Soils as the Skyscrapers - Soil: The Skin of the Planet Earth - page 79

Let us abandon for the time being the complicated description of pores and their
natural arrangement within soils. Dealing with soil porosity is much more practical
because its determination is relatively simple. Examining changes of soil porosity
owing to existing vegetation and man's activity has been the aim of many research
studies during the last two centuries. This accumulated knowledge of porosity is
therefore much more important in our practical life than studying the geometry of
pores. Hence, we shall spend more time and space on porosity than on approximate
studies about soil porous systems. Our intention is not to underestimate the utility
of soil porous system theories - to the contrary, where we fi nd the opportunity to
show illustrative results, we use them without going into detail of unnecessary
advanced theories and methods.

7.2 Soil Porosity

The volume of pores V P related to the total volume of soil V T in its natural confi gura-
tion of soil particles is denoted as porosity, as we have written in the previous sub-
chapter, P = V P /V T . Before we measure its parameters, it looks like a very simple task
until we have to decide exactly how large a volume of soil V T should be measured.
How we make this decision is illustrated in Fig. 7.5 . Notice in the top part of the
fi gure that when we initially choose a very small volume V T1 (e.g., the size of a sand
particle), everything depends upon the position of the center of the volume we sam-
ple. If a sample is taken from the center of a particle, we obtain porosity P = 0. If a
sample is taken from the center of a big pore, we obtain P = 1 (i.e., P = 100 %). If we
decide to sample a bigger volume V T2 with its center located on a solid particle, we
determine P = 0.33. Sampling the bigger volume V T2 with its center located on a
pore, we determine P = 0.72. For a still larger volume V T3 , P = 0.40 (centered to solid
particle) and P = 0.49 (centered to the pore). If we proceed to systematically increase
the sampling volume for two cases (1. centered on a solid particle and 2. centered
on a pore), we obtain two curves that differ substantially for relatively small sam-
pling volumes; see the graph at the bottom of Fig. 7.5 . As the volume of sample
continues to increase, the two curves fi nally merge to the same value of porosity P
regardless of where the sampling was centered. This volume is the representative
elementary volume, REV, and is theoretically applicable to exact studies within
prescribed limitations. In practical applications, decades of experience have proven
that a volume of about 100 cm 3 is suffi ciently adequate and precise in the great
majority of instances.
As a result, the determination of soil porosity is rather simple. We sample a natu-
ral fi eld soil using a metallic, cylindrical device having a diameter and height each
equal to about 6 cm. For special circumstances, the height is adjusted in accordance
to the aim of the study.
We push the device into the fi eld soil, remove the soil sample of known volume,
and oven-dry it in the laboratory. Weighing it after it becomes dry, we calculate the
soil bulk density. We next measure the specifi c density of the soil particles - another