well-drained, tephra-derived, glass-shard-rich Allophanic
Soils (Chap. 2). In Allophanic Soils allophane, ferrihydrite,
and imogolite form in situ (even though the process requires
desilication via leaching), but podzolization, in contrast,
always involves the translocation of organic and/or inorganic complexes, from the very acidic horizons in the upper
profile, and subsequent immobilisation and accumulation in
less-acidic lower horizons. The synthesis of allophane and
ferrihydrite via podzolization (also by strong leaching of
silicon as occurs in Allophanic Brown Soils) can thus take
place on parent materials that are devoid of any
tephra-derived volcanic glass. However, podzolization, and
the related processes that occur to form Allophanic Soils,
can occur together on tephra deposits, for example, in central
North Island where Podzol Soils are developed on Holocene
tephras.
The Densipan Podzol Soil (Wharekohe soil) of Northland
is formed on clays (formed from weathered banded sandstone and mudstone), which is globally unusual (although
they may not quite meet the full criteria for Spodosols, the
Soil Taxonomy equivalent to a Podzol Soil). Podzol Soils
generally form on permeable, unconsolidated materials that
support strong leaching, such as aeolian sands. The Podzol
Soils on clay are a reflection of the potency of the kauri tree
as a source of acids as well as the stability of the landscape,
with long periods without disturbance under long-lived trees,
during which the soil develops (see grey box). An unusual
feature of the kauri-derived podzol, in many places, is the
nature of the bleached horizon. Named a ‘densipan’ it has a
very high density without cementation. The densipan has
been recognised in Australia, the Caribbean, and England
but the Wharekohe soil would seem to represent its most
spectacular recorded example.
11.3 Soil-Landscape Relationships
11.3.1 Overview
At a national scale, Podzol Soils occur in areas of New
Zealand with high rainfall, a history of acid-forming vegetation, usually on unconsolidated, silica-rich, well-drained
parent material. Many Podzol Soils are on stable forested hill
country. They are also often found on sand dunes, including
in coastal Northland, Bay of Plenty, and Coromandel, and on
the West Coast of the South Island. On Holocene coastal
dune sequences, Podzol Soils are invariably formed on the
older, innermost (landward) dunes. Podzol Soils also occur
on tephra deposits at altitudes >*500 m, and are especially
common on the loose, highly silicic (rhyolitic) pumice
deposits emplaced by the Taupo eruption in AD 232 ± 10
(Fig. 11.5). On stable slopes, a succession of different Podzol Soil groups and subgroups can be identified
associated with podzolization and other processes operating
according to landscape position. A catenary relationship
may be identified with Orthic Podzol Soils on summits and
backslopes giving way to Groundwater-gley Podzol Soils on
foot slopes.
A unique feature of Podzol Soil’s soil-landscape relationships in New Zealand is the ‘egg-cup’ podzol (where the
pattern of E, Bh, and Bs horizons form a concave egg-cup
shape) associated with individual kauri trees in Northland.
Chronosequences (sequences of soils in similar landscape
positions, with similar climate, vegetation, and parent
materials but varying age) are of value in showing how soils,
such as the Podzol Soils, develop over time, including their
eventual retrogression. Chronosequences that include Podzol
Soils have been studied throughout New Zealand and have
contributed to the understanding of the genesis of both the
soils and landscapes. Here we report the Franz Joseph
chronosequence (Sect. 11.3.3).
11.3.2 The Egg-Cup Podzol Soils of Northland
A source of fascination, to both New Zealand and international soil aficionados, is the so-called ‘egg-cup’ Podzols of
Northland. In such soils, the E horizon and underlying Bh
and Bs horizons form a distinctive egg-cup shape
(Fig. 11.6). The egg-cup shape is thought to be derived from
greater leaching occurring immediately adjacent to a large,
long-lived tree (most likely a kauri tree) that has, over a long
period of time, provided plant-derived acids that have hastened the leaching near, and beneath, the tree. Because the
kauri lives for hundreds of years, large amounts of litter and
leachates are able to accumulate on a specific site. Egg-cup
podzols have been observed adjacent to more usual Podzol
Soils that have flat-lying soil horizons, or egg-cup podzols
may form a Podzol Soil ‘island’, a few metres across, surrounded by Ultic Soils. Such a pattern may relate to a mosaic
of trees evident in remnant kauri forests today (see grey
box).
