Environmental Engineering Reference
In-Depth Information
Table 19.1 The formation of soil horizons
Vertical redistribution of soil materials
Leaching of ions in the soil solution
Movement of clay-sized particles
Upward movement of water by capillarity
Surface deposition of dust and aerosols
Mixing processes
Organisms (e.g. cambisols, chernozems)
Cultivation of agricultural soils
Creep processes on slopes
Frost heave (cryoturbation)
Swelling and shrinkage of clays (e.g. vertisols)
Plate 19.1). 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 19.2. Some soils tend to show striking horizonation (e.g. podzols), whereas in
others the horizons are less distinct (e.g. vertisols). Table 19.1 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 are broad similarities and full details can be found in
their published soil memoirs (Soil Survey of England and Wales; Soil Survey of
Scotland; National Soil Survey of Ireland; Soil Conservation Service of the United
States). Internationally there are two commonly used soil classification schemes:
Soil
Taxonomy
of the US Department of Agriculture (USDA) and the system used by the UN
Food and Agriculture Organization and Educational Scientific and Cultural Organization
(FAO-UNESCO). The FAO system is the one used in this topic and details are given in
the Appendix.
THE SOIL
-
FORMING ENVIRONMENT
The nature of the soil profile at any particular place depends upon five main factors.
These are:
1 The past and present climate.
2 The physical and chemical characteristics of the parent material.
