Solonetzic Semiarid Soils—strongly developed argillic
horizons and high proportion of sodium on clay surfaces in
subsoils.
Soil subgroups, defined by the New Zealand Soil Classification, recognise soils that are related to other soil orders
or soil groups (Mottled), or soils that depart in some specific
way from the soil group or soil order (Weathered, Alkaline,
Thick, Saline, or Laminar). The central concept of the soil
group is the Typic subgroup.
The New Zealand Soil Classification expresses the
varying levels of salinity at the subgroup level by identifying
saline soils (with an electrical conductivity >0.8 mS cm
−1 ),
and alkaline soils (pH 8.6). Sodic soils (where sodium
ions dominate the cation exchange), with subsoil argillic
horizons that have dark-coloured coatings (dispersed soil
organic matter with clay), are recognised in the Solonetzic
group.
15.1.4 Origin of the Soil Order Name
‘Semiarid Soils’ are named for the semiarid climate which is
the dominant soil-forming factor that determines their
occurrence. However, Semiarid Soils are not directly identified by climate but by a number of soil properties that are
climate-related, including subsoil salt accumulation and low
organic matter contents in topsoils.
15.2 Soil Profile Genesis
15.2.1 Introduction
The formation of Semiarid Soils is dominated by the effects
of the climate, with a water deficit in most months of the
year (Fig. 1.4), hot summers, and cool winters. The lack of
water influences pedogenic processes (slowing weathering
and supporting salt accumulation) and restricts plant growth
(thus limiting the organic matter available to accumulate in
the soil). The major soil-forming processes that influence the
form of Semiarid Soils are the movement and accumulation
of salts and clay within the soil. With increasing age, the
soils form sequences that show increases in both salt and
clay accumulation. Many of the land surfaces in the dry
inland basins are old and so there has been time for advanced
soil development to occur. The parent materials are predominantly quartzo-feldspathic in origin, and include alluvium derived from local schist deposits and from greywacke
gravels carried from the mountain ranges and deposited in
the river terraces by the Clutha, Manuherikia, Taieri, and
other rivers. Locally derived colluvium, and loess blown off
river flood plains, also form soil parent materials. Thus, it is
likely that many of the Semiarid Soils reflect upbuilding
pedogenesis whereby new geological materials, such as
loess, are added to stable land surfaces, while at the same
time pedogenic processes are actively modifying the parent
materials. McCraw recognised that soils on high terraces and
flatter parts of hills in the area typically included a mantle of
loess which had accumulated incrementally, at a very low
rate, over thousands of years. Thus, pedogenesis would
continue (albeit at slow rates in the semiarid climate) unabated by the ongoing loess inputs.
15.2.2 Argillic Horizon Development
A characteristic of Semiarid Soils is the accumulation of clay
in the subsoil (forming argillic, or illuvial clay-enriched,
horizons) and the eventual reddening of the argillic horizons
over time. In the Clutha and Kawarau river valleys, the age
of river terraces and fan surfaces increases with height above
the river level. In the Clutha River valley, Michael Leamy
used reasoning similar to that outlined for the Rangitikei
River sequence (Chapter 10) to describe a chronosequence
of Semiarid Soils. The chronosequence displays the major
pedogenic changes over time, and the range of soils evident
in the Central Otago landscape. The chronosequence ranges
from an Immature Semiarid Soil through a Saline Argillic
Semiarid Soil and a Typic Argillic Semiarid Soil to a
Weathered Aged-argillic Semiarid Soil (Fig. 15.4).
The Immature Semiarid Soil, the youngest in the
sequence (although there are younger, Raw and Recent
Soils, on lowermost terraces and modern floodplains), has no
argillic horizon but subtle dark staining of the gravels
described (misleadingly) as an “incipient clay pan”. The
staining is probably organic matter mobilised under the
influence of weak sodium accumulation (with little or no
illuvial clay accumulated at this stage of soil development).
A salt-rich calcareous horizon (Ck) lies beneath the Bw.
The Saline Argillic Semiarid Soil, second in the
sequence, has a weakly developed argillic horizon stained by
soil organic matter, described as a brown clay pan, and
underlain by a calcareous horizon. The Typic Argillic
Semiarid Soil is third in the sequence with a red argillic
horizon described as a thin red clay pan, and underlain by a
calcareous horizon.
The Weathered Aged-argillic Semiarid Soil is fourth in
the sequence with a red argillic horizon identified by Leamy
as a thick red clay pan. It is inferred that the Weathered
Aged-argillic Semiarid Soil has developed through each of
the preceding three stages. If true, then the calcareous
horizon must have formed in the earlier stages and then been
subsequently removed by leaching. A feature of the materials of the first three stages of soils is that the spaces
between the stones and gravel are infilled with permeable
234
15 Semiarid Soils
horizons and high proportion of sodium on clay surfaces in
subsoils.
