infrastructure, as they tend to be sites of geological instability, subject to erosion or deposition. Where development
is proposed, careful regard may need to be taken to the soil
properties and position and specific engineering solutions
may need to be considered.
The ecosystems that develop on Raw Soils are often
unique as they provide an environment where pioneer, early
succession plants have a particular niche. There are many
introduced invasive weed species that will readily colonise
Raw Soils, preventing the natural ecological succession of
New Zealand native plants. Invasive plants that will readily
colonise Raw Soils include pampas (Cortaderia selloana),
wildling pines (often Pinus contorta), reed sweet grass
(Glyceria maxima, on water margins), and Hieracium pilosella (mouse-eared hawkweed, a plant that inhibits other
plants and invades bare ground in the South Island high
country). Constant vigilance is needed to prevent introduction of weeds, and rapid response to small invasions can
prevent a need for much larger efforts later. In some regions
major efforts are being, or have been, undertaken to try to
control the spread of such weeds.
Because of the large variety of Raw Soils, and their
potential uses, specific management requirement, and hazards, each soil group is considered separately in the following discussion.
Rocky Raw Soils are an important component of the
scenic landscapes that attract tourists to New Zealand. Many
are in parks or protected areas managed by the Department
of Conservation. Some underlie ski fields while others attract
hikers/trampers and mountain climbers. The main objectives
in managing Rocky Raw Soils are often to maintain the
natural environment and to protect users from hazards, for
example, potential rockfalls at sites where steep cliffs are
exposed.
At some sites, such as the Tongariro Alpine Crossing
Track, in the Tongariro National Park, trampling by humans
can greatly damage sensitive Raw Soils (Fig. 13.4). Up to
3000 tourists per day walk the Tongariro Crossing with a
total of about 125,000 in 2016. Human trampling of the soils
and associated flora and fauna (sparse as it is) can lead to
damage to rare ecosystems, compaction, and concentration
of runoff, and thus increased risk of rill and gully erosion.
Much effort is put into encouraging walkers to stay on
marked tracks, and to form and manage tracks to accommodate tourists and minimise damage to the wider
environment.
Both Fluvial and Gley Raw Soils are often in low-lying
areas and thus are at risk of flooding. With development of
flood protection works and drainage, such soils may be
developed for pastoral land use but expensive infrastructure
should generally be avoided unless special measures are in
place to provide protection from inundation. Gley Raw Soils
may also have very low bearing strength. Consequently,
heavy vehicles or buildings may risk sinking into the soil. In
areas such as tidal mudflats, with Fluid Gley Raw Soils, the
potential to sink into and be trapped in the soil extends to
people and animals trying to walk across the area. However,
Fluid Gley Raw Soils may be drained and reclaimed for
building infrastructure or agriculture development, and even
nature herself can make such a change. During the Napier
earthquake of 3 February 1931, an area including the
brackish Ahuriri Lagoon was raised about 1.5 m above its
previous level. Part of the Ahuriri Lagoon, supporting
Fluid-saline Gley Raw Soils, became dry. The salinity of the
uplifted soils varies over time, decreasing with leaching of
salt during wet seasons and increasing due to capillary rise
of saline groundwater during dry spells. The uplifted, formerly, Fluid Gley Soils are now Saline Recent Gley Soils.
When Fluid Gley Raw Soils are drained there is a risk
that they will become extremely acidic, forming soils known
globally as ‘acid sulphate soils’ (or Sulfaquents in Soil
Taxonomy). When the soil is drained, oxygen enters, and if
there is sulphur present (which is usual in brackish lakes,
coastal marine environments, landscapes comprising mine
tailings, or soils containing pyrite, FeS 2 ), then the oxygen
reacts with reduced forms of the sulphur (sulphate ions are
reduced to sulphide ions by bacteria) to form sulphuric acid:
FeS 2 þ 2H 2 O þ 3O 2 ! FeSO 4 þ H 2 SO 4 þ 1440 kJ
The soil pH is likely to drop to below 4, and can be as
low as 2, possibly the lowest pH for any mineral soil. Such
an acidic environment is capable of dissolving concrete in
structures such as bridge abutments, and rapidly corroding
steel pipelines and other infrastructure. Thus special engineering precautions are needed in such environments. The
low pH also allows toxic concentrations of soluble Al and Fe
to be released, together with heavy metals that may include
arsenic. The marked acidity inhibits most plant growth and
large inputs of lime are needed to provide for agricultural
development. Keeping the water table as high as possible
helps minimise oxygen infiltration and thus acid formation.
Jarrosite, a distinctive, yellow, iron-sulphate mineral often
forms in drained Fluid Gley Soils in coastal areas, and an
orange iron-sulphate mineral, schwertmannite, forms typically in wetland situations or on acid mine tailings between
pH 2 and 4.
There is potential for acid sulphate soils to develop in any
former tidal mudflat environments that are drained. Such
areas are particularly common adjacent to the harbours in the
northern North Island.
Many Hydrothermal Raw Soils, such as those at
Whakarewarewa in Rotorua and Craters of the Moon at
Wairakei, are managed primarily as tourist attractions and
sites of unique ecological and geological value. The main
212
13 Raw Soils
is proposed, careful regard may need to be taken to the soil
properties and position and specific engineering solutions
may need to be considered.
