Agriculture Reference
In-Depth Information
The contrasting soils that form on calcareous and acid rock parent materials are
discussed below.
Both climatic factors and the nature of the parent materials generally determine
the broad compositions of biological communities and pedogenesis in many areas may
largely result from the activities of organisms and the reactions induced by their
metabolites. This will exclude the areas of hot and cold deserts and those soils subject to
deep geochemical weathering where the biological influence on soil weathering is much
reduced. Whether occurring naturally or through man's activities, major structural
changes in the plant cover, as from forest to grassland, across sites with similar soils
usually lead to large changes in the soil environment. These include different litter and
sometimes mycorrhizal types and thus altered chemical weathering processes and
microclimatic regimes in the upper soil and the area immediately above it.
A major factor in pedogenesis across all zones is drainage status since soils differing
in this regard also differ strongly in chemical, physical and biological regimes.
Where drainage is inhibited, soils often have low redox status and hydromorphic
properties characterised by reduction and segregation of Fe (Section I.1.2.3) and a
reduced or inhibited rate of organic matter decomposition. A wide range of these soils
exists - permanently to intermittently saturated associated with poorly-drained coastal
and inland areas and such water bodies as rivers, ponds and lakes and areas flooded for
rice production. Apart from the coastal hydromorphic soils and the areas cultivated
for rice, such soils are distributed from sea level to the highest mountain ranges.
Whole regions may be influenced by inhibited drainage. In the tundra environment,
for example, seasonal melting of the upper permafrost layer leads to saturation of
the upper profile resulting in acid reducing conditions, reduced decomposition rates and
peat formation. Across the tropical world, peats (histosols) occur in poorly-drained
environments with the greatest areas being in South East Asia, Papua New Guinea and
Africa (Andriese, 1988).
Table II.6 summarises the dominant weathering and pedogenetic processes operating
across the earth's surface and the characteristic soils that result. It also lists the principal
processes involved in the transformation and neoformation of clays. Areas included in
the table do not include hot deserts and ice-covered areas where minimal chemical
weathering occurs or areas of carbonate-rich or tephritic parent materials.
4.2
Pedogenetic processes in tropical climates
Ferrallitisation is the dominant process in the hot, moist climates of the equatorial
regions covered by rainforests. Under such conditions, the original bedrock becomes
deeply and highly-weathered, irrespective of its initial composition. Hydrolysis is
the dominant process because of the acid soil solutions resulting from elevated
concentrations of dissolved carbon dioxide. Reactions with soil organic matter are
considered to be of lesser importance than in other environments and geochemical
weathering is considered to dominate in the freely-drained areas of the tropical regions
(Duchaufour, 1997).
