Agriculture Reference
In-Depth Information
II.4
MAJOR PROCESSES OF SOIL FORMATION
Soil formation is an enormously complex series of reactions and processes that may
continue to operate over long time periods. It encompasses weathering but also involves
the range of reorganisational and translocational processes discussed above, together
with the build up of soil organic matter and all that this entails in terms of biological
processes and bio-mineral interactions (see, for example, Ugolini, 1986a).
It is the interactions of the soil-forming factors (particularly climate) that determine
the general direction of weathering and pedogenesis in any given area and, as time
passes, the soil will develop towards a idealised transient steady state, the pedoclimax.
However, because of continuing weathering losses and changing environments, the
directions of pedogenesis will also change. The differing climatic conditions experienced
during the development of soils in the older landscapes will usually be reflected in their
morphological, chemical or physical properties.
Local factors may be important determinants of pedogenesis which normally alters
gradually along continua, except where abrupt changes of parent material, vegetation or
drainage occur (Ugolini, 1986a). These may lead to the formation of very different soils
within the same environment.
4.1
The pedoclimax concept and its limitations
Climate is the dominant factor in pedogenesis and early systems of soil classification
defined 'zonal soils' as the idealised pedoclimaxes of individual climatic zones. However,
Duchaufour (1982) considers that this concept should be restricted to processes related
to humification. Different major trends occur in pedogenesis, depending on the prevailing
climate: ferrallitisation in humid tropical soils and brunification in temperate regions.
Temperature and precipitation are the two major aspects of climate. Temperature is
important in that it determines the rates of chemical reactions and the metabolic rates of
most organisms. Precipitation is the major determinant of moisture regimes and water
fluxes, and hence of chemical weathering, pedogenesis and water supply to organisms.
As discussed earlier in this chapter (Section II.2.3), extended periods of time may be
required for soil development (see also Table II.2) and the changes that occur are mostly
gradual. However, the evolution of soils over time is not always continuous. Climatic or
other changes that affect ecosystem stability may lead to further phases of pedogenesis
in which an existing soil is replaced by one that differs in its mineral composition and
fabric (Lucas and Chauvel, 1992) (Figure II .9). Transformation fronts appear which may
progress laterally. In Central Amazonia for example, the lateritic cover developed
on Tertiary sediments is being replaced by spodosols (Lucas and Chauvel, 1992).
These transformations may be determined by rapid changes in plant cover following
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