9
Oxidic Soils
In the far north where it’s wet and warm
weathered old Oxidic Soils form.
In oxide clays just the tough survive
add nutrients to ensure plants thrive.
Abstract
Oxidic Soils occupy less than 1% of New Zealand,
occurring in small areas throughout Northland, but
concentrated near Kerikeri. New Zealand’s Oxidic Soils
represent a southern limit of the globally extensive
Oxisols which occur mainly in the tropics and subtropics.
Oxidic Soils form in New Zealand where conditions
favour the most intense weathering. Thus Oxidic Soils
occur mainly on readily weatherable, iron-rich, basaltic to
andesitic volcanic or mafic/ultramafic igneous rocks and
scoriaceous deposits on sites that have been stable,
potentially for millions of years, in the warm, humid
environment of Northland. The parent materials of Oxidic
Soils have been almost totally transformed to clay
minerals, oxides, and oxyhydroxides, including gibbsite,
goethite, haematite, and ferrihydrite. The soils have a
friable, fine polyhedral, structure although some may
include horizons comprising over 50% iron- or
aluminium-rich nodules giving a gravelly texture. The
soils are highly leached and low in nutrients, but with
good fertiliser management are used to successfully grow
citrus and other subtropical crops. The first grape vines in
New Zealand were probably planted on Oxidic Soils by
an associate of Samuel Marsden at Kerikeri in 1817, and
the first British-style farm was established on Oxidic Soils
inland from Kerikeri in 1830.
9.1 Important Features of Oxidic Soils
9.1.1 Concept and Key Features of the Soil
Order
Oxidic Soils (Fig. 9.1) form in warm, humid, geologically
stable environments in the far north of New Zealand on ironand magnesium-rich (mafic) volcanic rocks, dolerite (a mafic
igneous rock), ultramafic rocks (including peridotite), and
basaltic tephras (scoria and associated deposits). Such conditions favour weathering and thus large proportions of the
primary minerals have weathered mainly to oxides,
hydroxides, and oxyhydroxides (referred to as ‘oxides’ or
‘oxide clays’ hereafter for brevity).
Oxidic Soils are clay-rich soils that have high concentrations of iron and/or aluminium oxides which form oxidic
horizons (Bo). Such oxidic horizons must be at least 30 cm
thick and have an upper boundary between 20 and 60 cm
deep to define an Oxidic Soil in the New Zealand Soil
Classification. Soil aggregates in Bo horizons slake (fall
apart) readily, when immersed in water, to form stable microaggregates which show no, or only slight, dispersion. In
contrast, aggregates in similar horizons in the related
Granular or Ultic soils both slake and disperse more readily.
The Oxidic Soils, alongside some of the clay-rich Ultic
Soils (Chap. 16), are the most weathered soils in New
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
A. E. Hewitt et al., The Soils of Aotearoa New Zealand, World Soils Book Series,
https://doi.org/10.1007/978-3-030-64763-6_9
133
Oxidic Soils
In the far north where it’s wet and warm
weathered old Oxidic Soils form.
In oxide clays just the tough survive
add nutrients to ensure plants thrive.
Abstract
Oxidic Soils occupy less than 1% of New Zealand,
occurring in small areas throughout Northland, but
concentrated near Kerikeri. New Zealand’s Oxidic Soils
represent a southern limit of the globally extensive
Oxisols which occur mainly in the tropics and subtropics.
Oxidic Soils form in New Zealand where conditions
favour the most intense weathering. Thus Oxidic Soils
occur mainly on readily weatherable, iron-rich, basaltic to
andesitic volcanic or mafic/ultramafic igneous rocks and
scoriaceous deposits on sites that have been stable,
potentially for millions of years, in the warm, humid
environment of Northland. The parent materials of Oxidic
Soils have been almost totally transformed to clay
minerals, oxides, and oxyhydroxides, including gibbsite,
goethite, haematite, and ferrihydrite. The soils have a
friable, fine polyhedral, structure although some may
include horizons comprising over 50% iron- or
aluminium-rich nodules giving a gravelly texture. The
soils are highly leached and low in nutrients, but with
good fertiliser management are used to successfully grow
citrus and other subtropical crops. The first grape vines in
New Zealand were probably planted on Oxidic Soils by
an associate of Samuel Marsden at Kerikeri in 1817, and
the first British-style farm was established on Oxidic Soils
inland from Kerikeri in 1830.
9.1 Important Features of Oxidic Soils
9.1.1 Concept and Key Features of the Soil
Order
Oxidic Soils (Fig. 9.1) form in warm, humid, geologically
stable environments in the far north of New Zealand on ironand magnesium-rich (mafic) volcanic rocks, dolerite (a mafic
igneous rock), ultramafic rocks (including peridotite), and
basaltic tephras (scoria and associated deposits). Such conditions favour weathering and thus large proportions of the
primary minerals have weathered mainly to oxides,
hydroxides, and oxyhydroxides (referred to as ‘oxides’ or
‘oxide clays’ hereafter for brevity).
Oxidic Soils are clay-rich soils that have high concentrations of iron and/or aluminium oxides which form oxidic
horizons (Bo). Such oxidic horizons must be at least 30 cm
thick and have an upper boundary between 20 and 60 cm
deep to define an Oxidic Soil in the New Zealand Soil
Classification. Soil aggregates in Bo horizons slake (fall
apart) readily, when immersed in water, to form stable microaggregates which show no, or only slight, dispersion. In
contrast, aggregates in similar horizons in the related
Granular or Ultic soils both slake and disperse more readily.
The Oxidic Soils, alongside some of the clay-rich Ultic
Soils (Chap. 16), are the most weathered soils in New
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
A. E. Hewitt et al., The Soils of Aotearoa New Zealand, World Soils Book Series,
https://doi.org/10.1007/978-3-030-64763-6_9
133