Soil subgroups, defined by the New Zealand Soil Classification, recognise soils that are related to other soil orders
or soil groups (Mottled), or soils that depart in some specific
way from the soil group or soil order (Weathered, Alkaline,
Thick, Saline, or Laminar). The central concept of the soil
group is the Typic subgroup.
The New Zealand Soil Classification expresses the
varying levels of salinity at the subgroup level by identifying
saline soils (with an electrical conductivity >0.8 mS cm
−1 ),
and alkaline soils (pH 8.6). Sodic soils (where sodium
ions dominate the cation exchange), with subsoil argillic
horizons that have dark-coloured coatings (dispersed soil
organic matter with clay), are recognised in the Solonetzic
group.
15.1.4 Origin of the Soil Order Name
‘Semiarid Soils’ are named for the semiarid climate which is
the dominant soil-forming factor that determines their
occurrence. However, Semiarid Soils are not directly identified by climate but by a number of soil properties that are
climate-related, including subsoil salt accumulation and low
organic matter contents in topsoils.
15.2 Soil Profile Genesis
15.2.1 Introduction
The formation of Semiarid Soils is dominated by the effects
of the climate, with a water deficit in most months of the
year (Fig. 1.4), hot summers, and cool winters. The lack of
water influences pedogenic processes (slowing weathering
and supporting salt accumulation) and restricts plant growth
(thus limiting the organic matter available to accumulate in
the soil). The major soil-forming processes that influence the
form of Semiarid Soils are the movement and accumulation
of salts and clay within the soil. With increasing age, the
soils form sequences that show increases in both salt and
clay accumulation. Many of the land surfaces in the dry
inland basins are old and so there has been time for advanced
soil development to occur. The parent materials are predominantly quartzo-feldspathic in origin, and include alluvium derived from local schist deposits and from greywacke
gravels carried from the mountain ranges and deposited in
the river terraces by the Clutha, Manuherikia, Taieri, and
other rivers. Locally derived colluvium, and loess blown off
river flood plains, also form soil parent materials. Thus, it is
likely that many of the Semiarid Soils reflect upbuilding
pedogenesis whereby new geological materials, such as
loess, are added to stable land surfaces, while at the same
time pedogenic processes are actively modifying the parent
materials. McCraw recognised that soils on high terraces and
flatter parts of hills in the area typically included a mantle of
loess which had accumulated incrementally, at a very low
rate, over thousands of years. Thus, pedogenesis would
continue (albeit at slow rates in the semiarid climate) unabated by the ongoing loess inputs.
15.2.2 Argillic Horizon Development
A characteristic of Semiarid Soils is the accumulation of clay
in the subsoil (forming argillic, or illuvial clay-enriched,
horizons) and the eventual reddening of the argillic horizons
over time. In the Clutha and Kawarau river valleys, the age
of river terraces and fan surfaces increases with height above
the river level. In the Clutha River valley, Michael Leamy
used reasoning similar to that outlined for the Rangitikei
River sequence (Chapter 10) to describe a chronosequence
of Semiarid Soils. The chronosequence displays the major
pedogenic changes over time, and the range of soils evident
in the Central Otago landscape. The chronosequence ranges
from an Immature Semiarid Soil through a Saline Argillic
Semiarid Soil and a Typic Argillic Semiarid Soil to a
Weathered Aged-argillic Semiarid Soil (Fig. 15.4).
The Immature Semiarid Soil, the youngest in the
sequence (although there are younger, Raw and Recent
Soils, on lowermost terraces and modern floodplains), has no
argillic horizon but subtle dark staining of the gravels
described (misleadingly) as an “incipient clay pan”. The
staining is probably organic matter mobilised under the
influence of weak sodium accumulation (with little or no
illuvial clay accumulated at this stage of soil development).
A salt-rich calcareous horizon (Ck) lies beneath the Bw.
The Saline Argillic Semiarid Soil, second in the
sequence, has a weakly developed argillic horizon stained by
soil organic matter, described as a brown clay pan, and
underlain by a calcareous horizon. The Typic Argillic
Semiarid Soil is third in the sequence with a red argillic
horizon described as a thin red clay pan, and underlain by a
calcareous horizon.
The Weathered Aged-argillic Semiarid Soil is fourth in
the sequence with a red argillic horizon identified by Leamy
as a thick red clay pan. It is inferred that the Weathered
Aged-argillic Semiarid Soil has developed through each of
the preceding three stages. If true, then the calcareous
horizon must have formed in the earlier stages and then been
subsequently removed by leaching. A feature of the materials of the first three stages of soils is that the spaces
between the stones and gravel are infilled with permeable
234
15 Semiarid Soils