The repetition of geomorphic/geologic events, such as the
regular deposition of sediment by flooding, or tephra-fall,
every few decades or centuries, means that many Recent
Soils are formed on multi-layered deposits through
upbuilding pedogenesis, with the land surface slowly rising
at a rate that depends on the frequency and thickness of the
fresh geologic additions to the profile. Because soil weathering in Recent Soils is always in an early stage, and horizon
transformation is restricted to the uppermost part of the
profile, the character of the unaltered parent material has a
strong influence on the soil’s overall properties.
Regardless of the parent material and its origins, the key
process in Recent Soil development is the formation of an A
horizon by the input, and continuing accumulation, of soil
organic matter (a process known as melanisation). In more
developed Recent Soils, along with most other soil orders,
organic matter accumulation is balanced by decomposition
(humification) providing a quasi-equilibrium in soil organic
matter content.
Organic matter is accumulated in all mineral soils by four
paths:
1. growth and eventual death and decay of plant roots,
which are especially strong under grasslands;
2. fall of plant (leaves, branches, and bark), and/or animal
(urine, dung, and carcase) remains, to the soil surface
with consequent decay and incorporation by soil animals;
3. chemicals released from roots as ‘root exudates’, for
example, organic compounds that inhibit competing
organisms; and
4. soluble organic chemicals, such as polyphenols, washed
by rainwater from leaves and bark and channelled into
the soil (as leachate) in the leaching water.
Paths 3 and 4 may only contribute small quantities of
organic matter to the soil, but the compounds may be highly
active as soil-forming agents. For example, polyphenols,
washed from leaves and bark, act as potent chelating agents
carrying iron, aluminium, and phosphorus ions from
near-surface horizons, and releasing them deeper in the soil
(see Podzol Soils, Chap. 11).
Many Recent Soils have a weakly developed B horizon
where oxidation of iron-bearing minerals has led to the
accumulation of enough rust-coloured iron oxide material,
often goethite, to change the soil material from the colour of
the parent material to a warmer brown colour (a process
sometimes called brunification or rubification). Often the
iron oxides just form a thin coat over the rock particles or
mineral grains.
Fluvial Recent Soils are formed on river floodplains by
deposition of sediment from floods. The soil profile often
provides clues about the nature and frequency of past floods
that have delivered the sediment. Understanding the sediment history is useful to support the management of flood
hazards to life, crops, and infrastructure. Excellent examples
are provided in the detailed soil survey reports of Alan Pullar
in the Rangitaiki Plains (eastern Bay of Plenty) and in the
Gisborne Plains.
Where alluvial sediments have been deposited by repeated floods, then the silt and sand layers may be visible as
fine strata in the subsoil (e.g. Fig. 14.1). Thick layers of
material indicate floods with higher sediment loads. Layers
that include gravel-sized (or coarser) particles indicate
floods, or former river beds, that had high energy (high water
velocity and turbulence) to carry the larger material. On the
positive side, sediments can add fresh soil material to build
soil fertility.
Observing the thickness of the topsoil and any subsoil
layers darkened by humus can give insights into the frequency, and amount, of material laid down in, flood or
tephra deposition events (Fig. 14.4). Dark layers in the
profile are usually the topsoil of a buried soil. Generally, the
more strongly developed (thicker and darker coloured) the
buried soil horizons, the longer they must have had to
develop before the addition of new material cut them off
from near-surface soil processes.
The Rotomahana Mud, on which the uppermost horizons
are formed to a depth of 35 cm (in Fig. 14.4), provides an
example of how rapidly a soil may develop from a Raw Soil
to a Recent Soil. The A horizon has a clay content of up to
30%, which would normally indicate a period of considerable
weathering and clay synthesis. However, in this case, the clay
was ‘pre-weathered’ hydrothermal material, derived from
land near (pre-1886) Lake Rotomahana that was violently
engulfed and redeposited during the Tarawera eruption.
Vegetation was rapidly established on the fine material and
under the relatively cool, moist, climate organic matter was
able to accumulate to form an A horizon in about 100 years.
Soil surveyors have recognised three kinds of Fluvial or
Tephric Recent Soils: (a) non-accumulating Recent Soils
with no evidence of younger sediments or buried soil horizons at or near the soil surface; (b) rapidly accumulating
Recent Soils with evidence of buried topsoils or fresh sediment or thicker than normal topsoils at the soil surface; and
(c) an intermediate category of moderately accumulating
Recent Soils. The last two cases (b and c) exemplify retardant and developmental upbuilding pedogenesis (depending
on thickness and frequency of deposits).
