Geoscience Reference
In-Depth Information
Podzol) on wind-blown sand, Brandon, Suffolk. Surface black
organic layer (O), above pale-grey podzol horizon (Ea), above
black (Bh). Organic deposition in the B follows vertical
percolation zones, and horizontal black organic bands pick out
old land surfaces of the sands.
Photo: E. A. FitzPatrick
Eutric Vertisol) in Blue Nile alluvium, Sudan. The surveyor
recognized seven horizons in this 2 m soil section, with
gradual changes in structure from blocky at the top to massive
at the base being significant.
Photo: E. A. FitzPatrick
matter is decomposed by the soil's fauna and micro-
organisms to produce humus. The clay minerals and
hydrous oxides undergo further alteration, depending
on the amount of leaching and the type of ions in the
weathering solution; new minerals can crystallize and
previous ones alter. Ions in the weathering solution can
be thrown out of solution and precipitated in the solum
if chemical conditions allow (calcium carbonate, gypsum,
soluble salts, secondary quartz from silica).
The final stages of soil formation consist of the
processes of morphogenesis, i.e. the production of a
distinctive
soil profile
with its constituent layers or
horizons
(
Plate 18.1
).
The soil profile is the vertical section through
the soil; it is the fundamental unit for describing, sampling
and mapping soils. The soil horizons are the distinct
layers, roughly parallel to the surface, which differ in
colour, texture, structure and content of organic matter.
The clarity with which horizons can be recognized
depends upon the relative balance of the migration,
stratification, aggregation and mixing processes shown in
Figure 18.2
.
Some soils tend to show striking horizona-
tion, e.g. the podzol in
Plate 18.1
,
whereas in others the
horizons are less distinct, e.g. the vertisol in
Plate 18.2
.
Table 18.1
lists the processes which create and destroy
clear soil horizons. When horizons are studied they
are each given a letter symbol to reflect the genesis of
the horizon. There are many different schemes in use
by the major soil survey organizations in the world. There
