Geoscience Reference
In-Depth Information
Although a classification of soils is as necessary as it is for
rocks, plants, animals and any natural feature, 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.
International soil classifications
At the international level there are two main competing
schemes:
Soil Taxonomy
, developed by the Soil Con-
servation Service of the United States Department of
Agriculture (USDA) (Soil Survey Staff 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 of the 1: 5,000,000
Soil Map of the World, and
has been used increasingly for international communica-
tion ever since, with revisions 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 the 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
Table 18.3
together
with a brief description of their main features.
Revisions result from additional experience of working
with soils in the field and on agricultural projects. The
shallow, rock-dominated Lithosols, Rendzinas and
Rankers have been grouped into the new unit of Leptosols.
The group of Lixisols is used for soils with an argillic Bt
horizon with low-activity clays, and Luvisols are now soils
with argillic Bt 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 water-
logging and flooding. A new major group of Anthrosols
has been added to describe soils strongly influenced by
human activities.
Plate 18.22
Lateritic soil (FAO: Rhodic Ferralsol) in tropical
(Box) laterite. Intense leaching leaves only resistant ferric
oxides, kaolinite and quartz, with no weatherable minerals.
Section is 2 m.
Photo: J. H. Stevens
governments, multinational food corporations, inter-
national agricultural agencies and the farmers themselves
to 'improve' them.
Swidden
or shifting cultivation has
been much maligned in the past, for its burning of forests,
and low-input systems. However, it is in sympathy with
the tropical environment under low population densities,
and is still common, but is being superseded by perma-
nent farming systems in many areas.
Soil classification and soil nomenclature are perhaps the
most disputed and confused aspects of soil science.
