Geoscience Reference
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and break into smaller amorphous clods as the soil dries. Eventually, the cloddy
surface erodes into a rough, shapeless confi guration, and after a series of rainfalls
and intermittent drying conditions, it is transformed into a nearly impenetrable hard
crust. To prepare such a soil for crop production, the farmer follows his initial plow-
ing gesture with additional cultivation procedures specifi cally chosen to potentially
improve the meager tilth of his structureless soil. Even with all of his effort, condi-
tions within the soil are not the best for the growth of roots and generally do not
offer an optimal environment for profi table crop production.
Up to now we were dealing with soil structure easily visualized within the tilth
or top layer of a soil. A soil scientist's name for soil layers is horizons. The name
does not imply that the layers are strictly horizontal - they could be slightly inclined,
irregularly tilted, or undulating. But when we dig a pit at one location, we can rec-
ognize that soil color, texture, compactness, and the shape of structural aggregates
change with depth. By quantifying these morphological features that are typical for
various soil types, units of soil structure are identifi ed and named. We shall describe
them in the second half of our topic.
Now back to our horizons. The crumb structure is typical for the top A horizon
(humus horizon). In horizons at greater depths below the A horizon, there are differ-
ently shaped structural units of aggregates. In some instances, we fi nd differently
shaped aggregates even within the A horizon. Specifi c types of structure, defi ned
according to the shape of aggregates, may slightly differ between individual national
systems. Sometimes the term ped is used for aggregates originating strictly from
natural soil-forming processes. At other times, the existence of soil aggregates is attri-
buted to a combination of natural processes and man's activity, example in Fig. 6.1 .
If all three main axes of aggregates are roughly equal, the type of soil structure is
blocky. In some systems angular blocky structure (faces of aggregates intersect at
sharp angles) and subangular blocky structure (corners of faces are mostly rounded)
are recognized. If the aggregates are more or less rounded, we speak about granular
or crumb structure. Aggregates with horizontal axes shorter than the vertical are
typical for prismatic structure. Columnar structure is similar to prismatic structure,
but aggregates are rounded at the top. Aggregates with horizontal axes longer than
the vertical are characteristic of plates. In some systems this platy structure is sepa-
rated and called fl aky because of the very thin shapes of fl ake-like aggregates. A
honeycomb type of structure is also specifi ed. If aggregates are not formed and
individual soil grains are recognizable, then the structure is single grained, like in
sand. When no aggregates exist, the texture is loam or clay, and the soil breaks up
into big clods, the type of structure is named massive or amorphous. If such a mas-
sive or amorphous type of structure exists in the top A horizon, it is identical to a
structureless soil. The classifi cation schemes are not rigid and speaking less diplo-
matically, there is a free space for fabulosity and phantasm.
The degree of structure development and of aggregate stability is its grade. This
strength is described simply by words and by numbers from 0 to 3. Grade 0 indi-
cates that no soil aggregates exist, and the soil is structureless. The weak soil struc-
ture is denoted by 1, while 2 indicates medium structure. Lastly, 3 is for strong
structure. Strong structure can easily be seen in a soil pit, since the soil aggregates
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