Geoscience Reference
In-Depth Information
Fig. 6.1
The example of soil structure in the soil type consisting of A, B, and C horizons where A
horizon is rich in humus, B horizon originated by slight transport of silt and clay particles from A
horizon by percolating rain water. The shape and type of soil aggregates depend on soil evolution
and upon man's activity
have large gaps in between the aggregates. Weak soil structure is not easily seen in
a soil pit, and aggregates break into a discernible shape when they are manipulated
or squeezed. Medium is somewhere in between strong and weak. The determination
of the grade is rather subjective, especially grades 2 and 3.
The size of aggregates is not uniform for all types of aggregates. We describe one
of possible classifi cation systems in order to show the principle criteria. When all
three axes are roughly of the same length and the type is blocky, the size less than
5 mm is classifi ed as very fi ne, the range 5-10 mm is fi ne, 10-20 mm is medium,
20-50 mm is coarse, and above 50 mm the type is very coarse. Granular aggregates
have substantially smaller boundary values: less than 1, 1-2, 2-5, and 5-10 mm.
Very coarse does not exist. The height of platy aggregates is decisive and has bound-
ary values: less than 1, 1-2, 2-5, and 5-10 mm. Aggregates with extensive vertical
axes have these decisive heights: less than 10, 10-20, 20-50, 50-100, and more than
100 mm for very coarse aggregates.
The description of soil structure starts by its visual evaluation in the fi eld. A
chunk of topsoil extracted with a spade can be manually broken along “fracture”
lines into several big clods. With these naturally developed fracture lines being fre-
quently visible, each clod can be repeatedly crumbled to expose a series of smaller
and smaller aggregate units. The relative sizes, shapes, strengths, and spatial distri-
butions of all of the aggregate units and clods provide a basis for classifying and