14.2 Soil Profile Genesis
219
regular deposition of sediment by flooding, or tephra-fall,
every few decades or centuries, means that many Recent
Soils are formed on multi-layered deposits through
upbuilding pedogenesis, with the land surface slowly rising
at a rate that depends on the frequency and thickness of the
fresh geologic additions to the profile. Because soil weathering in Recent Soils is always in an early stage, and horizon
transformation is restricted to the uppermost part of the
profile, the character of the unaltered parent material has a
strong influence on the soil’s overall properties.
Regardless of the parent material and its origins, the key
process in Recent Soil development is the formation of an A
horizon by the input, and continuing accumulation, of soil
organic matter (a process known as melanisation). In more
developed Recent Soils, along with most other soil orders,
organic matter accumulation is balanced by decomposition
(humification) providing a quasi-equilibrium in soil organic
matter content.
Organic matter is accumulated in all mineral soils by four
paths:
1. growth and eventual death and decay of plant roots,
which are especially strong under grasslands;
2. fall of plant (leaves, branches, and bark), and/or animal
(urine, dung, and carcase) remains, to the soil surface
with consequent decay and incorporation by soil animals;
3. chemicals released from roots as ‘root exudates’, for
example, organic compounds that inhibit competing
organisms; and
4. soluble organic chemicals, such as polyphenols, washed
by rainwater from leaves and bark and channelled into
the soil (as leachate) in the leaching water.
Paths 3 and 4 may only contribute small quantities of
organic matter to the soil, but the compounds may be highly
active as soil-forming agents. For example, polyphenols,
washed from leaves and bark, act as potent chelating agents
carrying iron, aluminium, and phosphorus ions from
near-surface horizons, and releasing them deeper in the soil
(see Podzol Soils, Chap. 11).
Many Recent Soils have a weakly developed B horizon
where oxidation of iron-bearing minerals has led to the
accumulation of enough rust-coloured iron oxide material,
often goethite, to change the soil material from the colour of
the parent material to a warmer brown colour (a process
sometimes called brunification or rubification). Often the
iron oxides just form a thin coat over the rock particles or
mineral grains.
Fluvial Recent Soils are formed on river floodplains by
deposition of sediment from floods. The soil profile often
provides clues about the nature and frequency of past floods
that have delivered the sediment. Understanding the sediment history is useful to support the management of flood
hazards to life, crops, and infrastructure. Excellent examples
are provided in the detailed soil survey reports of Alan Pullar
in the Rangitaiki Plains (eastern Bay of Plenty) and in the
Gisborne Plains.
Where alluvial sediments have been deposited by repeated floods, then the silt and sand layers may be visible as
fine strata in the subsoil (e.g. Fig. 14.1). Thick layers of
material indicate floods with higher sediment loads. Layers
that include gravel-sized (or coarser) particles indicate
floods, or former river beds, that had high energy (high water
velocity and turbulence) to carry the larger material. On the
positive side, sediments can add fresh soil material to build
soil fertility.
Observing the thickness of the topsoil and any subsoil
layers darkened by humus can give insights into the frequency, and amount, of material laid down in, flood or
tephra deposition events (Fig. 14.4). Dark layers in the
profile are usually the topsoil of a buried soil. Generally, the
more strongly developed (thicker and darker coloured) the
buried soil horizons, the longer they must have had to
develop before the addition of new material cut them off
from near-surface soil processes.
The Rotomahana Mud, on which the uppermost horizons
are formed to a depth of 35 cm (in Fig. 14.4), provides an
example of how rapidly a soil may develop from a Raw Soil
to a Recent Soil. The A horizon has a clay content of up to
30%, which would normally indicate a period of considerable
weathering and clay synthesis. However, in this case, the clay
was ‘pre-weathered’ hydrothermal material, derived from
land near (pre-1886) Lake Rotomahana that was violently
engulfed and redeposited during the Tarawera eruption.
Vegetation was rapidly established on the fine material and
under the relatively cool, moist, climate organic matter was
able to accumulate to form an A horizon in about 100 years.
Soil surveyors have recognised three kinds of Fluvial or
Tephric Recent Soils: (a) non-accumulating Recent Soils
with no evidence of younger sediments or buried soil horizons at or near the soil surface; (b) rapidly accumulating
Recent Soils with evidence of buried topsoils or fresh sediment or thicker than normal topsoils at the soil surface; and
(c) an intermediate category of moderately accumulating
Recent Soils. The last two cases (b and c) exemplify retardant and developmental upbuilding pedogenesis (depending
on thickness and frequency of deposits).
14.2 Soil Profile Genesis
219