which are usually on terraces or fans elevated high in the
landscape. The age is probably not significant in terms of
fertility but it is relevant to the root environment. With age,
the clay is reddened and, in many sites, it clogs the subsoil
soil pores. This may limit the potential for roots to explore
the deeper soil for water and nutrients. Under irrigation,
there may be a risk of waterlogging in the subsoil.
Argillic Semiarid Soils have accumulations of clay in the
subsoil but, unlike Aged-argillic soils, the clay may not affect
the potential root depth and may boost the water holding
capacity. They are valued soils, especially on fan landforms
where deep schist alluvium provides good nutrients, rooting
conditions, and high water-holding capacity that facilitates
irrigation management. On fan soils, it is important to avoid
over-watering as drainage will run laterally, channelled by
the soil layers down the fan where it may cause waterlogging
or salt accumulation at the foot or toe of the fan.
The Immature Semiarid Soils have simple profiles but
have a wide range of soil materials and textures. Rapidly
permeable gravelly soils, and some strongly structured
(prismatic) soils, pose a risk of nutrient losses to water
tables, with consequences for water quality in lakes and
rivers. As for the other soils, droughty conditions usually
extend from spring through summer and autumn and so
irrigation is required for most crops. The stony profile is
poor for most crops not only because of the low rainfall but
also because the abundant gravels reduce the amount of fine
soil available to trap any rain that does fall. There is little
stored water for roots to tap into.
Stone fruit orcharding thrives on the Immature Semiarid
Soils of the Earnscleugh flats and Cromwell area (Molyneux
soils). The soils are gravelly sands and have not been
regarded as versatile soils because the stones/gravels limit
the profile available water capacity. However, the gaps
between the stones are important. They are filled with sand
material that enables the soil to drain rapidly. Stone fruit
blossom can be severely damaged by heavy frosts that spoil
the crops. An efficient method of counter-acting frost is to
spray the blossoming trees with water that coats the flowers
with ice, which provides insulation and the latent heat
released by water freezing is enough to prevent frost damage
(Fig. 15.11). On many soils, the large volume of
frost-fighting water accumulates and the soil becomes
waterlogged with consequent increased risk of Phytophthora
infection. However, the rapidly draining Molyneux soils
circumvent this risk. The Molyneux soils may not be highly
versatile but they are recognised as good specialist soils for
stone fruit production in frost-prone land where well monitored and managed micro-irrigation can keep the trees with
near-ideal soil moisture conditions.
The Solonetzic Semiarid Soils occur only as small patches and are infrequent enough that it is generally worth
avoiding developing them for productive use in order to
protect their rare native ecosystems (unique to the inland
basins of New Zealand). The Solonetzic Semiarid Soils
could, however, be rehabilitated for productive use with the
installation of drainage, application of calcium (gypsum) to
displace the sodium, and careful use of quality irrigation
water to displace the salts.
There has been a rapid expansion in growing grapes for
wine on Semiarid Soils since the 1990s (Fig. 15.9). A range
of soils are used and the dry climate means that it is relatively easy to control the water available to the crop to
ensure maximum quality grape and wine production.
The risk of bypass flow is high where there are permeable
or well-structured prismatic or columnar subsoils where
drainage water is channelled down the fissures, largely
avoiding the mass of the soil within structural units. Bypass
flow is a significant transport mechanism for water-borne
contaminants or nutrients. However, in this dry environment,
bypass flow can be prevented by avoiding soil saturation by
ensuring that irrigation is at rates slower than the saturated
hydraulic conductivity, and also by making sure water does
not pond on the soil surface. Thus, the old flood irrigation
systems should be avoided to prevent water and associated
contaminant loss to the groundwater.
Semiarid Soils have high slaking and dispersion potential.
Topsoil structures may break down under prolonged impact
by heavy machinery or by continuous tillage and, like most
soils, such risks are higher if they are wet. Weak soil
structures render the soils susceptible to erosion, especially
when exposed to water runoff or wind erosion aggravated
after particles are separated by frost-heave. Managing soils
to increase soil organic matter can help improve soil structural stability.
Many of the most intensively used soils, on the fans and
terraces, have low contents of clay and organic matter, and
thus, very low cation exchange and pH buffering capacities,
along with low oxalate-extractable iron and aluminium,
phosphate retention, absorbed sulphate, and aggregate stabilities. Gary Beecroft and Don Brash documented some of
the effects of low buffering capacity to alert land managers to
potential risks in managing low-buffering soils. They
included (1) lowering of soil pH due to application of
ammonium sulphate; (2) rapid topsoil compaction where
water is used to counter spring frosts in stone fruit orchards;
(3) herbicide penetration of topsoils in orchards affecting tree
roots; (4) serious wind erosion in topsoils under cultivation;
and (5) the development of Gley Soil features (such as
mottles and low-chroma colours indicative of prolonged
saturation) reported in Semiarid Soils under 50 years of
irrigation in Bannockburn.
When transferring management techniques from other
regions, it is important to be sure that soils at the place of
origin are comparable with those of the transfer site.