11.2 Soil Profile Genesis
169
Soils (Chap. 2). In Allophanic Soils allophane, ferrihydrite,
and imogolite form in situ (even though the process requires
desilication via leaching), but podzolization, in contrast,
always involves the translocation of organic and/or inorganic complexes, from the very acidic horizons in the upper
profile, and subsequent immobilisation and accumulation in
less-acidic lower horizons. The synthesis of allophane and
ferrihydrite via podzolization (also by strong leaching of
silicon as occurs in Allophanic Brown Soils) can thus take
place on parent materials that are devoid of any
tephra-derived volcanic glass. However, podzolization, and
the related processes that occur to form Allophanic Soils,
can occur together on tephra deposits, for example, in central
North Island where Podzol Soils are developed on Holocene
tephras.
The Densipan Podzol Soil (Wharekohe soil) of Northland
is formed on clays (formed from weathered banded sandstone and mudstone), which is globally unusual (although
they may not quite meet the full criteria for Spodosols, the
Soil Taxonomy equivalent to a Podzol Soil). Podzol Soils
generally form on permeable, unconsolidated materials that
support strong leaching, such as aeolian sands. The Podzol
Soils on clay are a reflection of the potency of the kauri tree
as a source of acids as well as the stability of the landscape,
with long periods without disturbance under long-lived trees,
during which the soil develops (see grey box). An unusual
feature of the kauri-derived podzol, in many places, is the
nature of the bleached horizon. Named a ‘densipan’ it has a
very high density without cementation. The densipan has
been recognised in Australia, the Caribbean, and England
but the Wharekohe soil would seem to represent its most
spectacular recorded example.
11.3 Soil-Landscape Relationships
11.3.1 Overview
At a national scale, Podzol Soils occur in areas of New
Zealand with high rainfall, a history of acid-forming vegetation, usually on unconsolidated, silica-rich, well-drained
parent material. Many Podzol Soils are on stable forested hill
country. They are also often found on sand dunes, including
in coastal Northland, Bay of Plenty, and Coromandel, and on
the West Coast of the South Island. On Holocene coastal
dune sequences, Podzol Soils are invariably formed on the
older, innermost (landward) dunes. Podzol Soils also occur
on tephra deposits at altitudes >*500 m, and are especially
common on the loose, highly silicic (rhyolitic) pumice
deposits emplaced by the Taupo eruption in AD 232 ± 10
(Fig. 11.5). On stable slopes, a succession of different Podzol Soil groups and subgroups can be identified
associated with podzolization and other processes operating
according to landscape position. A catenary relationship
may be identified with Orthic Podzol Soils on summits and
backslopes giving way to Groundwater-gley Podzol Soils on
foot slopes.
A unique feature of Podzol Soil’s soil-landscape relationships in New Zealand is the ‘egg-cup’ podzol (where the
pattern of E, Bh, and Bs horizons form a concave egg-cup
shape) associated with individual kauri trees in Northland.
Chronosequences (sequences of soils in similar landscape
positions, with similar climate, vegetation, and parent
materials but varying age) are of value in showing how soils,
such as the Podzol Soils, develop over time, including their
eventual retrogression. Chronosequences that include Podzol
Soils have been studied throughout New Zealand and have
contributed to the understanding of the genesis of both the
soils and landscapes. Here we report the Franz Joseph
chronosequence (Sect. 11.3.3).
11.3.2 The Egg-Cup Podzol Soils of Northland
A source of fascination, to both New Zealand and international soil aficionados, is the so-called ‘egg-cup’ Podzols of
Northland. In such soils, the E horizon and underlying Bh
and Bs horizons form a distinctive egg-cup shape
(Fig. 11.6). The egg-cup shape is thought to be derived from
greater leaching occurring immediately adjacent to a large,
long-lived tree (most likely a kauri tree) that has, over a long
period of time, provided plant-derived acids that have hastened the leaching near, and beneath, the tree. Because the
kauri lives for hundreds of years, large amounts of litter and
leachates are able to accumulate on a specific site. Egg-cup
podzols have been observed adjacent to more usual Podzol
Soils that have flat-lying soil horizons, or egg-cup podzols
may form a Podzol Soil ‘island’, a few metres across, surrounded by Ultic Soils. Such a pattern may relate to a mosaic
of trees evident in remnant kauri forests today (see grey
box).
11.2 Soil Profile Genesis
169