INTERNAL ENVIRONMENT, MICROCLIMATE AND RESOURCES - Soil Ecology - page 14

(Chapter II) (Soil Survey Staff, 1999); detailed descriptions of each order are to be found
in Wilding et al. (1983). Table I.4 presents examples of the particle size distributions,
carbon contents, parent materials, environments and locations of a diverse range of soils
classified at the level of Soil Order in Soil Taxonomy. It should also be recalled that
the limits of the various size classes of particles differ between systems.
The entisols show little profile development. The first of these soils was formed on a
beach ridge stabilised by tropical woodland vegetation and shows little horizon devel-
opment beyond a slight organic matter accumulation near the surface (Murtha, 1986).
In contrast, the second soil (Bockheim, 1980a) was formed on the terminal moraine of
a glacier in the cold desert of Antarctica; it supports no macrophytic vegetation and has
a negligible organic matter content. Despite its estimated age of 135,000 years, it has vir-
tually no clay; this is a legacy of the slow rates of chemical weathering that occur
in the Antarctic environment (Campbell and Claridge, 1987).
The andisols are a group of soils largely formed from volcanic ash parent materials
and occur principally in regions with a history of recent vulcanism. These soils are found
notably in countries of the Pacific rim but also occur in other continents and on certain
volcanic islands such as Martinique (West Indies) and Réunion (Mascaraigne Islands).
They are characterised by unusual physical properties such as low bulk densities and
irreversible shrinkage on drying (Maeda et al ., 1977), high soil organic matter contents
and the frequent presence of volcanic glass and the minerals allophane and imogolite.
They are also rich in active aluminium and/or iron and have a substantial capacity to fix
phosphorus in forms unavailable to plants (Wada, 1985). The andisol described by Bleeker
and Sageman (1990) is from Papua New Guinea and is located on the lower footslope
of an active volcano where it supports a vegetation of coconuts and cocoa. This soil is
periodically rejuvenated by volcanic ash showers, the most recent of which took place
during 1979.
The alfisol described by Coventry et al. (1983) is from a seasonally dry ( ca. 600 mm
annual average precipitation) tropical savanna environment in north-eastern Australia.
It is a coarse-textured soil with little organic matter, consistent with its climate and sparse
vegetation. High proportions of gravel occur throughout the profile; some gravel-sized
particles have been transported from elsewhere or were formed secondarily by precipitation
from solution and other processes, while further gravels at depth in the profile were inher-
ited from the parent rock. The clay content of the fine earth increases at depth, probably
due to the transport of clay-sized particles (illuviation) downward through the profile.
The alfisol described by Murtha (1982) from northern Australia was formed from
granitic alluvial and colluvial parent materials in a seasonally-dry tropical climate, with
an average annual rainfall of 1170 mm. It supports a tropical savanna vegetation with
a tree stratum largely dominated by species of the genus Eucalyptus (Myrtaceae) and
an herbaceous layer dominated by perennial grasses. As shown below, this soil is
characterised by a coarse-textured A horizon that changes abruptly to a finer B horizon
characterised by high concentrations of exchangeable sodium.
The ultisol described by Laffan (1988) was formed from metamorphic rock parent
materials in a humid (approximately 2000 mm annual average rainfall) environment in
tropical north-eastern Australia. It currently supports a montane rainforest through whose
canopy the taller stems of Eucalyptus species emerge. It has an appreciable carbon