exerted by plant roots during water deficits that average *150 mm over 4–5 months. Roger Parfitt and Derek
Milne proposed that the suction is transmitted through the
water in pores in the uniform silt-sized material, causing
shrinkage which is the equivalent of putting the soil under a
compressive stress, leading to an increase in bulk density.
The soils in drier areas will experience larger suctions and
thus attain higher bulk densities. The dense fragipans, being
well below the more porous, somewhat wetter, and biologically active surface horizons, will persist and possibly
increase in density over time.
Dispersion occurs where clay, silt, or sand particles are
separated from soil aggregates and behave as individual
particles or as much smaller aggregates. In many Pallic Soils
the particles are readily dispersed when the soil is saturated
(a process known as slaking) and then easily move with the
water. The relevance to fragipans is that dispersed particles
may clog soil pores, and so increase the over-all density of
the soil. Dispersion is counteracted in soils in which the
fabric is stabilised by any soil component that ‘glues’ the
particles together. Such components include soil organic
matter, secondary oxide weathering products, and lime
(calcium carbonate). The ‘glues’ are lacking in Pallic Soils
and so the soil fabric is vulnerable to dispersion.
Clay movement down the soil profile is evident in many
Pallic Soils, with clay skins often evident on aggregate
surfaces. Such clay skins also contribute to blockage of soil
pores and slowing of water movement through the soil
profile. The clay skins accumulate to form argillic horizons
(designated Bt, e.g. Fig. 10.4) including as thin subsoil clay
bands (lamellae).
Once a dense subsoil layer, or a fragipan, forms then root
and water penetration are limited. A perched water table
forms in wet months, resulting in anaerobic conditions and
reduction of iron minerals. Thus the pale grey colours of the
soil matrix form with rusty brown iron- and manganese-rich
mottles in areas associated with root channels or cracks
where
oxygen
is
available.
Under
pasture,
organic-matter-rich topsoils form with strongly developed
granular soil structures.
10.3 Soil-Landscape Relationships
10.3.1 Overview
Pallic Soils form mainly on the flattish to rolling landscapes
(downlands) of the lower North Island and eastern South
Island, in areas where loess mantles the landscape. Loess is
extensive in New Zealand. That the deposition of loess
occurs primarily during glacial periods has long been
recognised, with Timaru-based John Hardcastle (1847–
1927), the first in the world to make this climatic connection
in about 1890. Consequently, deep loess sections may show
a series of sediment-soil sheets with intervening
(more-strongly developed) paleosols (Fig. 10.6). With the
emplacement of each additional loess sheet on the landscape,
flattish summits grew higher while the valley floors
remained the same or lowered in response to base-level
lowering (Fig. 10.7). The outcome is the smooth, undulating
downland landscapes—for example, in the South Canterbury, Manawatu, and Whanganui areas (Fig. 10.1). Given
the uniform parent materials on the near-flat terrace surfaces,
the Pallic Soils usually form a relatively consistent soil
across a landscape. However, on rolling surfaces a soil
catena often prevails (Sect. 10.3.3).
On the youngest river terraces and flood plains, Pallic
Soils do not form, due to insufficient time and the generally
coarser alluvial parent material. At higher altitudes, and in
parts of Southland, the cooler, wetter, environment results in
formation of Brown Soils (Chap. 4). In the semiarid regions
of Central Otago, where soils remain dry for longer periods,
and are seldom saturated, semiarid, rather than Pallic Soils
are formed. Loess may be observed in road cuttings and
eroded faces (Fig. 10.6) as a prominent soil parent material
which stands out because its density protects it from erosion
of the cutting face, and it has a distinctive characteristic pale
colour.
In the fine sandy material, nearer the loess source, Pallic
Soils are not so dense, pores remain relatively open, and
roots and water movement are not significantly restricted.
New Zealand has some distinctive Pallic Soil soil-landscape
patterns. Two examples are discussed here, the Rangitikei
terrace sequence in the southern North Island, and the
Claremont catena of the Canterbury region.
10.3.2 Rangitikei Loess Chronosequence
Although loess had first been recognised at Banks Peninsula
by Julius (von) Haast in 1878, there was still much discussion in the 1950s about the source of the silt material that
mantled river terraces in the lower North Island—was it fine
homogenous alluvium, or loess? Des Cowie finally closed
that argument in 1964 when he showed that a pale layer
within the silt material was a tephra (the Aokautere Ash,
now known more broadly as the Kawakawa Tephra). The
Kawakawa Tephra was deposited by the Oruanui
super-eruption of Taupo volcano about 25,400 years ago
(sometimes referred to as the Kawakawa/Oruanui tephra).
The presence of the tephra-fall deposit provided conclusive
evidence that the encompassing silt material was in fact
loess, also carried by the wind. The tephra could be accurately dated giving a time-line within the loess deposit.
