Geoscience Reference
In-Depth Information
In steeper areas associated with the recent volcanic
chains, such as eastern Asia, Indonesia, New Guinea and
the Andes, mass wasting may become more significant.
Slides and flows can strip away vegetation and weathered
material to expose regolith and even bedrock. The effects
of these processes can be seen from satellite images of the
Amazon delta, where muddy waters from the Andean
foothills can be distinguished as they gradually mix with
the clean sea water.
In calcareous regions, solution is a highly significant
and rapid process which produces unusual landform types
such as the cockpit country in southern China and the
Caribbean. However, detailed studies in karst areas
indicate that the principal climatic factor affecting erosion
rate is the mean annual run-off. Whilst temperature has
some influence, it appears to operate through the degree
and type of soil and vegetation cover rather than as a direct
control of solution. Until the tools of process studies are
powerful enough to unravel the interaction of lithology,
erosion rate and time, it will not prove possible to
determine with certainty whether latitudinal variations of
climate have had more than a coincidental effect upon the
development of karstic landforms.
Soils
Within the humid tropics there are a group of distinctive
soil processes which are rarely found outside this zone.
These are rapid weathering and strong leaching, the
properties of a deep and highly weathered regolith and the
importance of organic matter in soil fertility and manage-
ment. The predominant minerals are kaolinitic clays and
hydrous oxides of iron and aluminium (sesquioxides)
which give the strikingly red colour to the soils. The
tropical climate is important in the operation of these
processes and provides the framework in which the soils
develop. In addition, lithological variations and relief play
an important part in the actual differentiation of soils.
Because organic matter is rapidly decomposed, the
main problem of soil utilization in the tropics is its
maintenance at suitable levels. The quantity of organic
matter lost from the soil during one year of cultivation in
the lowland tropics is of the order of two tonnes per
hectare (
Plate 27.1
).
To replace this is a major problem.
Research has shown that there is no practicable means of
maintaining organic matter under cultivation of annual
crops in the rain forest zone other than by an extended
fallowing system. It is almost impossible to replace organic
matter by fertilizers; they either become leached (N, Ca)
or fixed (P). Nutrient cycling in tropical rain forests was
discussed in
Chapter 21.
litter survives on the forest floor but the amount is small
compared with the annual fall.
Photo: Peter Smithson
Forests
The natural vegetation of much of the humid tropics is
forest. In the moister areas we find the true tropical rain
forest which covers about 13ยท2 per cent of the land surface
of Earth, about 17 M km
2
. These complex and variable
forests grow in lowland areas with over 1,700 mm of
annual rainfall and no distinct seasonality. When the
monthly rainfall drops below about 120 mm for longer
than one month the rain forest tends to be replaced by
tropical moist forest. When there is a strong seasonality
the trees become deciduous and so we find tropical
deciduous forest replacing tropical rain forest.
The soils associated with the forests are characterized
by intense and perhaps prolonged weathering, with active
leaching. Decomposition is so rapid that, despite high
inputs of plant debris, the soils rarely develop a distinct
organic surface layer. Moreover the intense weathering
