hypothesis was that loess may have settled on the Waiareka
soil material but its slow permeability caused overland flow
that would have eroded the loess.
During one Waiareka soil sampling effort, a large digger
was used to prepare a deep sampling pit. At a depth greater
than the usual sampling depth, the investigators were surprised to find buried loess at the base of the weathered
mafic-rock soil material. In the midst of the so-called loess
gap there was indeed loess—though deeply buried beneath
the Waiareka material. This prompted the alternative
hypothesis that the weathering mafic rock exposed on slopes
was shattered by freeze and thaw processes and transported
by solifluction downslope, covering the loess after its
deposition. Despite these findings the origin of the loess gap
remains largely unresolved and awaits a keen examination
of field evidence and paleoenvironmental reconstruction.
7.3.5 Southland Tuffaceous Greywacke
A series of hilly ridges divide the southern plains of
Southland from the northern Southland plains of the upper
Mataura River valley. The hills are generally known as the
Kaihiku-Hokonui region. The rocks are termed tuffaceous
greywacke. Unlike the greywacke rocks (Torlesse terrain)
and Otago schists further north, the primary materials were
lain down as sea-floor sediments while andesitic volcanoes
were active in the vicinity. Consequently, the sediments
incorporated andesitic eruptives. The soils, described by
Peter McIntosh, have higher iron, manganese, titanium,
potassium, aluminium, and magnesium than most other New
Zealand soils formed on greywackes and schists.
Two soil series in the Melanic Soil order have been
defined in this landscape. The Kaihiku soils (Argillic Orthic
Melanic Soils) and Stonycreek soils (Mottled Orthic Melanic
Soils) are both mapped on dry, north-facing hill slopes in
stony colluvial slope debris.
7.3.6 Nelson and Otago-Southland
Dunite/Serpentinite
An unusual Melanic Soil is found on ultramafic and mafic
rocks in a few places in New Zealand where such rocks,
associated with the Dun Mountain ophialite belt, occur at the
land surface. This ‘belt’ comprises a narrow zone that
extends through New Zealand where the upper mantle has
become exposed at the surface because of tectonism. The
ultramafic/mafic rocks form Mafic Melanic Soils on Dun
Mountain and Red Hills in the Nelson-Marlborough area,
West Dome in Southland, and in Otago and Northland. The
serpentinite rocks are also found in the Piopio area in King
Country but there the pervasive tephra mantle limits the
occurrence of genuine Mafic Melanic Soils.
The ultramafic parent rocks of Mafic Melanic Soils
comprise mainly dunite, serpentinite, peridotite, and associated basalts and dolerites, and they are enriched in minerals
such as olivine and pyroxene. They have low concentrations
of most major plant nutrients and high concentrations of
metals such as nickel, chromium, manganese, cobalt, and
magnesium, and a high pH. On Dun Mountain (for example,
Fig. 7.5), the metal enrichment has meant that the native
Nothofagus forest has been unable to colonise the soils. The
vegetation instead is dominated by stunted shrubs and
grasses, which can be toxic to sheep due to their high content
of metals. In a study on soils on an ultramafic scree in
northwest Otago, led by Bill Lee, it was found that plant
diversity and vegetation stature increase with declining
heavy metal availability and increasing soil nutrient status.
The boundary between the ultramafic and non-ultramafic
soils and associated vegetation is often sharp, a notable
exception to the usual gradational boundary paradigm
associated with many other soils in the landscape.
7.4 Key Soil Properties
7.4.1 Soil Composition
Most Melanic Soils have smectite clays or clay minerals
with inter-stratifications involving smectite. Melanic Soils
usually have smectitic, illitic, or kandic mineralogy groups.
‘Smectite’ is the name of a group of clay minerals which
includes montmorillonite and bentonite, which are usually
associated with soils with strong shrink/swell character, and
beidellite which is also found in some New Zealand Podzols.
Melanic Soils often also have the clay minerals illite,
kaolinite, and vermiculite interstratified with smectite.
7.4.2 Physical Properties
Melanic Soils have a wide range of clay contents (Fig. 7.6,
Table 7.1). Some soils, such as the Waiareka (Table 7.1),
have high (50–70%) clay contents and they are outside the
75th percentile illustrated in Fig. 7.5. However, all Melanic
Soils have sufficient clay to impact the soil properties and the
soil materials are sticky and plastic, and are sensitive to
water content. Melanic Soils generally have low readily
available, but high (20–25%) total available, water contents
reflecting the moisture-holding effect of the high clay content. Except for shallow soils on rock, or soils affected by
high water tables, potential rooting depths are relatively
large giving good overall profile available water.
