Environmental Engineering Reference
In-Depth Information
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 characteristics which are
rarely found outside this zone. These are the processes of rapid weathering and strong
leaching, the properties of a deep and highly weathered regolith and the importance of
organic matter in soil fertility and management. 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 28.1). 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 22.
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 million km 2 . These complex and variable forests grow in lowland areas with
over 1700 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 and leaching mean that the more soluble constituents are totally
removed. Iron and even silica may be mobilized. These soils have a low cation exchange
capacity and a limited supply of bases such as calcium and potassium (i.e. a low base
status). They possess high iron oxide contents which give red soil colours. In the FAO-
UNESCO soil classification scheme these soils are mainly ferralsols and acrisols. They
also include the ferricretes and laterites, which are frequently defined as tropical forest
soils.
Tropical rain forest vegetation is typically diverse both in species composition and in
structure. Whereas temperate forests may contain only three or four tree species per
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