Although low buffering is a feature of Semiarid Soils, it is by
15.7 Use and Management of Semiarid Soils
243
landscape. The age is probably not significant in terms of
fertility but it is relevant to the root environment. With age,
the clay is reddened and, in many sites, it clogs the subsoil
soil pores. This may limit the potential for roots to explore
the deeper soil for water and nutrients. Under irrigation,
there may be a risk of waterlogging in the subsoil.
Argillic Semiarid Soils have accumulations of clay in the
subsoil but, unlike Aged-argillic soils, the clay may not affect
the potential root depth and may boost the water holding
capacity. They are valued soils, especially on fan landforms
where deep schist alluvium provides good nutrients, rooting
conditions, and high water-holding capacity that facilitates
irrigation management. On fan soils, it is important to avoid
over-watering as drainage will run laterally, channelled by
the soil layers down the fan where it may cause waterlogging
or salt accumulation at the foot or toe of the fan.
The Immature Semiarid Soils have simple profiles but
have a wide range of soil materials and textures. Rapidly
permeable gravelly soils, and some strongly structured
(prismatic) soils, pose a risk of nutrient losses to water
tables, with consequences for water quality in lakes and
rivers. As for the other soils, droughty conditions usually
extend from spring through summer and autumn and so
irrigation is required for most crops. The stony profile is
poor for most crops not only because of the low rainfall but
also because the abundant gravels reduce the amount of fine
soil available to trap any rain that does fall. There is little
stored water for roots to tap into.
Stone fruit orcharding thrives on the Immature Semiarid
Soils of the Earnscleugh flats and Cromwell area (Molyneux
soils). The soils are gravelly sands and have not been
regarded as versatile soils because the stones/gravels limit
the profile available water capacity. However, the gaps
between the stones are important. They are filled with sand
material that enables the soil to drain rapidly. Stone fruit
blossom can be severely damaged by heavy frosts that spoil
the crops. An efficient method of counter-acting frost is to
spray the blossoming trees with water that coats the flowers
with ice, which provides insulation and the latent heat
released by water freezing is enough to prevent frost damage
(Fig. 15.11). On many soils, the large volume of
frost-fighting water accumulates and the soil becomes
waterlogged with consequent increased risk of Phytophthora
infection. However, the rapidly draining Molyneux soils
circumvent this risk. The Molyneux soils may not be highly
versatile but they are recognised as good specialist soils for
stone fruit production in frost-prone land where well monitored and managed micro-irrigation can keep the trees with
near-ideal soil moisture conditions.
The Solonetzic Semiarid Soils occur only as small patches and are infrequent enough that it is generally worth
avoiding developing them for productive use in order to
protect their rare native ecosystems (unique to the inland
basins of New Zealand). The Solonetzic Semiarid Soils
could, however, be rehabilitated for productive use with the
installation of drainage, application of calcium (gypsum) to
displace the sodium, and careful use of quality irrigation
water to displace the salts.
There has been a rapid expansion in growing grapes for
wine on Semiarid Soils since the 1990s (Fig. 15.9). A range
of soils are used and the dry climate means that it is relatively easy to control the water available to the crop to
ensure maximum quality grape and wine production.
The risk of bypass flow is high where there are permeable
or well-structured prismatic or columnar subsoils where
drainage water is channelled down the fissures, largely
avoiding the mass of the soil within structural units. Bypass
flow is a significant transport mechanism for water-borne
contaminants or nutrients. However, in this dry environment,
bypass flow can be prevented by avoiding soil saturation by
ensuring that irrigation is at rates slower than the saturated
hydraulic conductivity, and also by making sure water does
not pond on the soil surface. Thus, the old flood irrigation
systems should be avoided to prevent water and associated
contaminant loss to the groundwater.
Semiarid Soils have high slaking and dispersion potential.
Topsoil structures may break down under prolonged impact
by heavy machinery or by continuous tillage and, like most
soils, such risks are higher if they are wet. Weak soil
structures render the soils susceptible to erosion, especially
when exposed to water runoff or wind erosion aggravated
after particles are separated by frost-heave. Managing soils
to increase soil organic matter can help improve soil structural stability.
Many of the most intensively used soils, on the fans and
terraces, have low contents of clay and organic matter, and
thus, very low cation exchange and pH buffering capacities,
along with low oxalate-extractable iron and aluminium,
phosphate retention, absorbed sulphate, and aggregate stabilities. Gary Beecroft and Don Brash documented some of
the effects of low buffering capacity to alert land managers to
potential risks in managing low-buffering soils. They
included (1) lowering of soil pH due to application of
ammonium sulphate; (2) rapid topsoil compaction where
water is used to counter spring frosts in stone fruit orchards;
(3) herbicide penetration of topsoils in orchards affecting tree
roots; (4) serious wind erosion in topsoils under cultivation;
and (5) the development of Gley Soil features (such as
mottles and low-chroma colours indicative of prolonged
saturation) reported in Semiarid Soils under 50 years of
irrigation in Bannockburn.
When transferring management techniques from other
regions, it is important to be sure that soils at the place of
origin are comparable with those of the transfer site.
Although low buffering is a feature of Semiarid Soils, it is by
15.7 Use and Management of Semiarid Soils
243