Geoscience Reference
In-Depth Information
Table 7.1
The major soil orders of the arid and semi-arid regions (modified after Dregne, 1976).
Total global land
Percentage of global
Soil order
Primary characteristics
area occupied (km
2
)
land area occupied
16.6
×
10
6
Aridisols
Plant growth restricted by dryness and/or salinity all year
11.3
3.1
×
10
6
Alfisols
Moderate base saturation; an argillic horizon; some plant-
available water seasonally
2.1
19.2
×
10
6
Entisols
Almost no horizon differentiation; little-altered sedimentary
materials
13.1
5.5
×
10
6
Mollisols
Thick, dark, base- and organically-rich epipedon
3.7
×
10
6
Vertisols
Deep, cracking clay soils, with shrink-swell features common
1.9
1.3
features that are mentioned later in this chapter are not in-
cluded in the classification rules, and we thus sidestep soil
taxonomy, turning to the important issues of soil function
in the landscape.
erodible surfaces upwind, since these materials accumu-
late faster than they are eroded, especially during inter-
vals of dry and dusty climate. Local weathering products
and organic matter are also components of desert soils
(Marion
et al.
, 2008). As noted in the Introduction, partic-
ular surfaces within deserts do lose dusts to the global at-
mospheric transport system, and dust fallout and washout
in rain are known from widespread and remote locations,
and contribute to sedimentation over large areas of the
oceans (see Chapter 18). In the desert of western New
South Wales, Australia, it has been estimated that at the
end of the last glacial arid phase, regional landscapes may
have been blanketed by significant depths of windblown
materials from the Australian interior, with present-day
A-horizons more recently emplaced by slopewash pro-
cesses (see Chartres, 1982, 1983). Rates of dust acces-
sion to NSW soils decrease eastwards (downwind, see
Cattle, McTainsh and Elias, 2009). Tiller, Smith and
Merry (1987) cite aeolian dust accumulation rates from
sites in Australia of up to 32 t/km
2
a (equivalent to more
than 15 mm/ka). Rates of loess accumulation from sites
in China also cited by these authors are up to 70 mm/ka.
Windblown materials can be incorporated into desert soils,
providing allogenic minerals, salts and much more abun-
dant clay than could be produced in situ by ordinary
weathering processes. Thus, argillic horizons in desert
soils are often attributable to dust incorporation. In con-
trast, many humid zone soils are dominated by autogenic
clays weathered in situ from the parent materials, though
small accessions of dust are involved in their pedogenesis.
Because of the slow overall pace of landscape change,
desert soil-forming processes involve events over long
time periods. Materials such as calcium and bicar-
bonate ions, delivered in dilute solution in rainwater,
progressively accumulate in the soil. Subsoils, often al-
kaline because of accumulated carbonates, may slowly
develop carbonate enrichment to the point where fully in-
7.2.1
A note on terminology of near-surface
features in desert soils
Throughout this chapter, the term 'soil seal' is used to refer
to inorganic structures at and near the soil surface, since
the key hydrologic effect of these structures is partially
to seal the soil against water entry (and the reciprocal
escape of air from the soil pore spaces). The term 'soil
crust' is reserved for reference to organic or biological
soil crusts (BSCs), since these often form quite distinct
surface layers that can be separated from the underlying
soil in small flakes or sheets. This usage is not universal
in the published literature, but it is helpful for clarity of
expression to adopt distinct terminology. Some authors
refer to raindrop impact seals as 'seals' only when they
are wet and adopt the name 'crust' for the same features
when they have become air dry.
7.3
Some distinctive aspects of desert
soil development
Desert soils develop in a wide range of parent materials,
including the extensive fluviatile deposits of the pied-
mont slopes flanking upland areas, as well as fluvial and
aeolian materials of diverse origins and ages. Averaged
over regional scales, erosion rates in many desert areas
are low, and the intensity of leaching is low owing to
aridity. On balance, many desert soils are consequently
typified by
accumulations
of materials such as salts and
