Geoscience Reference
In-Depth Information
values, usually close to 2.7 g/cm
3
. The porosity is simply calculated from the two
densities.
Because individual pores in sandy soils can easily be observed with a naked eye,
it may seem as a surprise that sandy soils have the lowest porosity of all soils. The
arrangement of their individual particles resembles a combination of cubic and hex-
agonal packing held together by small admixtures of silt and clay particles. Their
porosity, about 40 %, may increase with increases of humus content, but it never
reaches the values of loams, clays, or other textures. The porosity of loamy soils
usually has a larger range of 52-64 %, because pores exist within three different
sized domains - between individual particles, within microaggregates, and within
macroaggregates. With the development of more aggregates, the improved soil
structure increases the total porosity. Even when the soil structure of a loamy soil
looks somewhat deteriorated or even destroyed, its microaggregation usually per-
sists to maintain its total porosity above that of sandy soils. The porosity of a loam
falls below 50 % whenever it is slushed after a rain or is more or less permanently
waterlogged. If this structureless puddled loam dries out on the soil surface, it keeps
its low porosity and forms a solid skin of topsoil. Clays and loamy clays usually
manifest a lower porosity than that of loams. However, the drying of structureless
clays and loamy clays is accompanied by shrinkage and the formation of fi ne and
coarse cracks causing an increase of porosity sometimes above the values of a
loamy compacted skin on the surface.
When we dig out a clod of fresh, moist soil and squeeze it in our palm, we fi nd
in addition to its changed shape that the overall size of the squeezed lump is smaller.
This reduction occurs because we compressed and transformed the biggest pores
into smaller pores. Consequently, the size distribution of pores can be decreased
when a soil is compressed, e.g., by traffi c or by heavy agricultural machinery and
vice versa.
7.3
Soil Pores Like a City Center
The variable shapes and size of soil pores continually change. It is not uncommon
for a pore of 1 mm in cross section to be drastically reduced to merely a micrometer
(1
m = 0.001 mm). The biggest pores display more or less parallel walls with dis-
tances up to 2 cm and are called cracks. Earthworm channels have the shape of
irregularly curved tubes with diameters of 4 mm or less. Pores originating after the
decay of grass roots have diameters ranging from 0.1 to 0.3 mm. Both are some-
times called biopores together with all pores originating under a direct and immedi-
ate infl uence of microfauna and fl ora. Pores between aggregates have most frequency
diameters between 10 and 200
μ
m, and in loose well-aggregated soil, they have
diameters up to 1 mm. Pores inside aggregates are substantially less - only 0.2-
10
μ
m. All of the pores between aggregates are sometimes denoted as macropores
and those inside aggregates are micropores. The range of pore size is very broad in
soils; see Fig.
7.6
.
μ
