34
WALLACE H. FULLER
other climatic factor. Thus, both temperature and precipitation have been
combined in various indices to define aridity of an area (Thornthwaite,
1948; Trewartha, 1954). Some excellent reviews of desert climates are
provided by Meigs (1953), McGinnies and Meadows (1968), Reitan and
Green (1967), and Ramaley (1952). Seasonal as well as daily temperatures vary more in arid than humid climates. Monthly means will differ
from — 12°C in coastal areas to over 15°C in interior basins. Diurnal temperatures range from — 1° to 10°C, except in the seacoast deserts where
ranges are much narrower. These temperature variations greatly affect
évapotranspiration, plant cover, and soil microbial activity.
In general, desert vegetation is sparse, short, and develops as individual
plants. Accumulation of soil organic matter is slow if at all. Ramaley
(1952) attempted to prepare a composite map of world desert boundaries
by surperimposing desert climate, desert vegetation, and desert soils maps
on a common base. There is a wide variation among each of the authors
as to the delineation of desert areas. The greatest differences occur in Asia
and, to a lesser extent, in North America. In Asia all three values vary
widely among authors whereas, in North America, climate and vegetation
vary the most. Desert soils appear to have a wider distribution than the
classificatory elements of climate and vegetation.
II. Desert Soil Surfaces and Classes
A. ARIDISOLS
Desert soils are classed "genetically" as Aridisols, i.e., soils of dry places.
They are variously described as having an ochric epipedon that is normally
soft when dry or that has a distinct structure. In addition they have one
or more of the following diagnostic horizons: argillic, natric, or cambic
horizons; calcic, gypsic, or salic horizons; or a duripan (U.S. Soil Conservation Service, 1967). An agricultural productive Aridisol is Mohave loam,
seen in Fig. 2. Soil genetists do not include shifting desert sand, recent
alluvial deposits, and shallow soils on bed rock with the Aridisols but as
Entisols, described later.
Wind plays an important role in Aridisol genesis. Even though North
American desert winds are less extreme than in other world deserts, dust
storms are common, sand and dust are continually moving, leaving polished desert pavements of pebbles and stones and forming and reforming
restless dunes or depositing loess (aeolian dust). Calcium carbonate and
other salts accumulate in Aridisols by the downward penetration of salts
WALLACE H. FULLER
other climatic factor. Thus, both temperature and precipitation have been
combined in various indices to define aridity of an area (Thornthwaite,
1948; Trewartha, 1954). Some excellent reviews of desert climates are
provided by Meigs (1953), McGinnies and Meadows (1968), Reitan and
Green (1967), and Ramaley (1952). Seasonal as well as daily temperatures vary more in arid than humid climates. Monthly means will differ
from — 12°C in coastal areas to over 15°C in interior basins. Diurnal temperatures range from — 1° to 10°C, except in the seacoast deserts where
ranges are much narrower. These temperature variations greatly affect
évapotranspiration, plant cover, and soil microbial activity.
In general, desert vegetation is sparse, short, and develops as individual
plants. Accumulation of soil organic matter is slow if at all. Ramaley
(1952) attempted to prepare a composite map of world desert boundaries
by surperimposing desert climate, desert vegetation, and desert soils maps
on a common base. There is a wide variation among each of the authors
as to the delineation of desert areas. The greatest differences occur in Asia
and, to a lesser extent, in North America. In Asia all three values vary
widely among authors whereas, in North America, climate and vegetation
vary the most. Desert soils appear to have a wider distribution than the
classificatory elements of climate and vegetation.
II. Desert Soil Surfaces and Classes
A. ARIDISOLS
Desert soils are classed "genetically" as Aridisols, i.e., soils of dry places.
They are variously described as having an ochric epipedon that is normally
soft when dry or that has a distinct structure. In addition they have one
or more of the following diagnostic horizons: argillic, natric, or cambic
horizons; calcic, gypsic, or salic horizons; or a duripan (U.S. Soil Conservation Service, 1967). An agricultural productive Aridisol is Mohave loam,
seen in Fig. 2. Soil genetists do not include shifting desert sand, recent
alluvial deposits, and shallow soils on bed rock with the Aridisols but as
Entisols, described later.
Wind plays an important role in Aridisol genesis. Even though North
American desert winds are less extreme than in other world deserts, dust
storms are common, sand and dust are continually moving, leaving polished desert pavements of pebbles and stones and forming and reforming
restless dunes or depositing loess (aeolian dust). Calcium carbonate and
other salts accumulate in Aridisols by the downward penetration of salts