Agriculture Reference
In-Depth Information
The soils that result from extended periods of weathering are known as palaeosols.
These soils possess characteristic suites of properties that depend on the properties
of their parent materials, the weathering environments to which they have been
subsequently exposed and any additions that have been made in the form of, for example,
dust or volcanic ash. Within their profiles, structures such as hardened layers or horizons
of silica (silcretes) or iron (ferricretes) may reflect pedogenetic processes that occurred
under climatic conditions that no longer exist. Two major ecological features of these
soils are their diminished capacity for biological production and their susceptibility to
the loss of a large proportion of their often-limited nutrient capital following disturbance.
Perhaps the world's most fertile soils are those associated with the geologically
active areas of the world which include such areas of recent vulcanism as the Pacific
'Rim of Fire', recently uplifted and glaciated areas and locations where recent
deposition has occurred. In contrast, highly-weathered and poorly-fertile soils are
abundant in stable continental areas of low relief, remote from active plate boundaries
(Fyfe et al., 1983). Such areas are widespread in the old landscapes of the world's
tropics, notably Australia, Africa and South America.
Rejuvenation of these landscapes is dependent on large-scale depositional or tectonic
events to provide a new parent material base for weathering into soil. A further source of
rejuvenation is large-scale erosion which may expose underlying, unweathered parent
materials.
Highly-weathered soils tend to be rich in neoformed clays because of the gradual
replacement of rock-forming minerals by those more stable in the near-surface environ-
ment. The dominant phyllosilicate clays present in these soils are those with low cation
exchange capacities and the mineralogy of many older tropical soils is dominated
by such 1:1 clays as kaolinite and halloysite together with the oxides of iron and
aluminium (Section I.1.1.1.2). Because of the oxide minerals and organic matter present,
the cation exchange complexes of these soils often possess an appreciable pH-dependent
charge component.
Concentrations of the biologically-important elements in such highly-weathered soils
are characteristically low. Phosphorus is a key element in both biological productivity
and in pedological development and is supplied very largely by the parent material
(Walker and Syers, 1976). However, Newman (1995) reports annual aerial deposition
rates of 0.07 to and some sites may be in approximate balance with
weathering losses. Nonetheless, in highly-weathered soils, concentrations are low and
a large proportion of that present occurs in organic form; depending on the pH, much
of the remainder may be occluded and therefore largely unavailable to organisms
(Section I.3.1.2.3). Deficiencies of phosphorus and other essential elements commonly
limit productivity in palaeosols.
Because of the low nutrient status of highly-weathered soils, aerial inputs of elements
in rainfall and dust (Section I.3.1.3) may become significant components of site nutrient
balances in areas where such soils occur (Artaxo et al., 1988). Jordan (1982) considered
that, in tropical Amazonia, the maintenance of the large evergreen rainforest biomass
is dependent on such inputs.
The biological implications of extended soil weathering are substantial. Since palaeosols
have a poor capacity to supply and retain plant nutrient elements, they naturally support
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