A horizons and dark brown to yellow brown colours in the B
