Geoscience Reference
In-Depth Information
Chapter 6
Crumbling of Soils
6.1
Soil Structure
Everybody has had many opportunities to observe breadcrumbs or the crumbs of a
cake. Within each crumb one can easily see coarse as well as tiny small holes or
pores. And between crumbs, the pores are really big. Whenever a bakery produces
bread without those big pores, the bread is heavy, has no tendency to crumble, and
is hard to swallow. People stop shopping at that bakery. Similarly, cakes without an
attractive pattern of large and small holes and crevices are heavy, appear like a piece
of concrete, and are unwanted in a bistro. Without satisfi ed customers the bakery
and the bistro are both on a path to bankruptcy.
A good soil should crumble like a good bread or cake. Soil crumbs are called
aggregates. They display a very broad distribution of pore sizes ranging from ultra-
microscopic pores up to pores recognized by a naked eye. Speaking scientifi cally,
there are pores of cross section from less than
m up to millimeters. Because such
a crumbling soil has a special structural arrangement of particles, we speak briefl y
about
soil structure
. Since there is the similarity with breadcrumbs, we fi nd the term
crumb structure in many languages, and this type of structure is considered as the
most favorable for plants. The comparison of soil to bread is by no means an essay-
istic style or manner. Although the term was introduced into the scientifi c world by
the famous Russian soil scientist Dokuchaev in the nineteenth century, he did not
invent it. He took it from farmers who distinguished between a good crumb struc-
ture of a high-yielding topsoil and a structureless, poor fertility soil. Structureless
soils were associated with low harvests and a lack of rye and wheat that often led to
famine. But we have to describe now the morphologic features of soils, and there-
fore we must resist temptation to write an essay on poor structureless soils and
famines.
A structureless soil in farmers' jargon does not crumble but forms big clods just
after plowing with the clods copying the shapes and numbers of the plow blades.
With the surface left idle and no longer tilled, the individual clods typically fracture
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