the weathering has advanced to the stage of forming oxides
(especially goethite, haematite, and gibbsite) as well as
subordinate kaolin-subgroup clays (halloysite and kaolinite).
The processes that form the soil may be viewed as a game
of ‘winners and losers’ and are centred around so-called
ferralitic weathering or ferralisation. The soil composition
depends on the availability, and relative mobility, of the
elements that make up the soil parent material. A range of
factors influence an element’s ability to either remain and
build up as a residual concentration (arguably a “winner”),
or be broken down and leached from the soil and so become
depleted (or “lost”). The fate of elements can thus be best
understood by focusing on the factors that promote mobilisation with consequent loss from the soil, or factors that
cause elements to be left behind and thus become concentrated in the soil as newly formed secondary minerals.
In the iron-rich soil-forming environment, the main elements of interest are iron, aluminium, silicon, and the
non-acid cations (also known as base cations, or bases),
namely, Ca
2+ , Mg
2+ , K
+
, and Na
+
. Where the parent material
does not contain calcium carbonate (natural lime), leaching
has the potential to increase soil acidity, especially where
soils are free draining and rainfall is high. At more acid
(lower) pH levels, silicon, along with the non-acid cations,
become dissolved in the soil solution and then are leached
from the soil profile, which, in turn, leads to a residual
concentration of mainly iron and aluminium then, if conditions are favourable, form less soluble oxides (sometimes
together with lesser amounts of kaolinite and halloysite) that
remain in the soil.
In some soils, the processes have led to the formation of
nodules that may comprise 50% of the soil volume in a
distinct coarse-textured, often gravelly, horizon. Analyses
show a nodule typically to contain *50% Fe oxides (haematite, goethite, ferrihydrite), *35% gibbsite, and *15%
kaolinite and/or halloysite. Significant organic matter is
found in nodules (*20% loss-on-ignition) with major elements dominated by Fe (34%), Al (13%), Si (3%), and Ti
(*1%). Iron- and manganese-rich concretions may also
form in Oxidic Soils where redox conditions prevail.
Thus, in Oxidic Soils, the major factors influencing soil
formation are an iron-rich parent material and strong weathering and acidity with the subsequent concentration of iron
and aluminium. In contrast to Oxidic Soils, Gley Soils
(Chap. 5) are controlled by acidity and biochemical reduction
which leads to the mobilisation, and potential loss by leaching,
of iron under reducing conditions. Podzol Soils (Chap. 11) are
controlled by marked acidity with iron, silicon, and organic
materials moved down and precipitated in the subsoil, leaving
a distinct, bleached E horizon near the soil surface.
Despite being formed mainly on basaltic lavas, which do
not contain quartz, Oxidic Soils may contain small amounts
of quartz, mainly in the upper profile. Bob Stewart showed,
using oxygen isotope analyses, that the quartz grains in an
Oxidic Soil were probably derived mainly from fallout of
distal rhyolitic tephra, such as Rotoehu Ash (c. 45,000 years
old) and Kaharoa Tephra (c. 700 years old), erupted from
central North Island volcanic centres. Sparse deposits of
these and other thin rhyolitic tephras have been recorded in
the sediments of Lake Omapere near Kaikohe. Small
amounts of the clay mineral vermiculite are thought to derive
from local loess or aeolian dust from Australia.
One opinion is that the well-formed structure of Oxidic
Soils is strongly influenced by bioturbation. This seems
likely in areas of high termite populations such as Australia,
but could such levels of bioturbation be relevant to New
Zealand Oxidic Soils? We do have native termites in
Northland but have they ever been sufficiently active to have
contributed to the friable fine structure that is characteristic
of our Oxidic Soils?
9.3 Soil-Landscape Relationships
The Oxidic Soils in New Zealand form at the end of the range
of tropical and subtropical Oxisols and so occur only in
conditions that maximise weathering to form mainly oxidic
clays. Thus Oxidic Soils form on flat to rolling, stable, land
surfaces on iron, magnesium, and aluminium-rich (readily
weatherable) parent materials, with (by New Zealand standards) a warm and humid climate. Oxidic Soils are associated
with the oldest mafic and ultramafic igneous rocks (including
dolerite, peridotite, and basaltic to andesitic volcanic lavas
and associated scoriaceous deposits) in the Northland and
Auckland landscapes. Oxidic Soils are formed on thick
saprolite (weathered rock) and the soil minerals mainly
reflect the mineralogy of the underlying rocks.
Leslie I. Grange, a pioneer of New Zealand soil science,
was the first person we have found to record a scientific
study related to the Oxidic Soils—he published a description
of the ‘ironstone soils’ (Typic Nodular Oxidic Soils) of
‘North Auckland’ in 1934. Norman Taylor had worked
closely with Grange and in about 1937 Taylor started trying
to understand the complex soil pattern of Northland,
including the soils now known as Oxidic Soils. Thus the first
soil survey of Northland by Taylor (along with Charles
Sutherland and Charles Wright) was undertaken between
1937 and 1951, resulting in a series of provisional 1:63 360
scale soil maps. From 1979 to 1985, Ted Cox updated the
soil maps of Northland which were published as part of the
New Zealand Land Inventory of Northland at a 1:100 000
scale. Alistair Wilson and Warwick McDonald sampled
some suites of soils in the 1980s with results available in the
National Soils Data Repository. Limited data are also
available in international field trip guidebooks. However,
research on the soil-landscape relationships of Oxidic Soils
136
9 Oxidic Soils
(especially goethite, haematite, and gibbsite) as well as
subordinate kaolin-subgroup clays (halloysite and kaolinite).
