Desert soils, paleosols and duricrusts - Climatic Change in Deserts - page 263

Reference Base for Soil Classification, with input from UNEP and the International
Union of Soil Sciences, culminating in the current World Reference Base for Soil
Resources (IUSS Working Group WRB ( 2007 ). It is interesting to compare Tabl e
15.1 distilled from the work of this group to the soils recognised in North America
( Table 15.2 ) by the Soil Survey Staff ( 2010 ) of the U.S. Department of Agriculture.
For ease of comparison, the equivalent or near-equivalent soils are listed in brackets
in Tables 15.1 and 15.2 . Certain of the Soil Orders recognised in North America do not
feature as such in the international classification (e.g., alfisols, gelisols, inceptisols),
just as certain of the international Major Soil Groups are not identified at this broad
level in the North American classification system, although they undoubtedly do occur
in North America (e.g., solonetz, solonchaks, rendzinas, kastanozems). Table 15.1 is
not the full list of thirty-two Major Soil Groups, but specifies the twenty soils most
commonly found in deserts and their margins, whether they are active or fossil. The
list of characteristics is gleaned from the key references given in the previous sections,
as well as the author's own experience of mapping and describing these soils in the
drier parts of Africa, Australia and Asia.
There has long been dispute among soil scientists as to whether soils should be
classified purely on the basis of observable morphology and physical field attributes
or whether they should be classified using both physical and chemical criteria. The
Australian soil scientist Keith Northcote developed what he described as a Factual
Key for classifying soils in the field (Northcote, 1971 ). He identified three broad
categories of soil profile on the basis of field soil texture: uniform, gradational and
texture-contrast. These then formed the basis for more detailed subdivision. In North
America and Europe, preference was given to including chemical properties in any
classification system. For example, the once widely popular Seventh Approximation
at a soil classification (Soil Survey Staff, 1960 ) used a combination of chemical and
physical criteria to define soil types. The Seventh Approximation suffered from using
unduly abstruse soil names concocted from a hybrid of Greek and Latin roots to
produce such forbiddingly opaque names as Natrargidic Mazustert , which simply
means an alkaline clay with a surface crust, and Orthic Grumustert for a non-saline,
non-alkaline cracking clay with a self-mulching surface. Aside from the obscure and
formidable terminology, which is really only a minor irritant, a fundamental weakness
of the Seventh Approximation, and of certain of its successors, is the uncritical use
of climatic zonalism as a genetic basis for classifying soils. The next section explains
this key point in more detail.
15.4 Soil-forming processes
Well-developed soil horizons have long been considered as diagnostic of mature
soil profiles (Joffe, 1949 ; Kubiena, 1950 ; Mohr and van Baren, 1959 ; Mohr et al.,
1972 ; Buol et al., 1973 ; Duchaufour, 1978 ; Paton et al., 1995 ; Birkeland, 1999 ;