areas of newly exposed, relatively unweathered, papa.
Initially, such areas would be classified as Raw Soils.
Because of the lack of soil material, they have low water
holding capacity and steep slopes so will be relatively
slow to re-establish vegetation and develop from Raw to
Recent Soils.
4. Translation zones. Translation zones are the mid-slopes
of the hillside over which landslide material has travelled. In the translation zone, the slip material reaches
maximum velocity, often forming a slurry that is channelled downwards. Some landslides may deepen by
stripping soil at the base of the slide down to a resistant
layer and removing regolith material. The turbulent
slough of material may over-flow the channel margins
leaving levee-like deposits. The force of soil and water
has great competency, sufficient to move large rocks and
trees. Within the translation zone, the landslide will leave
uneven surface erosion surfaces and deposited materials
from which Raw Soils are formed. Where sand- or
silt-dominated material is deposited the soil may be relatively fertile and pasture may be fairly quickly
re-established. Soils on such sites will usually develop
from Raw to Recent Soils within a few decades. On
eroded areas, the rate of soil development will depend on
the nature of the underlying material, but it may be many
decades before the topsoil becomes sufficiently established for soil to qualify as a Recent Soil.
5. Arrest zones. Movement of the slip may be arrested, at
least in part, where the land slope flattens, the slurry loses
water, or where it is captured by a stream. In many sites,
the base comprises disorderly debris of sediment, rock, or
regolith material and conveyed vegetation. In some places, large blocks may remain intact and so the deposited
soil may retain its original properties, which could range
from those of a Recent Soil to the predominant mature
soil in the region such as a Brown, Pallic, Allophanic, or
Ultic Soil. However, most materials in the arrest zone
will probably be highly disturbed and hence will likely
initially be classified as Raw Soils. In contrast in the
transition zone, topsoils may develop relatively quickly
and thus progress to Recent Soils. Often arrest zones are
hummocky and pockets of cut-off drainage will occur
where Gley Soils may develop.
6. Foot slopes. The foot slopes of the papa hill country are
usually fan-shaped, gently sloping areas. They may be
cut by steep gullies where streams come through. The
foot slopes are predominantly formed from colluvium,
some carried down and deposited in large mass movement events, some just as a result of minor surface erosion material being carried by overland flow. Often foot
slopes are marked by springs where groundwater seeps
from the adjacent hillside and they also receive runoff
water in heavy rainfall. Hence, the soils are often wet for
prolonged periods, as evidenced by rushes and the
presence of Gley Soils.
Thus, the soils of landscapes where soil slip erosion is
common are highly variable with a mosaic of soils formed in
a complex, but generally predictable, pattern depending on
landscape position and the time since the last erosion or
depositional event at any given site. One soil surveyor who
worked intensively in alluvial floodplain soils sought experience in eroded soft rock hill country. He reported that,
though alluvial floodplain soils had set a standard for complex variability, the soft rock terrain variability greatly
exceeded the soil complexity of alluvial terrain. For example, in a sample of 11 farms in the Pohangina Valley of the
Manawatu, a soil survey reconnaissance revealed at least 21
soil subgroups.
14.4 Key Soil Properties
14.4.1 Soil Composition
Because Recent Soils occur throughout New Zealand, in a
wide range of environments, their properties may vary
considerably from region to region. For example, soils in a
high rainfall climate may have higher soil organic matter
and greater acidity, than soils of comparable age with similar
parent materials in a semiarid climate.
The soil clay mineralogy varies with the inherited mineralogy of the parent material. For instance, in a landscape
dominated by Ultic Soils, the associated Recent Soils may
contain substantial kaolin subgroup clays. In less weathered
terrain, illite group minerals will be more likely, and in
tephric terrain, the primary materials will often be dominated
by volcanic glass. Soil development is mostly confined to
organic matter accumulation and humification in topsoils.
The B horizons, if present, may be expressed by a difference
in colour in comparison with that of the lower subsoil,
indicative of some oxidation of iron-bearing minerals, but the
limited time for the development of Recent Soils precludes
much formation of secondary minerals, thus the mineralogy
remains predominantly that of the parent materials.
14.4.2 Physical Properties
Recent Soils have high spatial variability in soil texture,
reflecting the texture of the parent materials. It is common to
have a stratification of contrasting materials in subsoils.
However, as a consequence of the limited time for weathering, clay contents in Recent Soils are generally less than
about 20% with silt- and sand-dominating soil textures
(Fig. 14.7, Table 14.1). An exception is the Rotomahana soil
224
14 Recent Soils
Initially, such areas would be classified as Raw Soils.