The ecosystems that develop on Raw Soils are often
unique as they provide an environment where pioneer, early
succession plants have a particular niche. There are many
introduced invasive weed species that will readily colonise
Raw Soils, preventing the natural ecological succession of
New Zealand native plants. Invasive plants that will readily
colonise Raw Soils include pampas (Cortaderia selloana),
wildling pines (often Pinus contorta), reed sweet grass
(Glyceria maxima, on water margins), and Hieracium pilosella (mouse-eared hawkweed, a plant that inhibits other
plants and invades bare ground in the South Island high
country). Constant vigilance is needed to prevent introduction of weeds, and rapid response to small invasions can
prevent a need for much larger efforts later. In some regions
major efforts are being, or have been, undertaken to try to
control the spread of such weeds.
Because of the large variety of Raw Soils, and their
potential uses, specific management requirement, and hazards, each soil group is considered separately in the following discussion.
Rocky Raw Soils are an important component of the
scenic landscapes that attract tourists to New Zealand. Many
are in parks or protected areas managed by the Department
of Conservation. Some underlie ski fields while others attract
hikers/trampers and mountain climbers. The main objectives
in managing Rocky Raw Soils are often to maintain the
natural environment and to protect users from hazards, for
example, potential rockfalls at sites where steep cliffs are
exposed.
At some sites, such as the Tongariro Alpine Crossing
Track, in the Tongariro National Park, trampling by humans
can greatly damage sensitive Raw Soils (Fig. 13.4). Up to
3000 tourists per day walk the Tongariro Crossing with a
total of about 125,000 in 2016. Human trampling of the soils
and associated flora and fauna (sparse as it is) can lead to
damage to rare ecosystems, compaction, and concentration
of runoff, and thus increased risk of rill and gully erosion.
Much effort is put into encouraging walkers to stay on
marked tracks, and to form and manage tracks to accommodate tourists and minimise damage to the wider
environment.
Both Fluvial and Gley Raw Soils are often in low-lying
areas and thus are at risk of flooding. With development of
flood protection works and drainage, such soils may be
developed for pastoral land use but expensive infrastructure
should generally be avoided unless special measures are in
place to provide protection from inundation. Gley Raw Soils
may also have very low bearing strength. Consequently,
heavy vehicles or buildings may risk sinking into the soil. In
areas such as tidal mudflats, with Fluid Gley Raw Soils, the
potential to sink into and be trapped in the soil extends to
people and animals trying to walk across the area. However,
Fluid Gley Raw Soils may be drained and reclaimed for
building infrastructure or agriculture development, and even
nature herself can make such a change. During the Napier
earthquake of 3 February 1931, an area including the
brackish Ahuriri Lagoon was raised about 1.5 m above its
previous level. Part of the Ahuriri Lagoon, supporting
Fluid-saline Gley Raw Soils, became dry. The salinity of the
uplifted soils varies over time, decreasing with leaching of
salt during wet seasons and increasing due to capillary rise
of saline groundwater during dry spells. The uplifted, formerly, Fluid Gley Soils are now Saline Recent Gley Soils.
When Fluid Gley Raw Soils are drained there is a risk
that they will become extremely acidic, forming soils known
globally as ‘acid sulphate soils’ (or Sulfaquents in Soil
Taxonomy). When the soil is drained, oxygen enters, and if
there is sulphur present (which is usual in brackish lakes,
coastal marine environments, landscapes comprising mine
tailings, or soils containing pyrite, FeS 2 ), then the oxygen
reacts with reduced forms of the sulphur (sulphate ions are
reduced to sulphide ions by bacteria) to form sulphuric acid:
FeS 2 þ 2H 2 O þ 3O 2 ! FeSO 4 þ H 2 SO 4 þ 1440 kJ
The soil pH is likely to drop to below 4, and can be as
low as 2, possibly the lowest pH for any mineral soil. Such
an acidic environment is capable of dissolving concrete in
structures such as bridge abutments, and rapidly corroding
steel pipelines and other infrastructure. Thus special engineering precautions are needed in such environments. The
low pH also allows toxic concentrations of soluble Al and Fe
to be released, together with heavy metals that may include
arsenic. The marked acidity inhibits most plant growth and
large inputs of lime are needed to provide for agricultural
development. Keeping the water table as high as possible
helps minimise oxygen infiltration and thus acid formation.
Jarrosite, a distinctive, yellow, iron-sulphate mineral often
forms in drained Fluid Gley Soils in coastal areas, and an
orange iron-sulphate mineral, schwertmannite, forms typically in wetland situations or on acid mine tailings between
pH 2 and 4.
There is potential for acid sulphate soils to develop in any
former tidal mudflat environments that are drained. Such
areas are particularly common adjacent to the harbours in the
northern North Island.
Many Hydrothermal Raw Soils, such as those at
Whakarewarewa in Rotorua and Craters of the Moon at
Wairakei, are managed primarily as tourist attractions and
sites of unique ecological and geological value. The main
212
13 Raw Soils