North Island, where most Ultic Soils occur, has the warmest
climate in New Zealand and hence rates of soil-forming
processes are relatively fast compared with those elsewhere.
The northern Ultic Soils form under generally humid conditions, although droughts are not uncommon. Because of
the warm climate and high solar radiation, evapotranspiration is high. Thus, the generally low monthly water balances
(rainfall/potential evaporation) tend to make the region
susceptible to drought when rainfall is only a little lower
than usual.
In terms of vegetation, pollen studies by Rewi Newnham
and others have shown that the Northland Peninsula
remained forested throughout the Last Glaciation, although
the forest was dominated by beech and ‘cool’ tree species
(rather than kauri and ‘warm’ species that characterise interglacials). During the Last Glaciation, mean annual temperatures in New Zealand were depressed by *6 °C.
Consequently, the Ultic Soils north of Auckland have
always been under forest, whereas the rest of New Zealand
was dominated mainly by shrubland-grassland vegetation
during the Last Glaciation (the Waikato region occupied an
intermediate position, comprising shrubland-grassland with
some beech forest and rare conifers).
Putting these factors all together means that the Ultic
Soils (in northern North Island) have been strongly weathered and hence clay formation (argillisation or clay synthesis) is well advanced with clay contents in lower subsoils
often around 50–60% or more.
Plant growth is rapid and melanisation, humification,
mineralisation, and nutrient recycling in Ultic Soils are also
rapid processes in warm, humid conditions. Soil mixing by
bioturbation, such as tree overturn and worm activity, have
been important and, as a result, the horizon boundaries in
many Ultic Soils are occluded or irregular, and indistinct to
diffuse.
Leaching has been strong despite many soils being
imperfectly to poorly drained (presumably because rainfall
has always been at least moderate to high, and land surfaces
are old, thus allowing plentiful time), and marked acidity is
therefore a key feature for the order. Ultic Soils are defined
as having a pH <5.5 for the major part of the profile to a
depth of 60 cm. Hand-in-hand with the leaching-induced
acidity is a low nutrient status.
Clay movement down the profile (illuviation or illimerisation) has occurred, shown by the common occurrence of
either an E horizon or a pale B horizon beneath the A
horizon and a complementary clay-enriched Bt (argillic)
horizon in the lower subsoil. Dark humus material is also
commonly translocated down cracks in the profile, coating,
and staining ped faces (which may be prismatic, coarse
blocky, or polyhedral) in the subsoil. The argillic horizon
slows permeability and therefore horizons above it typically
have redox and reductimorphic features, ranging from
orange mottles, Fe–Mn concretions, to low-chroma colours
sufficiently extensive to form Eg or Er horizons.
Although the Ultic Soils in Northland are formed by
topdown pedogenesis on a range of parent materials, mineralogical studies of sand fractions in the soils show that
minor amounts of volcanic glass also occur in the profiles.
Such glass shards were probably largely derived from tephra
blown directly from eruptions of volcanoes in far-off central
North Island. This inference is supported by the occurrence
of thin rhyolitic (silica-rich) tephra (volcanic ash) beds,
mainly comprising glass, preserved unaltered in lake sediments and peat bogs in Northland. Such tephras include a
*10-cm-thick layer of Rotoehu Ash (c. 45,000 years old) in
Lake Omapere near Kaikohe, along with dustings of
Kaharoa, Taupo, and other tephras.
In only one known case in North Island has an Ultic Soil
been formed by upbuilding (rather than topdown) pedogenesis, namely the tephra-derived Kainui soil (Sect. 16.3.3).
In Southland, an Ultic Soil formed on strongly weathered
alluvium also has a capping of loess and so it originated
mainly via topdown pedogenesis but with some upbuilding
pedogenesis in more recent times represented by incremental
loess accumulation.
16.3 Soil-Landscape Relationships
16.3.1 Introduction
The strongly weathered, strongly leached, clay-rich, and
acidic character of Ultic Soils is attributed to long periods of
land stability that provide sufficient time for soil-forming
process to operate. The land surfaces where Ultic Soils occur
are survivors of the major perturbations (volcanic activity,
tectonic uplift, and impacts related to glacial-interglacial
cycles including the Last Glaciation) that have strongly
influenced the soils and their parent materials in other
regions.
As well as their Northland-Auckland-Waikato homeland,
the Ultic Soils also occur in small isolated areas throughout
the rest of New Zealand where remnants of land surfaces
have remained stable. Thus, scattered Ultic Soils also occur
in:
– Wellington, where remnants of old land surfaces occur as
relatively stable ridge crests in hilly terrain.
– Marlborough Sounds, on spurs of low relief land near sea
level with slopes less than 25° that protrude into the inlets
and which have escaped burial by the active colluvium
on steeper slopes closer to the main ridges, specifically in
Port Underwood, and Tennyson Inlet. Beneath these
soils, the schist and schistose-greywacke saprolite is
deeply red weathered.
