Geoscience Reference
In-Depth Information
Chapter 7
Soils as the Skyscrapers
Whenever we observe a skyscraper, we immediately recognize an extremely tall
building usually made of concrete, steel, and glass towering above nearby buildings.
Our experience assures us that inside the huge structure, ample space provides a
healthy environment with human activities pulsating normally owing to sophisti-
cated forms of machinery operating within the remainder of the skyscraper's inter-
nal structure. On the other hand, when we look at a soil clod, we are completely
unaware that we have an ultramicroscopic skyscraper in our palm. Thinking of the
clod as a solid mass, we cannot imagine it as an ideal home for organisms living
within its cavities. It never occurs to us that its cavities, occupying about one-half
the volume of the clod, provide a healthy environment for its huge number of tiny
animals and plants living together.
7.1
Soil Pores
The space surrounded by individual solid soil particles is called a pore. Provided it
does not contain any solid soil material, such a cavity is designated as a pore regard-
less of its size and shape. Pores exist as coarse channels after roots have decayed
and as fi ssures after soils containing a high percentage of smectites become dry.
Moreover, pores exist within and between soil aggregates.
The relative volume of all pores in a specifi c volume of soil is defi ned as soil
porosity with symbol
P
and frequently denoted as percentage. If we write
P
= 45 %,
we mean that a 100-cm
3
volume of soil contains 45 cm
3
of pores and the remaining
volume of 55 cm
3
is fi lled by solid soil particles of either inorganic nature or organic
origin. Porosity is sometimes indicated by a decimal rather than a percentage. In our
example, its value would then be 0.45.
Soil pores are frequently denoted as voids in studies of mechanical properties of
soils. If the volume of voids is divided by the volume of solid particles in the soil,
we obtain the void ratio
e
, expressed as a decimal. For a clear illustration, we write
