SOIL FORMATION - Soil Ecology - page 154

II.3 THE STAGES OF SOIL FORMATION AND
DEVELOPMENT

The formation and evolution of soils involve a series of physical, chemical and biological
processes which act progressively over time, are controlled by climatic variables and
are greatly influenced by topography. Simonson (1978) divides such processes into
additions from without the soil system, removals or losses, translocations (or transfers)
within the system and transformations of contained materials.
The original parent material is transformed by in situ weathering into a mixture of
stable mineral components which blend intimately with organic materials to form the soil.
The parent material is first broken down into its primary minerals whose decomposition
products may be partially transformed into secondary or neoformed minerals.
From this early stage, the nutrients necessary for plant production and such other
essential components as Al and Fe accumulate progressively in the upper parts of the
incipient soils. Clay fractions are formed firstly through alteroplasmation i.e., transforma-
tion of primary minerals into clays with no subsequent modification of rock structure.
Pedoplasmation is a subsequent transformation whereby clay minerals acquire a
pedological structure and such specific properties as swelling and shrinkage.
The initiation of biological activities within the developing substrate leads to the
accumulation of organic matter. This organic matter mixes with the weathering mineral
components to form an A horizon that becomes an active source of further physico-
chemical changes in the underlying parent material to develop a C horizon. evolved
from decomposing organic matter also participates in the process. With further
development, weathering and downward transport of materials progressively modify
the deeper strata of the parent material and, depending on the processes operating, E and
B horizons may form. At this stage, translocation, biological transport and erosion
become the dominant processes in the evolution and differentiation of the soil profile.
Nutrient and other elements ( e.g., Si and Al, Cornu et al., 1997) and organic materi-
als are continually lost in solution and suspension through erosion and by
transport in sub-surface water flows. Such losses are expected to be greatest in incipient
soils with juvenile ecosystems and to diminish with ecosystem and soil development
(Odum, 1983).

3.1 Weathering

Weathering is the sum of the processes involved in the alteration of materials at and near
the surface through complex interactions between the lithosphere, the atmosphere,
the hydrosphere and the biosphere that occur over time. It may extend far below
the surface and includes all the physical and chemical processes responsible for rock
fragmentation and the production of dissolving ions. Weathering in the upper part of
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