16.2 Soil Profile Genesis
253
climate in New Zealand and hence rates of soil-forming
processes are relatively fast compared with those elsewhere.
The northern Ultic Soils form under generally humid conditions, although droughts are not uncommon. Because of
the warm climate and high solar radiation, evapotranspiration is high. Thus, the generally low monthly water balances
(rainfall/potential evaporation) tend to make the region
susceptible to drought when rainfall is only a little lower
than usual.
In terms of vegetation, pollen studies by Rewi Newnham
and others have shown that the Northland Peninsula
remained forested throughout the Last Glaciation, although
the forest was dominated by beech and ‘cool’ tree species
(rather than kauri and ‘warm’ species that characterise interglacials). During the Last Glaciation, mean annual temperatures in New Zealand were depressed by *6 °C.
Consequently, the Ultic Soils north of Auckland have
always been under forest, whereas the rest of New Zealand
was dominated mainly by shrubland-grassland vegetation
during the Last Glaciation (the Waikato region occupied an
intermediate position, comprising shrubland-grassland with
some beech forest and rare conifers).
Putting these factors all together means that the Ultic
Soils (in northern North Island) have been strongly weathered and hence clay formation (argillisation or clay synthesis) is well advanced with clay contents in lower subsoils
often around 50–60% or more.
Plant growth is rapid and melanisation, humification,
mineralisation, and nutrient recycling in Ultic Soils are also
rapid processes in warm, humid conditions. Soil mixing by
bioturbation, such as tree overturn and worm activity, have
been important and, as a result, the horizon boundaries in
many Ultic Soils are occluded or irregular, and indistinct to
diffuse.
Leaching has been strong despite many soils being
imperfectly to poorly drained (presumably because rainfall
has always been at least moderate to high, and land surfaces
are old, thus allowing plentiful time), and marked acidity is
therefore a key feature for the order. Ultic Soils are defined
as having a pH <5.5 for the major part of the profile to a
depth of 60 cm. Hand-in-hand with the leaching-induced
acidity is a low nutrient status.
Clay movement down the profile (illuviation or illimerisation) has occurred, shown by the common occurrence of
either an E horizon or a pale B horizon beneath the A
horizon and a complementary clay-enriched Bt (argillic)
horizon in the lower subsoil. Dark humus material is also
commonly translocated down cracks in the profile, coating,
and staining ped faces (which may be prismatic, coarse
blocky, or polyhedral) in the subsoil. The argillic horizon
slows permeability and therefore horizons above it typically
have redox and reductimorphic features, ranging from
orange mottles, Fe–Mn concretions, to low-chroma colours
sufficiently extensive to form Eg or Er horizons.
Although the Ultic Soils in Northland are formed by
topdown pedogenesis on a range of parent materials, mineralogical studies of sand fractions in the soils show that
minor amounts of volcanic glass also occur in the profiles.
Such glass shards were probably largely derived from tephra
blown directly from eruptions of volcanoes in far-off central
North Island. This inference is supported by the occurrence
of thin rhyolitic (silica-rich) tephra (volcanic ash) beds,
mainly comprising glass, preserved unaltered in lake sediments and peat bogs in Northland. Such tephras include a
*10-cm-thick layer of Rotoehu Ash (c. 45,000 years old) in
Lake Omapere near Kaikohe, along with dustings of
Kaharoa, Taupo, and other tephras.
In only one known case in North Island has an Ultic Soil
been formed by upbuilding (rather than topdown) pedogenesis, namely the tephra-derived Kainui soil (Sect. 16.3.3).
In Southland, an Ultic Soil formed on strongly weathered
alluvium also has a capping of loess and so it originated
mainly via topdown pedogenesis but with some upbuilding
pedogenesis in more recent times represented by incremental
loess accumulation.
16.3 Soil-Landscape Relationships
16.3.1 Introduction
The strongly weathered, strongly leached, clay-rich, and
acidic character of Ultic Soils is attributed to long periods of
land stability that provide sufficient time for soil-forming
process to operate. The land surfaces where Ultic Soils occur
are survivors of the major perturbations (volcanic activity,
tectonic uplift, and impacts related to glacial-interglacial
cycles including the Last Glaciation) that have strongly
influenced the soils and their parent materials in other
regions.
As well as their Northland-Auckland-Waikato homeland,
the Ultic Soils also occur in small isolated areas throughout
the rest of New Zealand where remnants of land surfaces
have remained stable. Thus, scattered Ultic Soils also occur
in:
– Wellington, where remnants of old land surfaces occur as
relatively stable ridge crests in hilly terrain.
– Marlborough Sounds, on spurs of low relief land near sea
level with slopes less than 25° that protrude into the inlets
and which have escaped burial by the active colluvium
on steeper slopes closer to the main ridges, specifically in
Port Underwood, and Tennyson Inlet. Beneath these
soils, the schist and schistose-greywacke saprolite is
deeply red weathered.
16.2 Soil Profile Genesis
253