Agriculture Reference
In-Depth Information
during drying to irregular, partially-accommodating surfaces between adjacent units to
pores with near cylindrical cross sections formed by root penetration and the burrowing
activities of soil animals. Structure is not a static feature of soils, the component units are
constantly being destroyed and renewed through the continuing and combined activities
of physical forces and the biota; structural changes may be apparent even at a seasonal
scale (Blackman, 1992). It is structure that largely defines the physical environment
within the soil in terms of its degree of aeration and the amount of water it will absorb,
retain and transmit. Following from this, it also determines the suitability of a soil as
a habitat for small invertebrates, and for plant growth.
1.3.1
COMPONENTS OF SOIL STRUCTURE
The basic unit of structure in soils is the aggregate, or ped, which is defined as an
association of soil particles that has a greater degree of internal, inter-particle cohesion
than externally to the particles surrounding it. Clods, in contrast to aggregates, are
larger structures (largest axis greater than approximately 25 mm) created by cutting
and compressing the soil during tillage operations. Aggregates are considered to be
formed through natural processes. Those created by physical processes are characterised
by angular-blocky or prismatic forms. In contrast, faunal activities create aggregates
with characteristically rounded shapes and many aggregates formed around a central
core of plant material have elongate shapes (Oades, 1993). A variety of aggregate types
will occur in most soils.
In most productive soils, aggregates occur in hierarchies of increasing size in which
small aggregates are bound together to form larger compound aggregates. Aggregates of
each size class in the hierarchy may possess properties distinct from those smaller and
larger and will be bound together with characteristic suites of binding agents; these also
differ between the size classes in the hierarchy.
Soil structure may also be considered from the viewpoint of the soil voids, or the spaces
between the structural aggregates. This approach is advantageous when considering
plant growth since the infiltration of water into the soil, its movement through the pro-
file both laterally and vertically and the total store and availability of water to plants all
depend on the presence and the size frequency distribution of the pores. Soil porosity
may be divided into textural and structural components. Textural porosity is the minimal
porosity resulting from the irregular distribution of the inorganic soil fragments; structural
porosity is that component of porosity due to the generally-larger interconnected pores.
Pores are classified on the basis of their equivalent cylindrical diameters (ECD)
although the boundaries of the size classes are somewhat arbitrary. Micropores are
defined as those pores sufficiently small (less than approximately 30 µm in diameter) to
retain water by capillarity and these contrast with the larger macropores which do not.
Figure I.18 contrasts the pore size distributions of an Amazonian oxisol supporting
respectively a primary forest and a pasture (Grimaldi
et al
., 1993). Table I.10 presents
examples of the typical size ranges of pores, biological and other structures commonly
found in soils.
