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individual soil particles. Such pore structures for a topsoil of loamy sand and of clay
loam are revealed by the solid curves in Fig.
7.7
. A peak of the most frequent pores
between the aggregates of the loamy sand occurs at a radius
r
= 20
μ
m, while the
most frequent pores inside its aggregates have a peak at
r
= 2.5
μ
m. Within the clay
loam the equivalent peaks occur at radii of 55 and 0.8
m, respectively. We note that
these broad ranges of double peaks are caused by the type and quality of soil struc-
ture coupled with variations of soil texture. In addition, soil fauna and decaying
roots and other deteriorating parts of fl ora infl uence the shape and maximal fre-
quency of soil pores in each of the two main components of the soil pore system.
The differences of the two soils are completely obvious when the log scale is used
for the equivalent pore radius. If we had used a linear scale, the fi rst peak of micro-
pores 2.5
μ
m (clay loam) would merge, and the difference
between pores inside the aggregates would have disappeared.
Pore-size distributions within soils are always sensitive and vulnerable to the
impacts of soil tillage and compression by machinery. Returning to Fig.
7.7
, we note
that the solid curve for each soil is also associated with a dotted curve derived from
measurements of porosity after each soil was individually subjected to a pressure of
300 kPa for a long time. The dotted curve for each soil is drastically changed from
its original uncompressed solid curve. The shifting of peaks and the changing mag-
nitudes of pore sizes are not the same for these two soils or for any other soil. In
other words, quantitative predictions of changing pore-size distributions remain a
challenge for future research.
Under the optics of a microscope, we see large pores existing next to narrow
pores, long pores ending abruptly, long pores merging to form big pores, big pores
connecting with tiny tubes, and all combinations of different sized tubes leading to
countless directions - collectively, we are inclined to see disorder or complete
chaos. It reminds us of a map of the center of a big city. There, the main traffi c,
administration centers of powerful companies, and top-quality shops are concen-
trated on the main avenue. Next to it are ordinary streets and narrow alleys and pas-
sageways leading one to another, while some passageways are blind or narrowed
just for foot passengers. We could say again, what a chaos! But it developed with
several principal aims: to enable the most effi cient transport of goods as well as of
citizens. This network offers a place for business, administration, destructive ele-
ments and law enforcement, and, even more generally, all important functions of the
city. In a similar way, the soil pore system looks chaotic, but it evolved to offer an
optimal network for the life and development of the type of soil we are observing
and soil scientists are studying.
When we compare all models to the reality that we attempt to study on thin sec-
tions cut from natural soil, we readily acknowledge that the models are crude esti-
mates regarding the variance and distribution of spatial pores. But even so, the
models help us understand the measured physical and chemical characteristics and
processes taking place within soil pores (Figs.
7.8
and
7.9
).
μ
m (loamy sand) and 0.8
μ
