on tephras. This because the glass-rich tephra parent materials that weather to allophane may also weather to halloysite
in less well drained soils (Sect. 2.2.2).
The parent materials of Allophanic Soils are mostly andesitic and/or rhyolitic tephras, with limited occurrences on
basaltic tephras (chiefly scoria). Allophanic Soil materials
may also form from indurated greywacke or schist rocks, or
their derivatives such as loess or alluvium, but the thickness
of the resulting allophanic soil material is usually too thin to
enable the soils to qualify as Allophanic Soils. Such soils
invariably have good drainage and are formed under precipitation sufficiently high to cause leaching and removal of
silicon. They are generally recognised as Allophanic Brown
Soils. The rock minerals most likely to weather to allophane
are volcanic glass and feldspars, and sometimes minerals
with a composition similar to kaolinite with a molar ratio of
Al:Si = 2. Rapidly weatherable volcanic glass and feldspars
dominate the sand fractions of soils in igneous (including
volcanic) parent materials and are the primary source of the
nanocrystalline minerals. Plagioclase feldspars are the most
likely primary source of allophanic soil material in
non-igneous parent materials.
2.4.2 Physical Properties
The physical properties of Allophanic Soils (Fig. 2.7,
Table 2.1) are excellent for supporting plant growth. Allophanic soil materials have exceptionally low soil dry bulk
densities (around 0.7 t m
−3 on average, potentially as low as
*0.5 t m
−3 ) and generally high to very high
macro-porosities. Thus most Allophanic Soils are well
drained as water passes readily through the highly permeable
soil. However, where there are slowly permeable subsoils,
mottled Bw(f) or B(g) horizons may occur lower in the soil,
in which case the soils would be regarded as moderately well
or imperfectly drained. In such cases, allophane predominates in upper profiles but may be subservient to halloysite,
or absent, lower in the profiles.
The clay contents of allophanic soil materials are generally reported as being *10–25%. However, particle size
measurement is difficult because the clay particles are tightly
aggregated (hence clay-sized material is underestimated and
sand-sized material overestimated) and so the ‘true’ or
‘primary clay content’ may be considerably higher than the
reported measurement. Also, irreversible changes in allophane clay aggregation may occur when soils are air dried,
hence allophanic soil materials should not be dried prior to
measurement of soil particle size. However, Brent Alloway
and colleagues reported a method for soils in Taranaki that
circumvented these problems of measurement. Allophanic
soil materials were dissolved using acidified ammonium
oxalate (enabling total clay content to be estimated
gravimetrically) and the particle-size distribution of the
inorganic residuum (sand and silt) was then able to be
measured free of clay aggregates.
Although Allophanic Soils are well drained they also
have particularly high soil-water retention. Plant roots move
readily through the friable, low penetration resistance, soil
and, for some plants such as kiwifruit, roots may extend to
depths of several metres. The high water-holding capacity
and deep potential rooting depths mean Allophanic Soils
have some of the greatest plant-available moisture storage of
any mineral soil.
Allophanic Soils have stable topsoils that resist puddling
or pugging under the impact of machinery or grazing animals in wet weather. The soil is readily dug and samples
crumble easily when crushed in the hand and are very friable. Drying may lead to changes in soil structure so
rewetting may not achieve the original volume.
Allophane or allophane-like soil clay minerals are
strongly expressed in the field-observed properties. The
minerals coat the sand and silt grains and maintain a porous,
low-density, soil structure with weak strength. The most
tactile property is the distinctly greasy feel when moistened
and rubbed firmly between the fingers. Sensitivity, an
important physical property in Allophanic Soils, is the loss
of soil strength that occurs when a soil sample is remoulded
by applying heavy pressure, shear, or mixing (a related
property is thixotopy). The sensitivity is the origin of the
distinctive ‘greasy’ feel the soil material has when moistened
and rubbed between the fingers, sometimes exuding droplets
of water in the process.
2.4.3 Chemical Properties
The most outstanding soil chemical property of Allophanic
Soils is their ability to adsorb anions (particularly phosphate
and sulphide) as evidenced by their very high anion storage
capacity or P retention (Fig. 2.8, Table 2.2) which may be as
high as 99%. Thus Allophanic Soils will retain a significant
proportion of applied fertiliser phosphorus in a form
unavailable to plants. Roger Parfitt and collaborators estimated that in 2001/02 about 11 kg ha
−1 of phosphorus was
gained in Waikato and Bay of Plenty topsoils that have high
P retention. The ongoing accumulation means that in areas
where fertiliser has been regularly used, some tonnes of P
are now ‘locked away’ in Allophanic Soils in a form not
easily available to plants.
Allophanic Soils provide excellent plant growing conditions due to their high water holding capacity and good
physical properties, thus supporting production of organic
matter. Organic matter is readily produced, and predominantly stored, in the topsoil. Moderate amounts are also
stored in buried (former) topsoil horizons (paleosols) within
2.4 Key Soil Properties
31
in less well drained soils (Sect. 2.2.2).