Because of the lack of soil material, they have low water
holding capacity and steep slopes so will be relatively
slow to re-establish vegetation and develop from Raw to
Recent Soils.
4. Translation zones. Translation zones are the mid-slopes
of the hillside over which landslide material has travelled. In the translation zone, the slip material reaches
maximum velocity, often forming a slurry that is channelled downwards. Some landslides may deepen by
stripping soil at the base of the slide down to a resistant
layer and removing regolith material. The turbulent
slough of material may over-flow the channel margins
leaving levee-like deposits. The force of soil and water
has great competency, sufficient to move large rocks and
trees. Within the translation zone, the landslide will leave
uneven surface erosion surfaces and deposited materials
from which Raw Soils are formed. Where sand- or
silt-dominated material is deposited the soil may be relatively fertile and pasture may be fairly quickly
re-established. Soils on such sites will usually develop
from Raw to Recent Soils within a few decades. On
eroded areas, the rate of soil development will depend on
the nature of the underlying material, but it may be many
decades before the topsoil becomes sufficiently established for soil to qualify as a Recent Soil.
5. Arrest zones. Movement of the slip may be arrested, at
least in part, where the land slope flattens, the slurry loses
water, or where it is captured by a stream. In many sites,
the base comprises disorderly debris of sediment, rock, or
regolith material and conveyed vegetation. In some places, large blocks may remain intact and so the deposited
soil may retain its original properties, which could range
from those of a Recent Soil to the predominant mature
soil in the region such as a Brown, Pallic, Allophanic, or
Ultic Soil. However, most materials in the arrest zone
will probably be highly disturbed and hence will likely
initially be classified as Raw Soils. In contrast in the
transition zone, topsoils may develop relatively quickly
and thus progress to Recent Soils. Often arrest zones are
hummocky and pockets of cut-off drainage will occur
where Gley Soils may develop.
6. Foot slopes. The foot slopes of the papa hill country are
usually fan-shaped, gently sloping areas. They may be
cut by steep gullies where streams come through. The
foot slopes are predominantly formed from colluvium,
some carried down and deposited in large mass movement events, some just as a result of minor surface erosion material being carried by overland flow. Often foot
slopes are marked by springs where groundwater seeps
from the adjacent hillside and they also receive runoff
water in heavy rainfall. Hence, the soils are often wet for
prolonged periods, as evidenced by rushes and the
presence of Gley Soils.
Thus, the soils of landscapes where soil slip erosion is
common are highly variable with a mosaic of soils formed in
a complex, but generally predictable, pattern depending on
landscape position and the time since the last erosion or
depositional event at any given site. One soil surveyor who
worked intensively in alluvial floodplain soils sought experience in eroded soft rock hill country. He reported that,
though alluvial floodplain soils had set a standard for complex variability, the soft rock terrain variability greatly
exceeded the soil complexity of alluvial terrain. For example, in a sample of 11 farms in the Pohangina Valley of the
Manawatu, a soil survey reconnaissance revealed at least 21
soil subgroups.
14.4 Key Soil Properties
14.4.1 Soil Composition
Because Recent Soils occur throughout New Zealand, in a
wide range of environments, their properties may vary
considerably from region to region. For example, soils in a
high rainfall climate may have higher soil organic matter
and greater acidity, than soils of comparable age with similar
parent materials in a semiarid climate.
The soil clay mineralogy varies with the inherited mineralogy of the parent material. For instance, in a landscape
dominated by Ultic Soils, the associated Recent Soils may
contain substantial kaolin subgroup clays. In less weathered
terrain, illite group minerals will be more likely, and in
tephric terrain, the primary materials will often be dominated
by volcanic glass. Soil development is mostly confined to
organic matter accumulation and humification in topsoils.
The B horizons, if present, may be expressed by a difference
in colour in comparison with that of the lower subsoil,
indicative of some oxidation of iron-bearing minerals, but the
limited time for the development of Recent Soils precludes
much formation of secondary minerals, thus the mineralogy
remains predominantly that of the parent materials.
14.4.2 Physical Properties
Recent Soils have high spatial variability in soil texture,
reflecting the texture of the parent materials. It is common to
have a stratification of contrasting materials in subsoils.
However, as a consequence of the limited time for weathering, clay contents in Recent Soils are generally less than
about 20% with silt- and sand-dominating soil textures
(Fig. 14.7, Table 14.1). An exception is the Rotomahana soil
224
14 Recent Soils