Soils as the Skyscrapers - Soil: The Skin of the Planet Earth - page 82

Fig. 7.6 A simplifi ed, ideal shape of a lognormal pore-size distribution in a structureless soil. The
horizontal axis is in the logarithmic scale. The peak of the curve indicates the most frequently
occurring pores (or cylindrical tubes of models). They have radii about 30-50 μ m

Because the surface of pores is not smooth, thin sections of soil observed by
microscope offer us fantastic images of pore formations. Sophisticated procedures
allow us to transform the images into quantitative analogues of geometric arrange-
ment and to better understand the term diameter used in the previous paragraph.
Let us fi rst simplify the shape of soil pores into vertical cylindrical tubes of radii
r ranging from 1 m (micrometer = 0.001 mm) to 1 mm. Next, if we arrange them

increasing from their smallest size ( r = 1

μ

m) on the left to their largest ( r = 1 mm)
on the right, we obtain a picture similar to the pipes of an organ. Designating the
length of each tube of radius r according to its percentage occurrence in the soil, we
obtain the relative number of tubes, i.e., their frequency. Plotting their frequency on
the vertical axis and their radial size in logarithmic scale on the horizontal axis, we
obtain a curve similar to a bell as illustrated in Fig. 7.6 . Most frequent are the pores
of radius 30

μ

m in our example, and the peak of the curve depends mainly upon the
soil texture. Lowest frequency belongs to the thinnest tube ( r = 1

μ

m) residing at the
left-hand side of the graph, while the fattest tube ( r = 1 mm) having nearly the lowest
frequency is at the far right-hand side of the graph.
Since the soil pores have irregular shapes, we compare the behavior of soil water
and fl ow of liquid through the soil to that through the parallel tubes in the model.
This parallel tube model provides a convenient means to easily explain the over-
whelming complexity of a soil by interpreting the radius of a tube as an equivalent
pore radius or simply pore radius.
We owe the readers an explanation of why we plot the pore radius in logarithmic
scale - the decadic with log 1 = 0, log 10 = 1, log 100 = 2, etc. Because 10 0 = 1,
10 1 = 10, etc., it follows that the length between 1 and 10

μ

μ m on the horizontal axis

is the same as that between 10 and 100 m. We

have the opportunity to follow the changes of the shape of the curves if the peak of
two studied curves is in the range of one-half of the order of magnitude as, for

μ m and also between 100 and 1,000 μ