tephra-mantled, sandier, well drained, soils on mounds or
low ridges with poorly drained, finer textured, silty soils in
the hollows or swales.
In the Hamilton Basin, and in parts of the
Matamata-Hauraki area, on alluvial plains and very
low-angle fans, with low ridge and swale topography, a
distinct soil pattern occurs with Orthic Allophanic Soils,
Impeded Allophanic Soils, and Tephric Gley Soils all
formed in close proximity, often within the same paddock
(Fig. 2.6). The soil pattern has sometimes been described
and mapped as a ‘soil complex’, suggesting that it is not
easy to predict which soil is where within the landscape.
However, a soil association is a better description as there is
a clear, predictable, relationship between the three soils at
the micro-topographic scale. The Allophanic Soils are
formed mainly on the mounds in an incrementallyaccumulating tephra mantle over well drained coarse volcanogenic alluvium (i.e. the soils have two parent materials). In the adjacent, low-lying, swales, Gley Soils are
formed in poorly drained fine-grained volcanogenic alluvium. In between the low ridge tops and the swale bottoms
is an intermediate soil, an Impeded Allophanic Soil, with a
modified tephra mantle on poorly drained volcanogenic
alluvium.
The properties of the soils are dominated by the thickness
of the tephra mantle on the alluvium, the small topographic
differences of the landsurface (mound vs. swale), and
changes in drainage, that together dictate the relative loss of
silicon through leaching and hence the types of clay formed
(Fig. 2.5). Similar soils occur in the Hinuera Valley and
Matamata-Hauraki Plains.
The tephra mantle is obvious on the low ridges but,
puzzlingly, equivalent tephra deposits are not evident in the
hollows, in which the silts seem entirely alluvial in origin,
suggesting either a different propensity for preservation
spatially or that some sort of reworking has taken place after
deposition. The mineral weathering sequence (Sect. 2.4.1)
led to formation of Typic Orthic Allophanic Soils (locally
known as Horotiu series in the Hamilton Basin and Waihou
series in the Matamata-Hauraki area) on the tephra-mantled,
sandy, well drained, low ridges with Typic Tephric Gley
Soil (Te Kowhai series in the Hamilton Basin and Waitoa
series in Matamata-Hauraki area) in the hollows or swales.
The two endpoints of the weathering sequence tend to get
the most attention, but the majority of the landscape comprises the intergrade between the two endpoints. The intergrade (known locally as Bruntwood series in the Hamilton
Basin, Piarere series around Matamata, and Te Puninga
series in the Hauraki area) has allophanic soil material at the
surface and a slowly permeable layer at depth and thus is
classified as a Typic Impeded Allophanic Soil (Fig. 2.6).
2.3.5 Southland Occurrences of Allophanic Soil
Materials
A few small, isolated, pockets of allophane-bearing soils
occur, particularly in northern Southland, in the Hokonui
Range, formed in areas where ancient andesitic tephra has
been preserved within the greywacke/argillite sequences.
The modern acid leaching regime has resulted in the loss of
silicon in soil solution and the formation of allophane on the
greywacke and andesitic material.
2.4 Key Soil Properties
2.4.1 Soil Composition
Allophane, the dominant clay in Allophanic Soils, is one of
the most reactive materials found in soil. Because allophane
has a huge influence on the properties of Allophanic Soils, it
is worth learning a little more about it. The structures of clay
minerals were mainly determined by measuring the way that
X-rays are diffracted by structural sheets in crystalline clay
minerals. However, allophane does not normally diffract
X-rays and so it was described as ‘amorphous to X-rays’.
Later it was discovered that allophane does in fact have
structural order in the form of very small hollow spherules
(3.5–5 nm in diameter). The structure has therefore been
termed ‘short-range order’ or, most recently, ‘nanocrystalline’ because the tiny spherules are structured within the
nanoscale range (1–100 nm). Individual spherules are about
the same size as a virus. Strictly, allophane qualifies as a
mineral because it has demonstrable crystallinity, albeit in
the nanoscale range (on the other hand, its common ‘parent’,
volcanic glass, is not a mineral because glass is amorphous
and therefore defined as a mineraloid). The chemical formula
of allophane is (1–2)SiO 2 ∙Al 2 O 3 ∙(2–3)H 2 O. The molar ratio
of silicon to aluminium varies but is generally about 1:2,
meaning that allophane contains approximately twice as
many atoms of Al as Si. Thus allophanes can be described as
either Al- or Si-rich.
In the North Island, most Allophanic Soils contain the
Al-rich variety of allophane, but Si-rich allophanes occur in
eastern North Island, including Hawke’s Bay where the
rainfall is lower and more seasonal in distribution. Allophane spherules comprise a defective outer gibbsitic
Al-octahedral sheet and an inner Si-tetrahedral sheet, a
structure described as ‘proto-imogolite’. The ‘broken bond
defects’ in the gibbsitic sheet wall provide perforations
where variable surface-charge characteristics arise because
of reactive hydroxyl groups [(OH)Al(OH 2 )] at these
sites. The gain of a proton generates a positive charge:
2.3 Soil-Landscape Relationships
29
low ridges with poorly drained, finer textured, silty soils in
the hollows or swales.