The parent materials of Allophanic Soils are mostly andesitic and/or rhyolitic tephras, with limited occurrences on
basaltic tephras (chiefly scoria). Allophanic Soil materials
may also form from indurated greywacke or schist rocks, or
their derivatives such as loess or alluvium, but the thickness
of the resulting allophanic soil material is usually too thin to
enable the soils to qualify as Allophanic Soils. Such soils
invariably have good drainage and are formed under precipitation sufficiently high to cause leaching and removal of
silicon. They are generally recognised as Allophanic Brown
Soils. The rock minerals most likely to weather to allophane
are volcanic glass and feldspars, and sometimes minerals
with a composition similar to kaolinite with a molar ratio of
Al:Si = 2. Rapidly weatherable volcanic glass and feldspars
dominate the sand fractions of soils in igneous (including
volcanic) parent materials and are the primary source of the
nanocrystalline minerals. Plagioclase feldspars are the most
likely primary source of allophanic soil material in
non-igneous parent materials.
2.4.2 Physical Properties
The physical properties of Allophanic Soils (Fig. 2.7,
Table 2.1) are excellent for supporting plant growth. Allophanic soil materials have exceptionally low soil dry bulk
densities (around 0.7 t m
−3 on average, potentially as low as
*0.5 t m
−3 ) and generally high to very high
macro-porosities. Thus most Allophanic Soils are well
drained as water passes readily through the highly permeable
soil. However, where there are slowly permeable subsoils,
mottled Bw(f) or B(g) horizons may occur lower in the soil,
in which case the soils would be regarded as moderately well
or imperfectly drained. In such cases, allophane predominates in upper profiles but may be subservient to halloysite,
or absent, lower in the profiles.
The clay contents of allophanic soil materials are generally reported as being *10–25%. However, particle size
measurement is difficult because the clay particles are tightly
aggregated (hence clay-sized material is underestimated and
sand-sized material overestimated) and so the ‘true’ or
‘primary clay content’ may be considerably higher than the
reported measurement. Also, irreversible changes in allophane clay aggregation may occur when soils are air dried,
hence allophanic soil materials should not be dried prior to
measurement of soil particle size. However, Brent Alloway
and colleagues reported a method for soils in Taranaki that
circumvented these problems of measurement. Allophanic
soil materials were dissolved using acidified ammonium
oxalate (enabling total clay content to be estimated
gravimetrically) and the particle-size distribution of the
inorganic residuum (sand and silt) was then able to be
measured free of clay aggregates.
Although Allophanic Soils are well drained they also
have particularly high soil-water retention. Plant roots move
readily through the friable, low penetration resistance, soil
and, for some plants such as kiwifruit, roots may extend to
depths of several metres. The high water-holding capacity
and deep potential rooting depths mean Allophanic Soils
have some of the greatest plant-available moisture storage of
any mineral soil.
Allophanic Soils have stable topsoils that resist puddling
or pugging under the impact of machinery or grazing animals in wet weather. The soil is readily dug and samples
crumble easily when crushed in the hand and are very friable. Drying may lead to changes in soil structure so
rewetting may not achieve the original volume.
Allophane or allophane-like soil clay minerals are
strongly expressed in the field-observed properties. The
minerals coat the sand and silt grains and maintain a porous,
low-density, soil structure with weak strength. The most
tactile property is the distinctly greasy feel when moistened
and rubbed firmly between the fingers. Sensitivity, an
important physical property in Allophanic Soils, is the loss
of soil strength that occurs when a soil sample is remoulded
by applying heavy pressure, shear, or mixing (a related
property is thixotopy). The sensitivity is the origin of the
distinctive ‘greasy’ feel the soil material has when moistened
and rubbed between the fingers, sometimes exuding droplets
of water in the process.
2.4.3 Chemical Properties
The most outstanding soil chemical property of Allophanic
Soils is their ability to adsorb anions (particularly phosphate
and sulphide) as evidenced by their very high anion storage
capacity or P retention (Fig. 2.8, Table 2.2) which may be as
high as 99%. Thus Allophanic Soils will retain a significant
proportion of applied fertiliser phosphorus in a form
unavailable to plants. Roger Parfitt and collaborators estimated that in 2001/02 about 11 kg ha
−1 of phosphorus was
gained in Waikato and Bay of Plenty topsoils that have high
P retention. The ongoing accumulation means that in areas
where fertiliser has been regularly used, some tonnes of P
are now ‘locked away’ in Allophanic Soils in a form not
easily available to plants.
Allophanic Soils provide excellent plant growing conditions due to their high water holding capacity and good
physical properties, thus supporting production of organic
matter. Organic matter is readily produced, and predominantly stored, in the topsoil. Moderate amounts are also
stored in buried (former) topsoil horizons (paleosols) within
2.4 Key Soil Properties
31