Agriculture Reference
In-Depth Information
horizon. Weathering is minimal in these youthful soils and most clays are inherited from
the parent materials (Ugolini, 1986a).
North of the tundra zone the soils are marked by decreasing biological inputs and
increasing dryness. Desert pavements become more apparent and A horizon development
is minimal with a brownish-yellow Bw horizon often present below the desert pavement
(Ugolini, 1986a). The soils are structureless, coarse-textured, saline and pH ranges from
neutral to alkaline (Campbell and Claridge, 1992).
Soils of the Antarctic cold desert are typically coarse-textured, with desert pavements
at the surface and an horizon of salt accumulation. Salinity is high and salt
efflorescences and calcite crusts are common. The salts present include nitrates and
sulphates of aerosolic origin inland and those derived from rock weathering; marine
aerosols deposit salts close to the sea. Soil reaction ranges from mildly acid to alkaline.
A permafrost typically occurs within 100 cm of the surface. Very limited chemical
weathering occurs although older soils may have an oxidised horizon comprising thin
oxide coatings over the mineral grains (Campbell and Claridge, 1992). While the soils
are generally of coarse texture, up to
ca.
5 % of clay sized materials occur within the older
soils, often largely comprising finely subdivided parent materials. However, limited
transformation of micas and illites has been noted and smectites have been recorded
from alkaline environments (Campbell and Claridge, 1992).
4.3.2
TEMPERATE CLIMATE PROCESSES
In temperate climates chemical weathering is greater than in the cold regions but is less
intense than in tropical areas. Consequently, for a specific landscape age, the parent
material influences are greater and the roles of plants and soil organisms and their
metabolites are of particular importance.
Pedogenesis follows very different pathways depending whether the parent materials
are calcareous (limestones) or acid (
e.g.,
granite, sandstone or gneiss).
4.3.2.1
Calcareous bedrock
The main weathering processes occurring in the softer calcareous rock parent materials
are the dissolution of carbonates by carbon dioxide (decarbonation), organic acids and
nitrates dissolved in the soil solution and accumulation of the remaining silicate clay
minerals. However, Al and Fe may originate from sources other than bedrock residues
including aeolian deposits (Cornu, 1995). Dissolved minerals are leached to
subterranean waters or accumulate in low-lying areas; such monovalent ions as
and
are leached much more rapidly than their divalent counterparts
Biological activity is intense and mineral element losses are reduced by the intimate
mixing of mineral and organic components which promotes the formation of stable
organo-mineral complexes. Under such conditions, clay mineral alteration is reduced
and the neoformation or the conservation of clays with high silica contents is favoured.
The soils that form on hard indurated limestones are of a very different character and
are often old polycyclic soils that have formed on heterogeneous parent materials and
undergone additions of other materials during development. In contrast to soils formed
