Agriculture Reference
In-Depth Information
Bockheim (1979) found several well-defined relationships between soil chemical
properties and age in Antarctic soils. For example, there was a linear relationship
between water-soluble salt content (as electrical conductivity, EC) in the horizon of
maximum salt content and age for soils ranging from 18 000 to 135 000 years:
The tundra occurs to the south of the polar desert zone and is characterised by a closed
vegetation (Ugolini, 1986a); organic components thus have a much greater organic
influence on soil development. Most soils in the tundra are formed in heavy-textured
alluvium in low-lying environments where the permafrost is close to the surface.
These soils occupy 90-99 % of the landscape and are largely saturated with meltwater
during the summer thaw. Soil processes are therefore dominated by reduction, gleying
and low rates of mineral weathering. The low rates of organic matter decomposition lead
to peat formation. At well drained sites, the weathering processes are transitional with
the southern tundra soils showing some of the acidolysis, acido-complexolysis and
translocation typical of spodosols. The northern tundra soils evidence little of this with
an A horizon in which the soil organic matter is stabilised in humus complexes
and a B horizon that shows little evidence of illuviated materials, apart from some silt
(Ugolini, 1986ab).
In cool climates with vegetation, decomposition is slow to intermediate and
water-soluble acid solutions are very active in promoting hydrolysis, acidolysis or acido-
complexolysis. Biological activity in temperate regions is normally high, and may lead
to a rapid immobilisation or insolubilisation of organic acids; partial hydrolysis becomes
the main weathering process. However, where the parent material is of low nutrient
status (for example, 'acid' materials high in silica) or the vegetation produces 'low quality'
litters resistant to decomposition, active biological communities may not develop which
leads to weathering conditions dominated by acidolysis or acido-complexolysis.
In contrast, weathering in hot and humid climates is considered to be dominated by
geochemical weathering processes that extend over very long periods (Duchaufour,
1997). In these climates, decomposition is so rapid that intermediate organic compounds
may have little effect beyond the surface horizons. Hydrolysis becomes massive because
of high temperatures and high carbon dioxide concentrations in the soil solution,
particularly at depth. concentrations normally increase with soil depth, sometimes
markedly (Section I.1.2.2). Concentrations found at depth in an Amazonian oxisol
(Lucas and Chauvel, pers. comm.) have been up to forty four times greater those of
the atmosphere and may accelerate the alteration of primary minerals by lowering the pH
(by up to 0.5 units).
Monosiallitisation and allitisation are the dominant processes of humid tropical climates,
unless drainage is impeded when neoformed smectitic 2:1 clay minerals may accumulate.
Where longer dry seasons occur, weathering is limited to mono- and bisiallitisation.
In hot deserts, physical weathering processes predominate although important chemical
weathering processes may still occur (Watson, 1992).