Cowie observed that the depth of loess above Kawakawa
Tephra increased from about 0.5 m at a distance from the
10.2 Soil Profile Genesis
151
Milne proposed that the suction is transmitted through the
water in pores in the uniform silt-sized material, causing
shrinkage which is the equivalent of putting the soil under a
compressive stress, leading to an increase in bulk density.
The soils in drier areas will experience larger suctions and
thus attain higher bulk densities. The dense fragipans, being
well below the more porous, somewhat wetter, and biologically active surface horizons, will persist and possibly
increase in density over time.
Dispersion occurs where clay, silt, or sand particles are
separated from soil aggregates and behave as individual
particles or as much smaller aggregates. In many Pallic Soils
the particles are readily dispersed when the soil is saturated
(a process known as slaking) and then easily move with the
water. The relevance to fragipans is that dispersed particles
may clog soil pores, and so increase the over-all density of
the soil. Dispersion is counteracted in soils in which the
fabric is stabilised by any soil component that ‘glues’ the
particles together. Such components include soil organic
matter, secondary oxide weathering products, and lime
(calcium carbonate). The ‘glues’ are lacking in Pallic Soils
and so the soil fabric is vulnerable to dispersion.
Clay movement down the soil profile is evident in many
Pallic Soils, with clay skins often evident on aggregate
surfaces. Such clay skins also contribute to blockage of soil
pores and slowing of water movement through the soil
profile. The clay skins accumulate to form argillic horizons
(designated Bt, e.g. Fig. 10.4) including as thin subsoil clay
bands (lamellae).
Once a dense subsoil layer, or a fragipan, forms then root
and water penetration are limited. A perched water table
forms in wet months, resulting in anaerobic conditions and
reduction of iron minerals. Thus the pale grey colours of the
soil matrix form with rusty brown iron- and manganese-rich
mottles in areas associated with root channels or cracks
where
oxygen
is
available.
Under
pasture,
organic-matter-rich topsoils form with strongly developed
granular soil structures.
10.3 Soil-Landscape Relationships
10.3.1 Overview
Pallic Soils form mainly on the flattish to rolling landscapes
(downlands) of the lower North Island and eastern South
Island, in areas where loess mantles the landscape. Loess is
extensive in New Zealand. That the deposition of loess
occurs primarily during glacial periods has long been
recognised, with Timaru-based John Hardcastle (1847–
1927), the first in the world to make this climatic connection
in about 1890. Consequently, deep loess sections may show
a series of sediment-soil sheets with intervening
(more-strongly developed) paleosols (Fig. 10.6). With the
emplacement of each additional loess sheet on the landscape,
flattish summits grew higher while the valley floors
remained the same or lowered in response to base-level
lowering (Fig. 10.7). The outcome is the smooth, undulating
downland landscapes—for example, in the South Canterbury, Manawatu, and Whanganui areas (Fig. 10.1). Given
the uniform parent materials on the near-flat terrace surfaces,
the Pallic Soils usually form a relatively consistent soil
across a landscape. However, on rolling surfaces a soil
catena often prevails (Sect. 10.3.3).
On the youngest river terraces and flood plains, Pallic
Soils do not form, due to insufficient time and the generally
coarser alluvial parent material. At higher altitudes, and in
parts of Southland, the cooler, wetter, environment results in
formation of Brown Soils (Chap. 4). In the semiarid regions
of Central Otago, where soils remain dry for longer periods,
and are seldom saturated, semiarid, rather than Pallic Soils
are formed. Loess may be observed in road cuttings and
eroded faces (Fig. 10.6) as a prominent soil parent material
which stands out because its density protects it from erosion
of the cutting face, and it has a distinctive characteristic pale
colour.
In the fine sandy material, nearer the loess source, Pallic
Soils are not so dense, pores remain relatively open, and
roots and water movement are not significantly restricted.
New Zealand has some distinctive Pallic Soil soil-landscape
patterns. Two examples are discussed here, the Rangitikei
terrace sequence in the southern North Island, and the
Claremont catena of the Canterbury region.
10.3.2 Rangitikei Loess Chronosequence
Although loess had first been recognised at Banks Peninsula
by Julius (von) Haast in 1878, there was still much discussion in the 1950s about the source of the silt material that
mantled river terraces in the lower North Island—was it fine
homogenous alluvium, or loess? Des Cowie finally closed
that argument in 1964 when he showed that a pale layer
within the silt material was a tephra (the Aokautere Ash,
now known more broadly as the Kawakawa Tephra). The
Kawakawa Tephra was deposited by the Oruanui
super-eruption of Taupo volcano about 25,400 years ago
(sometimes referred to as the Kawakawa/Oruanui tephra).
The presence of the tephra-fall deposit provided conclusive
evidence that the encompassing silt material was in fact
loess, also carried by the wind. The tephra could be accurately dated giving a time-line within the loess deposit.
Cowie observed that the depth of loess above Kawakawa
Tephra increased from about 0.5 m at a distance from the
10.2 Soil Profile Genesis
151