Environmental Engineering Reference
In-Depth Information
Appendix
Soil classification according to the FAO-
UNESCO
Soil
Map of the World
Soil classification is perhaps one of the most vexed and confused aspects of soil science.
Although a classification of soils is as necessary as it is for rocks, plants and animals,
agreement on a universally acceptable scheme has been elusive. This is partly because
soils are inherently difficult to classify, as they are a continuum, with all shades and
ranges of properties. It is also partly due to the emotional forces at work, as different
international and national soil survey organizations have sought to promote their own
schemes. At the international level there are two main competing schemes:
Soil
Taxonomy
, developed by the Soil Conservation Service of the US Department of
Agriculture (USDA 1975), and the
FAO-UNESCO Soil Map of the World
classification.
The latter is the scheme used in this volume for international correlation.
The FAO classification was first published in 1974 as the legend for the 1:5,000,000
Soil Map of the World
. It has been used increasingly for international communication
ever since. Revisions of the FAO soil classification were made in 1985 and 1988. With
some modifications the classification was used in 1985 for the 1:1,000,000
Soil Map of
the EEC countries
. The FAO classification consists of an amalgam of traditional names
(podzol, chernozem), newly coined names (lixisol, alisol) and borrowings from US
Soil
Taxonomy
(histosol, vertisol). In the 1988 revision there are twenty-eight soil groups,
subdivided at the second level into 153 soil units. The soil groups are listed in the table
together with a brief description of their main features.
The major revisions of the 1974 classification resulted from additional experience of
working with soils in the field and on agricultural projects. The traditional shallow soils
of Lithosols, Rendzinas and Rankers have been grouped into a new major unit of
Leptosols. The group of Lixisols is used for soils with an argillic B with low-activity
clays, and Luvisols are now soils with high/medium activity clays. Similarly Acrisols
have been split into Alisols, with aluminium and high-activity clays, and Acrisols with
low clay activity. The Yermosol and Aridisol groups have been deleted, and soils in dry
areas are now classified according to their profile characteristics. New units of Calcisols
showing calcium carbonate accumulation and Gypsisols showing gypsum accumulation
have been introduced. A new major group of Plinthosols has been introduced to cover
large areas of South America where plinthite causes surface waterlogging and flooding.
A new major group of Anthrosols has been added to describe soils strongly influenced by
human interference.