In the Hamilton Basin, and in parts of the
Matamata-Hauraki area, on alluvial plains and very
low-angle fans, with low ridge and swale topography, a
distinct soil pattern occurs with Orthic Allophanic Soils,
Impeded Allophanic Soils, and Tephric Gley Soils all
formed in close proximity, often within the same paddock
(Fig. 2.6). The soil pattern has sometimes been described
and mapped as a ‘soil complex’, suggesting that it is not
easy to predict which soil is where within the landscape.
However, a soil association is a better description as there is
a clear, predictable, relationship between the three soils at
the micro-topographic scale. The Allophanic Soils are
formed mainly on the mounds in an incrementallyaccumulating tephra mantle over well drained coarse volcanogenic alluvium (i.e. the soils have two parent materials). In the adjacent, low-lying, swales, Gley Soils are
formed in poorly drained fine-grained volcanogenic alluvium. In between the low ridge tops and the swale bottoms
is an intermediate soil, an Impeded Allophanic Soil, with a
modified tephra mantle on poorly drained volcanogenic
alluvium.
The properties of the soils are dominated by the thickness
of the tephra mantle on the alluvium, the small topographic
differences of the landsurface (mound vs. swale), and
changes in drainage, that together dictate the relative loss of
silicon through leaching and hence the types of clay formed
(Fig. 2.5). Similar soils occur in the Hinuera Valley and
Matamata-Hauraki Plains.
The tephra mantle is obvious on the low ridges but,
puzzlingly, equivalent tephra deposits are not evident in the
hollows, in which the silts seem entirely alluvial in origin,
suggesting either a different propensity for preservation
spatially or that some sort of reworking has taken place after
deposition. The mineral weathering sequence (Sect. 2.4.1)
led to formation of Typic Orthic Allophanic Soils (locally
known as Horotiu series in the Hamilton Basin and Waihou
series in the Matamata-Hauraki area) on the tephra-mantled,
sandy, well drained, low ridges with Typic Tephric Gley
Soil (Te Kowhai series in the Hamilton Basin and Waitoa
series in Matamata-Hauraki area) in the hollows or swales.
The two endpoints of the weathering sequence tend to get
the most attention, but the majority of the landscape comprises the intergrade between the two endpoints. The intergrade (known locally as Bruntwood series in the Hamilton
Basin, Piarere series around Matamata, and Te Puninga
series in the Hauraki area) has allophanic soil material at the
surface and a slowly permeable layer at depth and thus is
classified as a Typic Impeded Allophanic Soil (Fig. 2.6).
2.3.5 Southland Occurrences of Allophanic Soil
Materials
A few small, isolated, pockets of allophane-bearing soils
occur, particularly in northern Southland, in the Hokonui
Range, formed in areas where ancient andesitic tephra has
been preserved within the greywacke/argillite sequences.
The modern acid leaching regime has resulted in the loss of
silicon in soil solution and the formation of allophane on the
greywacke and andesitic material.
2.4 Key Soil Properties
2.4.1 Soil Composition
Allophane, the dominant clay in Allophanic Soils, is one of
the most reactive materials found in soil. Because allophane
has a huge influence on the properties of Allophanic Soils, it
is worth learning a little more about it. The structures of clay
minerals were mainly determined by measuring the way that
X-rays are diffracted by structural sheets in crystalline clay
minerals. However, allophane does not normally diffract
X-rays and so it was described as ‘amorphous to X-rays’.
Later it was discovered that allophane does in fact have
structural order in the form of very small hollow spherules
(3.5–5 nm in diameter). The structure has therefore been
termed ‘short-range order’ or, most recently, ‘nanocrystalline’ because the tiny spherules are structured within the
nanoscale range (1–100 nm). Individual spherules are about
the same size as a virus. Strictly, allophane qualifies as a
mineral because it has demonstrable crystallinity, albeit in
the nanoscale range (on the other hand, its common ‘parent’,
volcanic glass, is not a mineral because glass is amorphous
and therefore defined as a mineraloid). The chemical formula
of allophane is (1–2)SiO 2 ∙Al 2 O 3 ∙(2–3)H 2 O. The molar ratio
of silicon to aluminium varies but is generally about 1:2,
meaning that allophane contains approximately twice as
many atoms of Al as Si. Thus allophanes can be described as
either Al- or Si-rich.
In the North Island, most Allophanic Soils contain the
Al-rich variety of allophane, but Si-rich allophanes occur in
eastern North Island, including Hawke’s Bay where the
rainfall is lower and more seasonal in distribution. Allophane spherules comprise a defective outer gibbsitic
Al-octahedral sheet and an inner Si-tetrahedral sheet, a
structure described as ‘proto-imogolite’. The ‘broken bond
defects’ in the gibbsitic sheet wall provide perforations
where variable surface-charge characteristics arise because
of reactive hydroxyl groups [(OH)Al(OH 2 )] at these
sites. The gain of a proton generates a positive charge:
2.3 Soil-Landscape Relationships
29