Agriculture Reference
In-Depth Information
I.4
SOIL HETEROGENEITY
The complexity of the soil environment results from the interactions of four factors.
These are the types of energy input to the soil system, spatial and temporal patterns and
functional structures.
4.1
Diversity of energy types
Three different forms of energy dissipate within the soil.
Physical energy
linked to
gravity and the electrostatic forces of capillarity and attraction are responsible for
the movement of materials, in solution or suspension, which play such a predominant
role in soil formation. Another form of physical energy is the important release of
mechanical energy during mineral crystallisation (Pedro, 1964).
Chemical energy
may also play an important role through abiotic oxidation and reduction reactions.
Scharpenseel
et al.
(1984) have demonstrated that up to 20 % of the decomposition of
humic acids in sandy and silty soils may result from the abiotic oxidation of these
compounds.
Photosynthetic energy
becomes available to soil organisms as calorific
energy contained in the organic matter deposited on the soil surface and incorporated into
the mineral soil through such diverse biological activities as oxidation (respiration),
hydrolysis (digestion) and mechanical mixing (burrowing, transportation, construction).
The dissipation of these energies has quite separate effects. Chemical and physical
energies induce diffusive effects whereas the energy produced through photosynthesis
promotes the cycling of nutrient and other elements, and modification of the soil environment
by a specific group of invertebrates called “ecological engineers” (II.2.4 and Chapter IV).
4.2
Heterogeneity
Heterogeneity is a normal feature of ecosystem structure. It is not an expression of
disorder but originates from the diverse and multiple interactions between ecosystem
components. In soil, heterogeneity is expressed in the spatial and temporal distributions
of biotic and abiotic elements, and through their inclusion into different functional entities.
4.2.1
SPATIAL HETEROGENEITY
Spatial heterogeneity in soils occurs at widely different scales. At continental and
regional scales, the units shown in pedological maps reflect the effects of the soil forming
factors (largely climate, topography and the distributions of parent materials discussed
in Chapter II) on their distributions. At the scale of several hectares, the toposequences
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