Other factors that complicate the variability of fan sediments and soils include the weathering of the slope mantle of
rock and soils and their slope stability, the length of time
since the slope mantle was last eroded, and the proportion of
the slope mantle carried from the slopes and deposited onto
the fan. The processes also depend on the volume of water
flowing from the catchment and the turbulence of the
sediment-charged streams.
The processes generating such variability result in complex fan systems, driven by degrees of chaotic behaviour.
One mitigating factor that smooths out soil textural variability is the deposition of loess on older fans which provides a relatively homogeneous blanket of varying
thicknesses of silt and very fine sand. If the direction of loess
transport can be inferred, then it is usually thicker on the
downwind side of the fan. Such soils are formed by
upbuilding pedogenesis involving some complexity: episodic deposition of alluvium and the slow accretion of loess
during glacials takes place at the same time as weak soil
formation by topdown pedogenesis.
15.4 Key Soil Properties
15.4.1 Soil Composition
Semiarid Soils are mainly formed in coarse alluvial sediments that include boulders, cobbles, gravels, and sand, as
well as loess, derived from the non-calcareous greywacke,
schist, and mudstone rocks that dominate the semiarid Otago
and inland Canterbury regions. The parent materials are
dominated by quartz and feldspar, and by mica (which is
derived from schist). Weathering of the parent material is
generally limited due to the cool winters and generally dry
conditions, and in many cases, but not all, limited time. Thus
many of the soils tend to have the pale grey colours of the
parent materials (lithochromic).
Many Semiarid Soils are formed in the material deposited
from about 2000 to at least 30,000 years ago, and others on
high terraces (as noted earlier) may be an order of magnitude
older. The clay mineralogy reflects the limited soil weathering with clays dominated by hydrous mica or illite, plus
chlorites, mostly little altered from the original mica of the
parent material. Semiarid Soils generally have low concentrations of secondary oxides. However, on some high, older,
surfaces, and in areas of salt concentration, in Aged-argillic
and Solonetzic Semiarid Soils, kaolinite is common.
Some ‘amorphous’ (potentially nanocrystalline) hydrous
oxides of aluminium and iron also occur. Peter McIntosh
suggested in 1994 that small quantities of allophane (which
is a nanocrystalline mineral) were possibly present, coating
sand grains, in the old soils on high terraces at Pisa Flats near
Cromwell. This inference was based on reactions, recorded
as moderate to strong, for lower horizons using the NaF
(allophane ) field test. Despite the low rainfall, which would
normally seem to preclude any possibility of silicon being
sufficiently enleached to enable allophane to form, McIntosh
argued instead that the likely old age of the soil (hence long
time available), coupled with the subsoil’s highly permeable
character, generating strong acidity (pHs of 4.9 and 5.4),
would make up for this limitation. NaF does react with
calcium carbonate but this was absent from the soil examined by McIntosh (and the low pHs support such an
absence).
15.4.2 Physical Properties
While most of the Semiarid Soils are formed on gravelly
sand materials, on alluvial terraces, some are formed on
weathered rock materials and many contain loess (silty but
also sandy close to river bed sources). Thus, there is a wide
range of soil physical properties within the Semiarid Soil
order (Fig. 15.7, Table 15.1). Due to the slow rates of
Fig. 15.7 Median and upper and
lower quartiles of clay content,
soil dry bulk density (t m
−3
), and
total available water holding
capacity for Semiarid Soils in the
New Zealand Soil Data
Repository
238
15 Semiarid Soils
rock and soils and their slope stability, the length of time
since the slope mantle was last eroded, and the proportion of
the slope mantle carried from the slopes and deposited onto
the fan. The processes also depend on the volume of water
flowing from the catchment and the turbulence of the
sediment-charged streams.
The processes generating such variability result in complex fan systems, driven by degrees of chaotic behaviour.
One mitigating factor that smooths out soil textural variability is the deposition of loess on older fans which provides a relatively homogeneous blanket of varying
thicknesses of silt and very fine sand. If the direction of loess
transport can be inferred, then it is usually thicker on the
downwind side of the fan. Such soils are formed by
upbuilding pedogenesis involving some complexity: episodic deposition of alluvium and the slow accretion of loess
during glacials takes place at the same time as weak soil
formation by topdown pedogenesis.
15.4 Key Soil Properties
15.4.1 Soil Composition
Semiarid Soils are mainly formed in coarse alluvial sediments that include boulders, cobbles, gravels, and sand, as
well as loess, derived from the non-calcareous greywacke,
schist, and mudstone rocks that dominate the semiarid Otago
and inland Canterbury regions. The parent materials are
dominated by quartz and feldspar, and by mica (which is
derived from schist). Weathering of the parent material is
generally limited due to the cool winters and generally dry
conditions, and in many cases, but not all, limited time. Thus
many of the soils tend to have the pale grey colours of the
parent materials (lithochromic).
Many Semiarid Soils are formed in the material deposited
from about 2000 to at least 30,000 years ago, and others on
high terraces (as noted earlier) may be an order of magnitude
older. The clay mineralogy reflects the limited soil weathering with clays dominated by hydrous mica or illite, plus
chlorites, mostly little altered from the original mica of the
parent material. Semiarid Soils generally have low concentrations of secondary oxides. However, on some high, older,
surfaces, and in areas of salt concentration, in Aged-argillic
and Solonetzic Semiarid Soils, kaolinite is common.
Some ‘amorphous’ (potentially nanocrystalline) hydrous
oxides of aluminium and iron also occur. Peter McIntosh
suggested in 1994 that small quantities of allophane (which
is a nanocrystalline mineral) were possibly present, coating
sand grains, in the old soils on high terraces at Pisa Flats near
Cromwell. This inference was based on reactions, recorded
as moderate to strong, for lower horizons using the NaF
(allophane ) field test. Despite the low rainfall, which would
normally seem to preclude any possibility of silicon being
sufficiently enleached to enable allophane to form, McIntosh
argued instead that the likely old age of the soil (hence long
time available), coupled with the subsoil’s highly permeable
character, generating strong acidity (pHs of 4.9 and 5.4),
would make up for this limitation. NaF does react with
calcium carbonate but this was absent from the soil examined by McIntosh (and the low pHs support such an
absence).
15.4.2 Physical Properties
While most of the Semiarid Soils are formed on gravelly
sand materials, on alluvial terraces, some are formed on
weathered rock materials and many contain loess (silty but
also sandy close to river bed sources). Thus, there is a wide
range of soil physical properties within the Semiarid Soil
order (Fig. 15.7, Table 15.1). Due to the slow rates of
Fig. 15.7 Median and upper and
lower quartiles of clay content,
soil dry bulk density (t m
−3
), and
total available water holding
capacity for Semiarid Soils in the
New Zealand Soil Data
Repository
238
15 Semiarid Soils