106
7 Melanic Soils
soil material but its slow permeability caused overland flow
that would have eroded the loess.
During one Waiareka soil sampling effort, a large digger
was used to prepare a deep sampling pit. At a depth greater
than the usual sampling depth, the investigators were surprised to find buried loess at the base of the weathered
mafic-rock soil material. In the midst of the so-called loess
gap there was indeed loess—though deeply buried beneath
the Waiareka material. This prompted the alternative
hypothesis that the weathering mafic rock exposed on slopes
was shattered by freeze and thaw processes and transported
by solifluction downslope, covering the loess after its
deposition. Despite these findings the origin of the loess gap
remains largely unresolved and awaits a keen examination
of field evidence and paleoenvironmental reconstruction.
7.3.5 Southland Tuffaceous Greywacke
A series of hilly ridges divide the southern plains of
Southland from the northern Southland plains of the upper
Mataura River valley. The hills are generally known as the
Kaihiku-Hokonui region. The rocks are termed tuffaceous
greywacke. Unlike the greywacke rocks (Torlesse terrain)
and Otago schists further north, the primary materials were
lain down as sea-floor sediments while andesitic volcanoes
were active in the vicinity. Consequently, the sediments
incorporated andesitic eruptives. The soils, described by
Peter McIntosh, have higher iron, manganese, titanium,
potassium, aluminium, and magnesium than most other New
Zealand soils formed on greywackes and schists.
Two soil series in the Melanic Soil order have been
defined in this landscape. The Kaihiku soils (Argillic Orthic
Melanic Soils) and Stonycreek soils (Mottled Orthic Melanic
Soils) are both mapped on dry, north-facing hill slopes in
stony colluvial slope debris.
7.3.6 Nelson and Otago-Southland
Dunite/Serpentinite
An unusual Melanic Soil is found on ultramafic and mafic
rocks in a few places in New Zealand where such rocks,
associated with the Dun Mountain ophialite belt, occur at the
land surface. This ‘belt’ comprises a narrow zone that
extends through New Zealand where the upper mantle has
become exposed at the surface because of tectonism. The
ultramafic/mafic rocks form Mafic Melanic Soils on Dun
Mountain and Red Hills in the Nelson-Marlborough area,
West Dome in Southland, and in Otago and Northland. The
serpentinite rocks are also found in the Piopio area in King
Country but there the pervasive tephra mantle limits the
occurrence of genuine Mafic Melanic Soils.
The ultramafic parent rocks of Mafic Melanic Soils
comprise mainly dunite, serpentinite, peridotite, and associated basalts and dolerites, and they are enriched in minerals
such as olivine and pyroxene. They have low concentrations
of most major plant nutrients and high concentrations of
metals such as nickel, chromium, manganese, cobalt, and
magnesium, and a high pH. On Dun Mountain (for example,
Fig. 7.5), the metal enrichment has meant that the native
Nothofagus forest has been unable to colonise the soils. The
vegetation instead is dominated by stunted shrubs and
grasses, which can be toxic to sheep due to their high content
of metals. In a study on soils on an ultramafic scree in
northwest Otago, led by Bill Lee, it was found that plant
diversity and vegetation stature increase with declining
heavy metal availability and increasing soil nutrient status.
The boundary between the ultramafic and non-ultramafic
soils and associated vegetation is often sharp, a notable
exception to the usual gradational boundary paradigm
associated with many other soils in the landscape.
7.4 Key Soil Properties
7.4.1 Soil Composition
Most Melanic Soils have smectite clays or clay minerals
with inter-stratifications involving smectite. Melanic Soils
usually have smectitic, illitic, or kandic mineralogy groups.
‘Smectite’ is the name of a group of clay minerals which
includes montmorillonite and bentonite, which are usually
associated with soils with strong shrink/swell character, and
beidellite which is also found in some New Zealand Podzols.
Melanic Soils often also have the clay minerals illite,
kaolinite, and vermiculite interstratified with smectite.
7.4.2 Physical Properties
Melanic Soils have a wide range of clay contents (Fig. 7.6,
Table 7.1). Some soils, such as the Waiareka (Table 7.1),
have high (50–70%) clay contents and they are outside the
75th percentile illustrated in Fig. 7.5. However, all Melanic
Soils have sufficient clay to impact the soil properties and the
soil materials are sticky and plastic, and are sensitive to
water content. Melanic Soils generally have low readily
available, but high (20–25%) total available, water contents
reflecting the moisture-holding effect of the high clay content. Except for shallow soils on rock, or soils affected by
high water tables, potential rooting depths are relatively
large giving good overall profile available water.
106
7 Melanic Soils