The porosity of a soil depends on
several factors, including (1) packing density, (2) the breadth of the particlesize distribution (polydisperse vs. monodisperse), (3) the shape of particles,
and (4) cementing. Mathematically considering an idealized soil of packed
uniform spheres, φ must fall between 0.26 and 0.48, depending on the packing.
Spheres randomly thrown together will have φ near the middle of this range,
typically 0.30 to 0.35. A sand with grains nearly uniform in size (monodisperse) packs to about the same
porosity as spheres. In a polydisperse sand, the fit-ting of small grains
within the pores between large ones can reduce φ, conceivably below the 0.26
uniform-sphere minimum. Figure 2 illustrates this concept. The particular sort
of arrangement required to reduce φ to 0.26 or less is highly improbable,
however, so φ also typically falls within the 0.30-0.35 for polydisperse sands.
Particles more irregular in shape tend to have larger gaps between their
nontouching surfaces, thus forming media of greater porosity. In porous rock
such as sand-stone, cementation or welding of particles not only creates pores
that are different in shape from those of particulate media, but also reduces
the porosity as solid material takes up space that would otherwise be pore
space. Porosity in such a case can easily be less than 0.3, even approaching 0.
Cementing material can also have the opposite effect. In many soils, clay and
organic substances cement particles together into aggregates. An individual
aggregate might have a 0.35 porosity within it, but the medium as a whole has
additional pore space in the form of gaps between aggregates, so that φ can be
0.5 or greater. Observed porosities can be as great as 0.8 to 0.9 in a peat
(extremely high organic matter) soil.
Porosity is often conceptually partitioned
into two components, most commonly called textural and structural porosity. The
textural component is the value the porosity would have if the arrangement of
the particles were random, as described above for granular material without
cementing. That is, the textural porosity might be about 0.3 in a granular
medium. The structural component represents nonrandom structural influences,
including macropores and is arithmetically defined as the difference between
the textural porosity and the total porosity.
The texture of the medium relates in a
general way to the pore-size distribution, as large particles give rise to
large pores between them, and therefore is a major influence on the soil water
retention curve. Additionally, the structure of the medium, especially the
pervasive-ness of aggregation, shrinkage cracks, worm-holes, etc. substantially
influences water retention.
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