Page 29 - Soil Degradation, Conservation and Remediation

14 1 Soil Resources and Soil Degradation

There is low chemical weathering, low leaching, and sparse plant growth. Soil
organic matter content is low so that ochric horizons abundantly develop in
Aridisols. Salts (chlorides, sulfates, carbonates) released by limited chemical
weathering are not usually translocated to considerable depths but are accumulated
on the surface and, where there is some downward movement of water, in the B
horizon. Aridisols have one or more of the following within 100 cm of surface a
calcic, cambic, gypsic, natric, petrocalcic, petrogypsic, or salic horizon. An argillic
horizon is found in some Aridisols. This is believed to have developed under a
moister climate of the past. Aridisols are sparsely vegetated, mostly in xeric shrub
lands with xerophytes, cactus, and thorns. They may be cultivated if irrigation
can be given, but source of irrigation water is also scanty there. Aridisols com-
prise about 12 % of the world’s ice-free land surface. (The WRB equivalents of
Aridisols are Durisols, Gypsisols, and Solonchaks.) Aridisols have seven subor-
ders. They are:

Cryids: Cryids are the Aridisols of the cold climates. These soils are characteristi-
cally developed at high elevations, dominantly in the mountain and basin areas of
the USA and Asia and other parts of the world. Cryids commonly show evidence
of periglacial features.

Salids: Salids are Aridisols with accumulations that are more soluble than gypsum.

The most common form is sodium chloride, but sulfates and others may also
occur. These soils are common in depressions in the deserts or in closed basins
in wetter areas bordering deserts. Some salts may be brought to the upper hori-
zons by capillary rise of groundwater.
Durids: Durids are the Aridisols that have an accumulation of silica. There is a duri-

pan which is cemented partly with opal or chalcedony. The soils commonly have
calcium carbonate. The duripan restricts movement of water and penetration of
roots. These soils occur in the western part of the USA particularly in Nevada.
They are not known to occur outside the USA.

Gypsids: Gypsids are the Aridisols that have an accumulation of gypsum. These

soils occur in Iraq, Syria, Saudi Arabia, Iran, Somalia, West Asia, and in some of
the most arid regions of the USA. When the gypsic horizon occurs as a cemented
impermeable layer, it is recognized as the petrogypsic horizon.

Argids: Argids are the Aridisols that have accumulation of clay. These soils have an

argillic or natric horizon. The presence of an argillic horizon is commonly attrib-
uted to a moister paleoclimate. Most Argids occur in North America with a few
recognized in the deserts of North Africa or the Near East.
Calcids: Calcids are the Aridisols that have accumulation of residual calcium car-

bonate or was added as dryfall. Precipitation is inadequate to leach or move the
carbonates to great depths. These soils are extensive in the western USA and
other arid regions of the world.

Cambids: These are the Aridisols with the least degree of soil development. They

have a cambic horizon that has its upper boundary within 100 cm of the soil sur-
face. These soils are the most common Aridisols in the USA and other parts of the
world.