The processes that form the soil may be viewed as a game
of ‘winners and losers’ and are centred around so-called
ferralitic weathering or ferralisation. The soil composition
depends on the availability, and relative mobility, of the
elements that make up the soil parent material. A range of
factors influence an element’s ability to either remain and
build up as a residual concentration (arguably a “winner”),
or be broken down and leached from the soil and so become
depleted (or “lost”). The fate of elements can thus be best
understood by focusing on the factors that promote mobilisation with consequent loss from the soil, or factors that
cause elements to be left behind and thus become concentrated in the soil as newly formed secondary minerals.
In the iron-rich soil-forming environment, the main elements of interest are iron, aluminium, silicon, and the
non-acid cations (also known as base cations, or bases),
namely, Ca
2+ , Mg
2+ , K
+
, and Na
+
. Where the parent material
does not contain calcium carbonate (natural lime), leaching
has the potential to increase soil acidity, especially where
soils are free draining and rainfall is high. At more acid
(lower) pH levels, silicon, along with the non-acid cations,
become dissolved in the soil solution and then are leached
from the soil profile, which, in turn, leads to a residual
concentration of mainly iron and aluminium then, if conditions are favourable, form less soluble oxides (sometimes
together with lesser amounts of kaolinite and halloysite) that
remain in the soil.
In some soils, the processes have led to the formation of
nodules that may comprise 50% of the soil volume in a
distinct coarse-textured, often gravelly, horizon. Analyses
show a nodule typically to contain *50% Fe oxides (haematite, goethite, ferrihydrite), *35% gibbsite, and *15%
kaolinite and/or halloysite. Significant organic matter is
found in nodules (*20% loss-on-ignition) with major elements dominated by Fe (34%), Al (13%), Si (3%), and Ti
(*1%). Iron- and manganese-rich concretions may also
form in Oxidic Soils where redox conditions prevail.
Thus, in Oxidic Soils, the major factors influencing soil
formation are an iron-rich parent material and strong weathering and acidity with the subsequent concentration of iron
and aluminium. In contrast to Oxidic Soils, Gley Soils
(Chap. 5) are controlled by acidity and biochemical reduction
which leads to the mobilisation, and potential loss by leaching,
of iron under reducing conditions. Podzol Soils (Chap. 11) are
controlled by marked acidity with iron, silicon, and organic
materials moved down and precipitated in the subsoil, leaving
a distinct, bleached E horizon near the soil surface.
Despite being formed mainly on basaltic lavas, which do
not contain quartz, Oxidic Soils may contain small amounts
of quartz, mainly in the upper profile. Bob Stewart showed,
using oxygen isotope analyses, that the quartz grains in an
Oxidic Soil were probably derived mainly from fallout of
distal rhyolitic tephra, such as Rotoehu Ash (c. 45,000 years
old) and Kaharoa Tephra (c. 700 years old), erupted from
central North Island volcanic centres. Sparse deposits of
these and other thin rhyolitic tephras have been recorded in
the sediments of Lake Omapere near Kaikohe. Small
amounts of the clay mineral vermiculite are thought to derive
from local loess or aeolian dust from Australia.
One opinion is that the well-formed structure of Oxidic
Soils is strongly influenced by bioturbation. This seems
likely in areas of high termite populations such as Australia,
but could such levels of bioturbation be relevant to New
Zealand Oxidic Soils? We do have native termites in
Northland but have they ever been sufficiently active to have
contributed to the friable fine structure that is characteristic
of our Oxidic Soils?
9.3 Soil-Landscape Relationships
The Oxidic Soils in New Zealand form at the end of the range
of tropical and subtropical Oxisols and so occur only in
conditions that maximise weathering to form mainly oxidic
clays. Thus Oxidic Soils form on flat to rolling, stable, land
surfaces on iron, magnesium, and aluminium-rich (readily
weatherable) parent materials, with (by New Zealand standards) a warm and humid climate. Oxidic Soils are associated
with the oldest mafic and ultramafic igneous rocks (including
dolerite, peridotite, and basaltic to andesitic volcanic lavas
and associated scoriaceous deposits) in the Northland and
Auckland landscapes. Oxidic Soils are formed on thick
saprolite (weathered rock) and the soil minerals mainly
reflect the mineralogy of the underlying rocks.
Leslie I. Grange, a pioneer of New Zealand soil science,
was the first person we have found to record a scientific
study related to the Oxidic Soils—he published a description
of the ‘ironstone soils’ (Typic Nodular Oxidic Soils) of
‘North Auckland’ in 1934. Norman Taylor had worked
closely with Grange and in about 1937 Taylor started trying
to understand the complex soil pattern of Northland,
including the soils now known as Oxidic Soils. Thus the first
soil survey of Northland by Taylor (along with Charles
Sutherland and Charles Wright) was undertaken between
1937 and 1951, resulting in a series of provisional 1:63 360
scale soil maps. From 1979 to 1985, Ted Cox updated the
soil maps of Northland which were published as part of the
New Zealand Land Inventory of Northland at a 1:100 000
scale. Alistair Wilson and Warwick McDonald sampled
some suites of soils in the 1980s with results available in the
National Soils Data Repository. Limited data are also
available in international field trip guidebooks. However,
research on the soil-landscape relationships of Oxidic Soils
136
9 Oxidic Soils